Galactic star formation enhanced and quenched by ram pressure in groups and clusters

We investigate how ram pressure of intragroup and intracluster medium can influence the spatial and temporal variations of star formation (SF) of disc galaxies with halo masses (M-h) ranging from 10(10) to 10(12)M(circle dot) (i.e. from dwarf irregular to Milky Way-type) in groups and clusters with 10(13) <= M-h/M-circle dot <= 10(15) by using numerical simulations with a new model for time-varying ram pressure. The long-term evolution of SF rates and H alpha morphologies corresponding to the distributions of star-forming regions are particularly investigated for different model parameters. The principal results are as follows. Whether ram pressure can enhance or reduce SF depends on M-h of disc galaxies and inclination angles of gas discs with respect to their orbital directions for a given orbit and a given environment. For example, SF can be moderately enhanced in disc galaxies with M-h = 10(12)M(circle dot) at the pericentre passages in a cluster with M-h = 10(14)M(circle dot) whereas it can be completely shut down ('quenching') for low-mass discs with M-h = 10(10)M(circle dot). Ram pressure can reduce the H alpha-to-optical-disc-size ratios of discs and the level of the reduction depends on M-h and orbits of disc galaxies for a given environment. Disc galaxies under strong ram pressure show characteristic H alpha morphologies such as ring-like, one-sided and crescent-like distributions.