Discovery of a very extended emission-line region around the Seyfert 2 galaxy NGC 4388

We found a very large, similar to35 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 filaments, and extends northeastward from the galaxy. The typical Halpha luminosity L(Halpha) of the clouds is similar to10(37) ergs s(-1), and the total L(Halpha) of the region within 10 kpc of the nucleus is similar to2 x 10(38) ergs s(-1), which corresponds to an ionized gas mass of similar to10(5) M.. The map of the emission-line intensity ratio I([O III])/I(Halpha) indicates that the inner (r <12 kpc) 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.