``A New View of Cold Atomic Clouds in the Outer Galaxy''
S. J. Gibson, A. R. Taylor (University of Calgary),
& L. A. Higgs (HIA)
1999, Bull. A.A.S., 31, 1475, #72.06
A press release on the Canadian Galactic Plane Survey was associated with this poster.
Electronic Poster Contents
Though a major constituent of the interstellar medium, cold atomic gas, with T
<~ 100 K, is elusive. Maps of 21cm emission are dominated by warm HI, and most
observations of HI absorption against continuum sources are limited to discrete
points. However, HI self-absorption (HISA) against
warmer background HI can give a better view of the structure and distribution
of cold HI clouds in the Galaxy.
A systematic HISA study of cold Galactic HI requires broad angular coverage to
remain unbiased, as well as high angular resolution to detect small-scale
features which might otherwise be washed out. Our investigation is the first
to employ wide-field synthesis imaging to these ends. We are using Canadian Galactic Plane Survey
(Taylor et al. 2000; English et al. 1998) maps taken with the DRAO Synthesis
Telescope (Landecker et al. 1999). Our CGPS images have ~ 1' resolution with
0.8 km/s velocity channels over the region [147.3° > l >
74.2°, -3.6° < b < +5.6°].
Below are maps of HI emission, HISA, and
12CO emission in the eastern 25% of the survey, given as projections
along the velocity (Figure 1) and latitude (Figure 2) axes. We find:
- A rich and diverse population of HISA is revealed, with a wide variety
of shapes, sizes, and contrasts against the background HI. Some of these
features appear unresolved, and many would be invisible to a single-dish
- Despite sharing some of the compact, filamentary qualities of molecular
clouds, few of these HISA features appear associated with detected CO emission
(far-IR dust emission is found in some HISA features lacking CO, but not all).
Either the HISA is not probing the small fraction of HI in H2
clouds, as has been presumed in the past, or CO is not a good tracer of
H2 under some circumstaces.
- The bulk of the HISA occurs at velocities placing it in the Perseus
spiral arm (-35 to -60 km/s); Local gas contains a some faint HISA
features, but no strong absorption.
- The fainter HISA seems widely distributed in space and velocity, while
the darker HISA is organized into discrete complexes. One question we wish to
address is whether these dark complexes represent a particular aspect of arm
structure. For example, they might trace cold HI downstream of the Perseus
density wave shock prior to H2 condensation and star formation.
Figure 2 compares HISA velocities against Roberts'
(1972) spiral shock model (green line).
- If the HISA clouds lie in front of all the HI emission and have a spin
(excitation) temperature of 30 K, the total mass of cold HI they trace is
around 7.5 × 107 Mo. This value decreases for
lower spin temperatures, but may also be greater if part of the HI emission
originates in front of the HISA.
English, J., et al., 1998, Pub. Ast. Soc. Aust., 15, 56
Heyer, M. H., Brunt, C., Snell, R. L., Howe, J. E., Schoerb, F. P.,
J. M., 1998, Ap. J. Supp., 115, 241
Landecker, T. L., et al., 1999, A. & A., submitted
Roberts, W. W., 1972, Ap. J., 173, 259
Taylor, A. R., et al., 2000, A. J., submitted
Larger versions of each image below are available via links. PostScript copies
of Figures 1 & 2 are in many pieces to permit printing on regular
letter-sized pages; they are collected in sizeable gzipped tarfiles. Please
note assembling these panels after printing is a somewhat tedious job!
Figure 1: HI Survey Map (l,b)
HI 21cm brightness integrated over all velocity channels; white is bright and
black is faint. Blue contours show HISA column density (8 ×
1019 cm-2), computed for an assumed HISA spin temperature
of 30 K. Red contours show FCRAO 12CO J=1-0 brightness
integrated over the Perseus arm, from the Heyer et al. (1998) survey.
Full-size version: JPEG | GIF |
Figure 2: HI Survey Map (l,v)
As above, except integrated over latitude instead of velocity; HISA column
contours are for 0.5 and 1.0 × 1021 cm-2. Local gas
occurs near 0 km/s, and Perseus arm gas near -40 km/s. The green line marks
the position of the Roberts (1972) model HI spiral shock ridge in the Perseus
Full-size version: JPEG | GIF |
Figure 3: HI Brightness
Close view of a ``raw'' HI velocity channel with self-absorption. Brightness
ranges from 40 K (black) to 130 K (white). ON and OFF HI spectra, from the
cross and boxes respectively (visible in the full-scale image), are plotted for
one strong but compact HISA feature. The upper panel compares ON (solid) with
OFF (dashed), and the lower panel shows the ON-OFF temperature difference.
Figure 4: Velocity Search
We are developing automated methods of locating HISA in velocity cubes by
removing large-scale emission in space and velocity and flagging small-scale
dark residuals. This image shows (negative) temperatures of the dark residuals
found with our spectral search, ranging from 0 to -40 K.
Figure 5: Spatial Search
Here are residual (negative) temperatures from our spatial search. We
combine these results with those of the spectral search to flag suspected HISA
voxels (3-D pixels).
Figure 6: HISA Amplitude
Final ON-OFF temperature differences for all identified HISA voxels. The
voxels have been assembled into contiguous 3-D groups whose non-HISA spatial
and velocity edges are used to obtain the best estimate of HI brightness
behind the HISA feature. This is essential for determining its absorption
properties and mass.
- Steven Gibson,
University of Calgary
- Russ Taylor,
University of Calgary
- Lloyd Higgs, Herzberg Institute of Astrophysics (HIA)
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