Version 2 2023-06-12, 08:59Version 2 2023-06-12, 08:59
Version 1 2023-06-09, 16:39Version 1 2023-06-09, 16:39
journal contribution
posted on 2023-06-12, 08:59authored byRosemary Coogan, Mark Sargent, E Daddi, F Valentino, V Strazzullo, M Béthermin, R Gobat, D Liu, G Magdis
We study a population of significantly sub-solar enrichment galaxies at z=1.99, to investigate how molecular gas, dust and star-formation relate in low-metallicity galaxies at the peak epoch of star-formation. We target our sample with several deep ALMA and VLA datasets, and find no individual detections of CO[4-3], CO[1-0] or dust, in stark contrast to the >60% detection rate expected for solar-enrichment galaxies with these MS Halpha SFRs. We find that both low and high density molecular gas (traced by CO[1-0] and CO[4-3] respectively) are affected by the low enrichment, showing sample average (stacked) luminosity deficits >0.5-0.7 dex below expectations. This is particularly pertinent for the use of high-J CO emission as a proxy of instantaneous star-formation rate. Our individual galaxy data and stacked constraints point to a strong inverse dependence ?Z? of gas-to-dust ratios (G/D) and CO-to-H2 conversion factors (aco) on metallicity at z~2, with ?G/D<-2.2 and ?aCO<-0.8, respectively. We quantify the importance of comparing G/D and aco vs. metallicity trends from the literature on a common, suitably normalised metallicity scale. When accounting for systematic offsets between different metallicity scales, our z~2 constraints on these scaling relations are consistent with the corresponding relations for local galaxies. However, among those local relations, we favour those with a steep/double power-law dependence of G/D on metallicity. Finally, we discuss the implications of these findings for (a) gas mass measurements for sub-M* galaxies, and (b) efforts to identify the characteristic galaxy mass scale contributing most to the comoving molecular gas density at z=2.