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

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 ~10 37 ergs s -1 , and the total L (Hα) of the region within 10 kpc of the nucleus is ~2 × 10 38 ergs s -1 , which corresponds to an ionized gas mass of ~10 5 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.