A counter-rotating core in the dwarf elliptical galaxy VCC 510

Abstract

Aims. We present optical long-slit spectra of the Virgo dwarf elliptical galaxy VCC 510 at high spectral (σ ∼ 30 km s-1) and spatial resolution. The principal aim is to unravel its kinematical and stellar population properties. Methods. Heliocentric velocities and velocity dispersions as functions of galaxy radius are derived by deconvolving line-of-sight velocity distributions. The luminosity-weighted stellar population parameters age and element abundances are obtained by comparison of Lick absorption-line indices with stellar population models. Results. A maximum rotation v rot = 8 ± 2.5 km s-1 inside half the effective radius (re ≈20") and a mean, radially flat velocity dispersion σ = 44 ± 5 km ss-1 are measured. The core extending over the inner 2" (∼140 pc) is found to rotate in the opposite sense with vrotscore ≈ -1/2 vrot. VCC 510 (MB ∼ -15.7) is therefore by far the faintest and smallest galaxy with a counter-rotating core known. From the main body rotation and the velocity dispersion profile we deduce that VCC 510 is anisotropic and clearly not entirely supported by rotation. We derive an old luminosity-weighted age (10 ± 3 Gyr) and sub-solar metallicity ([Z/H] = -0.4 ± 0.1) inside the effective radius. There is tentative evidence that the counter-rotating core might be younger and less α/Fe enhanced. From the stellar population parameters we obtain a total stellar mass-to-light ratio of ∼3.6 (M/LB)⊙ which is significantly lower than a rough dynamical estimate obtained from the kinematics through the virial theorem (∼15). This discrepancy hints toward the possible presence of dark matter in the centre of VCC 510. Conclusions. We discuss the origin of the counter-rotating core and exclude fly-by encounters as a viable possibility. Gas accretion or galaxy merging provide more likely explanations. VCC 510 is therefore the direct observational evidence that such processes do occur in cluster satellite galaxies on dwarf galaxy scales. © ESO 2005.