Suppressed CO emission and high G/D ratios in z=2 galaxies with sub-solar gas-phase metallicity

Coogan, R. T.; Sargent, M. T.; Daddi, E.; Valentino, F.; Strazzullo, V.; Bethermin, M.; Gobat, R.; Liu, D. und Magdis, G. (2019): Suppressed CO emission and high G/D ratios in z=2 galaxies with sub-solar gas-phase metallicity. In: Monthly Notices of the Royal Astronomical Society, Bd. 485, Nr. 2: S. 2092-2105

Abstract

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 Atacama Large Millimeter/submillimeter Array and Very Large Array datasets, and find no individual detections of CO[4-3], CO[1-0], or dust, in stark contrast to the >60 per cent detection rate expected for solar-enrichment galaxies with these MS H alpha star formation rates (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 SFR. Our individual galaxy data and stacked constraints point to a strong inverse dependence alpha Z. of gas-to-dust ratios (G/D) and CO-to-H-2 conversion factors (alpha(CO)) on metallicity at z similar to 2, with gamma(G/D) <-2.2 and gamma(alpha CO) <-0.8, respectively. We quantify the importance of comparing G/D and aCO versus metallicity trends from the literature on a common, suitably normalized metallicity scale. When accounting for systematic offsets between different metallicity scales, our z similar to 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.

• Dokumententyp: Zeitschriftenartikel
• Fakultät: Physik
• Themengebiete: 500 Naturwissenschaften und Mathematik > 530 Physik
• ISSN: 0035-8711
• Sprache: Englisch