Effects of environmental gas compression on the multiphase ISM and star
  formation
  The cluster environment can affect galaxy evolution in different ways: via
ram pressure stripping or by gravitational perturbations caused by galactic
encounters. New IRAM 30m HERA CO(2-1) data of NGC 4501 and NGC 4567/68 are
presented. We find an increase in the molecular fraction where the ISM is
compressed. The gas is close to self-gravitation in compressed regions. This
leads to an increase in gas pressure and a decrease in the ratio between the
molecular fraction and total ISM pressure. The overall Kennicutt Schmidt
relation based on a pixel-by-pixel analysis at ~1.5 kpc resolution is not
significantly modified by compression. However, we detected continuous regions
of low molecular star formation efficiencies in the compressed parts of the
galactic gas disks. The data suggest that a relation between the molecular star
formation efficiency SFE_H2 and gas self-gravitation exists. Both systems show
spatial variations in the star formation efficiency with respect to the
molecular gas that can be related to environmental compression of the ISM. An
analytical model was used to investigate the dependence of SFE_H2 on
self-gravitation. The model correctly reproduces the correlations between
R_mol/P_tot, SFE_H2, and Toomre Q if different global turbulent velocity
dispersions are assumed for the three galaxies. We found that variations in the
N_H_2/I_CO conversion factor can mask most of the correlation between SFE_H2
and the Q parameter. Dynamical simulations were used to compare the effects of
ram pressure and tidal ISM compression. We conclude that a gravitationally
induced ISM compression has the same consequences as ram pressure compression:
(i) an increasing gas surface density, (ii) an increasing molecular fraction,
and (iii) a decreasing R_mol/P_tot in the compressed region due to the presence
of nearly self-gravitating gas. The response of SFE_H2 to compression is more
complex.
