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Context. Compact groups of galaxies are entities that have high densities of galaxies and serve as laboratories to study galaxy interactions, intergalactic star formation and galaxy evolution.

Aims. We create a catalogue of simulatedfossil groups and study their properties, in particular the merging histories of their first-ranked galaxies. We compare the simulatedfossil group properties with those of both simulatednon-fossil and observedfossil groups.

Context.HCG 31 is one of the most intriguing compact groups in Hickson's catalogue. It contains a central pair of interacting dwarf galaxies and other small members that are highly peculiar.

Aims.We analyse the photometric properties of the early-type Fornax cluster dwarf-galaxy population ($M_{\rm V}>-17$mag), based on a wide-field imaging study of the central cluster area in Vand Ibandpasses. We used the instrument/telescope combination IMACS/Magellan at Las Campanas Observatory, providing much larger light-collecting area and better image resolution than previous wide-field imaging surveys.

We report the discovery of Cl 2334+48, a rich cluster of galaxies in the Zone of Avoidance, identified in public images from the XMM-Newtonarchive. We present the main properties of this cluster using the XMM-NewtonX-ray data, along with new optical spectroscopic and photometric observations. Cl 2334+48is located at $z = 0.271$±0.001, as derived from the optical spectrum of the brightest member galaxy. Such redshift agrees with a determination from the X-ray spectrum (z= 0.263$^{+0.012}_{-0.010}$), in which an intense emission line is matched to the rest wavelength of the Fe Kαcomplex. Its intracluster medium has a plasma temperature of $4.92^{+0.50}_{-0.48}$keV, a sub-solar abundance of 0.38 ±$0.12~Z_{\odot}$, and a bolometric luminosity of 3.2 $\times$1044erg s-1. A density contrast $\delta = 2500$is reached in a radius of $0.5~h_{70}^{-1}$Mpc, and the corresponding enclosed mass is 1.5 $\times$$10^{14}~M_{\odot}$. Optical images show an enhancement of $g^\prime - i^\prime > 2.5$galaxies around the central galaxy, as expected if these were cluster members. The central object is a luminous E-type galaxy, which is displaced ~$40~h_{70}^{-1}$kpc from the cluster X-ray centre. In addition, it has a neighbouring arc-like feature (~$22\arcsec$or $90~h_{70}^{-1}$kpc from it), probably due to strong gravitational lensing. The discovery of Cl 2334+48emphasises the remarkable capability of the XMM-Newtonto reveal new clusters of galaxies in the Zone of Avoidance.

Context. Rotation curves of interacting galaxies often show that velocities are either rising or falling in the direction of the companion galaxy.Aims. We seek to reproduce and analyse these features in the rotation curves of simulated equal-mass galaxies suffering a one-to-one encounter as possible indicators of close encounters.Methods. Using simulations of major mergers in 3D, we study the time evolution of these asymmetries in a pair of galaxies during the first passage.Results. Our main results are: (a) the rotation curve asymmetries appear right at pericentre of the first passage, (b) the significant disturbed rotation velocities occur within a small time interval, of ?0.5?Gyrh-1, and, therefore, the presence of bifurcation in the velocity curve could be used as an indicator of the pericentre occurrence. These results are in qualitative agreement with previous findings for minor mergers and flybys.

Context.Fossil systems are defined to be X-ray bright galaxy groups (or clusters) with a two-magnitude difference between their two brightest galaxies within half the projected virial radius, and represent an interesting extreme of the population of galaxy agglomerations. However, the physical conditions and processes leading to their formation are still poorly constrained.

Context.The formation of ultra-compact dwarf galaxies (UCDs) is believed to be driven by interaction, and UCDs are abundant in the cores of galaxy clusters, environments that mark the end-point of galaxy evolution. Nothing is known about the properties of UCDs in compact groups of galaxies, environments where most of galaxy evolution and interaction is believed to occur and where UCDs in an intermediate stage in their evolution may be expected.