``The Hidden Realm of Cold Interstellar Hydrogen''
S. J. Gibson, A. R. Taylor (University of Calgary);
C. M. Brunt, P. E. Dewdney, & L. A. Higgs (HIA)
2001, 32nd Annual Meeting of the Canadian Astronomical Society, 73 [#P-066]
Electronic Poster Contents
POSTER TEXT
NOTE: This electronic poster contains more material than the
paper poster given at the CASCA meeting. I have added a few extra images from
my talk at the Green Bank
IGPS meeting, with explanatory captions. I have also changed the figure
order from the paper version.
INTRODUCTION
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
Galaxy.
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 1-3 show some of the HISA mapped to date,
while Figures 4-6 show measured and derived HISA gas
properties and an analysis of the relation between HISA and CO emission.
PROPERTY DERIVATION METHOD
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
second equation
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 we determine delta-s from estimated characteristic angular
scales of features and likely distances based on velocity and a simple spiral
arm model (e.g., gas near L=140, V=-40 is in the Perseus arm, with a
distance of ~ 2 kpc, for which 1 arcminute corresponds to 0.6 pc).
We use a canonical ISM pressure of 4000 cm-3 K and consider two
extreme values for fn of 1.0 and 0.01.
RESULTS
- 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 peaks at 0.8 and 1.6 km/s correspond to 1 and 2 channels,
respectively; many of our features may be undersampled in velocity. Likewise,
most features have angular widths close to the 1' CGPS beam, suggesting there
is finer structure we are missing.
- 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
temperatures.
- Since the H I column density is proportional to
both TS and tau, it drops precipitously for low
fn. In the fn = 0.01 case, the H I columns should be multiplied by ~200 to obtain the full atomic
+ molecular value. The total HISA mass in this 5 x 5 degree sample is ~10^5
M_sun for fn = 1.
- HISA strength shows no correlation with 12CO brightness, but
when this apparent scatter is converted into a cumulative 2-D histogram of HISA
stronger than one threshhold matching CO stronger than another threshhold, the
resulting diagram shows a degree of HISA-CO association which significantly
exceeds the random probability of association, especially for strong HISA and
weak CO. Consequently, there are many HISA clouds containing CO, but either
the atomic/molecular fraction is not constant in these objects, or the CO is
not tracing all the H2.
POSTER FIGURES
Larger versions of each image below are available via links.
Figure 1: HISA Survey Map (l,v)
Longitude-velocity projection of the HISA currently mapped over the full CGPS,
from l = 147 to 74 degrees; data between l = 114 and 87 degrees
have yet to be processed. Contours of latitude-integrated HISA
TON - TOFF contrast are shown on top of H I emission from the Leiden-Dwingeloo survey. Red lines mark
approximate velocity boundaries between gas in the Local, Perseus, and Outer
spiral arms.
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Figure 2a: HISA Survey Map - East (l,b)
Top:
Sky projection of detected HISA over a 28-degree section of the CGPS, showing
velocity-integrated TON - TOFF values. Darker
features have stronger absorption.
Bottom: 12CO emission in the Perseus spiral arm, where
most of the HISA is found for this longitude range. Note: In the CASCA poster, this CO map was a transparency overlaid on the HISA map.
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BONUS FEATURE: This figure was not in the CASCA poster!
Figure 2b: HISA Survey Map - West (l,b)
HISA found in a 12-degree section on the west end of the CGPS, looking down the
tangent of the Local spiral arm in Cygnus. Unfortunately, no CO data is
available here for comparison, but the long sightline column gives a very rich
HISA field, with a covering factor approaching unity in places.
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Figure 3: HISA Survey Coverage and Completeness
A cumulative count of the number of image voxels containing HISA, normalized by
the number of voxels with bright enough H I for HISA to be
detected. About 8% of the voxels in which HISA might be found appear to have
HISA at some level, but there may be more HISA which is too faint to be seen in
our data.
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Figure 4: HISA - CO Correlation
HISA TON - TOFF contrast and 12CO
brightness temperature show no obvious correlation, in contradiction to the
traditional expectation of HISA tracing a small, fixed fraction of atomic gas
in molecular clouds. The intensity scale of the plot is logarithmic.
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Figure 5: HISA - CO Association
Despite the lack of quantitative correlation, many HISA clouds do contain CO at
some level, and vice versa. Here we plot the number of image voxels with HISA
above a given contrast and CO above a given brightness, normalized by the
number of voxels which would be associated in a purely random distribution.
The intensity scale is linear. The red contours mark S/N levels of 3,
10, 30, and 100 for the normalized association statistic.
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BONUS FEATURE: This figure was not in the CASCA poster!
Figure 6a: Observed HISA Properties
Before invoking the ideal gas and line integral assumptions to derive the HISA
temperature and opacity, we consider what can be measured directly from the
raw data. The 4-component radiative transfer equation
can be re-written in linear terms as
where the first term in square brackets [ ] is the slope and the second
[ ] term is the intercept. TON - TOFF,
TOFF, and TC are observed, while
TS, tau, and p are unknowns. If all three of
the latter were constant for all HISA features (TC is
typically insignificant), then a plot of TON -
TOFF vs. TOFF would show a single, linear
correlation.
From left to right, the four plots below show the raw distribution of points
from the CGPS MW1 mosaic, then the same distribution normalized by the number
of points in each TOFF bin, then the same data with a set of
comparison slopes (ranging from 0.1 to 1.0 in steps of 0.1) with an arbitrary
zero intercept, and finally a similar plot for the MN1 mosaic. Intensity
scales are logarithmic.
Clearly there is no single linear relation describing these data, which exhibit
considerable scatter. HISA contrast does not correlate well against off-HISA
brightness, and most of the HISA is quite low-contrast regardless of
TOFF. If the faint HISA comprising the bulk of the scattered
points has any coherent slope, it is no steeper than 0.1; if the points are
distributed homogeneously, so that on average p ~ 0.5, then tau
< 0.2 for most HISA.
But leaving aside the faint HISA points in the scatter core, many of the
stronger HISA features comprising the upper envelopes do exhibit a limited
degree of linearity, with slopes of 0.2 or steeper (occasionally ~ 1.0). Since
p <= 1, these must have tau > 0.2 (occasionally > 1.0), as might
be expected. But even here, the linearities are only partial, breaking down
for TOFF > ~ 90-100 K. This suggests either tau or
p (or TS via the intercept term), is changing with
TOFF for the stronger HISA and brighter off-HISA emission --
in other words, while weak HISA may be homogeneously distributed temperature
fluctuations in the ambient H I, the strong HISA cannot
be, since its properties and/or sightline geometry vary with
TOFF. One possibility is that the strong HISA is
preferentially associated with spiral structure in some way.
Figure 6b: Derived HISA Properties
These histograms show the value distributions of a number of properties. The
top row of TON - TOFF, delta-theta, and
delta-v are all directly measured quantities. The two lower rows give
derived values for TS, tau, and NHI,
using the method outlined above with
fn = 1 in the middle row and fn = 0.01 in
the bottom row. The group of plots on the left is linearly scaled, while those
on the right are log-scaled.
BONUS FEATURE: This figure was not in the CASCA poster!
Figure 6c: HISA Property Correlations and Limits
When one has properties for large numbers of objects, one might ask whether
these correlate in any interesting way. The answer appears to be no. The
figure on the left shows feature angular size vs. linewidth, while that on the
right shows TS vs. tau.
While some property values are preferred over others, there are no obvious
correlations (the linear features are sampling artifacts), and in fact there a
general scatter over the full range of parameter values which are permissible
by the limits of the telescope and the image processing software. No size vs.
linewidth relation is seen in the HISA (perhaps indicating the clouds are not
gravitationally bound), and the bimodality in the characteristic temperatures
and optical depths results from Local arm gas being systematically closer than
Perseus arm gas, which translates to smaller delta-s sightline
dimensions in the property solution method (the HISA we detect has similar
angular size regardless of distance). In effect, this survey is seeing the gas
which it is able to see (which is colder in the local arm), but by no means the
full range of what is actually there.
CREDITS
- Authors
- Steven Gibson,
University of Calgary
- Russ Taylor,
University of Calgary
- Chris Brunt, Herzberg Institute of Astrophysics (HIA)
- Peter Dewdney, Herzberg Institute of Astrophysics (HIA)
- Lloyd Higgs, Herzberg Institute of Astrophysics (HIA)
- Data
- Funding
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