J/A+A/682/A161 H2 in the diffuse ISM (Skalidis+, 2024)
Constraining the H2 column densities in the diffuse interstellar medium using
dust extinction and H I data.
Skalidis R., Goldsmith P.F., Hopkins P.F., Ponnada S.B.
<Astron. Astrophys. 682, A161 (2024)>
=2024A&A...682A.161S 2024A&A...682A.161S (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; H I data ; Carbon monoxide ; Extinction
Keywords: methods: data analysis - ISM: abundances - dust, extinction -
ISM: structure - Galaxy: abundances - local interstellar matter
Abstract:
Carbon monoxide (CO) is a poor tracer of H2 in the diffuse
interstellar medium (ISM), where most of the carbon is not
incorporated into CO molecules, unlike the situation at higher
extinctions.
We present a novel, indirect method for constraining H2 column
densities (NH2) without employing CO observations. We show that
previously recognized nonlinearities in the relation between the
extinction, AV(H2), derived from dust emission and the HI column
density (NHI) are due to the presence of molecular gas.
We employed archival NH2 data, obtained from the UV spectra of
stars, and calculated AV(H2) toward these sight lines using 3D
extinction maps. A third-degree polynomial fits the data well and can
be used to estimate NH2 using NHI and AV(H2). In 95% of the cases,
the estimates produced by the fitted equation have deviations of less
than a factor of 3.5. We constructed a NH2 map of our Galaxy and
compared it to the CO integrated intensity (WCO) distribution.
We find that the average ratio (XCO) between NH2 and WCO is
approximately equal to 2x1020cm-2(K.km/s)-1^, consistent with
previous estimates. However, we find that the XCO factor varies by
orders of magnitude on arcminute scales between the outer and the
central portions of molecular clouds. For regions with
NH2≥1020cm-2, we estimate that the average H2 fractional
abundance is 0.25. Multiple (distinct) largely atomic clouds are
likely found along high-extinction sightlines (AV≥1mag), hence
limiting fH2 in these directions. More than 50% of the lines of
sight with NH2≥1020cm-2^ are untraceable by CO with a J=1-0
sensitivity limit WCO=1K.km/s.
Description:
We constructed NH2 maps of our Galaxy using HI, and dust extinction
data. The HI data were obtained from the HI4PI survey, while for dust
extinction, we used two different maps: 1) the extinction map of
Schlegel et al. (1998ApJ...500..525S 1998ApJ...500..525S), normalized as suggested by
Schlafly et al. (2010ApJ...725.1175S 2010ApJ...725.1175S), and 2) the extinction map of
the Planck Collaboration. We provide a copy of both NH2 maps. The
fits files NH2mapschlegel.fits, and NH2mapplanck.fits correspond
to our inferred NH2 maps obtained using the Schlegel and Planck data
respectively. We compared both NH2 maps with the CO (J=1-0)
integrated intensity (WCO) from Dame et al. (2001ApJ...547..792D 2001ApJ...547..792D) to
construct XCO (XCO=NH2/WCO) maps of our Galaxy. We also provide a
copy of the XCO maps following the same name convention.
The FITS files contain H2 column densities and XCO sampled on a
HEALPix grid (nside=1024, Galactic coordinates, Ring indexing scheme).
In the NH2 maps, pixels with NH2<10^19, which is the sensitivity
limit of our method, have been masked. In the XCO maps, pixels with
NH2<1019cm-2 and WCO<1K.km/s (this is the sensitivity limit in
the CO survey of Dame et al., 2001ApJ...547..792D 2001ApJ...547..792D), have been masked.
In all maps, the values are shown on a logarithmic scale, while the
angular resolution of the maps is 16 arcminutes. See Sects. 2 and 3 of
the manuscript for more details.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 95 4 List of fits maps
fits/* . 4 Individual fits maps
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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- 4 I4 --- Nx Number of pixels along X-axis
6- 10 I5 --- Ny Number of pixels along Y-axis
12- 16 I5 Kibyte size Size of FITS file
18- 38 A21 --- FileName Name of FITS file, in subdirectory fits
40- 95 A56 --- Title Title of the FITS file
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Acknowledgements:
Raphael Skalidis, skalidis(at)caltech.edu
(End) Patricia Vannier [CDS] 11-Jan-2024