ALMA Lensing Cluster Survey: Molecular Gas Properties of Line-Emitting Galaxies from a Blind Survey

TL;DR

This study uses ALMA data to conduct a blind search for line-emitting galaxies, detecting seven sources including CO, [CII], and CH lines, revealing molecular gas properties and the first detection of CH in a galaxy via a blind survey.

Contribution

It presents the first detection of CH in a galaxy through a blind survey and demonstrates the effectiveness of gravitational lensing in probing faint molecular gas emissions.

Findings

Detected seven line emitters, including CO, [CII], and CH lines.

Most emitters follow known molecular gas scaling relations.

First detection of CH in a galaxy at cosmological distances via a blind survey.

Abstract

We present results of a blind search for line-emitting galaxies using ALMA Lensing Cluster Survey data. We detected seven line emitters, one of which is [C\,{\sc ii}] at $z = 6.071$, four are CO at $z = 0.8$--1.1, and the remaining two are possibly CO or [C\,{\sc i}] within photometric redshift ranges. Three of the four CO emitters are multiple images of the same galaxy. Compared to previous line-emitter searches in ALMA deep fields, our sample probes molecular gas masses $\sim$1 dex below the lower bound, thanks to gravitational lensing (typically $\mu \sim 4$, up to $\sim$30 in extreme cases). Most emitters are located in a region similar to normal star-forming galaxies in the star formation rate (SFR) versus molecular gas mass plane. To reduce dependence on SFR and stellar mass, we analyzed the molecular gas fraction and depletion timescale as a function of distance from the star-formation main sequence. We found that most emitters broadly follow the scaling relations from previous studies, consistent within the intrinsic scatter. In addition, we serendipitously detected the CH $N = 1$, $J = 3/2 \rightarrow 1/2$ $\Lambda$-doublet transition from one CO emitter at $z = 1.142$, representing the first detection of CH from an individual galaxy at cosmological distances through a blind survey. The CH/CO column density ratio of $\sim$$10^{-4}$ is comparable to that of local AGN-host galaxies, suggesting that CH traces molecular gas associated with AGN activity, possibly irradiated by X-rays.