Physical conditions of the molecular gas in metal-poor galaxies

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 H<SUB>2</SUB>. 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<SUB>☉</SUB> 0.3), two detections of <SUP>13</SUP>CO 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 <SUP>12</SUP>CO(2-1)/<SUP>12</SUP>CO(1-0) (R<SUB>21</SUB>) and <SUP>12</SUP>CO(3-2)/<SUP>12</SUP>CO(1-0) (R<SUB>31</SUB>) line ratios of our sample with galaxies from the literature and find that only NGC 1140 shows extreme values (R<SUB>21</SUB> R<SUB>31</SUB> 2). Fitting physical models to the <SUP>12</SUP>CO and <SUP>13</SUP>CO emission in NGC 1140 suggests that the molecular gas is cool (kinetic temperature T<SUB>kin</SUB> ≲ 20 K), dense (H<SUB>2</SUB> volume density n<SUB>H2</SUB> ≳ 10<SUP>6</SUP> cm<SUP>-3</SUP>), with moderate CO column density (N<SUB>CO</SUB> 10<SUP>16</SUP> cm<SUP>-2</SUP>) and low filling factor. Surprisingly, the [<SUP>12</SUP>CO]/[<SUP>13</SUP>CO] 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 <SUP>13</SUP>CO 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 <SUP>12</SUP>CO and [Ci](1-0) emission in NGC 1140 gives an unusually low [<SUP>12</SUP>CO]/[<SUP>12</SUP>C] abundance ratio, suggesting that in this galaxy atomic carbon is at least 10 times more abundant than <SUP>12</SUP>CO. <P />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.