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

Gan Luo; Di Li; Ningyu Tang; J. R. Dawson; John M. Dickey; L. Bronfman; Sheng Li Qin; Steven J. Gibson; Richard Plambeck; Ricardo Finger; Anne Green; Diego Mardones; Bon Chul Koo; Nadia Lo

doi:10.3847/2041-8213/ab6337

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

Gan Luo, Di Li, Ningyu Tang, J. R. Dawson, John M. Dickey, L. Bronfman, Sheng Li Qin, Steven J. Gibson, Richard Plambeck, Ricardo Finger, Anne Green, Diego Mardones, Bon Chul Koo, Nadia Lo

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Abstract

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 τ^CO_rms ∼ 0.002 and τ^HCO+_rms ∼ 0.0008) submillimeter molecular absorption line observations toward 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to A_v < 0.32 mag and HCO⁺ was detected down to A_v < 0.2 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 T_ex 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_HCO+ and N_H2 (Pearson correlation coefficient P ∼ 0.93) than that of N_CO and N_H2 (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) × 10²⁰ cm^-2 (K km s^-1) ^-1 is approximately six times larger than the average value found in the Milky Way.

Original language	English
Article number	L4
Pages (from-to)	1-7
Number of pages	7
Journal	Astrophysical Journal Letters
Volume	889
Issue number	1
DOIs	https://doi.org/10.3847/2041-8213/ab6337
Publication status	Published - 20 Jan 2020

Bibliographical note

Copyright 2020 the American Astronomical Society. First published in Astrophysical Journal Letters, volume 889, issue 1, article L4. The original publication is available at https://doi.org/10.3847/2041-8213/ab6337, published by IOP Publishing. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.

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Luo, G., Li, D., Tang, N., Dawson, J. R., Dickey, J. M., Bronfman, L., Qin, S. L., Gibson, S. J., Plambeck, R., Finger, R., Green, A., Mardones, D., Koo, B. C., & Lo, N. (2020). Revealing the CO X-factor in dark molecular gas through sensitive ALMA absorption observations. Astrophysical Journal Letters, 889(1), 1-7. Article L4. https://doi.org/10.3847/2041-8213/ab6337

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title = "Revealing the CO X-factor in dark molecular gas through sensitive ALMA absorption observations",

abstract = "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 τCOrms ∼ 0.002 and τHCO+rms ∼ 0.0008) submillimeter molecular absorption line observations toward 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to Av < 0.32 mag and HCO+ was detected down to Av < 0.2 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 NHCO+ and NH2 (Pearson correlation coefficient P ∼ 0.93) than that of NCO and NH2 (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) × 1020 cm-2 (K km s-1) -1 is approximately six times larger than the average value found in the Milky Way.",

author = "Gan Luo and Di Li and Ningyu Tang and Dawson, {J. R.} and Dickey, {John M.} and L. Bronfman and Qin, {Sheng Li} and Gibson, {Steven J.} and Richard Plambeck and Ricardo Finger and Anne Green and Diego Mardones and Koo, {Bon Chul} and Nadia Lo",

note = "Copyright 2020 the American Astronomical Society. First published in Astrophysical Journal Letters, volume 889, issue 1, article L4. The original publication is available at https://doi.org/10.3847/2041-8213/ab6337, published by IOP Publishing. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.",

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T1 - Revealing the CO X-factor in dark molecular gas through sensitive ALMA absorption observations

AU - Luo, Gan

AU - Li, Di

AU - Tang, Ningyu

AU - Dawson, J. R.

AU - Dickey, John M.

AU - Bronfman, L.

AU - Qin, Sheng Li

AU - Gibson, Steven J.

AU - Plambeck, Richard

AU - Finger, Ricardo

AU - Green, Anne

AU - Mardones, Diego

AU - Koo, Bon Chul

AU - Lo, Nadia

N1 - Copyright 2020 the American Astronomical Society. First published in Astrophysical Journal Letters, volume 889, issue 1, article L4. The original publication is available at https://doi.org/10.3847/2041-8213/ab6337, published by IOP Publishing. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.

PY - 2020/1/20

Y1 - 2020/1/20

N2 - 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 τCOrms ∼ 0.002 and τHCO+rms ∼ 0.0008) submillimeter molecular absorption line observations toward 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to Av < 0.32 mag and HCO+ was detected down to Av < 0.2 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 NHCO+ and NH2 (Pearson correlation coefficient P ∼ 0.93) than that of NCO and NH2 (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) × 1020 cm-2 (K km s-1) -1 is approximately six times larger than the average value found in the Milky Way.

AB - 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 τCOrms ∼ 0.002 and τHCO+rms ∼ 0.0008) submillimeter molecular absorption line observations toward 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to Av < 0.32 mag and HCO+ was detected down to Av < 0.2 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 NHCO+ and NH2 (Pearson correlation coefficient P ∼ 0.93) than that of NCO and NH2 (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) × 1020 cm-2 (K km s-1) -1 is approximately six times larger than the average value found in the Milky Way.

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M3 - Article

AN - SCOPUS:85080039642

SN - 2041-8205

VL - 889

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EP - 7

JO - Astrophysical Journal Letters

JF - Astrophysical Journal Letters

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ER -

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

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Shining Light on the Dark Milky Way (ARC Discovery Early Career Researcher Award)

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

Abstract

Bibliographical note

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Shining Light on the Dark Milky Way (ARC Discovery Early Career Researcher Award)

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