CO(1-0) line imaging of massive star-forming disc galaxies at z=1.5-2.2

TL;DR

This study presents CO(1-0) line imaging of massive star-forming galaxies at z~1.5-2.2, revealing sub-thermal excitation and spatially resolved gas structures, enhancing understanding of molecular gas properties in early universe galaxies.

Contribution

First detection of CO(1-0) in these galaxies combined with excitation analysis and spatial mapping, providing new insights into molecular gas distribution and excitation at high redshift.

Findings

Average line brightness temperature ratios support sub-thermal excitation.

Most molecular gas is not confined to giant clumps, suggesting diffuse distribution.

Spatially resolved maps reveal clumpy structures and extended gas regions.

Abstract

We present detections of the CO(J= 1-0) emission line in a sample of four massive star-forming galaxies at z~1.5-2.2 obtained with the Karl G. Jansky Very Large Array (VLA). Combining these observations with previous CO(2-1) and CO(3-2) detections of these galaxies, we study the excitation properties of the molecular gas in our sample sources. We find an average line brightness temperature ratios of R_{21}=0.70+\-0.16 and R_{31}=0.50+\-0.29, based on measurements for three and two galaxies, respectively. These results provide additional support to previous indications of sub-thermal gas excitation for the CO(3-2) line with a typically assumed line ratio R_{31}~0.5. For one of our targets, BzK-21000, we present spatially resolved CO line maps. At the resolution of 0.18 arcsec (1.5 kpc), most of the emission is resolved out except for some clumpy structure. From this, we attempt to identify molecular gas clumps in the data cube, finding 4 possible candidates. We estimate that <40 % of the molecular gas is confined to giant clumps (~1.5 kpc in size), and thus most of the gas could be distributed in small fainter clouds or in fairly diffuse extended regions of lower brightness temperatures than our sensitivity limit.