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

Abstract

We present evidence for a counter-rotating core in the low-luminosity (M_B = -18.2) elliptical galaxy NGC 770 based on internal stellar kinematic data. This counter-rotating core is unusual as NGC 770 is not the primary galaxy in the region and it lies in an environment with evidence of on-going tidal interactions. We discovered the counter-rotating core via single-slit Keck/ESI echelle spectroscopy; subsequent integral field spectroscopy was obtained with the Gemini/GMOS IFU. The counter-rotating region has a peak rotation velocity of 21 km/s as compared to the main galaxy's rotation speed of greater than 45 km/s in the opposite direction. The counter-rotating region extends to a radius of 4'' (0.6 kpc), slightly smaller than the half-light radius of the galaxy which is 5.3'' (0.8 kpc) and is confined to a disk whose scale height is less than 0.8'' (0.1 kpc). We compute an age and metallicity of the inner counter-rotating 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 counter-rotating 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.