ABSTRACT

The nearby elliptical galaxy NGC 4278 (D = 8.2 h-1 Mpc) has long been known to harbor an active LINER, radio nucleus as well as an extended gas disk. Observations of NGC 4278 in the 21 cm HI line using the VLA and recent deep, long-slit optical spectroscopy of the emission line gas are discussed. The atomic gas disk shows regular rotation and extends to over eleven times the half-light radius Re. Noncircular motions in the central regions can be satisfactorily fit by a triaxial model for the galaxy density distribution in which the gas moves on increasingly elliptic orbits towards the nucleus. These elliptic orbits may help feed gas into the active nuclear source in NGC 4278. The high gas rotational velocities in the outer parts give evidence for a massive, dark halo surrounding this active elliptical galaxy.

INTRODUCTION: WHAT CAN WE LEARN FROM GAS KINEMATICS IN ELLIPTICAL GALAXIES?

Contrary to the early definitions of elliptical galaxies as old stellar systems lacking gas and dust, many early-type systems are now routinely detected in all tracers of the cool interstellar medium. About 60% of ellipticals have optical emission lines from ionized gas (Phillips et al. 1986), 45% show thermal dust emission at 60–100 μ (Knapp et al. 1989), 40% show optical absorption by dust patches (Sadler and Gerhard 1985), and about 20% are detected in HI or CO emission (Knapp 1987; Lees et al. 1991).