TY - JOUR
T1 - Suppressed CO emission and high G/D ratios in z = 2 galaxies with sub-solar gas-phase metallicity
AU - Coogan, R. T.
AU - Sargent, M. T.
AU - Daddi, E.
AU - Valentino, F.
AU - Strazzullo, V.
AU - Béthermin, M.
AU - Gobat, R.
AU - Liu, D.
AU - Magdis, G.
N1 - Publisher Copyright:
© 2019 The Author(s)
PY - 2019/2/15
Y1 - 2019/2/15
N2 - 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 γ G/D <−2.2 and γαCO <−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.
AB - 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 γ G/D <−2.2 and γαCO <−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.
KW - Galaxies: ISM
KW - Galaxies: abundances
KW - Galaxies: evolution
KW - Galaxies: high-redshift
KW - Galaxies: star formation
UR - http://www.scopus.com/inward/record.url?scp=85067027911&partnerID=8YFLogxK
U2 - 10.1093/mnras/stz409
DO - 10.1093/mnras/stz409
M3 - Article
AN - SCOPUS:85067027911
SN - 0035-8711
VL - 485
SP - 2092
EP - 2105
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 2
ER -