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 2galaxy NGC 4388, using deep narrow-band imaging with the prime focus camera(Suprime-Cam) of the Subaru telescope. This region consists of many faint gasclouds or filaments, and extends northeastwards from the galaxy. The typicalH-alpha luminosity L(H-alpha) of the clouds is 10^37 erg s^-1, and the totalL(H-alpha) of the region within 10 kpc from the nucleus is 2 x 10^38 erg s^-1,which corresponds to an ionized gas mass of 10^5 M_solar. The map of theemission-line intensity ratio I([OIII])/I(H-alpha) indicates that the inner (r< 12 kpc) region of the VEELR may be excited by nuclear ionizing radiation. Theexcitation mechanism of the outer (r > 12 kpc) region is unclear, but it islikely 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 inthe VEELR are (i) the tidal debris resulting from a past interaction with agas-rich dwarf galaxy, i.e., a minor merger, or (ii) the interstellar medium ofNGC 4388, stripped by the ram pressure of the hot intracluster medium of theVirgo cluster.Comment: 15 pages, 7 figures, ApJ accepte