A census of star-formation and gas mass tracers in two lensed $z \sim 4$ dusty star-forming galaxies

TL;DR

This study uses ALMA observations of two lensed dusty star-forming galaxies at z~4 to compare various gas and star formation tracers, revealing differences in spatial extent and proposing a new proxy for gas depletion timescale.

Contribution

It introduces the [C I](2-1)/CO(7-6) flux ratio as a new observable proxy for gas depletion timescale in high-redshift galaxies.

Findings

[C II] emission is more extended than other tracers.

The [C I](2-1)/CO(7-6) ratio correlates with gas depletion time.

[C II] may not reliably trace molecular gas or SFR in high-z galaxies.

Abstract

We present new and archival Atacama Large Millimeter/submillimeter Array (ALMA) observations of two strongly lensed dusty star-forming galaxies (DSFGs) selected from the South Pole Telescope survey, SPT0418-47 $(z = 4.225)$ and SPT2147-50 $(z = 3.760)$. We study the [C II], CO(7-6), [C I](2-1), and, in SPT0418-47, $p$-H$_2$O emission, which along with the underlying continuum (rest-frame 160 $\mu$m and 380 $\mu$m) are routinely used as tracers of gas mass and/or star-formation rate (SFR). We perform a pixel-by-pixel analysis of both sources in the image plane to study the resolved Kennicutt-Schmidt relation, finding generally good agreement between the slopes of the SFR versus gas mass surface density using the different tracers. Using lens modeling methods, we find that the dust emission is more compact than the line emission in both sources, with CO(7-6) and [C I](2-1) similar in extent and [C II] the most extended, reminiscent of recent findings of extended [C II] spatial distributions in galaxies at similar cosmic epochs. We develop the [C I](2-1) / CO(7-6) flux density ratio as an observable proxy for gas depletion timescale ($\tau_{\rm dep}$), which can be applied to large samples of DSFGs, in lieu of more detailed inferences of this timescale which require analysis of observations at multiple wavelengths. Furthermore, the extended [C II] emission in both sources, compared to the total continuum and line emission, suggests that [C II], used in recent years as a molecular gas mass and SFR tracer in high-$z$ galaxies, may not always be a suitable tracer of these physical quantities.