A search for counter-rotating stars in S0 galaxies

Abstract

We have obtained high signal-to-noise spectra along the major axes of 28 S0 galaxies in order to search for the presence of disc stars on retrograde orbits. Full line-of-sight velocity distributions are extracted from the data, and the velocity distributions are modelled as arising from the superposition of populations of stars on prograde and retrograde orbits. We find no new cases in which a significant fraction of disc stars lie on retrograde orbits; an identical analysis of NGC 4550 does reveal the previously known counter-rotating stellar disc in this system. Upper limits determined for each object indicate that no more than ∼ 5 per cent of the observed disc star light could arise from counter-rotating stellar components. These results suggest that previously discovered disc galaxies with counter-rotating stars are exceptional, and that (at 95 per cent confidence) less than 10 per cent of S0 galaxies contain significant counter-rotating populations. The most likely value for the fraction of such S0 galaxies lies closer to 1 per cent. This result contrasts with the prevalence of counter-rotating gas in these systems; combining our new observations with existing data, we find that 24 ± 8 per cent (1 σ error) of the gas discs in S0 galaxies counter-rotate relative to their stellar components.