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 disk stars on retrograde orbits. Full line-of-sight velocity distributions were extracted from the data, and the velocity distributions were 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 disk stars lie on retrograde orbits; an identical analysis of NGC 4550 does reveal the previouslyknown counter-rotating stellar disk in this system. Upper limits determined for each object indicate that no more than ∼ 5% of the observed disk star light could arise from counter-rotating stellar components. These results suggest that previously-discovered disk galaxies with counter-rotating stars are exceptional and that (at 95% confidence) at most 10% of S0 galaxies contain significant counter-rotating populations. The most likely value for the fraction of such S0 galaxies lies closer to 1%. 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% (1-σ error) of the gas disks in S0 galaxies counter-rotate relative to their stellar components.

Molecular gas and dust in NGC 4550

“…Once the gas is consumed, NGC 3593 will look like an S0 galaxy with a counterrotating stellar population comprising ∼25% of the total stellar disk mass. Although less than in NGC 4550, it is still significant in comparison to the limits obtained for counterrotating stellar components in the sample of 28 S0 galaxies by Kuijken et al (1996).…”

“…Despite dedicated searches (cf. Kuijken et al 1996), there are only two known galaxies with large counterrotating stellar disks; NGC 4550 (Rubin et al 1992) and NGC 7217 (Merrifield & Kuijken 1994). There are a few cases which do contain counterrotating stellar disk components, but where the secondary component is much less extended than the primary one and is confined to the central region: NGC 3593 (Bertola et al 1996), NGC 4138 (Jore et al 1996) and NGC 7331 (Prada et al 1996).…”

“…There are a few cases which do contain counterrotating stellar disk components, but where the secondary component is much less extended than the primary one and is confined to the central region: NGC 3593 (Bertola et al 1996), NGC 4138 (Jore et al 1996) and NGC 7331 (Prada et al 1996). On the other hand Bertola et al (1992) and Kuijken et al (1996) report that ∼20-25% of all gas disks found in S0 galaxies counterrotate with respect to the stars. If this gas would form stars in such numbers, that more than 5% of the stellar disks were counterrotating, a much higher fraction of galaxies with counterrotating stellar components would be observed.…”

Structure and kinematics of candidate double-barred galaxies

“…According to Kuijken et al (1996), the gaseous disks in lenticular galaxies demonstrate a counter-rotation phenomenon in 24 ± 8% of all cases. Therefore, this is a frequent kinematic feature in early-type galaxies.…”

Making Spirals with Counter-Rotating Disks: The Case of NGC 4550

“…Many other cases of counter-rotation are now known in ellipticals and also in spirals, but what makes the case of NGC 4550 particular (Kuijken, Fisher, & Merrifield 1996) is that the mass ratio of the counter-rotating disks is nearly 1/1. In the case of ellipticals counter-rotation is so frequent (e.g.…”