Physical conditions of the molecular gas in metal-poor galaxies^⋆

¹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
e-mail: hunt@arcetri.astro.it
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Astronomy Department, King Abdulaziz University, PO Box 80203, Jeddah, Saudia Arabia
⁴ Observatoire de Paris, LERMA, Collège de France, CNRS, PSL, Sorbonne University UPMC, 75014 Paris, France
⁵ Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
⁶ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁷ Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Avenue, Cambridge CB3 0HE, UK
⁸ ESO, Karl Schwarzschild str. 2, 85748 Garching bei München, Germany
⁹ Vittja 64, 74793 Alunda, Sweden

Received: 18 April 2017
Accepted: 8 August 2017

Abstract

Studying the molecular component of the interstellar medium (ISM) in metal-poor galaxies has been challenging because of the faintness of carbon monoxide emission, the most common proxy of H₂. Here we present new detections of molecular gas at low metallicities, and assess the physical conditions in the gas through various CO transitions for 8 galaxies. For one, NGC 1140 (Z/Z_⊙ ~ 0.3), two detections of ¹³CO isotopologues and atomic carbon, [Ci](1–0) and an upper limit for HCN(1–0) are also reported. After correcting to a common beam size, we compared ¹²CO(2–1)/¹²CO(1–0) (R₂₁) and ¹²CO(3–2)/¹²CO(1–0) (R₃₁) line ratios of our sample with galaxies from the literature and find that only NGC 1140 shows extreme values (R₂₁ ~ R₃₁ ~ 2). Fitting physical models to the ¹²CO and ¹³CO emission in NGC 1140 suggests that the molecular gas is cool (kinetic temperature T_kin ≲ 20 K), dense (H₂ volume density n_H2 ≳ 10⁶ cm^-3), with moderate CO column density (N_CO ~ 10¹⁶ cm^-2) and low filling factor. Surprisingly, the [¹²CO]/[¹³CO] abundance ratio in NGC 1140 is very low (~8−20), lower even than the value of 24 found in the Galactic Center. The young age of the starburst in NGC 1140 precludes ¹³CO enrichment from evolved intermediate-mass stars; instead we attribute the low ratio to charge-exchange reactions and fractionation, because of the enhanced efficiency of these processes in cool gas at moderate column densities. Fitting physical models to ¹²CO and [Ci](1–0) emission in NGC 1140 gives an unusually low [¹²CO]/[¹²C] abundance ratio, suggesting that in this galaxy atomic carbon is at least 10 times more abundant than ¹²CO.