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Small Galaxy Groups
ASP Conference Series, Vol. 3  10 8
, 1999
M. Valtonen and C. Flynn, eds.
Minibars in the Centers of Normal Galaxies
A. V. Zasov and O. K. Sil'chenko
Sternberg Astronomical Institute, University av. 13, Moscow 119899, Rus-
sia
Abstract. Analysis of ionized gas velocity elds in the circumnuclear
regions of 13 normal spiral galaxies has shown that about a half of them
reveal discrepancy between kinematical axis at R < 2" and photometric
or kinematical axes of outer regions. It evidences for high frequency of
non-axisymmetric gravitational potential occurence in inner few hundred
parsecs of galaxies. The possible nature of these minibars is discussed.
Nuclear bars, or minibars observed in the very centers of some spiral galax-
ies, are poorly understood features of the galactic structure. A majority of them
were found by visual inspection of galaxy images (see Buta and Crocker 1993
and references therein). It is generally accepted that they are related to the
nuclear rings formed in barred galaxies at the inner Linblad resonance (ILR),
although according to Buta and Crocker (1993), in some cases they were found
in non-barred spirals, so in general a presence of large-scale non-axisymmetric
gravitational potential is not a rigid condition for the nuclear bar to be formed.
Typical length of nuclear bars is 1 - 2 kpc for H 0 = 75km=s=Mpc. But the
shape of the nuclear bar size distribution over small lengths (L < 1 kpc) is cer-
tainly strongly aected by selecton eects. Here we consider the presence of the
smallest bar-like congurations which we call minibars in the centers of galaxies.
Perhaps the most direct way to reveal bar-like pattern in the central regions
of galaxies is to analyse velocity data instead of brightness distribution to be
sure that the rotation of gas (or stars) bears signs of non-axisymmetrical poten-
tial. Below we describe results of our measurements of ionized gas line-of-sight
velocities in the inner (R < 10") regions of nearby spiral galaxies obtained at 6m
re ector of the Special Astrophysical observatory of Russian Academy of Sci-
ences mainly in 1993-1994 with two-dimensional spectral devices which allow to
investigate simultameously velocity elds and morphology (isophote ellipticity
and orientation).
Our observational program included 13 barred and non-barred nearby galax-
ies without noticeable nuclear activity. All galaxies were chosen after preliminary
analysis of our long-slit data obtained earlier which has revealed presence of lo-
cal features on their rotation curves in the circumnuclear regions such as local
maxima or small plateau in the center. Three dierent spectral devices were
used: Long-Slit Spectrograph (LS), Multi-Pupil Field Spectrograph (MPFS)
and Fabri-Perot scanning interferometer (FPI). Raw spectra were detected by
TV Photon Counting System 512  512 pixels (1px = 0:7" 1:2"). Typical see-
ing during the observations was 1:5" 2". Line-of sight velocity gradients along
dierent position angles (PA) were measured from the positions of H and [NII]
emission lines in the range R = 1:5" 2:3" from the nucleus. We have used these
1

2
estimates to verify kinematical symmetry of velocity eld with respect to the
kinematical or photometric major axes found for the outer parts. Central line-
of-sight velocity gradient of circularly rotating gas alters with the PA following
cosine-like law and reaching its extrema at the line of nodes. Presence of bar or
oval orbit distortion results in the shift of the PA o .
Table 1. Program galaxies
NGC Type Spectral device Presence of minibar
23 SBb LS, MPFS, FP No
497 SBbc LS, MPFS, FP Yes
615 Sb LS, MPFS No
895 Sc LS, MPFS Yes
972 Sb LS, MPFS Yes
4100 Sc LS Yes
4536 SBc LS, MPFS No
6181 SBc LS, MPFS, FP Yes
6643 Sc LS No
7013 SO/a LS No
7171 SBb LS, MPFS Yes (?)
7217 Sb MPFS Yes
7331 Sbc LS No
crrrr
Minibar is assumed to be detected if the discrepancy between kinematical
major axis positions in the inner and in the outer regions exceeds 5 7 ô . As
one can see from Table 1, about half of the galaxies under consideration might
have minibars. They may not to be real optical high-contrast bars: we state
only the gas motions on elliptical orbits due to barred potential. Only three of
these seven galaxies possess large-scale bars, so we may guess that there is no
direct relation between two features. In general, although our results are not
free from observational selection, they give rst evidence of high frequency of
minibar occurence among normal spiral galaxies.
Table 2. Galaxies with bar-like distortions of circumnuclear velocity
elds
NGC
PA o R, kpc Comments ( M b
M d
) R=1kpc
497 202 ô 1 counterrotation? 1.7
895 44 ô 0.3 inclined disk? > 1
972 45 ô 0.2 8
10 ô 0.9
4100 26 ô 0.15 nuclear ring R = 6" > 1
6181 28 ô 0.3 1.4
8 ô 1.0
7171 190 ô :: 0.35 counterrotation? 2.3
7217 130 ô 0.15 nuclear ring R = 10" 2.0

3
Dierences between the inner and outer kinematical major axes are pre-
sented in Table 2. Linear radii corresponding to the given inner PA o are also
given. The scales of the bar-like orbit distortions in these galaxies are among the
smallest ever observed in galaxies beyond the Local Group (in most cases they
are less than 0.5 kpc). Optical images of all these galaxies look quite normal
in their inner parts. Two of them (NGC 4100 and NGC 7217) have nuclear
rings less than 1 kpc by radius (Buta and Crocker, 1993), but none has optical
nuclear bar mentioned in the literature (although detailed photometry of their
nuclear regions is still highly desirable). In some cases (NGC 497, NGC 7171,
NGC 7217) the dierence of PA o 's is between 90 ô and 270 ô that mimics nuclear
gas counterrotation under long-slit spectral observations.
Fig. 1 presents azimuthal dependencies of line-of-sight velocity gradients in
the nuclear regions of some galaxies which are found to have minibars. Their
maxima are noticeably shifted from the lines of nodes dened by the outer parts.
Angular dierences between the expected and the observed maxima depend both
on the amplitude of bar potential and on its orientation - parameters which are
unknown. In principle, the observed shift of cosine-like curve maxima may be
caused not only by an oval orbit distortion, but also by the inclination of in-
ner disc with respect to the main disk of galaxy. For several galaxies whose
inner isophotes demonstrate orientations of the major axis in agreement with
the kinematical central PA o (NGC 497, NGC 895, and NGC 972) this possibil-
ity cannot be excluded. However the azimuthal dependence of central velocity
gradient for NGC 972 (Fig. 1) resemble poorly cosine law looking asymmetri-
cal, so in this galaxy the presence of inclined, circularly rotating nuclear disk is
quite improbable. In addition there are other cases, e.g. NGC 6181 and 7217,
where the inner isophotes are turned with respect to the line of nodes in the
sense opposite to that of kinematical major axis twist (Fig. 2 - our broad-band
observations for NGC 6181 and combined data for NGC 7217), and here the
presence of bar-like distortions is obvious. For two galaxies which were observed
with FPI (NGC 972 and NGC 6181) we could retrace the change of kinematical
major axis orientation along the R to obtain a boundary of the abnormal axis
orientation region. In both cases it appears to be well resolved. The radius
where (PA) o slows down or stops its variation is about 5" 8" (0.6-0.9 kpc) for
NGC 972 and about 5" (0.8 kpc) for NGC 6181. Beyond these radial distances
the in uence of minibar on the gaseous disk rotation is negligible.
The galaxies where minibars are found do not dier much from non-barred
galaxies by the general shapes of their rotation curves. No colour peculiarities
of nuclear regions were also found for galaxies where multiaperture UBV -data
are available (Longo and Vaucouleurs, 1983, 1985).
One may expect that the main stellar disc does not control gravitational
potential of galaxies under consideration at R < 1 kpc. To verify it we modelled
rotation curves of the galaxies within their growing parts (several kpc from
the center) in a frame of two-component model (exponential disc + sphere) to
separate these components (the extended rotation curve of NGC 7217 was taken
from Rubin et al., 1985, others are ours). Photometric scale-lengths of the discs
were taken from the literature, then we extrapolated density distribution law of
the disc onto the central region. Note that in many cases this procedure may lead
to overestimation of the central density, because discs often show photometric

4
prole attening near the centers or central density depressions (see for example
Kent, 1986, 1987). Presence of dark haloes does not in uence the results because
we restricted ourselves by the inner regions of galaxies. The masses ratios of
spherical and disc components for R = 1 kpc are shown in the last column of
the Table 2. It follows that in the central regions of the galaxies gravitation of
the stellar disc is suppressed by the bulge. This circumstance allows to restrict
possible mechanisms of minibar formation.
Large relative mass of spherical component prevents the development of
classical bar-mode disc instability. Two possibilities left instead. The rst one
is the presence of massive and dynamically cool nuclear discs in the inner several
hundred parsecs, which are independent from the main stellar discs being far
more dense. The similar subsystems were found by us earlier in some galaxies
with axisymmetric central rotation (Afanasiev, Sil'chenko, and Zasov, 1989).
The second possibility is to consider the observed inner bars as slowly rotat-
ing bars developed from the hot stellar population through the elongated orbit
instability (the mechanism proposed by Lynden-Bell, 1979, and later developed
by Polyachenko with coauthors - see Polyachenko, 1994, or Polyachenko and
Polyachenko, 1994). In this case the process of sticking of elongated orbits to-
gether leads to the formation of bar from hot stellar population of inner bulge
or nuclear stellar disc. It is expected that slow bar ends up at the ILR, so it
may naturally explain the position of nuclear rings where they encircle nuclear
bars. If both - normal and slow bar - coexist in a galaxy, there must not be any
correlation between their orientations because of the dierent angular velocities,
which agrees with observations of galaxies with nuclear bars.
In both cases the presence of large-scale "normal" bars is not necessary for
minibars to be formed, although indirectly a large bar may provoke a creation
of inner one owing to the accumulation of a bar-driven interstellar gas onto the
central disc (if it exists) or to the dynamical heating of stars in the central region
of galaxy, which is essential for the formation of slow bar.
Acknowledgments. We are grateful to our colleagues from the Special As-
trophysical Observatory of Russian Academy of Science V.Afanasiev, A.Burenkov,
S.Dodonov, and V.Vlasyuk for the invaluable help in the carrying out the ob-
servational program.
This work has been partly supported by the Russian Fund of Fundamental
Researches (grant 93-02-17060).
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