Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/people/Baerbel.Koribalski/min_lecture/min_lecture_final.ps.gz Äàòà èçìåíåíèÿ: Sun Nov 18 00:32:12 2001 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 17:14:29 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: arp 220

The Relation between the Large­scale Gas Dynamics,
Nuclear Kinematics, and Activity in Spiral Galaxies
B¨arbel Koribalski
Australia Telescope National Facility (CSIRO), P.O.Box 76, Epping,
NSW 2121, Australia
Abstract. Most of the spirals discussed in the following are well­known
starburst galaxies (M 82, NGC 253, etc.). In addition to their nuclear
activity, which makes them so popular, the selected galaxies have several
other phenomena in common, such as bars, gas outflow, and possibly
nuclear rings. The relationship between those phenomena is discussed in
the following. I will show that the gas dynamics in the outer and inner
parts of these galaxies are strongly related to their starburst activity.
Galaxy interactions as well as bars play an important role as they affect
the gas flow within the disk, driving gas inwards and outwards where it
accumulates at preferred resonance orbits. The inflowing gas provides
the supply for the central starburst region from which gas is eventually
ejected into the halo. I compare the group of nearby starburst galaxies
with distant ultra­luminous galaxies (Arp 220, NGC 6240, etc.), in which
strong tidal interactions cause the radial gas inflow. In both groups high­
resolution H i absorption measurements reveal fast­rotating nuclear rings.
1. Introduction
In this lecture I will discuss some aspects of the relation between the large­scale
gas dynamics, nuclear kinematics, and activity in spiral galaxies. The original
idea which led to this project was the possibility of using H i absorption measure­
ments as a tracer of nuclear rings in galaxies. The two keys which may help to
understand the relationship are bars in galaxies and galaxy­galaxy interactions.
A successful approach to this issue involves observations and theoretical N­body
simulations of galaxies which complement each other.
The best tracer for the overall gas distribution and kinematics in galaxies
is the 21­cm emission line of the neutral hydrogen atom (H i). Although the
H i content and global spectrum of a galaxy can be measured with a single­dish
telescope, for this project interferometers such as the Australia Telescope Com­
pact Array (ATCA) 1 in the Southern Hemisphere and the Very Large Array
1
The Australia Telescope Compact Array is part of the Australia Telescope National Facil­
ity (ATNF), a division of the Commonwealth Scientific and Industrial Research Organisation
(CSIRO).
1

2 Koribalski
Heliocentric
Velocity
(km/s)
RIGHT ASCENSION (B1950)
05 06 10 05 00 05 55 50 45
1200
1150
1100
1050
1000
950
900
850
800
2
4
6
8
10
12
Figure 1. Major­axis position­velocity diagram of the starburst
galaxy NGC 1808 (Koribalski, Dickey & Mebold 1993). We assumed
a position angle of 320 ffi (see also Fig. 5 and 6). The contour lev­
els are --4.5, --3, --1.5, 1.5, 3, 4.5, 6, 7.5, and 9 mJybeam \Gamma1 . These
data were taken with the VLA BnC array; the resolution is about
15 00 \Theta 20 00 \Theta 5 km s \Gamma1 . --- Note that both H i emission (solid lines) and
absorption (dotted lines) cover about the same velocity range.

Activity in Spiral Galaxies 3
(VLA) 2 in the Northern Hemisphere are required to map galaxies in detail. For
the brightest galaxies we can also use the H i absorption line to study the gas
motions close to the centre by using the largest configurations of the existing
interferometers. This method has several advantages: 1) at high angular resolu­
tion the H i emission is resolved out leaving absorption only (see Dickey 1979),
2) the H i absorption unambiguously traces the gas in front of the continuum
sources, and 3) the velocity resolution is usually very high (a few km s \Gamma1 ), much
higher than in the studies of optical emission lines. The obvious disadvantage is
that absorption studies are limited to bright galaxies, mostly starburst or other­
wise active galaxies. And even in those galaxies one has to keep in mind that the
absorption depends directly on the distribution and strength of the continuum
sources in the nuclear region.
A description of one of the first interferometric studies of H i absorption
lines is given by Clark, Radhakrishnan & Wilson (1962) and Clark (1965).
2. The Nuclear Region: H i Absorption Measurements
Absorption by neutral atomic hydrogen in luminous spiral galaxies provides an
excellent tool to study the gas kinematics in their nuclear regions. Whereas from
emission studies it is difficult to interpret the observed radial velocities, absorp­
tion measurements have the advantage that the detected lines must come from
the near side of the nucleus. So, red­shifts or blue­shifts relative to the systemic
velocity of the galaxy, v sys , unambiguously show infall or outflow of gas, respec­
tively. Since the absorption lines are intrinsically narrow as a result of the low
temperatures of the optically thick gas, typically 30 to 150 K (Dickey, Salpeter &
Terzian 1979), it is surprising that numerous galaxies show H i absorption over a
broad range of velocities (a few hundred km s \Gamma1 ); see e.g., Fig. 1. Some examples
are found in the absorption surveys by Mirabel (1982) and Mirabel & Sanders
(1988), using the Arecibo telescope, and by Dickey (1982, 1986), obtained with
the large configurations of the VLA.
Interferometric studies of individual bright galaxies, e.g., NGC 253, NGC 660,
NGC 1068, NGC 1365, NGC 1808, M 82, NGC 3079, NGC 3628, NGC 4945,
etc. reveal H i emission and absorption over about the same velocity range. And
many more galaxies can be added to the sample (see Table 1). It is remarkable
that these are all barred galaxies with high central activity and far­infrared (FIR)
luminosities of a few 10 10 L fi (see Tables 2 and 3), often revealing outflow of gas
from the central region. They also frequently contain megamasers and Seyfert
nuclei although the starburst activity usually dominates the total power output.
S'ersic & Pastoriza (1965) had already noted that galaxies with peculiar
nuclei, e.g., NGC 1097, NGC 1365, NGC 1808, and NGC 7552 (most of them
containing `hot spots', i.e., extremely bright H ii regions), are barred (SB or
SAB), strongly indicating a relationship between the nuclei of galaxies and their
whole structure. My main aim is to investigate further the nature of this rela­
tionship.
2
The Very Large Array is a facility of The Very Large Array is a facility of the National Radio
Astronomy Observatory (NRAO), which is operated by Associated Universities, Inc., under
Cooperative agreement with the National Science Foundation.

4 Koribalski
To understand the nuclear kinematics in these galaxies --- which are prob­
ably very much alike --- sensitive high­resolution H i observations need to be
carried out which can at least partly resolve the central continuum. A sum­
mary of our high­resolution ATCA observations is given in Section 3. Section 4
contains a literature review of the sample of galaxies described above, including
some of our new results, and a comparison with strongly interacting galaxies
and others with broad H i absorption lines. As a summary I present a global
model in Section 5.
Table 1. Some properties of the H i absorption and emission lines in
the selected galaxies
Name v abs range \Deltav abs v em range \Deltav em gradient Ref.
[km s \Gamma1 ] [km s \Gamma1 ] [km s \Gamma1 arcsec \Gamma1 ]
NGC 253 75--330 255 75--400 325 (a)
NGC 660 640--1050 410 675--1050 375 87 (b)
NGC 1068 760--1300 540 900--1300 400 (c)
NGC 1365 1473--1786 313 1432--1840 408 (d)
NGC 1808 820--1180 360 810--1190 380 13 (e)
NGC 3034 66--375 309 50--380 330 (f)
NGC 3079 840--1420 580 860--1420 560 500 (g)
NGC 3628 650--1080 430 670--1020 350 27 (h)
NGC 4945 380--740 360 355--750 395 (j)
Circinus 350--550 200 285--590 295 (k)
NGC 6221 1300--1650 350 (l)
NGC 7552 1500--1700 200 1460--1760 300 (l)
NGC 7582 1380--1780 400 1380--1810 430 (l)
Column (1): Galaxy name. Columns (2)--(5): The velocity range observed in H i ab­
sorption and emission, respectively. Column (6): Lower limit to the velocity gradient of
the H i absorption feature, when resolved. Column (7): References. (a) Dickey, Brinks
& Puche 1992; (b) Gottesmann & Mahon 1990; Baan, Rhoads & Haschick 1992; (c)
Gallimore et al. 1994; Brinks et al. 1995; (d) J¨ors¨ater & van Moorsel 1995; (e) Ko­
ribalski, Dickey & Mebold 1993; (f) Weliachew, Fomalont & Greisen 1984; Crutcher,
Rogstad & Chu 1978; (g) Irwin & Seaquist 1991; Baan & Irwin 1995; (h) Schmelz, Baan
& Haschick 1987; Wilding, Alexander & Green 1993; (j) Ables et al. 1987; (k) Koribalski
& Whiteoak 1996; (l) Koribalski, Lavezzi, Dickey & Whiteoak 1996.
2.1. The Puzzle
There are mainly two questions to be addressed: 1) What causes the large ve­
locity width (several hundred km s \Gamma1 ) of the absorption lines detected against
the nuclear regions of galaxies ? 2) Why are the absorption line widths ob­
served against the nuclear radio continuum similar to those measured in emission
over the whole extent of the galaxy ? The problem is illustrated well in Fig. 1
which shows a typical H i position­velocity diagram of a starburst galaxy (here
NGC 1808); both the H i emission and absorption lines cover nearly 400 km s \Gamma1 .

Activity in Spiral Galaxies 5
To address these question one should first examine the conditions for observing
neutral hydrogen gas in absorption 3 : we need cold atomic hydrogen gas (T ¸ !
150 K) in front of a rather strong continuum source. Then, there are mainly
two explanations for the large velocity range observed in absorption. The gas
is either A) in turbulent motion or B) in a regular Keplerian orbit around the
nucleus. Case A) means there are numerous cold gas clouds falling into and
ejected out of the nuclear region. To explain the similar velocity range for emis­
sion and absorption the clouds are required to have maximum velocities similar
to the rotation amplitude of the galaxy. This model is, e.g., strongly favoured by
Mirabel & Sanders (1988) who in their paper conclude that ``... in the central re­
gions of the most luminous infrared galaxies there must be high concentrations of
turbulent atomic gas enshrouding the nuclear radio­continuum source.'' Case B)
requires rather high rotational velocities close to the nucleus and was therefore
often rejected. But now that interferometers and VLBI techniques are used to
resolve the nuclear continuum structure in galaxies those high rotation velocities
have been found in nearly all cases (see Section 4). Most of the large absorption
line widths can be reproduced if the rotation amplitude in the nuclear region is
similar to that in the outer region of the galaxy. This requires either a rather flat
rotation curve or two components with similar amplitude as for example present
in the Galactic rotation curve (Dame et al. 1987). In some cases the inner part
of the rotation curve has to rise toward the nucleus to explain absorption line
widths much larger than the observed velocity range in emission. The extreme
rotational velocities of the nuclear maser emission in NGC 4258 shows what
might be happening in the centre of many starburst galaxies (see Section 4.3).
A more detailed discussion of the two cases is given by Koribalski, Dickey
& Mebold (1993). In several galaxies both high rotational velocities and either
infall or outflow of gas are observed.
3. Observations
In addition to detailed studies of individual galaxies like NGC 253 (Koribal­
ski, Whiteoak & Houghton 1995), NGC 1808 (Koribalski, Dickey & Mebold
1993; Koribalski 1993), NGC 4945 (Ables et al. 1987; Ott 1995), and Circinus
(K. Jones et al. 1996, in prep.; Koribalski & Whiteoak 1996), a snap­shot sur­
vey of the brightest nearby galaxies (F 100¯m ¸ ? 20 Jy; v sys ¸ ! 3000 km s \Gamma1 ) has
been carried out at declinations below \Gamma35 ffi using the largest configuration of
the ATCA (Koribalski et al. 1996). The 20­cm radio continuum emission and
H i were observed simultaneously. Because of the high angular resolution ( ¸ ?6 00 )
the H i emission is resolved out in most cases and only H i absorption against
the strongest background sources is detected. After Hanning smoothing the ve­
locity resolution is 6.6 km s \Gamma1 . A large variety of nuclear structures was found
3 H i absorption studies are used in many other areas of astronomy, e.g., in Galactic work to
determine distances for pulsars, H ii regions, supernova remnants, etc., and spin temperatures
of the interstellar medium (ISM). In extragalactic astronomy, H i and Lyman ff absorption line
measurements against bright quasars can help to uncover the amount and properties of the
gas in the line of sight, often out to very large distances corresponding to the early stages of
galaxy evolution.

6 Koribalski
(multiple sources, such as NGC 1792; double sources, such as NGC 3883; point
sources plus extended emission, such as NGC 2442, etc.) as well as broad H i
absorption lines against the strongest nuclear sources.
To investigate the relationship between the processes occurring in the nu­
clear regions of galaxies and their large­scale dynamics follow­up observations
have been carried out of several of those galaxies showing H i absorption in the
snap­shot survey. For that project the rather compact 375­m configuration (to
pick up faint extended emission, e.g., tidal tails and bridges between neigh­
bouring galaxies) and the intermediate 1.5­km configuration of the ATCA were
chosen; some of the preliminary results are presented here. The angular resolu­
tion varies and is given in each of the figure descriptions whereas the velocity
resolution is the same as before.
4. Galaxies with Broad H i Absorption Lines
4.1. The Group of Nearby Starburst Galaxies
This section describes a group of relatively nearby spiral galaxies showing a
variety of activities. The original selection criterion for this sample was the
presence of broad H i absorption lines --- a quite unusual criterion at first glance,
but a very valuable one in the later interpretation of the overall kinematics in
these galaxies. Such broad absorption lines are quite peculiar, because they
indicate a large range of velocities both blue­ and red­shifted in a very tiny
region just in front of the radio continuum emission. Since such a broad H i
absorption line was detected in NGC 1808 (see below) I wanted to know its
origin and the relation to the surrounding medium in the galaxy.
The galaxies with broad H i absorption lines can be divided into two different
groups, one consisting of relatively nearby luminous spirals (see Tables 1, 2 and
3) and the other of distant ultra­luminous mergers (see Table 4). In the following
I will take a closer look at the members of the first group.
Starbursts are thought to be the result of gas accretion toward the central
regions of galaxies. There are mainly two mechanisms known which can trans­
port mass into this region: either tidal interactions or bars (see e.g., Combes
1988). I suggest that the broad H i absorption lines in the central region of most
luminous spirals (see Table 1) are caused by a fast­rotating ring of cold gas.
The accretion of gas in the inner region (probably near the inner Lindblad reso­
nances) could have been induced by the bar potential of these galaxies (see e.g.,
Combes & Gerin 1985). Since outflow of gas from the central region is another
phenomenon observed in this group of galaxies I propose a model in which the
neutral gas is fuelling the nuclear starburst where it is partly ionized and then
ejected by supernova explosions and stellar winds (see Section 5).
NGC 253 is a nearby edge­on galaxy and the brightest member of the Sculp­
tor group. Its nuclear region is particularly active, revealing numerous compact
radio sources (Antonucci & Ulvestad 1988), as well as bright optical and infrared
emission lines. The strong and extended radio emission makes it especially suit­
able for H i absorption measurements (see e.g., Combes, Gottesman & Weliachew
1977; Dickey, Brinks & Puche 1992; Koribalski, Whiteoak & Houghton 1995).

Activity in Spiral Galaxies 7
Figure 2. (top) H i distribution of the starburst galaxy NGC 253.
Figure 3. (bottom) H i mean velocity field of NGC 253 (for a full
description see Koribalski, Whiteoak & Houghton 1995). The data
were taken with three different arrays of the ATCA (12 h each).

8 Koribalski
Gardner & Whiteoak (1974) also measured broad H 2 CO and OH absorption
lines. NGC 253 reveals a bar which has been detected in the near­infrared
(Scoville et al. 1985; Forbes & DePoy 1992), optical (Pence 1980), and --- on
a much smaller scale --- in the CO emission (Canzian, Mundy & Scoville 1988;
see also Mauersberger et al. 1996). Various authors have also indicated the pos­
Table 2. Some basic parameters of the selected galaxies
Name ff(2000) ffi (2000) Type i v 21
[ h m s ] [ ffi 0 00 ] [ ffi ] [km s \Gamma1 ]
NGC 253 00 47 33.1 --25 17 18 .SXS5.. 78 251
NGC 660 01 43 01.4 +13 38 37 .SBS1P. 65 853
NGC 1068 02 42 40.2 --00 00 48 RSAT3.. 40 1137
NGC 1365 03 33 36.6 --36 08 17 .SBS3.. 60 1662
NGC 1808 05 07 42.8 --37 30 51 RSXS1.. 57 1005
NGC 3034 09 55 54.0 +69 40 57 .I.0../ 82 203
NGC 3079 10 01 58.2 +55 40 43 .SBS5./ 85 1125
NGC 3628 11 20 16.3 +13 35 22 .S..3P/ 82 847
NGC 4945 13 05 26.2 --49 28 15 .SBS6*/ 78 560
Circinus 14 13 10.2 --65 20 20 .SAS3*. 60 438
NGC 6221 16 52 46.7 --59 12 59 .SBS5.. 43 1482
NGC 7552 23 16 10.9 --42 35 01 PSBS2.. 28 1585
NGC 7582 23 18 23.3 --42 22 11 PSBS2.. 64 1575
Column (1): Galaxy name. Columns (2) & (3): Galaxy position. Column (4): Galaxy
type. Column (5): Approximate galaxy inclination. Column (6): Mean heliocentric
radial velocity derived from H i observations.
References: All parameters except the inclination are taken from the Third Reference
Catalog (RC3) by de Vaucouleurs et al. (1991). The galaxy inclination has either been
taken from the H i catalog by Huchtmeier & Richter (1989) or references mentioned in
the summary of the individual galaxies.
sibility of large­scale outflow of ionized gas into the halo (see Schulz & Wegner
1992, and references therein). Extended X­ray emission has been observed by
Fabbiano & Trinchieri (1984) and Pietsch & Tr¨umper (1993) using the Einstein­
and ROSAT satellites, respectively.
Figures 2 and 3 show the H i gas distribution and velocity field of NGC 253,
respectively. The emission is distributed rather similar to the optical light
and shows a rather regularly rotating spiral galaxy (Koribalski, Whiteoak &
Houghton 1995). The H i absorption against the central continuum sources
causes the prominent `hole' in the centre of the distribution. The major­axis
position­velocity diagram of NGC 253 is rather similar to that of NGC 1808
(Fig. 1), with one major difference: here the absorption spectrum is not quite
as broad as the emission spectrum and it is asymmetric with respect to the sys­
temic velocity (see Table 1). Whereas the large width of the absorption is most
likely caused by a fast­rotating nuclear ring, the asymmetry of the line has been

Activity in Spiral Galaxies 9
interpreted as due to gas outflow from the nuclear region. A detailed analysis
of the H i data is in progress.
Table 3. Far­infrared flux, luminosity and derived star­formation rate
D F FIR L FIR SFR
[Mpc] [10 \Gamma11 Wm \Gamma2 ] [10 9 L fi ] [M fi yr \Gamma1 ]
NGC 253 3.4 5.59 (a) 20 5.2
NGC 660 11.2 0.35 (a) 14 3.6
NGC 1068 15.2 0.97 (b) 71 18.4
NGC 1365 20.5 0.51 (a) 68 17.6
NGC 1808 10.9 0.57 (c) 21 5.5
NGC 3034 3.25 5.84 (a) 19 4.9
NGC 3079 15.6 0.25 (b) 19 4.9
NGC 3628 10.2 0.31 (a) 10 2.6
NGC 4945 6.7 3.70 (a) 52 13.5
Circinus 3.6 1.21 (d) 5 1.3
NGC 6221 17.7 0.22 (d) 22 5.6
NGC 7552 20.9 0.36 (d) 50 12.9
NGC 7582 20.7 0.25 (d) 34 8.8
Column (1): Galaxy name. Column (2): Adopted distance, assuming H 0 =
75km s \Gamma1 Mpc \Gamma1 . Column (3): Far­infrared flux as calculated from the IRAS 60¯m
and 100¯m fluxes. References: (a) Rice et al. 1988, (b) Young et al. 1989, (c) Danks,
Perez & Altner 1990, (d) IRAS Point Source Catalog 1985. Column (4): Far­infrared
luminosity (/ D 2 ). Column (5): Star­formation rate: SFR (M – 0:1 M fi ) = 0.26 \Lambda
column (4), see Hunter et al. (1986).
NGC 660 is a rather peculiar galaxy for which two kinematical systems exist:
a nearly edge­on disk (PA = 46 ffi ; i ú 80 ffi ) and an inclined polar ring or strongly
warped outer disk (PA = \Gamma7 ffi ; i ¸ ! 60 ffi ) (Gottesman & Mahon 1990; Baan,
Rhoads & Haschick 1992; Combes et al. 1992). The 6­cm continuum map by
Condon et al. (1982) shows quite extended emission along the inner disk with two
peaks roughly 4 00 apart. H i position­velocity diagrams obtained by Gottesman
& Mahon along the major axes of both systems show a broad absorption line
the varying column density of which resembles the (here unresolved) continuum
structure. This and the map of the integrated mean velocity dispersion shown
by the authors hints at a fast­rotating nuclear ring at PA = 46 ffi , thus belonging
to the disk. Further, there are indications for a stellar bar (Young, Kleinmann
& Allen 1988) and very tentatively for nuclear outflow (Gottesman & Mahon
1990). Baan, Rhoads & Haschick (1992) obtained both high­resolution H i and
OH absorption measurements with the VLA. They found that a central disk
(diameter ¸ 6 00 or 300 pc) with a large velocity gradient (see Table 1) dominates
the absorption signature. The companion UGC 01195, an irregular, distorted
galaxy is located 22 0 or 72 kpc away.

10 Koribalski
DECLINATION
(J2000)
RIGHT ASCENSION (J2000)
03 33 38s 37s 36s 35s 34s
NGC 1365
(ATCA)
­36 08'00"
15"
30"
45"
DECLINATION
(J2000)
RIGHT ASCENSION (J2000)
02 46 22s 21s 20s 19s 18s 17s 16s
NGC 1097
(ATCA)
­30 16'00"
15"
30"
45"
17'00"
Figure 4. 20­cm radio continuum emission of the starburst galaxies
NGC 1365 and NGC 1097. These data were taken with the largest
array of the ATCA (snap­shot). The beam has been convolved to 8 00 .
NGC 1068 is a barred galaxy with a very complex nuclear region. Although
it is an archetype Seyfert 2, at least half of the IR emission arises from star
formation (Young, Kleinmann & Allen 1988). High­resolution (¸2 00 ) VLA H i
absorption measurements carried out by Gallimore et al. (1994) reveal several
blue­ and red­shifted features against the SW radio lobe and a jet as well as broad
(FWHM = 128\Sigma25 km s \Gamma1 ) double absorption lines against the radio nucleus.
The absorption lines cover a total velocity range of ¸540 km s \Gamma1 , from ¸760 to
1300 km s \Gamma1 . The two lines against the core are centred at +66\Sigma12 km s \Gamma1 and
--295\Sigma9 km s \Gamma1 , resulting in a mean offset from v sys of --181 km s \Gamma1 . Because
of this large offset Gallimore et al. reject circular orbits (which are favoured for
many of the galaxies presented here), but prefer a model with two distinct, near­
nuclear cloud populations, one falling into and the other flowing out of the radio
core, both associated with the fuelling of and exhaust from the central engine.
In case of a fast­rotating nuclear ring I would expect the radius to coincide with
the inner Lindblad resonance (ILR) induced by the bar potential. It is possible
that the absorption lines observed in NGC 1068 are caused by outflow and a
nuclear ring as suggested for NGC 253. For comparison see also the summary
of H i absorption measurements against the Seyfert galaxy NGC 4151 and the
peculiar galaxy NGC 4258 in Section 4.3.
A detailed discussion of NGC 1068 and NGC 4151 is given in the chapter
by Brinks & Mundell in this volume.
NGC 1365 is a southern, strongly barred spiral galaxy with no apparent
companions. VLA H i observations by Ondrechen & van der Hulst (1989) show
the overall gas extent of the galaxy. The central region contains a Seyfert nu­
cleus as well as circum­nuclear radio continuum and Hff emission, indicating
star formation (Saikia et al. 1994; Sandqvist, J¨ors¨ater & Lindblad 1995). On­
drechen & van der Hulst find a strong absorption feature centred at 1570 km s \Gamma1

Activity in Spiral Galaxies 11
Figure 5. (top) H i distribution of the starburst galaxy NGC 1808.
Figure 6. (bottom) H i mean velocity field of NGC 1808 (for a full
description see Koribalski 1993). These data were taken with the BnC
and AnB arrays of the VLA (5 h each).

12 Koribalski
with a width of only 33 km s \Gamma1 . But their position­velocity diagrams indicate a
much broader (\Deltav ¸ ? 200 km s \Gamma1 ) feature which is confirmed by J¨ors¨ater & van
Moorsel (1995) who find a total velocity width of 313 km s \Gamma1 for the H i absorp­
tion. Our own ATCA snap­shot observations of NGC 1365 and also NGC 1097,
another strongly barred galaxy, clearly reveal H i absorption over a broad ve­
locity range. The broad absorption features are most likely caused by a rapidly
rotating ring/disk of neutral gas. For NGC 1365 a similar disk of ionized matter
with a radius of 7 00 has already been inferred by Lindblad (1978). NGC 1097
is in many respects very similar to NGC 1365 (see Ondrechen, van der Hulst &
Hummel 1989). The continuum sources of both galaxies are displayed in Fig. 4.
NGC 1808 is a peculiar southern spiral galaxy and rather nearby example of
nuclear starburst activity (L FIR ú 2 \Theta 10 10 L fi ); for a comprehensive overview see
Koribalski (1993). Well known are the prominent dust filaments which emerge
nearly perpendicular to its central disk (V'eron­Cetty & V'eron 1985; Laustsen,
Madsen & West 1987), similar to those observed in M 82, indicating contin­
uing disk­halo interactions. The nuclear region contains a number of bright
`hot spots', which are mostly interpreted as very bright H ii regions (S'ersic &
Pastoriza 1965; Alloin & Kunth 1979; Koribalski & Dettmar 1993). In the ac­
tual nucleus, however, V'eron­Cetty & V'eron drew attention to the presence of
a broad Seyfert­like component of Hff with FWHM about 550 km s \Gamma1 . Radio
observations at 6 cm reveal a number of very small (! 1 00 ) compact sources in
the central region, most likely to be supernovae and supernova remnants (Saikia
et al. 1990).
VLA measurements of its nuclear region reveal that the broad (\Deltav ú
360 km s \Gamma1 ) H i absorption line seen at low angular resolution is a much narrower
line which shifts its centre velocity over the face of the continuum (Koribalski,
Dickey & Mebold 1993). This is interpreted as a torus of cold, dense gas with
a rotation velocity of ¸250 km s \Gamma1 and radius 500 pc. The gravitational mass
required to explain this fast rotation is a few times 10 9 M fi . Figures 5 and 6
display the overall H i distribution and velocity field of the galaxy NGC 1808.
The central `hole' is caused by H i absorption.
The radius and velocity of the nuclear ring are comparable to the distri­
bution and kinematics of the optically visible `hot spots'. The possibility of
a molecular gas ring has been considered by Dahlem et al. (1990) and is also
indicated in new CO(2--1) data by Aalto et al. (1994).
A stellar bar (length ¸ 3 0 or 6 kpc), which was discovered in the Hff line
(Phillips 1993; Koribalski & Dettmar 1993), might be causing the gas to accumu­
late at the inner and outer Lindblad resonances. The inner Lindblad resonance
as determined from the bar pattern speed coincides with the ring radius; the
outer resonance lies just outside the pseudo­ring which is formed by the outer
spiral arms.
NGC 3034 (M 82) is a typical and probably the most well­known starburst
galaxy and has been extensively studied in nearly all wavelength ranges. Al­
though M 82 is classified as a dwarf irregular galaxy, it is very similar to many
of the starburst galaxies discussed here, in particular NGC 1808. High­resolution
observations by Kronberg, Bierman & Schwab (1985) show that the continuum

Activity in Spiral Galaxies 13
emission is concentrated into a large number of compact sources, identified as
very young supernova remnants. A cylindrical outflow of dense molecular clouds
and ionized gas has been observed, ejected from the disk into the halo at several
hundred km s \Gamma1 (Nakai et al. 1987; Bland & Tully 1988). H i absorption lines
shifting over the continuum source(s) clearly indicate a fast­rotating nuclear ring
(Weliachew, Fomalont & Greisen 1984; Yun 1992). Telesco et al. (1991) report
the detection of a bar ¸1 kpc long; the nuclear ring (radius ¸ 300 pc) lies
roughly between the ILR(s) which are found at 40 and 600 pc.
NGC 3079 is a remarkable edge­on spiral galaxy. It is optically disturbed
and dusty, has a Seyfert 2/LINER spectrum, an active nucleus, and starburst
activity (for a summary see Baan & Irwin 1995, and references therein). Two
radio lobes extending approximately along the minor axis of the galaxy --- very
similar to those in Circinus (see below) --- suggest the presence of outflow (e.g.,
Duric & Seaquist 1988; Hummel, van Gorkom & Kontanyi 1983). This is also
indicated by the Hff images which reveal a giant loop (Ford et al. 1986) and,
on deeper images, several filaments on kpc scales (Armus, Heckmann & Miley
1990). The nuclear activity may be fuelled by an inner molecular disk (Young,
Claussen & Scoville 1988) which has a radius of ¸400 pc. Broad H i absorption
lines detected by Irwin & Seaquist (1991) probably indicate a fast­rotating ring
of similar size. High­resolution VLA H i and OH absorption measurements by
Baan & Irwin (1995) reveal numerous components, one of which is a rapidly
rotating compact disk, extended by about 100 pc or 1: 00 25 (see also Gallimore et
al. 1994). NGC 3079 also contains the most luminous known H 2 O megamaser.
NGC 3628 is a peculiar edge­on galaxy which belongs to the Leo Triplet, a
nearby complex of three interacting spirals. H i observations by Rots (1978) and
Haynes, Giovanelli & Roberts (1979) reveal emission in all three galaxies as well
as a plume and bridge between NGC 3627 and NGC 3628. CO observations
of the group have been published by Young, Tacconi & Scoville (1983). Neu­
tral hydrogen absorption in NGC 3628 was first reported by Dickey (1982); he
quotes a line width of 93 km s \Gamma1 (although there is a hint of faint absorption
over a much larger velocity range) centred at 882 km s \Gamma1 , close to the systemic
velocity of the galaxy. VLA observations against the extended continuum of
NGC 3628 by Schmelz, Baan & Haschick (1987) reveal about 10 individual H i
absorption features over a velocity range of ¸350 km s \Gamma1 . The velocity gra­
dient of several strong components has been interpreted as a circular, solidly
rotating disk. A very detailed study of both the H i emission and absorption in
NGC 3628 has been carried out by Wilding, Alexander & Green (1993). Zhang,
Wright & Alexander (1993) obtained high­resolution H i and CO measurements
of NGC 3627. We obtained ATCA 20­cm radio continuum and H i snap­shot
observations, but the angular resolution is rather low.
NGC 4945 is a nearby, southern, edge­on galaxy with an extremely large
optical extent. Giant complexes of gas and dust in the disk give it a patchy
appearance similar to NGC 253. The presence of a bar was indicated by de
Vaucouleurs (1964), but further measurements are necessary to support this
finding. The `active' nucleus exhibits both starburst and Seyfert characteristics

14 Koribalski
DECLINATION
(J2000)
RIGHT ASCENSION (J2000)
14 14 30s 00s13 30s00s12 30s00s
­65 10'
15'
20'
25'
30'
Figure 7. H i distribution of the Circinus galaxy (courtesy: K. Jones,
B. Koribalski, M. Elmouttie, R. Haynes). Contour levels are 0.042,
0.063, 0.084, 0.105, 0.126, 0.169, 0.211 Jy beam \Gamma1 km s \Gamma1 . This is a
very preliminary map was taken with the 750­m array of the ATCA (12
h). The angular resolution is ¸40 00 . Please note that the H i emission
is extended over a much larger field than displayed.
(Whiteoak 1986). Nakai (1989) indicated the presence of optical filaments on
one side of the galaxy emerging from the nuclear region toward the halo. The
fan­like morphology of the outflow has a scale­height of 2 kpc and extends ¸10
kpc along the plane. A broad H i absorption line has been detected, but not
resolved, by Whiteoak & Gardner (1976) and Ables et al. (1987). Similar broad
absorption lines have also been found at other wavelengths (see Whiteoak &
Wilson 1990, and references therein). Several authors find evidence for a fast­
rotating molecular ring: in CO(1--0) by Bergman et al. (1992), in H 2 (2.2¯m)
by Koornneef (1993), and in CO(2--1) by Dahlem et al. (1993). The nuclear
ring is quite thick and has a mean radius of about 100--200 pc and a rotational
velocity of 200--250 km s \Gamma1 . A detailed study of the H i emission and absorption
in NGC 4945 has recently been carried out with the ATCA by Ott (1995).
Circinus is a nearby spiral galaxy only 4 ffi below the Galactic plane. It was
found while inspecting a Schmidt plate and shortly afterwards observed in H i
with the Parkes 64­m telescope. Freeman et al. (1977) measured a half­width

Activity in Spiral Galaxies 15
CIRCINUS RA: 14 13 10 DEC: ­65 20 21.6
flux
(mJy/beam)
heliocentric velocity (km/s)
200 300 400 500 600 700
0
­1
­2
­3
­4
­5
­6
Figure 8. H i absorption spectrum toward the nuclear region of the
Circinus galaxy. These data were taken with the largest array of the
ATCA (Koribalski & Whiteoak 1996).
of at least 32 0 \Theta 15 0 for the H i extent of Circinus, much larger than the optical
diameter (¸10 0 ) of the galaxy. K. Jones et al. (1996, in prep.) and Koribalski
& Whiteoak (1996) only recently mapped Circinus with several configurations
of the ATCA; a few preliminary results are presented here. Fig. 7 shows part of
the overall H i distribution, clearly outlining the two spiral arms. The elongated
central disk (length ¸5 0 ) indicates a bar, and the central `hole' is caused by
H i absorption against the bright central region of Circinus (see Fig. 8). The
nuclear activity is caused by star formation, a Seyfert 2 nucleus and giant radio
lobes (perhaps associated with outflow), very similar to those in NGC 3079.
Harnett et al. (1990) observed Circinus with Parkes at 1665 and 1667 MHz and
found strong OH absorption lines over a velocity range of nearly 200 km s \Gamma1 .
High­resolution ATCA measurements reveal H i absorption over about the same
velocity range. The data show a shift of the line over the face of the continuum,
indicating a fast­rotating nuclear ring (Koribalski & Whiteoak 1996). This is
supported by Hff images taken with AAT Taurus Fabry­Perot interferometer
(Fig. 9) and with the ESO New Technology Telescope (Marconi et al. 1994)
showing a broken ring or spiral arms at ¸200 pc from the nucleus. Marconi et
al. also found an O iii emission cone toward the NW indicating highly ionized
gas flowing out from the nuclear region.

16 Koribalski
DECLINATION
(B1950)
RIGHT ASCENSION (B1950)
14 09 22s 20s 18s 16s 14s
­65 05'45"
06'00"
15"
30"
45"
07'00"
Figure 9. Hff emission from the nuclear region of the Circinus galaxy.
This is a very preliminary image taken with the TAURUS­2 Fabry­
Perot instrument at the Anglo­Australian Telescope (courtesy: B. Ko­
ribalski, K. Taylor, J. Whiteoak). The angular resolution is ¸1: 00 5.
NGC 6221 is a barred spiral galaxy which is interacting with its brightest
neighbour, NGC 6215, and possibly also with two newly discovered low­surface
brightness galaxies (see Fig. 10). The galaxy group lies only ¸ 10 ffi below the
Galactic plane which explains the rather crowded field in the optical. Recent
measurements with the ATCA 375­m array revealed an H i bridge between the
two major galaxies and several extensions of the gas envelope of NGC 6221
which can be attributed to tidal forces between these galaxies (B. Koribalski
et al., in prep.). Detailed studies of the Hff line emission in NGC 6221 by
Pence & Blackman (1984) revealed very large non­circular motions of the ionized
gas, possibly as a result of tidal interactions and streaming motions along the
bar. The H i distribution of NGC 6221 (Fig. 11) is quite similar to the optical
emission; the central depression is caused by H i absorption which is observed
over several hundred km s \Gamma1 . A nuclear ring is the most likely explanation
regarding the similarity of this galaxy to others discussed here, but further data
analysis is needed to confirm this idea. The H i velocity field looks much more
regular than the Hff field which was reconstructed from several long­slit spectra.
The nuclear activity is characterised by star formation and a weak Seyfert 2
nucleus.

Activity in Spiral Galaxies 17
DECLINATION
(J2000)
RIGHT ASCENSION (J2000)
16 54 30 00 53 30 00 52 30 00 51 30 00
­59 00
05
10
15
20
25
5
10
15
NGC6221
NGC6215
BK_1
BK_2
Figure 10. H i distribution (contours) of NGC 6221 and neighbour­
ing galaxies overlaid onto the optical emission (greyscale) from the
Digitized Sky Survey (DSS). The contour levels are 0.15, 0.3, 0.6, 1.2,
2.25, 4.5, and 9 Jy beam \Gamma1 km s \Gamma1 . Please note that no primary beam
correction has been applied here. A tidal H i bridge is visible between
NGC 6221 and its brightest neighbour, NGC 6215 (v sys = 1555 km s \Gamma1 ).
The two low­surface brightness galaxies (BK 1: v sys ¸ 1645 km s \Gamma1 and
BK 2: v sys ¸ 1510 km s \Gamma1 ) are newly discovered members of this in­
teracting group of galaxies. These data have been obtained with the
375­m array of the ATCA (12 h). The angular resolution is about 1: 0 5.

18 Koribalski
NGC6221
(ATCA)
Jy/beam
km/s
DECLINATION
(J2000)
RIGHT ASCENSION (J2000)
16 53 00 52 55 50 45 40 35 30
­59 11
12
13
14
15
0.0
0.5
1.0
1.5
2.0
RIGHT ASCENSION (J2000)
16 53 00 52 55 50 45 40 35 30
1350
1400
1450
1500
km/s
1550
1600
1650
1700
Figure 11. Left: H i distribution of NGC 6221. The contour levels
are 0.15, 0.3, 0.6, 0.9, 1.2, and 1.5 Jy beam \Gamma1 km s \Gamma1 . Right: Mean
H i velocity field of NGC 6221. The contour levels range from 1315
to 1615 km s \Gamma1 , step 15 km s \Gamma1 . These data have been obtained with
the 1.5­km array of the ATCA (12 h). The resolution is 30 00 \Theta 25 00 \Theta
6.6 km s \Gamma1 .
Not much is known about NGC 6215, which is classified as a non­barred
spiral galaxy of type SAS5 (RC3). Its H i emission extends from ¸1450 to
1650 km s \Gamma1 (Fig. 10).
NGC 7552 is a prominent starburst galaxy; no active nucleus has been re­
ported. Because of its nearly face­on inclination the strong bar is easy to iden­
tify. Although it has no close neighbours, the group of galaxies associated with
NGC 7582 (see below) at a distance of ¸30 0 plays an important role. Forbes
et al. (1994) found a 1­kpc starburst ring in NGC 7552 consisting of supernova
remnants and optical `hot spots'. We observe H i absorption against the ring
and find a velocity shift of ¸120 km s \Gamma1 between its eastern and western parts.
NGC 7582 is a barred spiral which is interacting with several bright neigh­
bours, namely NGC 7590, NGC 7599 (see Fig. 12), and most likely also NGC 7552
(see above). Recent ATCA measurements reveal several H i bridges between the
group members (B. Koribalski et al., in prep.). NGC 7582 has a Seyfert 2 nu­
cleus, but it is clearly dominated by starburst activity as shown by strong radio
emission, which probably originates from many supernova remnants, and by the
presence of numerous H ii regions (Morris et al. 1985). TAURUS Fabry­Perot ob­
servations of the circum­nuclear Hff emission suggest a fast­rotating disk of H ii
regions (diameter ¸1 kpc) in the plane of the galaxy. Morris et al. also suggest

Activity in Spiral Galaxies 19
DECLINATION
(J2000)
RIGHT ASCENSION (J2000)
23 19 30 15 00 18 45 30 15 00 17 45 30
­42 12
14
16
18
20
22
24
26
28
5
10
15
20
NGC7582
NGC7599
NGC7590
Figure 12. H i distribution (contours) of NGC 7582 and two nearby
galaxies, NGC 7590 (v sys = 1596 km s \Gamma1 ) and NGC 7599 (v sys =
1654 km s \Gamma1 ) overlaid onto the optical emission (greyscale) from the
Digitized Sky Survey (DSS). The contour levels are 0.15, 0.3, 0.6, 1.2,
2.25, and 4.5 Jy beam \Gamma1 km s \Gamma1 . Please note that no primary beam
correction has been applied here. Several tidal tails are visible near
NGC 7582, pointing toward the neighbours in the east and toward
NGC 7552 which lies at a projected distance of ¸30 0 . These data have
been obtained with the 375­m array of the ATCA (12 h). The angular
resolution is about 1: 0 7 \Theta 1: 0 3.

20 Koribalski
DECLINATION
(J2000)
RIGHT ASCENSION (J2000)
23 18 30s 25s 20s 15s
­42 20'00"
30"
21'00"
30"
22'00"
30"
23'00"
30"
24'00"
30"
0.0
0.5
1.0
1.5
2.0
Jy/beam
km/s
RIGHT ASCENSION (J2000)
23 18 30s 25s 20s 15s
1450
1500
1550
1600
1650
1700
1750
1800
km/s
Figure 13. H i distribution and mean velocity field of NGC 7582.
These data were taken with the 1.5­km array of the ATCA. The angular
resolution is 38 00 \Theta 23 00 .
that blue­shifted O iii line emission originates from high excitation gas expand­
ing outwards from the nucleus in a cone. The H i distribution of NGC 7582 (see
Fig. 13) is rather asymmetric (because of tidal interaction) and extends over a
velocity range of at least 400 km s \Gamma1 . The absorption lines cover nearly the same
range, but are ¸50 km s \Gamma1 blue­shifted with respect to the emission. We find
the H i absorption line clearly shifting over the face of the continuum emission
indicating a fast­rotating ring. Circumnuclear Hff emission, suggesting a similar
feature (see above), hints at a mixture of gas phases in the nuclear region.
Recent high­resolution 8.64­GHz observations with the ATCA by Norris &
Forbes (1996) suggest a double nucleus.
4.2. Strongly Interacting Galaxies
Table 4 lists a few strongly interacting or merging galaxies which also show broad
H i absorption lines. These galaxies happen to be some of the nearest and more
famous examples of very luminous FIR­galaxies with far­infrared luminosities
of more than 10 12 L fi (see also Heckman et al. 1990). They are the sites of
exceedingly powerful bursts of star formation. The RC3­Type is added to show
how peculiar these systems are. Several of these galaxies have also been detected
in hydroxyl absorption (see e.g., Schmelz et al. 1986).
I could easily add a few more galaxies to this group (see e.g., Dickey 1982,
1986; Mirabel 1982; Heckman et al. 1983). Mirabel & Sanders (1988) find that
18 out of their 92 luminous far­infrared galaxies show H i absorption with line
widths of 100--600 km s \Gamma1 , although some cases are very weak (e.g., U 9618,
Mrk 331), and many of the detections were known before (see the references

Activity in Spiral Galaxies 21
in Table 4). Interferometric studies need to be carried out to separate the H i
absorption from the emission to determine more accurate line widths and centre
velocities. For example, the study fails to pick up the broad absorption in the
nearby polar­ring galaxy NGC 660, and there may well be others.
Table 4. Strongly interacting pairs of spirals
Name D RC3­Type abs. line width References
[Mpc] [km/s]
NGC 520 30 .P..... 100 1,5,7
Mkn 231 160 .SAT5$P 150 1
NGC 2623 74 .P..... 525 2,5,7
NGC 3256 37 .P..... 300 10
NGC 3690/IC 694 42 .IB.9P. 300 1,5,8
IC 4553 (=Arp 220) 72 .S?.... 743 2,(4),5,6,7
IC 883 92 .I..9*P 460 3,7
NGC 6240 100 .I.0.*P 700 1,3,(9)
References: (1) Dickey 1982; (2) Mirabel 1982; (3) Heckman et al. 1983; (4) Norris et
al. 1985; (5) Dickey 1986; (6) Baan et al. 1987; (7) Mirabel & Sanders 1988; (8) Baan
& Haschick 1990; (9) van der Werf et al. 1993; (10) English 1994.
What is happening in these galaxies ? Are they scaled­up versions of the
group of nearby luminous galaxies discussed in the previous section ? I think
the answer is yes. But in these galaxies the cold gas which feeds the starburst
is driven inwards by strong tidal interaction or merging between galaxies (see
e.g., Rieke et al. 1985), whereas in the nearby galaxies, bars and/or weak galaxy
interactions may be responsible.
Are those broad absorption lines also caused by fast­rotating nuclear rings ?
Interferometer maps of the integrated CO emission have shown that cold gas is
strongly concentrated toward the `active' nuclei (starburst nuclei or AGN) of
those systems (Sargent & Scoville 1991; Scoville et al. 1991). The kinematics,
which are more clearly revealed by high­resolution H i absorption measurements,
indicate that at least part of the gas rotates around the nuclei (see e.g., Baan &
Haschick 1990). A very nice example is the peculiar galaxy IC 4553, where the
H i and OH absorption measurements clearly indicate a fast­rotating edge­on
disk close to the nucleus; the total velocity shift across the source is 145 km s \Gamma1
over a distance of 1: 00 9 or ¸700 pc (Baan et al. 1987, and references therein). But
there are also inward and outward motions of the cold gas. The central activity
of IC 4553, which is also the best studied OH megamaser galaxy, is caused by a
Seyfert 2 or starburst nucleus (see e.g., Heckman et al. 1983).
The gas distribution and kinematics in interacting systems could provide
information about the merger evolution. As the interaction progresses, a large
fraction of the interstellar medium (ISM) of each galaxy is expected to sink
into a common centre of mass. I suggest that the cold gas is accumulated in a
ring near the central source from where it is spiralling inwards, eaten up by the
hungry starburst, and then partly ejected into the halo.

22 Koribalski
4.3. Other Galaxies with H i Absorption Lines
Seyfert Galaxies. NGC 4151 is a weakly­barred spiral galaxy (type PSXT2).
It is also one of the best­studied examples of a nearby active galactic nucleus
(AGN), and one of the original nuclei noted by Seyfert (1943). The Seyfert type
is possibly intermediate between 1 and 2. Pedlar et al. (1992) investigated the
large­scale H i structure of the galaxy which covers a velocity range of about
920--1080 km s \Gamma1 in emission and absorption; the galaxy is viewed nearly face­
on (i ¸ 20 ffi ). They find that the most striking feature of the H sc i emission
structure is a `fat' bar, or oval, which occupies the central 5 kpc. The distortions
of the velocity field are most likely caused by the non­circular gas flows in the
bar. No evidence for tidal interactions with the companions at a distance of ¸ ?20 0
has been found. Pedlar et al. suggest that the massive, gas­rich bar of NGC 4151
could have caused the inflow of gas to the centre (similar to the model I discuss
for the nearby starburst galaxies in Section 4.1), and that the neutral hydrogen
gas present within the few hundred parsec of the nucleus (as shown by the H i
absorption measurements) constitutes a fuelling reservoir for the AGN.
Using the VLA B­array H i absorption toward the nuclear region of NGC 4151
was detected by Dickey (1986), with a centre velocity of 997 km s \Gamma1 and width
of 52\Sigma15 km s \Gamma1 . To resolve the nuclear region Mundell et al. (1995) obtained
Merlin H i absorption measurements with an angular resolution of 0: 00 15. They
found absorption (FWHM ¸90 km s \Gamma1 , centred at 993\Sigma6 km s \Gamma1 ) against the
nucleus only. Several other, slightly weaker continuum sources which are part
of a jet pointing toward us show no absorption. The authors therefore conclude
that the neutral hydrogen disk in which the nucleus is embedded can be no
thicker than 50 pc.
The H i survey of Seyfert galaxies by Heckman, Balick & Sullivan (1978)
shows how the low angular resolution of even the biggest single­dish telescopes
makes it nearly impossible to detect H i absorption against the nuclear contin­
uum sources of galaxies. Out of 58 Seyfert and Seyfert­like galaxies, 25 were
detected in H i emission and only three in H i absorption.
The peculiar galaxy NGC 4258: A black hole candidate. NGC 4258 is
a nearby (D = 6.4 Mpc) active spiral galaxy with high luminosity H 2 O maser
emission. VLBI observations by Greenhill et al. (1995) and Miyoshi et al. (1995)
find the masers residing in a very fast­rotating (v sys \Sigma900 km s \Gamma1 ), nearly edge­
on toroid/disk of radius 0.2 pc. This provides the most compelling evidence so
far for a black hole in the centre of a galaxy. --- The masers are probably formed
in relatively dense gas clouds that are forming new stars.
H i observations of NGC 4258 (van Albada 1978, 1980) reveal a rather flat
rotation curve (v rot ¸200 km s \Gamma1 ) in the outer parts of the galaxy, but a steep
rise in the nuclear region. The latter prompts van Albada (1978) to speculate
``that the rotational velocities near the nucleus may be significantly higher than
measured, implying a larger central mass concentration.'' Streaming motions in
the H i velocity field are attributed to the weak bar in the disk of NGC 4258
(type SXS4, see Table 2).
NGC 4258 is also well known for its anomalous arms, visible only in Hff and
radio continuum, which are usually interpreted in terms of collimated nuclear

Activity in Spiral Galaxies 23
outflow or jets (Cecil, Wilson & Tully 1992; Dettmar & Koribalski 1990, and
references therein).
As NGC 4258, the galaxies NGC 3079, NGC 1068, NGC 4945, and Circi­
nus each host an H 2 0 megamaser (apparent isotropic luminosity ? 10 L fi ). A
position­velocity diagram of the brightest H 2 O masers in the central parsec of
NGC 1068 (Gallimore et al. 1996) shows a similar, but much less extreme phe­
nomenon than NGC 4258. Other megamaser galaxies are currently investigated.
Weaker H 2 0 masers are also known in NGC 253, M 82 and a few other galaxies
(see Greenhill et al. 1990, and references therein).
Elliptical Galaxies. The nearest and most prominent radio elliptical galaxy is
NGC 5128 (Cen A). H i, OH, and H 2 CO absorption features have been detected
near the systemic velocity of the galaxy (550 km s \Gamma1 ). The H i absorption line
extends from about 500 to 700 km s \Gamma1 , whereas the H i emission is much broader,
extending rather symmetrically over ¸500 km s \Gamma1 (Gardner & Whiteoak 1976,
and references therein). High­resolution VLA observations by van der Hulst,
Golisch & Haschick (1983) show three rather narrow absorption features, one at
the systemic velocity of Cen A and the other two at +26 and +46 km s \Gamma1 . The
red­shifted components possibly arise from clouds falling into the nucleus where
they fuel the central engine; this has also been observed in several other radio
elliptical galaxies (see van Gorkom et al. 1990).
Several narrow absorption features, both at the systemic velocity and at
red­shifted velocities, have been observed against the nucleus of the early­type
galaxy NGC 1052 (van Gorkom et al. 1986).
Other radio elliptical galaxies detected in H i absorption are NGC 315,
NGC 1275, NGC 3894, NGC 5363, 3C 236, and 4C 31.06, bringing the num­
ber of detections up to eight (van Gorkom et al. 1989, and references therein).
Quasars. The first detection of H i in a quasar spectrum was made by Brown
& Roberts (1973), who detected a narrow absorption line against 3C 286 at a
red­shift of z = 0.69. H i absorption has also been detected against the QSO
PKS 1157+014 (Wolfe, Briggs & Jauncey 1981) and the BL Lac object AO
0235+164 (Wolfe, Davis & Briggs 1982). For a summary see e.g., Giovanelli &
Haynes (1988). Another very interesting study is that of H i absorption lines
against strong radio continuum sources, many of them quasars, in the vicinity
of spiral galaxies; see e.g., Corbelli & Schneider (1990) and Dickey, Brinks &
Puche (1992).
Others. VLBI observations of H i absorption (3420--3590 km s \Gamma1 ) against the
active nucleus (0: 00 016 or 5 pc) of the highly luminous spiral galaxy NGC 5793
(v sys = 3521 km s \Gamma1 ; D = 70 Mpc) reveal several components (Gardner et al.
1992). The authors claim that these are probably caused by various clouds
moving outwards, and not by a single rotating cloud complex surrounding the
nucleus. But a higher sensitivity study and larger bandwidth are definitely
needed to solve the issue.
Several strong absorption lines in the radio galaxy 3C 293 (z = 0.045) have
been detected by Haschick & Baan (1985). They can resolve the broad (\Deltav =
480 km s \Gamma1 ) H i absorption spectrum into more than 10 components, which can

24 Koribalski
be attributed to two different structures: ---a) the strongest features are part of
a rapidly rotating disk surrounding the nucleus of the galaxy and ---b) several
high­velocity features which are either red­ or blue­shifted are interpreted as
clouds falling into and being expelled from the nucleus of 3C 293. The observed
velocity gradient across the whole 2: 00 2 continuum structure is 179 km s \Gamma1 .
Recent VLA observations of CygnusA, the nearest powerful FR ii galaxy
(z = 0.0565), by Conway & Blanco (1995) also revealed a broad (\Deltav = 270 km s \Gamma1 )
absorption line against its 15­pc nucleus. It most likely consists of two compo­
nents arising from a rotating nuclear torus.
Another radio galaxy, NGC 4261 (3C 270), shows a rather narrow (\Deltav =
65 km s \Gamma1 ) H i absorption feature against its nucleus, near the systemic velocity
at ¸2200 km s \Gamma1 (Jaffe & McNamara 1994).
5. A Global Model
As a summary I present a probable model for the evolution of the nearby star­
burst galaxies by taking into account observations of their large­scale and nuclear
gas dynamics as well as numerical simulations of gas flows in galaxies.
Since bars, which strongly influence the gas kinematics in the disk of a
galaxy, can be triggered or enhanced by galaxy interactions (Athanassoula
1990; Gerin, Combes & Athanassoula 1990), it is necessary to study the en­
vironment of galaxies as well as their individual gas dynamics. Combes et al.
(1990) show that the growing time of a bar instability (which is severely af­
fected by various galaxy properties like e.g., the gas mass and the bulge­to­disk
mass ratio) is at least 0.5 10 9 yr. When the bar has settled down, after about
1--1.5 10 9 yr, it rotates with a high velocity. With time, the bar length grows
and its pattern speed slows down slightly. All of the nearby starburst galaxies
discussed here are barred and most of them show signs of tidal interaction (e.g.,
disturbed velocity fields, tidal tails, H i bridges). Some of the best examples
are NGC 3034 (M 82), NGC 6221, and NGC 7582. Thus, it is very likely that
galaxy interaction plays an important role in the formation of bars.
Whereas in the distant ultra­luminous galaxies like Arp 220, NGC 6240,
and others (see Table 4) strong tidal interactions or merging are responsible for
the central activity, the less violent starbursts in nearby galaxies like NGC 253
and NGC 1808 (see Table 2) are possibly caused by bars, or oval distortions, in
the disk (Combes 1988; Heckman 1990).
The best way to analyse the overall gas dynamics in galaxies is by observing
the neutral hydrogen gas. The H i velocity field, in particular, can be used to
study the streaming motions around the bar. A good example is the barred
starburst galaxy NGC 1808. Its bar is very prominent in the Hff line, which is
concentrated in numerous clumps along a nearly straight line extending ¸3 kpc
to both sides of the nucleus. Whereas it is relatively easy to obtain velocities for
the individual clumps, it is difficult to measure the velocity field of the diffuse
ionized gas surrounding the bar. Although the bar is not as prominent in the
distribution of the neutral hydrogen gas, the deviations from circular motions
caused by the bar are immediately visible in the H i velocity field. In fact, one
can nearly see the gas flowing toward the galaxy centre where it fuels the nuclear
starburst.

Activity in Spiral Galaxies 25
Theoretical studies have shown that the gas orbits within a barred po­
tential are highly elliptical and change their shape and size near the location
of resonances (see e.g., Contopoulos & GrosbÜl 1989). A characteristic radius
in a barred galaxy is the co­rotation radius (CR) which appears to lie near or
slightly beyond the end of the bar. Numerical simulations (e.g., Combes & Gerin
1985; Combes 1988) show that the gas is streaming outwards from CR to the
outer Lindblad resonance (OLR) and inwards from CR to the inner Lindblad
resonance (ILR). Outer (pseudo)­rings of galaxies, usually formed by the spiral
arms, indicate the accumulation of gas at the OLR. The ILR regularly lies within
the central few hundred parsec of a galaxy and high­resolution observations are
needed to resolve the nuclear gas kinematics. H i absorption measurements with
interferometers such as the ATCA and VLA have revealed ample evidence for
such rings in starburst galaxies, some of which is summarised in the previous
sections. The nuclear rings or tori are found to be rotating at very large speed,
often at least as high as in the outer parts of the galaxy.
One unsolved problem for numerical simulations of gas flows in galaxies is
the transport of matter into the star­forming region. Whereas the gas flow from
the disk to the nuclear ring, is well explained by the presence of a bar, there has
been no explanation on how the gas gets inside the inner Lindblad resonance
where observations clearly show the existence of large amounts of molecular and
cold atomic gas. (For a thorough discussion on numerical simulations of gas
flows in galaxies see the chapter by Jan Palous in this volume.) To fuel, e.g., an
M 82­class starburst interstellar masses of at least 10 8 M fi are needed (Heck­
man 1990). Thus, some process must allow a substantial fraction of the ISM of
a galaxy to flow right into the nuclear region where it generates and sustains the
star formation activity and perhaps, in a more evolved stage, feeds an active
nucleus (or a black hole). Stellar winds and supernova explosions eventually
lead to the formation of chimneys and fountains, where ionized gas breaks out
of the galactic plane, dragging neutral gas and dust (and magnetic field lines)
out to large scale­heights. The outflows in NGC 253, M 82 and NGC 1808 are
well studied examples.
Rieke, Lebofsky & Walker (1988) suggest a sequence for the evolution of a
nuclear starburst in which star formation occurs first at the nucleus and then
spreads over the whole nuclear region, accompanied by gas flowing out from the
centre. In this sequence NGC 253 is the prototype of `phase 3' where the nuclear
star formation has ended and superbubbles break into the halo. In `phase 4' the
circum­nuclear star formation continues and the given prototype is M 82. At
the end of the sequence, when star formation has ceased, stand galaxies like
M 31 (Andromeda) where the emission is dominated by old stellar populations.
M 31 is also a promising candidate for a black hole in the centre as the rotation
velocity and the velocity dispersion strongly increases in the central few pc (see
Dressler & Richstone 1988; Kormendy 1988, 1990). Maybe our Galaxy is also
at the end of this sequence showing little star­formation activity, but large ve­
locities in the nuclear disk (see Dame et al. 1987).

26 Koribalski
Acknowledgments.
ffl I thank my collaborators in the various projects mentioned for making
available the data before publication. Most of all I like to thank Tracy
Lavezzi and several other students (Sarah Maddison, Cheryl Frost, Brett
Hennig, Sally Houghton, Kate Brooks, Ann­Maree Tabone) for their help
during the observations and with part of the data reduction.
ffl This research made use of the NASA/IPAC Extragalactic Database (NED)
which is operated by the Jet Propulsion Laboratory, Caltech, under con­
tract with the National Aeronautics and Space Administration.
ffl The Digitized Sky Survey (DSS) was produced by the Space Telescope
Science Institute (STScI) and is based on photographic data from the UK
Schmidt Telescope, the Royal Observatory Edinburgh, the UK Science and
Engineering Research Council, and the Anglo­Australian Observatory.
References
Aalto, S., Booth, R.S., Black, J.H., Koribalski, B., Wielebinski, R. 1994, A&A,
286, 365
Ables, J.G., Forster, J.R., Manchester, R.N., Rayner, P.T., Whiteoak, J.B.,
Mathewson, D.S., Kalnajs, A.J., Peters, W.L., Wehner, H. 1987, MN­
RAS, 226, 157
Alloin, D., Kunth, D. 1979, A&A, 71, 335
Antonucci, R.R.J., Ulvestad, J.S. 1988, ApJ, 330, L97
Armus, L., Heckman, T.M., Miley, G.K. 1990, ApJ, 364, 471
Athanassoula, E. 1990, IAU Colloquium 124 on ``Paired and Interacting Galax­
ies'', ed. J. Sulentic, W. Keel & C. Telesco (Washington, NASA), 505
Athanassoula, E. 1992, MNRAS, 259, 328 and 345
Baan, W.A., Haschick, A.D. 1990, ApJ, 364, 65
Baan, W.A., Rhoads, J., Haschick, A.D. 1992, ApJ, 401, 508
Baan, W.A., van Gorkom, J.H., Schmelz, J.T., Mirabel, I.F. 1987, ApJ, 313,
102
Baan, W.A., Irwin, J.A. 1995, ApJ, 446, 602
Bergman, P., Aalto, S., Black, J.H., Rydbeck, G. 1992, A&A, 265, 403
Bland, J., Tully, R.B. 1988, Nature, 334, 43
Brinks, E., Skillman, E.D., Terlevich, R.J., Terlevich, E. 1996, in prep.
Brown, R.L., Roberts, M.S. 1973, ApJ, 184, L7
Canzian, B., Mundy, L.G., Scoville, N.Z. 1988, ApJ, 333, 157
Cecil, G., Wilson, A.S., Tully, R.B. 1992, ApJ, 390, 365
Clark, B.G. 1965, ApJ, 142, 1398
Clark, B.G., Radhakrishnan, V., Wilson, R.V. 1962, ApJ, 135, 151
Combes, F. 1988, in ``Galactic and Extragalactic Star Formation'', ed. R.E. Pu­
dritz & M. Fic (Kluwer Academic Publishers), 475

Activity in Spiral Galaxies 27
Combes, F., Gerin, M. 1985, A&A, 150, 327
Combes, F., Gottesman, S.T., Weliachew, L. 1977, A&A, 59, 181
Combes, F., Debbasch, F., Friedli, D., Pfenniger, D. 1990, A&A, 233, 82
Combes, F., Braine, J., Casoli, F., Gerin, M., van Driel, W. 1992, A&A, 259,
L65
Condon, J.J., Condon, M.A., Gisler, G., Puschell, J.J. 1982, ApJ, 252, 102
Contopoulos, G., GrosbÜl, P. 1989, A&AR, 1, 261
Conway, J.E., Blanco, P.R. 1995, ApJ, 449, L131
Corbelli, E., Schneider, S.E. 1990, ApJ, 356, 14
Crutcher, R.M., Rogstad, D.H., Chu, K. 1978, ApJ, 225, 784
Dahlem, M., Aalto, S., Klein, U., Booth, R., Mebold, U., Wielebinski, R., Lesch,
H. 1990, A&A, 240, 237
Dahlem, M., Golla, G., Whiteoak, J.B., Wielebinski, R., H¨uttemeister, S.,
Henkel, C. 1993, A&A, 270, 29
Dame, D.M., Ungerechts, H., Cohen, R.S., de Geus, E.J., et al. 1987, ApJ, 322,
706
Danks, A.C., Perez, M.R., Altner, B. 1990, in ``Bulges of Galaxies'', ed. B.J.
Jarvis & D.M. Terndrup (M¨unchen: ESO), 243
Dettmar, R.­J., Koribalski, B. 1990, A&A, 240, L15
de Vaucouleurs, G. 1964, ApJ, 139, 899
de Vaucouleurs, G., de Vaucouleurs, A., Corwin Jr., H.G., Buta, R.J., Paturel,
G., Fouqu'e, P. 1991, Third Reference Catalogue of Bright Galaxies (New
York: Springer Verlag), [RC3]
Dickey, J.M. 1979, ApJ, 233, 558
Dickey, J.M. 1982, ApJ, 263, 87
Dickey, J.M. 1986, ApJ, 300, 190
Dickey, J.M., Brinks, E., Puche D. 1992, ApJ, 385, 501
Dickey, J.M., Salpeter, E.E., Terzian, Y. 1979, ApJS, 36, 77
Dressler,A., Richstone, D. 1988, ApJ, 324, 701
Duric, N., Seaquist, E.R. 1988, ApJ, 326, 574
English, J. 1994, Ph.D. Thesis, Australian National University
Fabbiano, G., Trinchieri, G. 1984, ApJ, 286, 491
Forbes, D., DePoy, D.L. 1992, A&A, 259, 97
Forbes, D., Norris, R.P., Williger, G.M., Smith, R.C. 1994, AJ, 107, 984
Ford, H.C., Dahari, O., Jacoby, G.H., Crane, P.C., Ciardullo, R. 1986, ApJ, 311,
L7
Freeman, K.C., Karlsson, B., Lyngša, G., Burrell, J.F., van Woerden, H., Goss,
W.M., Mebold, U. 1977, A&A, 55, 445
Gallimore, J.F., Baum, S.A., O'Dea, C.P., Brinks, E., Pedlar, A. 1994, ApJ,
422, L13
Gallimore, J.F., Baum, S.A., O'Dea, C.P., Brinks, E., Pedlar, A. 1996, ApJ, in
press
Gardner, F.F., Whiteoak, J.B. 1974, Nature, 247, 56

28 Koribalski
Gardner, F.F., Whiteoak, J.B. 1976, Proc. ASA, 3, 63
Gardner, F.F., Whiteoak, J.B., Norris, R.P., Diamond, P.J. 1992, MNRAS, 258,
296
Gerin, M., Combes, F., Athanassoula, E. 1990, A&A, 230, 37
Giovanelli, R., Haynes, M.P. 1988, in ``Galactic and Extragalactic Radio Astron­
omy'', ed. G.L. Verschuur & K.I. Kellermann (Springer Verlag), 555
Gottesman, S.T., Mahon, M.E. 1990, IAU Colloquium 124 on ``Paired and In­
teracting Galaxies'', ed. J. Sulentic, W. Keel & C. Telesco (Washington,
NASA), 209
Greenhill, L.J., Moran, J.M., Reid, M.J., Gwinn, C.R., Menten, K.M., Eckart,
A., Hirabayashi, H. 1990, ApJ, 364, 513
Greenhill, L.J., Jiang, D.R., Moran, J.M., Reid, M.J., Lo, K.Y., Claussen, M.J.
1995, ApJ, 440, 619
Haschick, A.D., Baan, W.A. 1985, ApJ, 289, 571
Harnett, J., Whiteoak, J.B., Reynolds, J.E., Gardner, F.F., Tzioumis, A. 1990,
MNRAS, 244, 130
Haynes, M.P., Giovanelli, R., Roberts, M.S. 1979, ApJ, 229, 83
Heckman, T.M., Balick, B., Sullivan III, W.T. 1978, ApJ, 224, 745
Heckman, T.M., Balick, B., van Breugel, W.J.M., Miley, G.K. 1983, AJ, 88, 583
Heckman, T.M. 1990, IAU Colloquium 124 on ``Paired and Interacting Galax­
ies'', ed. J. Sulentic, W. Keel & C. Telesco (Washington, NASA), 359
Heckman, T.M., Armus, L., Miley, G.K. 1990, ApJS, 74, 833
Huchtmeier, W.K., Richter, O.G. 1989, A General Catalog of H i Observations
of Galaxies (Springer Verlag)
Hummel, E., van Gorkom, J.J., Kontanyi, C.G. 1983, ApJ, 267, L5
Hunter, D.A., Gillet, F.C., Gallagher, J.S., Rice, W.L., Low, F.J. 1986, ApJ,
303, 171
Irwin, J., Seaquist, E.R. 1991, ApJ, 371, 111
Jaffe, W., McNamara, B.R. 1994, ApJ, 434, 110
J¨ors¨ater, S., van Moorsel, G.A. 1995, AJ, 110, 2037
IRAS Point Source Catalog (PSC) 1985, Joint IRAS Working Group (Wash­
ingthon, DC: GPO)
Koornneef, J. 1993, ApJ, 403, 581
Koribalski, B. 1993, Ph.D. Thesis, University of Bonn
Koribalski, B., Dettmar, R.­J. 1993, The ESO Messenger, 71, 37
Koribalski, B., Dickey, J.M, Mebold, U. 1993, ApJ, 402, L41
Koribalski, B., Whiteoak, J.B., Houghton, S. 1995, Publ. ASA, 12, 20
Koribalski, B., Whiteoak, J.B. 1996, in prep.
Koribalski, B., Lavezzi, T.E., Dickey, J.M., Whiteoak, J.B. 1996, in prep.
Kormendy, J. 1988, ApJ, 325, 128
Kormendy, J. 1990, in ``High Energy Neutrino Astrophysics'', ed. V.J. Stenger
et al. (Singapure: World Scientific), 196
Kronberg, P.P., Bierman, P.L., Schwab, F.R. 1985, ApJ, 291, 693

Activity in Spiral Galaxies 29
Laustsen, S., Madsen, C., West, R.M. 1987, (ESO) Exploring the Southern Sky,
Springer Verlag, Berlin, Heidelberg, p.30
Lindblad, P.O. 1978, in Publ. Copenhagen Univ. Obs., Astronomical Papers
Dedicated to Bengt Str¨omgren, ed. A. Reiz & T. Andersen T., 403
Marconi, A., Moorwood, A.F.M., Origlia, L., Olivia, E. 1994, ESO Messenger,
78, 20
Mauersberger, R., Henkel, C., Wielebinski, R., Reuter, H.­P. 1996, A&A, in
press
Mirabel, I.F. 1982, ApJ, 260, 75
Mirabel, I.F., Sanders, D.B. 1988, ApJ, 335, 104
Miyoshi, M., Moran, J., Herrnstein, J. , Greenhill, L., Nakai, N., Diamond, P.,
Inoue, M. 1995, Nature, 373, 127
Morris, S., Ward, M., Whittle, M., Wilson, A.S., Taylor, K. 1985, MNRAS, 216,
193
Mundell, C.G., Pedlar, A., Baum, S.A., O'Dea, C.P., Gallimore, J.F., Brinks,
E. 1995, MNRAS, 272, 355
Nakai, N. 1989, PASJ, 41, 1107
Nakai, N., Hayashi, M., Handa, T., Sofue, Y., Haegawa, T. 1987, PASJ, 39, 685
Norris, R.P., Baan, W.A., Haschick, A.D., Diamond, P.J., Booth, R.S. 1985,
MNRAS, 213, 821
Norris, R.P., Forbes, D. 1996, in prep.
Ondrechen, M.P., van der Hulst, J.M., Hummel, E. 1989, ApJ, 342, 39
Ondrechen, M.P., van der Hulst, J.M. 1989, ApJ, 342, 29
Ott, M. 1995, Ph.D. Thesis, University of Bonn
Pedlar, A., Howley, P., Axon, D.J., Unger, S.W. 1992, MNRAS, 259, 369
Pence, W.D. 1980, ApJ, 239, 54
Pence, W.D., Blackman, C.P. 1984, MNRAS, 207, 9
Phillips, A.C. 1993, AJ, 105, 486
Pietsch, W., Tr¨umper, J. 1993, Advances in Space Research, 13, 171
Rice, W.L., Lonsdale, C.J., Soifer, B.T., Neugebauer, G., Kopan, E.L., Lloyd,
L.A., de Jong, T., Habing, H.J. 1988, ApJS, 68, 91
Rieke, G.H., Cutri, R.M., Black, J.H., Kailey, W.F., et al. 1985, ApJ, 290, 116
Rieke, G.H., Lebofsky, M.J., Walker, C.E. 1988, ApJ, 325, 679
Rots, A.H. 1978, AJ, 83, 219
Saikia, D.J., Unger, S.W., Pedlar, A., Yates, G.J., Axon, D.J., Wolstencroft,
R.D., Taylor, K., Gyldenkerne, K. 1990, MNRAS, 245, 397
Saikia, D.J., Pedlar, A., Unger, S.W, Axon, D.J. 1994, MNRAS, 270, 46
Sandqvist, Aa., J¨ors¨ater, S., Lindblad, P.O. 1995, A&A, 295, 585
Sargent, A., Scoville, N. 1991, ApJ, 366, L1
Schmelz, J.T., Baan, W.A., Haschick, A.D., Eder, J. 1986, AJ, 92, 1291
Schmelz, J.T., Baan, W.A., Haschick, A.D. 1987, ApJ, 315, 492
Schulz, H., Wegner, G. 1992, A&A, 266, 167

30 Koribalski
Scoville, N.Z., Soifer, B.T., Neugebauer, G., Young, J.S., Matthews, K., Yerka,
J. 1985, ApJ, 289, 129
Scoville, N.Z., Sargent, A.I., Sanders, D.B., Soifer, B.T. 1991, ApJ, 366, L5
S'ersic, J.L., Pastoriza, M. 1965, PASP, 77, 287
Seyfert, C.K. 1943, ApJ, 97, 28
Telesco, C.M., Campins, H., Joy, M., Dietz, K., Decher, R. 1991, ApJ, 369, 135
van Albada, G.D. 1978, Ph.D. Thesis, Sterrewacht Leiden
van Albada, G.D. 1980, A&A, 90, 123
van der Hulst, J.M., Golisch, W.F., Haschick, A.D. 1983, ApJ, 264, L37
van der Werf, P.P., Genzel, R., Krabbe, A., Blietz, M., et al. 1993, ApJ, 405,
522
van Gorkom, J.H., Knapp, G.R., Raimond, E., Faber, S.M., Gallagher, J.S.
1986, AJ, 91, 791
van Gorkom, J.H., Knapp, G.R., Ekers, R.D., Ekers, D.D., Laing, R.A., Polk,
K.S. 1989, AJ, 97, 708
van Gorkom, J.H., van der Hulst, J.M., Haschick, A.D., Tubbs, A.D. 1990, AJ,
99, 1781
Ve'ron­Cetty, M.­P., Ve'ron, P. 1985, A&A, 145, 425
Weliachew, L., Fomalont, E.B., Greisen, E.W. 1984, A&A, 137, 335
Whiteoak, J.B. 1986, Proc. ASA, 6, 467
Whiteoak, J.B., Gardner, F.F. 1976, Proc. ASA, 3, 71
Whiteoak, J.B., Wilson, W.E. 1990, MNRAS, 245, 665
Wilding, T., Alexander, P., Green, D.A. 1993, MNRAS, 263, 1075
Wolfe, A.M., Briggs, F.H., Jauncey, D.L. 1981, ApJ, 248, 460
Wolfe, A.M., Davis, M.M., Briggs, F.H. 1982, ApJ, 259, 495
Young, J.S., Tacconi, L.J., Scoville, N.Z. 1983, ApJ, 269, 136
Young, J.S., Claussen, M.J., Scoville, N.Z. 1988, ApJ, 324, 115
Young, J.S., Kleinmann, S.G., Allen, L.E. 1988, ApJ, 334, L63
Young, J.S., Xie, S., Kenney, J.D.P., Rice, W.L. 1989, ApJS, 70, 699
Yun, M.S. 1992, Ph.D. Thesis, Harvard Univ., Cambridge, MA
Zhang, X., Wright, M., Alexander, P. 1993, ApJ, 418, 100