Angular momentum-related probe of cold gas deficiencies

Abstract

Recent studies of neutral atomic hydrogen (HI) in nearby galaxies found that all field disk galaxies are HI saturated, in that they carry roughly as much HI as permitted before this gas becomes gravitationally unstable. By taking this HI saturation for granted, the atomic gas fraction $f_{\rm atm}$ of galactic disks can be predicted as a function of the stability parameter $q=j\sigma/(GM)$, where $M$ and $j$ are the baryonic mass and specific angular momentum of the disk and $\sigma$ is the HI velocity dispersion Obreschkow et al. 2016. The log-ratio $\Delta f_q$ between this predictor and the observed atomic fraction can be seen as a physically motivated `HI deficiency'. While field disk galaxies have $\Delta f_q \approx0$, objects subject to environmental removal of HI are expected to have $\Delta f_q>0$. Within this framework, we revisit the HI deficiencies of satellite galaxies in the Virgo cluster and in clusters of the EAGLE simulation. We find that observed and simulated cluster galaxies are HI deficient and that $\Delta f_q$ slightly increases when getting closer to the cluster centres. The $\Delta f_q$ values are similar to traditional HI deficiency estimators, but $\Delta f_q$ is more directly comparable between observations and simulations than morphology-based deficiency estimators. By tracking the simulated HI deficient cluster galaxies back in time, we confirm that $\Delta f_q\approx0$ until the galaxies first enter a halo with $M_{\rm halo}>10^{13} {\rm M_{\odot}}$, at which moment they quickly lose HI by environmental effects. Finally, we use the simulation to investigate the links between $\Delta f_q$ and quenching of star formation.