Gas, dust, and the CO-to-molecular gas conversion factor in low-metallicity starbursts^⋆

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: leslie.hunt@inaf.it
² Princeton University Observatory, Peyton Hall, Princeton, NJ, 08544-1001 USA
³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
⁵ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁶ Observatoire de Paris, LERMA, Collège de France, CNRS, PSL, Sorbonne University, 75014 Paris, France
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Astronomy Department, King Abdulaziz University, PO Box 80203, Jeddah, Saudia Arabia
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
¹⁰ INAF – Istituto di Radioastronomia, Via Piero Gobetti 93/3, 40129 Bologna, Italy
¹¹ Department of Physics – University of Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹² INFN – Istituto Nazionale di Fisica Nucleare, Largo B. Pontecorvo 3, 56127 Pisa, Italy
¹³ INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
¹⁴ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham, DH1 3LE UK
¹⁵ Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham, DH1 3LE UK
¹⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹⁷ Dipartimento di Astronomia e Scienza dello Spazio, Università degli Studi di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
¹⁸ AURA for the European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218 USA

Received: 27 December 2022
Accepted: 10 May 2023

Abstract

The factor relating CO emission to molecular hydrogen column density, X_CO, is still subject to uncertainty, in particular at low metallicity. In this paper, to quantify X_CO at two different spatial resolutions, we exploited a dust-based method together with ALMA 12-m and ACA data and H I maps of three nearby metal-poor starbursts, NGC 625, NGC 1705, and NGC 5253. Dust opacity at 250 pc resolution was derived based on dust temperatures estimated by fitting two-temperature modified blackbodies to Herschel PACS data. By using the HI maps, we were then able to estimate dust-to-gas ratios in the regions dominated by atomic gas, and, throughout the galaxy, to infer total gas column densities and H₂ column densities as the difference with HI. Finally, from the ACA CO(1–0) maps, we derived X_CO. We used a similar technique with 40 pc ALMA 12-m data for the three galaxies, but instead derived dust attenuation at 40 pc resolution from reddening maps based on VLT/MUSE data. At 250 pc resolution, we find X_CO ∼ 10²² − 10²³ cm⁻²/K km s⁻¹, 5–1000 times the Milky Way value, with much larger values than would be expected from a simple metallicity dependence. Instead, at 40 pc resolution, X_CO again shows large variation, but is roughly consistent with a power-law metallicity dependence, given the Z ∼ 1/3 Z_⊙ metal abundances of our targets. The large scatter in both estimations could imply additional parameter dependence, which we have investigated by comparing X_CO with the observed velocity-integrated brightness temperatures, I_CO, as predicted by recent simulations. Indeed, larger X_CO is significantly correlated with smaller I_CO, but with slightly different slopes and normalizations than predicted by theory. Such behavior can be attributed to the increasing fraction of CO-faint (or dark) H₂ gas with lower spatial resolution (larger beams). This confirms the idea the X_CO is multivariate, depending not only on metallicity but also on the CO brightness temperature and beam size. Future work is needed to consolidate these empirical results by sampling galaxies with different metal abundances observed at varying spatial resolutions.