``The Dark Side of the Galaxy: a Skiagraphic Panorama of Cold H I''
S. J. Gibson, A. R. Taylor (University of Calgary);
L. A. Higgs, C. M. Brunt, & P. E. Dewdney (HIA)
2002, 33rd Annual Meeting of the Canadian Astronomical Society
Electronic Poster Contents
POSTER TEXT
INTRODUCTION
A skiagram is
an image made up of shadows or outlines. Much of the cold atomic hydrogen in
the interstellar medium can only be studied skiagraphically as H I self-absorption (HISA) shadows against background H I
21cm emission. Using the DRAO Synthesis Telescope (Landecker et al. 2000), the
Canadian Galactic Plane
Survey (CGPS;
Taylor et al. 2002) has captured a multitude of HISA features, the majority of
which are invisible in prior surveys at lower resolution. We have developed
automated methods of detecting and analyzing HISA that allow us to study the
cloud population systematically in the CGPS. Our HISA search algorithms filter
out all H I emission having large angular scales or broad
linewidths. The significant negative features which remain are flagged as
HISA. The brightness of the H I behind the HISA is
estimated from emission adjacent to the HISA features in space and velocity,
and this background intensity is then used to find the amount of absorption and
various gas properties. Since the HISA gas temperature must be less than the
background brightness temperature, all the H I shown in
this poster has T ~ 100 K.
PANORAMAS
Figure 1 provides panoramic views of the cold H
I visible as HISA in the CGPS. The upper panel is a
longitude-latitude projection integrated over all gas velocities, while the
lower panel is a longitude-velocity projection integrated over all latitudes.
Both give an integral of the quantity TON - TOFF
for the HISA, where darker blue colors indicate greater absorption. The pink
is integrated 12CO J=1-0 emission from the FCRAO Outer Galaxy Survey
(Heyer et al. 1998). Since the vast majority of HISA within the 102-141 degree
longitude range of the FCRAO survey is in the Perseus arm, only Perseus CO is
shown in the (l,v) projection. The HISA and CO line strengths correlate
poorly even when HISA and CO features coincide, which is also rare. This
contradicts of the traditional view of HISA arising from trace atomic gas in
molecular clouds.
The strongest HISA appears to congregate in filamentary complexes. Most of
these are at Perseus spiral arm velocities near -40 km/s in the eastern (left)
half of the survey, and they track the velocity curve for H I gas just downstream of the Perseus spiral shock (Roberts 1972)
rather closely. By contrast, the western (right) end of the survey is
dominated by HISA in the Local spiral arm. All our HISA detections are limited
to areas with bright H I backgrounds, so the absence of
HISA in the maps does not necessarily indicate an absence of cold H I gas. Since the brightest backgrounds are found in the Perseus
arm and along the Local arm tangent near L ~ 80 degrees, it is natural
to see more HISA in these locations.
STRONG VS. WEAK FEATURES
While strong HISA appears confined to discrete structures in space and
velocity, weaker HISA is found everywhere that the H I
background is sufficiently bright. Figure 2
illustrates this point. The panel on the left shows integrated HISA optical
depth, emphasizing the concentrated distributions of strong HISA. The panel on
the right shows the maximum optical depth at each position (among all latitudes
along the projected axis). The opacity of the weaker HISA does not drop off at
the edges of the areas where HISA is detectable: instead, the weak HISA has a
relatively constant strength wherever it can be seen.
We infer from this behavior that the weak HISA traces a low-level ambient
population of cold H I. This material may exist
throughout the interstellar medium, but it only appears as HISA where the
viewing geometry is favorable. Quite likely, the cold atomic gas revealed by
HISA is only the tip of the iceberg. The same may well be true for the strong
HISA -- other large, organized, cold H I complexes may
exist outside of the areas we can examine skiagraphically.
POSTER FIGURES
Larger versions of each image below are available via links.
Figure 1: CGPS HISA + CO Panoramas
In the poster shown at the meeting, these two figures consisted of large,
multipanel maps of the HISA (blue) with overlay transparencies of the CO
(pink). Links are provided below for these PS panel files as well as single,
lower-resolution GIFs. See text for further
details.
(l,b) Projection:
GIFs: HISA + CO
| HISA only
| CO only
+
PS panels: HISA 1
| HISA 2
| HISA 3
| HISA 4
| CO 1
| CO 2
(l,v) Projection:
GIFs: HISA + CO
| HISA only
| CO only
+
PS panels: HISA 1
| HISA 2
| HISA 3
| HISA 4
| CO 1
| CO 2
Figure 2: HISA Optical Depth vs. Velocity
HISA distribution in velocity space for strong and weak features, with
Roberts' (1972) spiral shock ridge shown again for comparison. See text for further details.
CREDITS
- Authors
- Steven Gibson,
University of Calgary
- Russ Taylor,
University of Calgary
- Lloyd Higgs, Herzberg Institute of Astrophysics (HIA)
- Chris Brunt, Herzberg Institute of Astrophysics (HIA)
- Peter Dewdney, Herzberg Institute of Astrophysics (HIA)
- Data
- Funding
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