NGC 770: A Counterrotating Core in a Low-Luminosity Elliptical Galaxy

Abstract

We present evidence for a counterrotating core in the low-luminosity (MB = -18.2) elliptical galaxy NGC 770 based on internal stellar kinematic data. This counterrotating core is unusual, as NGC 770 is not the primary galaxy in the region and lies in an environment with evidence of ongoing tidal interactions. We discovered the counterrotating core via single-slit Keck ESI echelle spectroscopy; subsequent integral field spectroscopy was obtained with the Gemini Multiobject Spectrograph. The counterrotating region has a peak rotation velocity of 21 km s-1 as compared to the main galaxy's rotation speed of greater than 45 km s-1 in the opposite direction. The counterrotating region extends to a radius of ~4'' (0.6 kpc), slightly smaller than the half-light radius of the galaxy, which is 53 (0.8 kpc). The photometry and two-dimensional kinematics suggest that the counterrotating component is confined to a disk whose scale height is less than 08 (0.1 kpc). The major axis of counterrotation is misaligned with that of the outer galaxy isophotes by 15°. We compute an age and metallicity of the inner counterrotating region of 3 ± 0.5 Gyr and [Fe/H] = 0.2 ± 0.2 dex based on Lick absorption-line indices. The lack of other large galaxies in this region limits possible scenarios for the formation of the counterrotating core. We discuss several scenarios and favor one in which NGC 770 accreted a small gas-rich dwarf galaxy during a very minor merging event. If this scenario is correct, it represents one of the few known examples of merging between two dwarf-sized galaxies.