ALLSMOG: an APEX Low-redshift Legacy Survey for MOlecular Gas – I. Molecular gas scaling relations, and the effect of the CO/H2 conversion factor

Abstract

We present ALLSMOG, the APEX Low-redshift Legacy Survey for MOlecular Gas. ALLSMOG is a survey designed to observe the CO(2 -1) emission line with the APEX telescope, in a sample of local galaxies (0.01 < z < 0.03), with stellar masses in the range 8.5 < log(M * /M ) < 10. This paper is a data release and initial analysis of the first two semesters of observations, consisting of 42 galaxies observed in CO(2 -1). By combining these new CO(2 -1) emission line data with archival H I data and SDSS optical spectroscopy, we compile a sample of low-mass galaxies with well-defined molecular gas masses, atomic gas masses, and gas-phase metallicities. We explore scaling relations of gas fraction and gas consumption time-scale, and test the extent to which our findings are dependent on a varying CO/H 2 conversion factor. We find an increase in the H 2 /H I mass ratio with stellar mass which closely matches semi-analytic predictions. We find a mean molecular gas fraction for ALLSMOG galaxies of M H2 /M * = (0.09-0.13), which decreases with stellar mass. This decrease in total gas fraction with stellar mass is in excess of some model predictions at low stellar masses. We measure a mean molecular gas consumption time-scale for ALLSMOG galaxies of 0.4-0.7 Gyr. We also confirm the non-universality of the molecular gas consumption time-scale, which varies (with stellar mass) from 100 Myr to 2 Gyr. Importantly, we find that the trends in the H 2 /H I mass ratio, gas fraction, and the non-universal molecular gas consumption time-scale are all robust to a range of recent metallicity-dependent CO/H 2 conversion factors.