Molecular Gas Detections in Eight Faint DSFGs with Red NIR Colors at z = 1.2-2.5

TL;DR

This study uses NOEMA to detect molecular gas in eight faint DSFGs at redshifts 1.2-2.5, revealing consistent depletion times and potential overestimation of gas masses in low-mass starburst galaxies.

Contribution

First CO detections in faint DSFGs at these redshifts using NIR color selection, providing insights into molecular gas properties and scaling relations.

Findings

Seven new CO detections and one [C I] detection.

Constant depletion times of ~500 Myr across the sample.

Low-mass starburst galaxies show higher gas fractions and depletion times than predicted.

Abstract

We present a NOEMA survey of CO(3-2), CO(4-3), and [C I]($^3$P$_1$-$^3$P$_0$) in eight faint (average $S_{\rm 850 \mu m} = 2.3$ mJy) dusty star-forming galaxies (DSFGs) at $z = 1.2-2.5$. We used a NIR flux-color cut to match faint SCUBA-2 sources to red stellar counterparts with existing spectroscopic redshifts, allowing us to target CO lines at known frequencies. We obtained seven new CO detections and a serendipitous [C I] detection in an off-axis source, and measured molecular gas masses of $M_{\rm mol} = (6-22)\times10^{10}~(\alpha_{\rm CO}/3.6)~{\rm M}_\odot$ from these lines. We performed UV-to-mm SED fits to measure the SFRs and stellar masses of our sample, and compared these with two other $z = 1-3$ CO samples from the literature. The CO detections have constant depletion times of $t_{\rm dep} \sim 500$ Myr, with no evidence for correlation between $t_{\rm dep}$ and redshift or main-sequence offset. We find that low-mass ($M_\star \lesssim 10^{11}~{\rm M}_\odot$), starbursting galaxies have gas fractions and depletion times twice as high as predicted by molecular gas scaling relations, which may indicate that $M_{\rm mol}$ is systematically over-estimated in this population, possibly due to decreased $\alpha_{\rm CO}$ or increased CO excitation compared to the well-studied massive and/or main-sequence DSFG population.