  In a 5 hour Halpha exposure of the N-W region of the Coma cluster with the
2.1m telescope at SPM (Mx) we discovered a 65 kpc cometary emission of ionized
gas trailing behind the SBab galaxy NGC 4848. The tail points in the opposite
direction of the cluster center, in the same direction where stripped HI has
been detected in previous observations. The galaxy shows bright HII regions in
an inner ring-like pattern, where the star formation takes place at the
prodigious rate of 8.9 Msun/yr. From the morphology of the galaxy and of the
trailing material, we infer that the galaxy is suffering from ram pressure due
to its high velocity motion through the cluster IGM. We estimate that 4 x 10^9
Msun of gas is swept out from the galaxy forming the tail. Given the ambient
conditions in the Coma cluster (rho = 6.3 x 10^-27 g/cm^3; sigma_vel = 940
km/s) simulations predict that the ram pressure mechanism is able to remove
such an amount of gas in less than 200 Myr. This, combined with the geometry of
the interaction, indicating radial infall into the cluster, leads to the
conclusion that NGC 4848 is caught in its first passage through the dense
cluster environment.
