Ultraviolet tails and trails in cluster galaxies: A sample of candidate
  gaseous stripping events in Coma
  We have used new deep observations of the Coma cluster from GALEX to identify
13 star-forming galaxies with asymmetric morphologies in the ultraviolet. Aided
by optical broad-band and H-alpha imaging, we interpret the asymmetric features
as being due to star formation within gas stripped from the galaxies by
interaction with the cluster environment. The selected objects display a range
of structures from broad fan-shaped systems of filaments and knots
(`jellyfish') to narrower and smoother tails extending up to 100 kpc in length.
Some of the features have been discussed previously in the literature, while
others are newly identified here. As an ensemble, the candidate stripping
events are located closer to the cluster centre than other star-forming
galaxies; their radial distribution is similar to that of all cluster members,
dominated by passive galaxies. The fraction of blue galaxies which are
undergoing stripping falls from 40% in the central 500 kpc, to less than 5%
beyond 1 Mpc. We find that tails pointing away from (i.e. galaxies moving
towards) the cluster centre are strongly favoured (11/13 cases). From the small
number of `outgoing' galaxies with stripping signatures we conclude that the
stripping events occur primarily on first passage towards the cluster centre,
and are short-lived compared to the cluster crossing time. Using infall
trajectories from simulations, the observed fraction of blue galaxies
undergoing stripping can be reproduced if the events are triggered at a
threshold radius of ~1 Mpc and detectable for ~500 Myr. HST images are
available for two galaxies from our sample and reveal compact blue knots
coincident with UV and H-alpha emission, apparently forming stars within the
stripped material. Our results confirm that stripping of gas from infalling
galaxies, and associated star formation in the stripped material, is a
widespread phenomenon in rich clusters.
