J/A+A/675/A145 IRAM spectra of 34 radio sources (Liszt+, 2023)
The dark neutral medium is (mostly) molecular hydrogen.
Liszt H., Gerin M.
<Astron. Astrophys. 675, A145 (2023)>
=2023A&A...675A.145L 2023A&A...675A.145L (SIMBAD/NED BibCode)
ADC_Keywords: Radio sources ; Spectroscopy ; Interstellar medium
Keywords: ISM: clouds - ISM: structure - ISM: molecules - ISM: atoms
Abstract:
More gas is sometimes inferred in molecular cloud complexes than is
represented in HI or CO emission, and this is called dark neutral
medium (DNM).
Our aim is to extend a study of DNM along 13 directions in the
outskirts of Chamaeleon by determining the atomic or molecular
character of the DNM along a larger sample of sightlines.
We acquired ALMA ground rotational state absorption profiles of HCO+
and other molecules toward 33 compact extragalactic continuum
background sources seen toward the Galactic anticenter, deriving
N(H2)=N(HCO+)/3 10-9 as before. We observed J=1-0 CO emission
with the IRAM 30m telescope in directions where HCO+ was newly
detected.
HCO+ absorption was detected in 28 of 33 new directions and CO
emission along 19 of those 28. The five sightlines lacking detectable
HCO+ have three times lower <EB-V> and <N(DNM)>. Binned in EB-V,
N(H2) and N(DNM) are strongly correlated and vary by factors of
50=100 over the observed range EB-V∼0.05-1mag, while N(HI)
varies by factors of only 2-3. On average N(DNM) and N(H2) are
well matched, and detecting HCO- absorption adds little to no H2
in excess of the previously inferred DNM. There are five cases where
2N(H2)<N(DNM)/2 indicates saturation of the HI emission profile.
For sightlines with WCO≥1K-km/s, the CO-H2 conversion
factor N(H2)/WCO=2-3x1020cm-2/(1K-km/s) is higher
than is derived from studies of resolved clouds in gamma-rays.
Our work sampled primarily atomic gas with a mean H2 fraction ∼1/3,
but the DNM is almost entirely molecular. CO fulfills its role as an
H2 tracer when its emission is strong, but large-scale CO surveys
are not sensitive to H2 columns associated with typical values
N(DNM)=2-6x1020cm-2. Lower XCO values from gamma-ray studies
arise in part from different definitions and usage. Sightlines with
WCO≥1K-km/s represent 2/3 of the H2 detected in HCO+ and
detecting 90% of the H2 would require detecting CO at levels
Wco~=0.2-0.3K-km/s.
Description:
There are 61 files: 28 files *.12CO.fits and 33 files *.HCOP.fits
The .12CO.fits files contain 12CO J=1-0 emission spectra from the
IRAM 30m telescope under project 03-19 in units of Kelvins on the main
beam brightness scale.
The .HCOP.fits files contain HCO+ J=1-0 absorption spectra from
ALMA project 2018.1.00115.S The units are line/continuum-1. Most of
the HCO+ profiles can be retrieved from the ALMA science archive but
many were not reduced as part of the batch processing and were
obtained by running the data reduction pipeline offline.
Files are named by source, slightly abbreviated:
J0203P113 -> J0203+1134
The data are spectra in 16 bit integer fits format with the horizontal
axis in units of LSRK velocity.
The unit of intensity is the line/continuum ratio for the HCO+
absorption spectra, or the main beam brightness in Kelvins for the
12CO emission data.
Degenerate single pixel axes give the J2000 coordinates and rest
frequency defining the 0-velocity origin.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 149 61 List of fits spectra
fits/* . 61 Individual fits spectra
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 31 A8 "DD-MM-YY" Obs.date Observation date
33- 44 E12.6 m/s bVELO-LSR Lower value of VELO-LSR interval
46- 56 E11.6 m/s BVELO-LSR Upper value of VELO-LSR interval
58- 64 F7.1 m/s dVELO-LSR VELO-LSR resolution
66- 76 E11.6 Hz Freq Observed frequency
78 I1 Kibyte size Size of FITS file
80- 98 A19 --- FileName Name of FITS file, in subdirectory fits
100-149 A50 --- Title Title of the FITS file
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Acknowledgements:
Harvey Liszt, hliszt(at)nrao.edu
(End) Patricia Vannier [CDS] 17-May-2023