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 disk
galaxies with halo masses (M_h) ranging from 10^10 M_sun to 10^12 M_sun (i.e.,
from dwarf irregular to Milky Way-type) in groups and clusters with 10^13 <
M_h/M_sun < 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
disk galaxies and inclination angles of gas disks with respect to their orbital
directions for a given orbit and a given environment. For example, SF can be
moderately enhanced in disk galaxies with M_h=10^12 M_sun at the pericenter
passages in a cluster with M_h=10^14 M_sun whereas it can be completely shut
down (`quenching') for low-mass disks with M_h=10^10 M_sun. Ram pressure can
reduce the H_alpha-to-optical-disk-size ratios of disks and the revel of the
reduction depends on M_h and orbits of disk galaxies for a given environment.
Disk galaxies under strong ram pressure show characteristic H_alpha
morphologies such as ring-like, one-sided, and crescent-like distributions.
