``Cold H I in the Outer Galaxy:
Properties and Distribution''
S. J. Gibson, A. R. Taylor (University of Calgary);
L. A. Higgs, P. E. Dewdney, & C. M. Brunt (HIA)
2000, Bull. A.A.S., 32, 1401, #7.02
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 H I, and most observations of H I absorption
against continuum sources are limited to discrete points. However, H I self-absorption (HISA) against
warmer background H I can give a better view of the
structure and distribution of cold H I clouds in the
A systematic HISA study of cold Galactic H I 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. 2001) maps taken with the DRAO
Synthesis Telescope. 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°].
This poster gives preliminary results for a full-fledged analysis of the gas
properties and distribution of HISA features in the CGPS. We employ several
automated algorithms to identify self-absorption within the H I data, estimate the background brightness being absorbed, and
compute its physical properties from a limited set of assumptions. Figures 1-4 illustrate this process over a small part
of the survey, while Figures 5-6 plot the property
A simple radiative transfer model representing a HISA cloud with foreground and
background H I emission and background continuum emission
is described by the expression
where TON and TOFF are observed
brightnesses on and off the HISA feature, TS is the spin or
excitation temperature of the HISA gas, tau is its optical depth,
TC is the continuum intensity, and p is
the fraction of H I emission lying behind the HISA
feature. We measure TON, TOFF, and
TC, and assume a likely value for p, but
TS and tau remain unknown. To constrain these two
variables, we make use of line integral and ideal gas relations to derive a
which gives TS in terms of the line center opacity
tau0, linewidth delta-v, the physical thickness of the
HISA feature along the sightline delta-s, and the partial pressure of
the atomic gas, P fn. With reasonable values applied to
these new parameters, TS and tau can be obtained by
solving the two equations together (see Gibson
et al. 2000 ApJ 540, 851 for details).
For this poster, we assume p = 1, the most favorable value for seeing
HISA, and delta-s = 0.6 pc, which corresponds to 1' in the Perseus arm,
and may serve as a rough average scale of ``solidity'' for the HISA we detect.
We use a canonical ISM pressure of 4000 cm-3 K and consider two
extreme values for fn of 1.0 and 0.01.
The plots below give a number of results related to
the properties we have begun to explore; these are taken from a 5° x
5° portion of the CGPS. We find most properties cluster around common
- The HISA features have a temperature contrast of only 10% against
typical H I backgrounds of ~100 K. Most are too subtle to
be seen in lower-resolution searches; the crowding of features near zero
contrast suggests many may be missed by our survey as well.
- Most features have very narrow linewidths, indicating quiescent gas.
The strong peak at 1.6 km/s corresponds to 2 channels in the CGPS; many of our
features may be undersampled in velocity.
- If the cold gas traced by HISA is purely atomic (fn =
1), it is at the cold end of the normal H I temperature
range, but warmer than molecular gas. If however the gas is mostly
H2 (fn = 0.01), it has typical molecular cloud
- Though it often spatially anticorrelates with TS, the
optical depth also drops significantly with TS for low
fn. The low-tau tail of the distribution represents
the warmer, barely-discernable HISA.
- The H I column density is proportional to both
TS and tau and drops more than either for low
fn; the gas mass is similarly affected. The second mass peak
represents Local arm gas, with the main peak corresponding to Perseus arm gas.
In the fn = 0.01 case, these columns and masses should be
multiplied by ~200 to obtain the full atomic + molecular value. The total HISA
mass in this 5° x 5° sample is ~105 solar masses for
fn = 1.
- The spatial and velocity distribution of tau(HISA) tracks the H
I background, but incompletely; most, but not all, HISA
may be mixed homogeneously with H I emission. Our
investigation of the spatial distribution of HISA is ongoing.
Larger versions of each image below are available via links.
Figure 1: H I Brightness
Close view of a ``raw'' H I velocity channel with
self-absorption. Brightness ranges from 40 K (black) to 120 K
(white). ON and OFF H I spectra, from the cross
and boxes respectively, are given for one strong but compact HISA feature
marked in the intensity map. The upper spectrum panel compares ON
(solid) with OFF (dashed), and the lower spectrum panel shows the
ON-OFF temperature difference.
Figure 2: HISA Amplitude
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 H I
brightness behind the HISA feature. This is essential for determining its
absorption properties and mass.
Figure 3: HISA Temperature
Spin temperatures obtained at all points of the map containing HISA; for cold H
I, these are similar to the gas kinetic temperature. The
intensity scale ranges from 30 K (black) to 110 K (white), at
which point the gas is too warm to self-absorb. The cooler sightlines
frequently occur in cloud interiors, but this is not a general rule.
Figure 4: Optical Depth
tau values obtained simultaneously with TS. The
intensity scale ranges from 0 (white) to 1.4 (black). Optical
depth and temperature anticorrelate to a large degree; the more opaque regions
tend to be colder.
Figure 5: Feature Properties
These histograms show the value distributions of a number of properties.
TON - TOFF and delta-v are both
measured quantities, independent of other assumptions (they are shown in both
sets of plots). TS and tau are calculated for
fn = 1 on the left and for fn = 0.01 on the
right; NHI and MHI are derived from these
values. HISA in the Local spiral arm is generally less massive than that in
the Perseus arm at this longitude.
Figure 6: HISA and H I Emission Velocities
This longitude - velocity projection shows the relative distributions of HISA
and H I emission over a larger area. The background image
is H I emission intensity, with contours of weak
(yellow) and strong (red) HISA opacity overlaid. Both weak and
strong HISA trace the background H I to a limited extent,
but also exhibit a degree of independence indicating the HISA gas is not
homogeneously mixed with the ambient H I.
- Steven Gibson,
University of Calgary
- Russ Taylor,
University of Calgary
- Lloyd Higgs, Herzberg Institute of Astrophysics (HIA)
- Peter Dewdney, Herzberg Institute of Astrophysics (HIA)
- Chris Brunt, Herzberg Institute of Astrophysics (HIA)
Return to the Neutral Hydrogen Self-Absorption page.