12CO(1-0) observations of NGC 4848: A Coma galaxy after stripping

We study the molecular gas content and distribution in the Coma cluster spiral galaxy NGC 4848 which is HI deficient and where the remaining atomic gas is on one side of the galaxy, presumably because of the strong ram-pressure in the rich Coma cluster. Plateau de Bure interferometric CO(1-0) observations reveal a lopsided H₂ distribution with an off-center secondary maximum coincident with the inner part of the HI. NGC 4848 is not at all deficient in molecular gas as it contains M_{H_2} ~ 4 x 10⁹ M_sun in the central and inner disk regions. As predicted by earlier calculations, ram-pressure has little influence on the dense molecular gas in the inner disk, which appears dynamically normal at our 2 kpc resolution. At the interface between the CO and HI emission regions, about 8 kpc NW of the center, however, strong star formation is present as witnessed by Hα and radio continuum emission. From the radio and Hα fluxes we estimate the star formation rate in the northern emission region to be close to 0.3 M_sun yr^-1, nearly that of an average quiescent spiral. This is the region in which earlier Fabry-Pérot observations revealed a double-peaked Hα line, indicating gas at two different velocities at the same sky position. In order to understand these observations, we present the results of numerical simulations of the galaxy-cluster ICM interaction. We suggest that NGC 4848 already passed through the center of the cluster about 4x 10⁸ years ago. At the observed stage ram pressure has no more direct dynamical influence on the galaxy's ISM. We observe the galaxy when a fraction of the stripped gas is falling back onto the galaxy. Ram pressure is thus a short-lived event with longer-lasting consequences. The combination of ram-pressure and rotation results in gas at different velocities colliding where the double-peaked Hα line is observed. Ram-pressure can thus result, after re-accretion, in displaced molecular gas without the H₂ itself being pushed efficiently by the intracluster medium. This process, however, requires strong stripping, such that only galaxies with radial orbits can be affected as much as NGC 4848. A scenario where two interactions take place simultaneously is also consistent with the available data but less probable on the basis of our numerical simulations.