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 - Funding Information:
This paper makes use of the following ALMA data: ADS/JAO.ALMA#2012.1.00885.S and ADS/JAO.ALMA#2015.1.01355.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ.
Funding Information:
We thank the anonymous referee, whose comments improved the clarity and quality of this paper. RTC acknowledges support from the Science and Technology Facilities Council (grant ST/N504452/1) and the University of Sussex. MTS was supported by a Royal Society Leverhulme Trust Senior Research Fellowship (LT150041).
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
VL - 485
SP - 2092
EP - 2105
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 2
ER -