ALMACAL V: Absorption-selected galaxies with evidence for excited ISMs

TL;DR

This study analyzes molecular gas in absorption-selected galaxies at z~0.5, revealing excited ISM conditions similar to luminous infrared galaxies or AGN, highlighting the importance of CO SLEDs for accurate gas mass estimates.

Contribution

It presents the first detailed CO SLED analysis of absorption-selected galaxies, showing their ISM conditions can differ significantly from typical star-forming galaxies.

Findings

CO SLEDs are distinct from typical star-forming galaxies.

Absorption-selected systems may preferentially include interacting galaxies.

ISM conditions in these galaxies resemble those of luminous infrared galaxies or AGN.

Abstract

Gas-rich galaxies are selected efficiently via quasar absorption lines. Recently, a new perspective on such absorption-selected systems has opened up by studying the molecular gas content of absorber host galaxies using ALMA CO emission line observations. Here, we present an analysis of multiple CO transitions ($L'_{\rm CO} \sim 10^9$ K km s$^{-1}$) in two $z \sim 0.5$ galaxies associated with one Ly$\alpha$ absorber towards J0238+1636. The CO spectral line energy distribution (CO SLED) of these galaxies appear distinct from that of typical star-forming galaxies at similar redshifts and is comparable with that of luminous infrared galaxies or AGN. Indeed, these galaxies are associated with optically identified AGN activity. We infer that the CO line ratios and the $\alpha_{\rm CO}$ conversion factor differ from the Galactic values. Our findings suggest that at least a fraction of absorption selected systems shows ISM conditions deviating from those of normal star-forming galaxies. For a robust molecular gas mass calculation, it is therefore important to construct the CO SLED. Absorption-line-selection identifies systems with widely distributed gas, which may preferentially select interacting galaxies, which in turn will have more excited CO SLEDs than isolated galaxies. Furthermore, we raise the question whether quasar absorbers preferentially trace galaxy overdensities.