EGALISM

Project 1 :   Cormier et al. 2018

¹³CO emission in nearby discs

The goal of the study is to better understand the emission of ¹³CO and its use as a tracer of molecular gas mass in galaxies. Carbon monoxide (CO) provides crucial information about the molecular gas properties of galaxies. While ¹²CO has been targeted extensively, isotopologues such as ¹³CO have the advantage of being less optically thick and observations have recently become accessible across full galaxy discs. This study presents a comprehensive new dataset of ¹³CO(1-0) observations with the IRAM 30-m telescope of the full discs of 9 nearby spiral galaxies from the EMPIRE survey at a spatial resolution of 1.5 kpc (see figure above). We analyse the ¹²CO(1-0)-to-¹³CO(1-0) ratio as a function of galactocentric radius and other physical parameters. That ratio varies by a factor of 2 at most within and amongst galaxies with a median value of 11 and larger variations in the galaxy centres than in the discs. Those variations can be explained by optical depth effects, most likely due to changes in the mixture of diffuse/dense gas, and abundance gradients. Overall, we find that ¹³CO(1-0) does not appear to be a good predictor of the bulk molecular gas mass in normal galaxy discs due to the presence of a large diffuse phase, but it may be a better tracer of the mass than ¹²CO(1-0) in the galaxy centres where the fraction of dense gas is larger.

The goal of the study is to characterize the physical conditions of (unresolved) nearby low-metallicity galaxies. The Spitzer, Herschel (and now SOFIA) observatories have provided exquisite photometry and spectroscopy data for 50 galaxies of the Dwarf Galaxy Survey. Using the spectral synthesis code Cloudy, we are modeling the cooling lines that originate from the star-forming regions in those galaxies (HII region and PDR). Those models allow us to constrain the dust and gas physical conditions, the phase filling factors, and to investigate key questions regarding the phase origin of [CII] or mass of CO-dark gas as a function of metallicity and SFR. This first paper presents 2-component HII region and PDR models for each galaxy of the DGS survey. We show that: (1) lower-metallicity galaxies are typified by harder radiation fields; (2) [CII] emission arises mainly from the neutral gas, not from the ionized gas; (3) the covering factor of PDRs is correlated with metallicity, facilitating the escape of ionizing photons. A second paper, following Madden & Cormier, will report on the mass of CO-dark gas in those galaxies.