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 extended emission-line region (VEELR) found serendipitously around the Seyfert 2 galaxy NGC 4388 in the Virgo Cluster using the Subaru Telescope. The Hα recession velocities of most of the filaments of the region observed are highly blueshifted with respect to the systemic velocity of the galaxy. The velocity field is complicated, and from the kinematic and morphological points of view, there seem to be several streams of filaments: low-velocity filaments, with radial velocities v with respect to the systemic velocity of NGC 4388 of roughly -100 km s^-1; high-velocity (v ∼ -300 km s^-1) filaments; and a very high velocity (v ∼ -500 km s^-1) cloud complex. The emission-line ratios of the VEELR filaments are well explained by power-law photoionization models with solar abundances, suggesting that the Seyfert nucleus of NGC 4388 is the dominant ionization source of the VEELR and that the VEELR gas has moderate metallicity. In addition to photoionization, shock heating probably contributes to the ionization of the gas. In particular, the filaments outside the ionization cone of the Seyfert nucleus are mainly excited by shocks. The predicted shock velocity is ∼200–300 km s^-1, which is comparable to the velocities of the filaments. We conclude that the VEELR was formerly the disk gas of NGC 4388, which has been stripped by ram pressure due to the interaction between the hot intracluster medium and the galaxy. The velocity field and the morphology of the VEELR closely resemble snapshots from some numerical simulations of this process. In the case of NGC 4388, the ram pressure stripped gas, which is normally seen as extended H I filaments, happens to be exposed and ionized by the radiation from the active galactic nucleus, and so can be seen as optical emission-line gas.