Physical conditions of the molecular gas in metal-poor galaxies⋆
1 INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
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
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 Tkin ≲ 20 K), dense (H2 volume density nH2 ≳ 106 cm-3), with moderate CO column density (NCO ~ 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.
Key words: galaxies: starburst / galaxies: dwarf / galaxies: star formation / galaxies: ISM / ISM: molecules / radio lines: ISM
Based on observations carried out with the IRAM 30 m and the Atacama Pathfinder Experiment (APEX). IRAM is supported by the INSU/CNRS (France), MPG (Germany), and IGN (Spain), and APEX is a collaboration between the Max-Planck-Institut fur Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.
© ESO, 2017