The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Molecular Gas Reservoirs in High-redshift Galaxies

Abstract

We study the molecular gas properties of high-z galaxies observed in the ALMA Spectroscopic Survey (ASPECS) that targets an ∼1 arcmin<SUP>2</SUP> region in the Hubble Ultra Deep Field (UDF), a blind survey of CO emission (tracing molecular gas) in the 3 and 1 mm bands. Of a total of 1302 galaxies in the field, 56 have spectroscopic redshifts and correspondingly well-defined physical properties. Among these, 11 have infrared luminosities {L}<SUB>{IR</SUB>}\gt {10}<SUP>11</SUP> {L}<SUB>☉ </SUB>, I.e., a detection in CO emission was expected. Out of these, 7 are detected at various significance in CO, and 4 are undetected in CO emission. In the CO-detected sources, we find CO excitation conditions that are lower than those typically found in starburst/sub-mm galaxy/QSO environments. We use the CO luminosities (including limits for non-detections) to derive molecular gas masses. We discuss our findings in the context of previous molecular gas observations at high redshift (star formation law, gas depletion times, gas fractions): the CO-detected galaxies in the UDF tend to reside on the low-{L}<SUB>{IR</SUB>} envelope of the scatter in the {L}<SUB>{IR</SUB>}{--}{L}<SUB>{CO</SUB>}<SUP>\prime </SUP> relation, but exceptions exist. For the CO-detected sources, we find an average depletion time of ∼1 Gyr, with significant scatter. The average molecular-to-stellar mass ratio ({M}<SUB>{{H</SUB>}2}/M <SUB>*</SUB>) is consistent with earlier measurements of main-sequence galaxies at these redshifts, and again shows large variations among sources. In some cases, we also measure dust continuum emission. On average, the dust-based estimates of the molecular gas are a factor ∼2-5× smaller than those based on CO. When we account for detections as well as non-detections, we find large diversity in the molecular gas properties of the high-redshift galaxies covered by ASPECS.