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

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 ongoing 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 −1 as compared to the main galaxy's rotation speed of greater than 45 km s −1 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 – 2 – galaxy which is 5.3 ′′ (0.8 kpc). The photometry and two-dimensional kinematics suggest that the counter-rotating component is confined to a disk whose scale height is less than 0.8 ′′ (0.1 kpc). The major-axis of counter-rotation is misaligned with that of the outer galaxy isophotes by 15 •. 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.