The present-day gas content of simulated field dwarf galaxies

We examine the gas content of field dwarf galaxies in a high-resolution cosmological simulation. In agreement with previous work, we find that galaxies inhabiting dark matter haloes with mass below a critical value, M200 ≲ Mcrit ≈ 5 × 109 M☉, are quiescent at the present day. The gas content of these galaxies is thus insensitive to feedback from evolving stars. Almost half of these quiescent systems today have gas masses much smaller than that expected for their mass. We find that gas-deficient galaxies originate from (1) past interactions with massive hosts, in which a dwarf loses gas and dark matter via tidal and ram-pressure forces; and (2) from hydrodynamic interactions with the gaseous filaments and sheets of the cosmic web, in which a dwarf loses gas via ram pressure. We refer to these systems as ‘flybys’ and ‘COSWEBs’. Flybys locate in high-density regions, tracing the location of the most massive galaxies in the simulation. In contrast, COSWEBs are dispersed throughout the volume and trace the cosmic web. For sub-critical systems, M200 < Mcrit, the fraction of COSWEB galaxies can be as high as 35 per cent, and much higher for flybys, which make up 100 per cent of the galaxies with M200 < 3 × 108 M☉. The deficit of gas caused by these mechanisms may preclude the detection of a large fraction of field dwarfs in future H I surveys. For galaxies inhabiting haloes with mass M200 > Mcrit, we find that cosmic web stripping, on average, shuts down star formation in more than 70 per cent of the affected systems.