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