Subaru Deep Spectroscopy of the Very Extended Emission-Line Region of NGC 4388: Ram Pressure Stripped Gas Ionized by Nuclear Radiation

We report here the results of deep optical spectroscopy of the very extendedemission-line region (VEELR) found serendipitously around the Seyfert 2 galaxyNGC 4388 in the Virgo cluster using the Subaru Telescope. The H-alpha recessionvelocities of most of the filaments of the region observed are highlyblue-shifted with respect to the systemic velocity of the galaxy. The velocityfield is complicated, and from the kinematic and morphological points of view,there seem to be several streams of filaments: low velocity (v = -100 km/s)filaments, high velocity (v = -300 km/s) filaments, and a very high velocity (v> -500 km/s) cloud complex. The emission-line ratios of the VEELR filaments arewell explained by power-law photoionization models with solar abundances,suggesting that the Seyfert nucleus of NGC 4388 is the dominant ionizationsource of the VEELR and that the VEELR gas has moderate metallicity. Inaddition to photoionization, shock heating probably contributes to theionization of the gas. In particular, the filaments outside the ionization coneof the Seyfert nucleus are mainly excited by shocks. We conclude that the VEELRwas formerly the disk gas of NGC 4388, which has been stripped by ram pressuredue to the interaction between the hot intra-cluster medium (ICM) and thegalaxy. The velocity field and the morphology of the VEELR closely resemblesnapshots from some numerical simulations of this process. In the case of NGC4388, the ram pressure-stripped gas, which is normally seen as extended HIfilaments, happens to be exposed and ionized by the radiation from the AGN, andso can be seen as optical emission-line gas.Comment: 20 pages, LaTeX, Accepted by Astron.