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Revealing the CO X-factor in dark molecular gas through sensitive ALMA absorption observations


Luo, G and Li, D and Tang, N and Dawson, JR and Dickey, JM and Bronfman, L and Qin, SL and Gibson, SJ and Plambeck, R and Finger, R and Green, A and Mardones, D and Koo, BC and Lo, N, Revealing the CO X-factor in dark molecular gas through sensitive ALMA absorption observations, Astrophysical Journal Letters, 889, (1) pp. 1-7. ISSN 2041-8205 (2020) [Refereed Article]

Copyright Statement

2020. The American Astronomical Society. All rights reserved.

DOI: doi:10.3847/2041-8213/ab6337


Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be underabundant and/or subthermally excited. We performed one of the most sensitive (down to ${\tau }_{\mathrm{rms}}^{\mathrm{CO}}\sim 0.002$ and ${\tau }_{\mathrm{rms}}^{{\mathrm{HCO}}^{+}}\sim 0.0008$) submillimeter molecular absorption line observations toward 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to ${A}_{{\rm{v}}}\lt 0.32\,\mathrm{mag}$ and HCO+ was detected down to ${A}_{{\rm{v}}}\lt 0.2\,\mathrm{mag}$, where atomic gas and dark molecular gas start to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures Tex of 4.1 and 2.7 K, for CO and for HCO+, respectively, which are close to the cosmic microwave background and explain their being undercounted in emission surveys. A stronger linear correlation was found between ${N}_{{\mathrm{HCO}}^{+}}$ and ${N}_{{{\rm{H}}}_{2}}$ (Pearson correlation coefficient P~0.93) than that of NCOand${N}_{{{\rm{H}}}_{2}}$ (P~0.33), suggesting HCO+ is a better tracer of H2 than CO in diffuse gas. The derived CO-to-H2 conversion factor (the CO X-factor) of (14+-3)x1020 cm-2 (K $\mathrm{km}\,{{\rm{s}}}^{-1}$)-1 is approximately six times larger than the average value found in the Milky Way.

Item Details

Item Type:Refereed Article
Keywords:carbon-monoxide, clouds, emission, chemistry, ratio, dust, HCO+
Research Division:Physical Sciences
Research Group:Astronomical sciences
Research Field:Galactic astronomy
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the physical sciences
UTAS Author:Dawson, JR (Dr Joanne Dawson)
UTAS Author:Dickey, JM (Professor John Dickey)
ID Code:152816
Year Published:2020
Web of Science® Times Cited:4
Deposited By:Physics
Deposited On:2022-08-24
Last Modified:2022-09-19

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