2002ApJ...567..118Y

We found a very large, ∼35 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 Hα luminosity L(Hα) of the clouds is ∼10³⁷ ergs.s^–1, and the total L(Hα) of the region within 10 kpc of the nucleus is ∼2x10³⁸ ergs.s^–1, which corresponds to an ionized gas mass of ∼10⁵ M_☉. The map of the emission-line intensity ratio I([O III])/I(Hα) 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.