The Molecular Gas Reservoirs of z ∼ 2 Galaxies: A Comparison of CO(1−0) and Dust-based Molecular Gas Masses

Abstract

We test the use of long-wavelength dust continuum emission as a molecular gas tracer at high redshift, via a unique sample of 12, z ∼ 2 galaxies with observations of both the dust continuum and CO(1-0) line emission (obtained with the Atacama Large Millimeter Array and Karl G. Jansky Very Large Array, respectively). Our work is motivated by recent, high redshift studies that measure molecular gas masses (M mol ) via a calibration of the rest-frame 850µm luminosity (L 850µm,rest ) against the CO(1-0)-derived M mol of star-forming galaxies. We hereby test whether this method is valid for the types of high-redshift, star-forming galaxies to which it has been applied. We recover a clear correlation between the rest-frame 850µm luminosity, inferred from the singleband, long-wavelength flux, and the CO(1-0) line luminosity, consistent with the samples used to perform the 850µm calibration. The molecular gas masses, derived from L 850µm,rest , agree to within a factor of two with those derived from CO(1-0). We show that this factor of two uncertainty can arise from the values of the dust emissivity index and temperature that need to be assumed in order to extrapolate from the observed frequency to the rest-frame at 850µm. The extrapolation to 850µm therefore has a smaller effect on the accuracy of M mol derived via single-band dust-continuum observations than the assumed CO(1-0)-to-M mol conversion factor. We therefore conclude that single-band observations of long-wavelength dust emission can be used to reliably constrain the molecular gas masses of massive, star-forming galaxies at z 2.

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: CO Excitation and Atomic Carbon in Star-forming Galaxies at z = 1–3

“…Sources are indicated by the 3mm.ID (except 1mm.C07 and 1mm.C14a). The black lines show the best-fit empirical relations from Scoville et al (2016; assuming both a constant and dust-luminosity dependent dust-to-gas conversion factor), while the gray triangles show their calibration sample as well as more recent observations from Kaasinen et al (2019). Figure 10.…”

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: The Nature of the Faintest Dusty Star-forming Galaxies

“…Various studies have shown that the cold dust emission from distant galaxies in the Rayleigh-Jeans (RJ) long-wavelength regime can be used as a reliable tracer of the ISM molecular gas mass, under reasonable assumptions on the dust properties (Scoville et al 2014(Scoville et al , 2016Hughes et al 2017;Kaasinen et al 2019). Although the method comes in different flavors, the dust temperatures are expected to have little effect on the dust content measurements, and it is assumed to be fixed at T d =25 K, which corresponds to the typical value for local star-forming galaxies.…”

Dust and gas content of high-redshift galaxies hosting obscured AGN in the Chandra Deep Field-South

“…However, they point out that a α CO value several times smaller is often assumed for ULIRGs and SMGs, and that such a low α CO would be more appropriate to describe the very nuclear regions. In addition, in Kaasinen et al (2019), where the Scoville et al (2016) relation is exploited to test the use of long-wavelength dust emission as a gas mass tracer, the authors conclude that the main contribution to the uncertainty in the derived molecular masses (from either CO(1-0) or dust continuum) remains the α CO factor. Given this large uncertainty, we additionally derive the gas masses assuming the same Galactic α CO of Scoville et al (2016), in order to provide a range of masses that fairly takes this uncertainty into account.…”