Suppressed CO emission and high G/D ratios in z = 2 galaxies with sub-solar gas-phase metallicity
Date Issued
2019
Author(s)
Coogan, R. T.
•
Sargent, M. T.
•
Daddi, E.
•
Valentino, F.
•
•
Béthermin, M.
•
Gobat, R.
•
Liu, D.
•
Magdis, G.
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 α 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 ∝ Zγ of gas-to-dust ratios (G/D) and CO-to-H2 conversion factors (αCO) on metallicity at z ∼ 2, with γG/D <-2.2 and γ _{α _CO}< -0.8, respectively. We quantify the importance of comparing G/D and αCO versus metallicity trends from the literature on a common, suitably normalized 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.
Volume
485
Issue
2
Start page
2092
Issn Identifier
0035-8711
Ads BibCode
2019MNRAS.485.2092C
Rights
open.access
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