Abstract
The origin of the high star formation rates (SFR) observed in high-redshift dusty star-forming galaxies is still unknown. Large fractions of dense molecular gas might provide part of the explanation, but there are few observational constraints on the amount of dense gas in high-redshift systems dominated by star formation. In this paper, we present the results of our Atacama large millimeter array (ALMA) program targeting dense-gas tracers (HCN(5-4), HCO+(5-4), and HNC(5-4)) in five strongly lensed galaxies from the South Pole Telescope (SPT) submillimeter galaxy sample. We detected two of these lines (S/N > 5) in SPT-125-47 at z = 2.51 and tentatively detected all three (S/N ∼ 3) in SPT0551-50 at z = 3.16. Since a significant fraction of our target lines is not detected, we developed a statistical method to derive unbiased mean properties of our sample taking into account both detections and non-detections. On average, the HCN(5-4) and HCO+(5-4) luminosities of our sources are a factor of ∼1.7 fainter than expected, based on the local L′HCN(5-4) − LIR relation, but this offset corresponds to only ∼2σ if we consider sample variance. We find that both the HCO+/HCN and HNC/HCN flux ratios are compatible with unity. The first ratio is expected for photo-dominated regions (PDRs) while the second is consistent with PDRs or X-ray dominated regions (XDRs) and/or mid-infrared (IR) pumping of HNC. Our sources are at the high end of the local relation between the star formation efficiency, determined using the LIR/[CI] and LIR/CO ratios, and the dense-gas fraction, estimated using the HCN/[CI] and HCN/CO ratios. Finally, in SPT0125-47, which has the highest signal-to-noise ratio, we found that the velocity profiles of the lines tracing dense (HCN, HCO+) and lower-density (CO, [CI]) molecular gas are similar. In addition to these lines, we obtained one robust and one tentative detection of 13CO(4-3) and found an average I 12CO(4-3)/I13CO(4-3) flux ratio of 26.1−3.5+4.5, indicating a young but not pristine interstellar medium. We argue that the combination of large and slightly enriched gas reservoirs and high dense-gas fractions could explain the prodigious star formation in these systems.
Highlights
The molecular gas in high-redshift galaxies has been probed by observations of the rotational lines of CO (e.g., Solomon & Vanden Bout 2005)
We present the results of our Atacama large millimeter array (ALMA) program targeting dense-gas tracers (HCN(5-4), HCO+(5-4), and HNC(5-4)) in five strongly lensed galaxies from the South Pole Telescope (SPT) submillimeter galaxy sample
As explained in the Introduction, the [12C]/[13C] abundance ratio has been proposed as a diagnostic of the evolutionary stage of a galaxy, since different nuclear reactions produce 12C and 13C; the former is produced via triple alpha nuclear processes in young massive stars, while the latter is produced in the CNO cycle in evolved asymptotic giant branch (AGB) stars (Wilson & Rood 1994)
Summary
The molecular gas in high-redshift galaxies has been probed by observations of the rotational lines of CO (e.g., Solomon & Vanden Bout 2005). Observations of Milky Way clumps in HCN(1-0), as well as in a variety of other dense gas tracers, by Wu et al (2005, 2010) found a roughly linear relation between the SFR and the densegas mass (Mdense) consistent with the galaxy-integrated measurements. Bigiel et al (2016) showed that the increase in densegas fraction towards the center of M 51 leads to a decrease in the dense-gas star formation efficiency (∝LIR/LHCN), since there will be fewer overdense regions able to gravitationally collapse if the average density is higher This interpretation, as well as the claimed linearity of the SFR-dense gas relation is debated in the literature. We assume a Planck Collaboration XIII (2016) cosmology and a Chabrier (2003) initial mass function (IMF)
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