Discovery of a Very Extended Emission-Line Region around the Seyfert 2 Galaxy NGC 4388

We found a very large, D35 kpc, emission-line region around the Seyfert type 2 galaxy NGC 4388, using deep narrowband imaging with the prime focus camera (Suprime-Cam) of the Subaru Telescope. This region consists of many faint gas clouds, or —laments, and extends northeastward from the galaxy. The typical Ha luminosity L (Ha) of the clouds is D1037 ergs s~1, and the total L (Ha) of the region within 10 kpc of the nucleus is D2 ] 1038 ergs s~1, which corresponds to an ionized gas mass of D105 The map of the emission-line intensity ratio I([O III])/I(Ha) indicates that the inner (r \ 12 kpc) M _ . region of the very extended emission-line region (VEELR) may be excited by nuclear ionizing radiation. The excitation mechanism of the outer (r [ 12 kpc) region is unclear, but it is likely that the nuclear radiation is also a dominant source of its ionization. We discuss the origin of the ionized gas. Two plausible origins of the gas in the VEELR are (1) the tidal debris resulting from a past interaction with a gas-rich dwarf galaxy, i.e., a minor merger, and (2) the interstellar medium of NGC 4388, stripped by the ram pressure of the hot intracluster medium of the Virgo Cluster.