Neutral hydrogen associated with shells and other fine structure in NGC 2865: A dynamically young elliptical?

Abstract

We report the discovery of neutral hydrogen in a second elliptical galaxy with shells, NGC 2865. Very Large Array images reveal an association between the neutral hydrogen (H I) and the fine structure (shells, tails, and loops) in the galaxy. Similar to what we previously observed in NGC 5128 (Centaurus A), most of the 6 x 10^8^ h^-2^ M_sun_ of cold gas is found in a broken ring in the outer regions of NGC 2865 (beyond 1/2 D_25_) and is displaced to the outside of the shells and loops. The measured velocities cover a range of 500 km s^-1^ around the systemic velocity. The velocity field of the outer H I has the same sense and magnitude (and line of nodes) as that of the stars in the elliptical body. Although NGC 2865 appears to be a relaxed elliptical galaxy, deep images, photometry, and spectroscopy suggest that the galaxy might be the recent (less than 7 Gyr) product of a major disk-to-disk merger- a "dynamically young elliptical." Our HI data support this hypothesis. Nevertheless, the association between gas and stellar fine structure, with gas displaced outward from the start in projected position, implies gas motions not predicted by any of the current merger scenarios. Using the H I ring and assuming nearly circular motion, we measure M/L_B_ at large radii (4 x 1/2 D_25_). We find M/L_B_ = 33 +/- 4 h, a factor of 5 greater than the value of M/L_B_ found for the central regions, indicating the presence of a dark halo.