Kinematics and ionization of extended gas in active galaxies. IV - The Seyfert 2 galaxy NGC 4388

Abstract

We present the results of direct CCD imaging and long-slit, medium-dispersion spectroscopy of the extended narrow-line-emitting gas in the nearby Seyfert 2 galaxy NGC 4388, type SB(s)b pec. The spectra have been used to investigate the distribution, kinematics, and ionization structure of the excited gas in the regions surrounding the nucleus out to a distance of ~ 4.8 kpc. Our data support earlier suggestions that the galaxy, which appears to lie near the core of the Virgo cluster, has been tidally distorted, possibly by the nearby cluster core galaxies M84 or IC 3303. Much of the highly ionized gas in NGC 4388 appears to be distributed in two wide cones originating from the nucleus and extending above and below the disk. We observe line emission of this gas out to a projected height of ~ 1.5 kpc, although it appears to extend even higher. The overall velocity field of this gas and that lying within the disk is somewhat irregular, although a component of normal rotation about the nucleus is evident. Two isolated systems of high-ionization gas whose velocity are significantly discrepant from this velocity field are inferred to be material also lying well above the disk of the galaxy. These regions appear, however, to have different origins: the isolated patches of emission extending above the eastern spiral arm are likely the remnants of one or more recent supernovae, while the region situated directly to the north of the nucleus is interpreted as material which may have been ejected from the nucleus as it also coincident with a radio feature oriented perpendicular to the disk and extending 1.8 kpc from the optical nucleus. We offer three separate explanations of the presence of the remainder of the gas above and below the disk: (1) ejection from the nucleus, (2) removal from the disk by ram pressure and/or other processes arising from the interaction of the galaxy with the intracluster medium, and (3) accretion from a less massive galaxy during a recent tidal encounter. There are problems with each of these models individually and we feel the observed distribution and kinematics of the gas may be the result of a combination of these processes acting together.