Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.naic.edu/~gibson/hisa/poster/1999/poster.ps.gz Äàòà èçìåíåíèÿ: Mon Jan 11 22:03:38 1999 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 01:26:27 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: dust

OVERVIEW
HI self­absorption (HISA) against background HI 21cm emission
reveals cold atomic gas structures in the interstellar medium. These often
correlate spatially with molecular gas and dust (e.g., Knapp 1974; Peters
& Bash 1987), though certainly not always; the exact physical relationship
between HISA and H 2 is not well known. HISA features as a group are
poorly understood, due to the high angular resolution required for accurate
estimates of background spectra from adjacent sightlines, and to the lack
of unbiased high­resolution searches for such objects.
We have begun a systematic study of HISA within a 73 ffi \Theta9 ffi region (Fig.
2) mapped by the Dominion Radio Astrophysical Observatory's Synthesis
Telescope (DRAO­ST) for the ongoing Canadian Galactic Plane Survey
(CGPS; Taylor et al. 1999; English et al. 1998). The ¸1 0 DRAO­ST beam
reveals considerable substructure within detected clouds (see Figs. 1, 3
& 5). Our investigation has uncovered a wealth of remarkable features
in both the Local and Perseus Arms. Some have clear 12 CO and dust
counterparts, while many others do not.
Below, we investigate properties of sample CGPS HISA features and also
consider aspects of the population as a whole. Their partial correlation
with CO and dust, narrow linewidths, and intermediate appearance be­
tween diffuse HI and compact CO morphologies suggest the HISA features
may represent HI in the act of molecular condensation. This possibility
is augmented by a substantial abundance of HISA at the same velocities
where molecular clouds are likely to form after encountering the Perseus
Arm density wave shock.

PHYSICAL PROPERTIES
We wish to obtain physical parameters for our HISA objects: spin temperature
T SPIN , optical depth Ü HISA , column and volume densities N HISA and n HISA , and mass
M HISA . This requires solving the radiative transfer equation. We consider the im­
plicit 4­component formulation of Feldt (1993), with a HISA feature, warm, op­
tically thin HI emission in front of and behind it, and a continuum background
T CONT
:
T ON = T OFF + (T SPIN \Gamma p \Delta T OFF \Gamma T CONT ) \Delta
i
1 \Gamma e \GammaÜ HISA
j
:
Here T ON and T OFF are continuum­subtracted ON and OFF brightnesses, and p is the
fraction of T OFF emission lying behind the HISA feature. Constraining p leaves two
unknowns, Ü HISA
and T SPIN
. Inclusion of other relations, such as that for Gaussian
line column density, uniform gas density, and an ideal gas law then produce the
transcendental equation
T SPIN =
v u u u u t
5:2 \Theta 10 \Gamma19 \Delta Lk \Delta P THERM =k
\Deltav FWHM \Delta
`
\Gamma ln
Ÿ
T SPIN + (1\Gammap) \Delta T OFF \Gamma (T ON + T CONT )
T SPIN + (1\Gammap) \Delta T OFF \Gamma (T OFF + T CONT )
–' ;
where Lk is the HISA line­of­sight pathlength, and a canonical pressure such as
P THERM =k = 4000 cm \Gamma3 K is used (only thermal pressure is relevant).
In Table 1, we estimate properties for the objects in Figures 3 & 5. Both have
T CONT
= 5 K from the general Galactic synchrotron background. Representative
T ON
& T OFF
values were taken from Figures 4 & 6. In more sophisticated future
analyses, these will be measured separately for each HISA voxel, with T OFF
taken
from a volume interpolation of non­HISA voxels surrounding the feature in space
and velocity.
We assume the smallest high­contrast filaments in each object are roughly cylin­
drical (within a factor of 2) to obtain Lk; the pathlength will be that of a single
filament if their filling factor inside the object is low, which typically appears to be
the case. The ¸2 kpc distance to the Perseus feature is based on the sightline ve­
locity model of Roberts (1972), while the 200­500 pc distance for the Local feature
is constrained merely to lie within the Local Arm, but outside the Local Bubble.
p=1 was used for both objects; if p ! 1, T SPIN
, Ü HISA
, and related values become lower
limits. For the Perseus feature, M HISA
refers only to the central 10 0 \Theta45 0 component;
the entire complex may be 10 \Gamma 100 times more massive.

Table 1: Sample Feature Properties
Quantity Value
Perseus Local
l 139.46 ffi 145.10 ffi
b +0:91 ffi \Gamma2:01 ffi
v LSR \Gamma41:0 km s \Gamma1 +1:0 km s \Gamma1
Direct T ON
80 K 60 K
Measurements T OFF ¸110 K ¸62 K
T CONT
5 K 5 K
Angular Size 10 0 \Theta 45 0 20 0 \Theta 60 0
\Deltav FWHM 3.5 km s \Gamma1 3.5 km s \Gamma1
d ¸2000 pc ¸200 \Gamma 500 pc?
p 1.0 1.0
T SPIN
5:2 \Theta 10 \Gamma19 \Delta N HISA =(\Deltav FWHM \Delta (Ü HISA
) cent )
Assumptions P THERM =k 4000 cm \Gamma3 K
Lk ¸(L?) min
N HISA n HISA \Delta Lk
n HISA (P THERM =k)=T SPIN
Projected Size ¸5:8 \Theta 26 pc ¸1:2 \Gamma 2:9 \Theta 3:5 \Gamma 8:7 pc
Lk ¸6 pc ¸0:6 \Gamma 6:0 pc
Resulting T SPIN ¸70 K ¸56 \Gamma 65 K
Measurements Ü HISA ¸1 ¸0:2 \Gamma 1:8
N HISA ¸5 \Theta 10 20 cm \Gamma2 ¸7 \Theta 10 19 \Gamma 8 \Theta 10 20 cm \Gamma2
n HISA ¸60cm \Gamma3 ¸72 \Gamma 62 cm \Gamma3
M HISA ¸600 M fi ¸2:4 \Gamma 160 M fi
Though our methods of estimating distances and background spectra are still crude
at this stage, the values obtained serve as useful rough measures of the class of cold
atomic structures revealed by our ongoing investigation.

OPACITY DISTRIBUTION
The true power of a large survey is the ability to study ensembles of objects sta­
tistically. We are working to develop automated techniques for the identification
of HISA features in velocity cubes and the measurement of their properties, which
can then be used to examine group characteristics and correlations with other ISM
constituents, e.g., CO emission. The complex shapes and sheer numbers of HISA
features require the task of locating HISA voxels and their non­HISA neighbors
(for background estimation) to be automated.
The darker HISA features are quite easy to identify by eye but less so by computer,
due to confusion imposed by noise and complex background HI emission structure.
An initial method which traces at least the darkest HISA is to smooth each velocity
channel in the cube spatially to improve S=N , and then convolve each sightline
spectrum with a narrow (FWHM=2km s \Gamma1 ) 1­D ``Mexican Hat'' wavelet function
to pick out sharp local spectral minima. We have used this technique to flag the
darkest HISA voxels in the velocity cube, interpolating non­HISA neighbors in
the surrounding volume to estimate T OFF . Resulting aggregate properties for each
channel are shown in Figure 7.
The top panel shows the average T OFF \GammaT ON
contrast. For some HISA voxels this can
be 40 K or more, but for most it is only a few kelvins. Since the limit of detectability
is 1\Gamma2 K, this suggests many more HISA features may exist which are too optically
thin to see. The middle panel shows the average of T OFF itself, with ! TB (HI) ?
overplot for comparison. Though !T OFF ? is a measure of average HISA neighbor
brightness rather than average total HI brightness, a correlation between the two
is apparent. Comparison of ! TB (HI) ? with ! T OFF \Gamma T ON ? above shows most
detected features have a contrast of a few percent of background. The minimum
!T OFF ? value is ¸40 K (values of zero occur in channels where no HISA was found).
Consideration of the radiative transfer equation suggests this may represent a lower
limit to T SPIN
, as features become invisible when T SPIN
= p \Delta T OFF
+ T CONT
. 40 K is
warmer than most HISA detected in previous searches of molecular clouds, but
cooler than canonical temperatures of 80 K for ``cold'' neutral hydrogen. We use
T SPIN
=60 K to compute optical depths integrated over each channel in the bottom
panel. This serves as an estimate of HISA mass vs. velocity. Since our present
algorithm flags only the darkest HISA voxels, it uses many of their less­dark HISA
neighbors to find T OFF . This underestimation of background brightness makes all
current contrast, optical depth, and mass measurements lower limits.

A SPIRAL SHOCK SIGNATURE?
A key question about HISA concerns the nature of its physical environment. On
small scales, we are interested in whether HISA occurs inside, around, or indepen­
dent of molecular gas. Our investigation so far shows a mixture of these cases,
perhaps indicating an evolutionary relationship. On large scales, the spatial dis­
tribution of HISA clouds gives clues relating their formation to global processes in
the ISM. We wish to learn whether HISA is distributed in a homogeneous ``plum
pudding'' fashion, or is more concentrated in specific regions.
Figure 7 shows a significant peak of integrated HISA opacity at a radial velocity
of ­41 km s \Gamma1 LSR. This peak coincides with others in mean CO and HI emission
brightness, both of which in fact correspond to gas constituents of the Perseus
Spiral Arm. A plum pudding distribution would also show HISA peaks coincident
with background HI peaks, since the fraction of HISA to HI is constant. However
the HISA peak at ­41 km s \Gamma1 is much greater relative to the general HISA level than
the HI emission peak is to other HI, arguing against a homogeneous distribution.
Instead, the peak in integrated opacity suggests a substantial abundance in HISA
in the Perseus Arm, in full agreement with visual impressions of the amount of
HISA in different channels of the velocity cube. While most velocities appear to
have a certain low level of HISA, perhaps indicating a general faint homogeneous
population, Perseus velocities show a major enhancement.
These velocities are the same predicted by Roberts (1972) to contain gas encounter­
ing the density wave shock of the Perseus Arm. Gas falling into the arm's gravita­
tional potential slows abruptly upon colliding with gas already there, and picks up
speed again on its way out, appearing to slow once more at larger distances, due to
the perspective effects of a given sightline . The shock itself induces cloud compres­
sion, leading to the condensation of HI into H 2 , and the eventual formation of new
stars. We believe we may be seeing evidence of this initial phase transformation.
The radiative transfer requirement of additional HI emission beyond the HISA is
satisfied by the Roberts spiral shock geometry, which places the HI leaving the arm
further along the sightline at the same velocity as gas in the shock. Though our
measurements are currently only lower limits tracing the darkest of the HISA, we
feel it is likely that a more thorough census will produce the same result.

REFERENCES
Cao, Y., Terebey, S., Prince, T. A., & Beichman, C. A., 1997, Ap.J. Supp., 111,
387
Dame, T.M., Ungerechts, H., Cohen, R.S., de Geus, E., Grenier, I.A., May, J.,
Murphy, D.C., Nyman, L.A., & Thaddeus, P., 1987, Ap.J., 322, 706
English, J., et al., 1998, Pub. Ast. Soc. Aust., 15, 56
Feldt, C., 1993, A.&A., 276, 531
Heyer, M. H., Brunt, C., Snell, R. L., Howe, J. E., Schoerb, F. P., & Carpenter,
J. M., 1998, Ap.J. Supp., 115, 241
Knapp, G. R., 1974, A.J., 79, 527
Laustsen, S., Madsen, C., & West R, 1987, Exploring the Southern Sky,
Springer­Verlag: Berlin
Peters, W. L., & Bash, F. N., 1987, Ap.J., 317, 646
Roger, W. W., 1972, Ap.J., 173, 259
Taylor, A. R., et al., 1999, A.J., in preparation

Figure 1: Large Field View: Left: CGPS DRAO HI 21cm brightness at \Gamma41 km s \Gamma1 , ranging from
20 (black) to 130 (white) K. Numerous HISA features appear as dark blue filaments. Center: 12 CO
J =1\Gamma0 emission at the same velocity, with a display ceiling of 3 K. Data from Heyer et al. (1998).
Right: IRAS HIRES 60¯m dust emission, from 2 ¸30MJy sr \Gamma1 . Data from Cao et al. (1997).
Figure 2: Location in the Milky Way: Optical Milky Way, showing the 73 ffi \Theta9 ffi CGPS coverage
(yellow) with the area of Figure 1 marked. B1950 coordinates (green) and constellations (red) are
also shown. Image from a mosaic of scanned photographs by Laustsen, Madsen, & West (1987),
courtesy of the NASA Astrophysical Data Facility.
Figure 3: Perseus Feature Detail: Close­up of a HISA complex in the Perseus Arm at \Gamma41 km s \Gamma1 ,
with brightness ranging from 50 \Gamma 135 K. Yellow CO contours (Heyer et al.) are at 1, 3, & 5 K,
and red 60¯m contours (Cao et al.) are at 15, 17, & 19 MJy sr \Gamma1 . ON (white) and OFF (black)
spectrum boxes used in Fig. 4 are shown.
Figure 4: Perseus Feature Spectra: ON (solid) and OFF (dashed) HI and CO representative
spectra for the Perseus HISA feature, extracted from boxes marked in Fig. 3.
Figure 5: Local Feature Detail: A much fainter feature in the Local Arm at +1km s \Gamma1 , with
brightness from 55 \Gamma 80 K. Yellow CO contours (Dame et al. 1987) are at 0.33, 0.67, 1.0, and 1.33
K. No correspondence in dust emission was found. ON (white) and OFF (black) spectrum boxes
used in Fig. 6 are shown.
Figure 6: Local Feature Spectra: ON (solid) and OFF (dashed) HI and CO representative
spectra for the Local HISA feature, extracted from boxes marked in Fig. 5. The CO detection is
marginal but suggestive, being limited in S=N by low spatial resolution.
Figure 7: HISA Velocity Distribution: