Gas compression and stellar feedback in the tidally interacting and
  ram-pressure stripped Virgo spiral galaxy NGC 4654
  NGC 4654 is a Virgo galaxy seen almost face-on, which undergoes nearly
edge-on gas ram pressure stripping and a fly-by gravitational interaction with
another massive galaxy, NGC 4639. NGC 4654 shows a strongly compressed gas
region near the outer edge of the optical disk, with HI surface densities
(HSDR), exceeding the canonical value of 10-15 Msun/pc2. New IRAM 30m HERA
CO(2-1) data of NGC 4654 are used to study the physical conditions of the ISM.
The CO-to-H$_2$ conversion factor was estimated and found to be one to two
times the Galactic value with significant decrease in the ratio between the
molecular fraction and the total ISM pressure in the HSDR, self-gravitating
gas, a Toomre parameter below $Q=1$ and star-formation efficiency 1.5-2 times
higher. Analytical models were used to reproduce radial profiles of the SFR and
the atomic and molecular surface densities. A Toomre parameter of $\rm Q \sim
0.8$ combined with an increase in the velocity dispersion of 5 km/s are
necessary conditions to simultaneously reproduce the gas surface densities and
the SFR. A dynamical model was used to reproduce the gas distribution of NGC
4654. The comparison between the velocity dispersion given by the moment 2 map
and the intrinsic 3D velocity dispersion from the model were used to
discriminate between regions of broader linewidths caused by a real increase in
the velocity dispersion and those caused by an unresolved velocity gradient
only. We found that the 5 km/s increase in the intrinsic velocity dispersion is
compatible with observations. During a period of gas compression through
external interactions, the gas surface density is enhanced, leading to an
increased SFR and stellar feedback. Under the influence of stellar feedback,
the gas density increases only moderately. The stellar feedback acts as a
regulator of star-formation, increasing the turbulent velocity within the
region.
