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We present a new scenario for the origin of the counter-rotating stars in disc galaxies, defined as stars that have a negative tangential velocity. This scenario involves a merger between two gas-rich disc galaxies that have comparable masses, are nearly coplanar, and are rotating in the same direction. The merger results in an intense starburst, during which a significant fraction of the gas is converted to stars. The system then settles into an equilibrium configuration consisting of a thick disc and a bulge partly supported by velocity dispersion and a thin disc supported by rotation. Star formation proceeds until most of the gas supply is exhausted. Stars formed during the starburst have tangential velocities ranging from $-600$ to $600\, {\rm km\, s^{-1}}$. Stars formed afterward in the thick disc and bulge have high eccentricities and low tangential velocities, typically in the range $-100$ to $100\, {\rm km\, s^{-1}}$, while stars formed in the thin disc have large, positive velocities. All fast, counter-rotating stars ($V\lt -200\, {\rm km\, s^{-1}}$) are old, metal-poor, with very low dispersion in ages and metallicities. By contrast, fast, corotating stars ($V\gt 200\, {\rm km\, s^{-1}}$) have a wide range of ages and metallicities. The average abundances ratios $\rm [O/H]$ and [Fe/H] for fast, corotating stars typically exceed the corresponding ratios for fast, counter-rotating stars by $0.1-0.4\,\mathrm{ dex}$, while the dispersion in the values of NFe/NH are larger by factors between 2 and 14. This provides an observational signature of major, gas-rich mergers at high redshift.

Accretion in the nuclei of active galaxies may occur chaotically. This can produce accretion discs which are counter‐rotating or strongly misaligned with respect to the spin of the central supermassive black hole (SMBH), or the axis of a close SMBH binary. Accordingly we consider the cancellation of angular momentum in accretion discs with a significant change of plane (tilt) between inner and outer parts. We estimate analytically the maximum accretion rate through such discs and compare this with the results of Smoothed Particle Hydrodynamics (SPH) simulations. These suggest that accretion rates on to supermassive black holes may be larger by factors& 100 if the disc is internally tilted in this way rather than pl anar. This offers a natural way of driving the rapid growth of supermassive black holes, and the coalescence of SMBH binaries.

We probe the HI properties and the gas environments of three early-type barred galaxies harbouring counter-rotating ionized gas, NGC 128, NGC 3203 and NGC 7332. Each system has one or more optically-identified galaxy, at a similar or as yet unknown redshift within a 50 kpc projected radius. Using HI synthesis imaging data, we investigate the hypothesis that the counter-rotating gas in these galaxies has been accreted from their neighbours. In NGC 128 and NGC 3203, we find 9.6e7 and 2.3e8 Msun of HI, respectively, covering almost the entire stellar bodies of dwarf companions that appear physically connected. Both the HI morphology and kinematics are suggestive of tidal interactions. In NGC 7332, we do not find any directly-associated HI. Instead, NGC 7339, a neighbour of a comparable size at about 10 kpc, is found with 8.9e8 Msun of HI gas. More recently in a single dish observation, however, another group discovered a large HI structure which seems to be an extension of NGC 7339's HI disc and also covers NGC 7332. All these observations thus suggest that HI gas is being accreted in these three galaxies from their companions, which is likely responsible for the kinematically-decoupled gas component present in their central region. Considering the incompleteness of existing studies of the faint dwarf galaxy population both in the optical and in HI, accretion from cold gas blobs, presumably gas-rich dwarfs, is expected to occur even more frequently than what is inferred from such cases that have been observed to date.

When integrals in the standard Tremaine-Weinberg method are evaluated for the case of a realistic model of a doubly barred galaxy, their modifications introduced by the second rotating pattern are in accord with what can be derived from a simple extension of that method, based on separation of tracer's density. This extension yields a qualitative argument that discriminates between prograde and retrograde inner bars. However, the estimate of the value of inner bar's pattern speed requires further assumptions. When this extension of the Tremaine-Weinberg method is applied to the recent observation of the doubly barred galaxy NGC 2950, it indicates that the inner bar there is counter-rotating, possibly with the pattern speed of -140 +/- 50 km/s/arcsec. The occurrence of counter-rotating inner bars can constrain theories of galaxy formation.

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 previously-known counter-rotating stellar disk in this system. Upper limits determined for each object indicate that no more than about 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-sigma error) of the gas disks in S0 galaxies counter-rotate relative to their stellar components., Accepted for publication in MNRAS. Latex file, using mn.sty and epsf.sty, including postscript figures except fig.2. Figure 2 can be obtained from http://www.astro.rug.nl/~secr/