From simple arguments, the interaction with the ICM may also be expected to increase temporarily the star formation rate, particularly on the forward side of the galaxy where intracluster material may be accreted by the galaxy and where the ram pressure will compress the interstellar clouds. Evidence that such is the case comes from studies both of the gas distributions and star formation indicators. Indeed, Cayatte et al. (1989) find that the distribution of HI is generally asymmetric with the side away from M87 being the more extensive. Kenney et al. (1989) have mapped the CO emission in the HI poor Sa NGC 4419 and find the molecular distribution itself to be significantly asymmetric, suggesting that the molecular as well as the atomic gas in this central located, high velocity galaxy is currently undergoing a strong interaction with the ICM in Virgo.
Gavazzi and Jaffe (1986) have conducted a radio continuum survey of galaxies in the Coma / A1367 supercluster with the VLA and find that the cluster spirals, although HI deficient, have actually stronger radio continuum emissivity. In particular, they find a number of HI poor, radio loud, rather blue objects just at the edge of the X-ray distribution around A1367, and suggest that the interaction of the hot X-ray gas with the cool interstellar clouds in the galaxy leads to a collapse of the molecular clouds and an enhancement of the star formation rate.
It is generally assumed that the molecular clouds are the precursors of
the massive
star formation regions. Interestingly,
Kennicutt (1989)
has concluded from a comparison
of the distributions of H
emission, HI and CO in 15 nearby galaxies that the disk
averaged H surface brightness
is well correlated with the mean atomic and total gas
surface densities and only weakly so with the mean molecular surface
density derived
from the CO observations. This finding implies that the coupling between
the active
star formation and the molecular clouds is not as strong as one might
have expected.
Rubin at this meeting has presented preliminary results on rotation
curves derived for Virgo cluster spirals in which the
H emission cuts off inside the
optical disk, at the
same radius where the HI disk truncates. Hence we see that the massive
star formation
and the HI are both seemingly affected whereas the dense molecular component
remains intact. The rates of massive star formation derived from the
H, radio continuum
and FIR emission are a factor of two or three lower in the HI deficient
objects. Thus as discussed by
Kenney and Young
(1989),
the retention of the molecular component
maintains at least a moderate rate of massive star formation. An
interesting point
presented by Kenney and Young is that, while the outer disks are very HI
deficient,
the inner ones are at least moderately so, and it is actually possible
that some of the
inner HI has been compressed into the molecular stage. The increased
mass surface
density of the molecular clouds and their central location are able to
defend them from the effects of ram pressure.
Several of the most severely stripped galaxies in Virgo show signs of peculiar nuclear activity or strong gas asymmetries that are consistent with a picture of gas infall or induced star formation (Cayatte et al. 1989). In the past, it is likely that such gas sweeping events were even more prevalent. Thus, it is of interest to study current epoch stripping with an eye on the expectations for observations of clusters at higher redshifts. As discussed by Bothun and Dressler (1986), star formation induced by galaxy-galaxy or galaxy ICM interactions could be important at earlier epochs and the present existence of some blue disk galaxies in the Coma cluster is consistent with a ram pressure induced process of star formation.
While the picture of sweeping is attractive, one should keep in mind the numerous enigmas that have been long known and remain unexplained. NGC 1961 mapped at Westerbork by Shostak et al. (1982) seems to have suffered a gas removal event, yet it is not located in a particularly dense environment.