# ALMA constraints on star-forming gas in a prototypical z=1.5 clumpy galaxy: the dearth of CO(5-4) emission from UV-bright clumps

## TL;DR

This study uses deep ALMA CO(5-4) observations of a z=1.5 clumpy galaxy to investigate the distribution of star-forming gas, revealing strong nuclear emission but no detection from UV-bright clumps, implying diverse star formation efficiencies.

## Contribution

First detailed ALMA CO(5-4) analysis linking stellar properties to gas distribution in a high-redshift clumpy galaxy, highlighting differences between nucleus and clumps.

## Key findings

- CO(5-4) emission is concentrated in the galaxy nucleus.
- UV-bright clumps show no detectable CO(5-4) emission.
- Clump star formation rates vary significantly depending on the method used.

## Abstract

We present deep ALMA CO(5-4) observations of a main sequence, clumpy galaxy at z=1.5 in the HUDF. Thanks to the ~0.5" resolution of the ALMA data, we can link stellar population properties to the CO(5-4) emission on scales of a few kpc. We detect strong CO(5-4) emission from the nuclear region of the galaxy, consistent with the observed $L_{\rm IR}$-$L^{\prime}_{\rm CO(5-4)}$ correlation and indicating on-going nuclear star formation. The CO(5-4) gas component appears more concentrated than other star formation tracers or the dust distribution in this galaxy. We discuss possible implications of this difference in terms of star formation efficiency and mass build-up at the galaxy centre. Conversely, we do not detect any CO(5-4) emission from the UV-bright clumps. This might imply that clumps have a high star formation efficiency (although they do not display unusually high specific star formation rates) and are not entirely gas dominated, with gas fractions no larger than that of their host galaxy (~50%). Stellar feedback and disk instability torques funnelling gas towards the galaxy centre could contribute to the relatively low gas content. Alternatively, clumps could fall in a more standard star formation efficiency regime if their actual star-formation rates are lower than generally assumed. We find that clump star-formation rates derived with several different, plausible methods can vary by up to an order of magnitude. The lowest estimates would be compatible with a CO(5-4) non-detection even for main-sequence like values of star formation efficiency and gas content.