We discuss recent developments on the age and metallicity distribution for early type galaxies in different environments.

We argue that multiple stellar arcs are formed by single beamed super-explosions or long-standing jets produced by objects connected with Gamma-ray bursts.

We present optical V, R, and I-band observations on the structure and stellar populations of the M81 Group and IC342/Maffei Complex galaxies. The images have been obtained with the 6m BTA and 2.5m Nordic optical telescopes. Stellar photometry of several galaxies in these groups have been performed. The resulting colour-magnitude diagrams reveal the morphology of different stellar populations in these systems. Distances to the galaxies have been estimated from the tip of the red giant branch (TRGB), or from the brightest blue and red stars. For some galaxies the reddening, metallicity and surface profile parameters have been measured. We describe in more detail the results for four galaxies: IC 2574, Holmberg I, DDO 53 and IC 342. A more comprehensive project is underway.

Hα luminosities are presented in order to study the Star Formation Rates (SFRs) of a sample of galaxies in compact groups from Hickson's (1982) catalogue. Although the comparison of the SFRs of the disk galaxies in our sample with those of a sample of field galaxies yielded no difference between the average SFRs for disk galaxies in compact groups and in the field, environmental effects seem to influence the Hα luminosities of late and early-type galaxies in compact groups. No relationship was found between the total normalized Hα luminosities of the groups and some dynamical parameters, indicating that the dynamical state of the group does not influence the SFR of the group. The lack of dominant interaction induced starbursts in our sample is compatible with a scenario for compact groups of galaxies in which the dark matter of the group is arranged in a common halo, thereby preventing a fast collapse of the galaxies.

Using the equivalent width (EW) of [O II]λ3727, I analyse the star formation properties of Las Campanas Redshift Survey galaxies in different environments. The star formation activity in compact groups is found to be depressed relative to that in loose groups and the field.

We compare the characteristics of stellar populations with those of dark halos. Dark matter around galaxies, and in groups, clusters and voids is discussed. Modern data suggest that the overall density of matter in the Universe is ΩM = 0.3 ± 0.1, about 80 % of this matter is nonbaryonic dark matter, and about 20 % is baryonic, mostly in the form of hot intra-cluster and intragroup gas, the rest in stellar populations of galaxies. All bright galaxies are surrounded by dark matter halos of external radii 200 − 300 kpc; halos consist mostly of non-baryonic matter with some mixture of hot gas. The Universe is dominated by dark energy (cosmological constant) term. Dark matter dominates in the dynamical evolution of galaxies in groups and clusters.

Triple systems of galaxies with the typical mass ∼ 10h13M⊙ and crossing time about the Hubble time provide evidence for ongoing collapse on the space scale ∼ 1 Mpc. This is confirmed by a set of computer simulations with the use of free-fall models.

New redshift surveys of galaxies in the field of compact groups have discovered a population of faint galaxies which act as satellites orbiting in the potential well of the bright group. Here we analyze the mass distribution of the groups by comparing the mass derived from the bright members and the mass obtained from the satellite galaxies. Our analysis indicates the presence of a dark halo around the main group with a mass roughly four times that measured for the dominant galaxies of the compact group.

We found that heavier halos are ruled out by the observations when comparing the distribution of positions and redshifts of the satellite galaxies with the distribution of satellites of isolated spiral galaxies. The results agree with a picture where compact groups may form a stable system with galaxies moving in a common dark halo.

Available observational data allow us to discriminate between the visible matter and the dark matter in M 31 and thus to determine the most important parameters of the dark halo (the mass, the radius and the outer extent).

Small, bound satellites of larger spiral galaxies are employed in the determination of the statistical presence of mass beyond the rotation disk of the largest (primary) galaxy. Optical and neutral hydrogen data are combined to develop mass estimates using the small bound groups. The resulting distribution of the internal/external mass ratios provides limits on halo mass models, and a glimpse at this mass estimation technique.

Numerical simulations of the dynamics of triplets of galaxies show that virial estimations of the individual masses of triplets are unreliable because of strong nonsteadiness of these groups and projection effects. However, the mass of a typical small galaxy group can be estimated statistically, from data on a whole homogeneous ensemble of groups such as, for example, Karachentsev’s Catalogue of compact galaxy triplets. The characteristic total mass of a triplet from this catalogue is 4 - 7 times greater than the visible mass when the standard mass-to-light ratio is assumed. This suggests the presence of some considerable amount of dark matter even in these compact triplets. In wide triplets of galaxies the amount of dark matter may be significantly larger. The presence of distributed dark matter makes motions of galaxies in triplets more stochastic. In this case the existence of long-lived hierarchical configurations (temporary binaries) in triple systems becomes much less probable than without dark matter. The analysis of internal motions of galaxies in triplets and their configurations may be interpreted as additional non-dynamical evidence of the presence of dark matter in galaxy triplets.

A major uncertainty in studying small groups of galaxies is the total masses of the individual galaxies themselves. I discuss here recent measurements of the mass of the Milky Way disk and new constraints on possible constituents of the putative dark halo.

In some external edge-on spiral galaxies, such as NGC 891, 4244, 4565 and 5907, the observed light distribution exhibits a sharp truncation in the outer parts (e.g., Casertano 1983, hereafter C83). As a rule, such galaxies are known to be warped in their gas disk, the deformation starting at about their optical cut-off radius. These facts suggest a truncation in the disk density distribution. Moreover, the rotation velocity of these galaxies starts decreasing at about the same radius, which can also be considered as the "signature" of truncation (see C83).

Both giant spiral galaxies in the Local Group, the Milky Way and M31, have a dip in the average rotation velocity which can be produced by a truncation of the galactic disk. Ninković & Petrovskaya (1992) constructed from Haud’s (1981) rotation curve for M31 a model including, like C83’s for NGC 5907, a truncated exponential disk and a halo. The truncation distance was found to be close to the radius beyond which warping of the hydrogen layer of M31 begins.

Kinematic data from neutral hydrogen observations provide information to solve the interdependent problems of the determination of the main Galactic constants (the Solar-Galactic center distance R0, the Oort constant A and others) and the Galactic rotation curve (Nikiforov & Petrovskaya 1994, hereafter NP94, and references therein). However, in the standard method for finding R0 by comparing the rotations of HI clouds and some other objects (typically HII regions/CO clouds), the kinematic model, constructed typically solely from HI data, is considered to be the same for both galactic subsystems (e.g. Merrifield 1992). In practice a discrepancy between their rotation curves can produce strongly erroneous results (Merrifield 1992, NP94). Establishing the common rotation law from HI plus HII/CO data in NP94 is only a part of attacking the problem.

We have obtained new optical spectra of the radio galaxy 3C 129 and the giant galaxy close to it. From these spectra we deduce a relative radial velocity of 710 km s−1 between the two galaxies. Using the orbit calculations of Byrd & Valtonen (1978) and the new observations we obtain a new value, 3.3 × 1014M⊙, for the mass of the system.

Cosmological many-body clustering agrees with the spatial and velocity distribution functions of galaxies at low redshifts, and it can be extended to high redshifts z ≈ 3 or more. The high redshift distribution functions are predicted to have a particular form. In the simplest case, there are no free parameters in this prediction, but the degree of clustering depends sensitively on Ω0. Current observations of small groups at high redshifts suggest that Ω0 = 0.3 ± 0.2 for Einstein-Friedmann cosmologies.

The predominance of minor mergers, i.e. the non-locality of mergings, results in a mass function slope (“Shechter” index) intermediate between a power-law index of a spectrum with constant mass flux and that of constant flux of the number of massive galaxies. The index proves to be close to that of the faint end of the galaxy luminosity function.

Using high resolution N-body simulations we study the formation and evolution of clusters and groups in a ΛCDM cosmological model. Groups of galaxies already form before z = 4. Merging of groups and accretion leads to cluster formation at z ≲ 2. Some of the groups merge into large isolated halos.

For the first time, the combination of semi-analytic modelling of galaxy formation and N-body simulations of cosmic structure formation enables us to model, at the same time, both the photometric and the clustering properties of galaxies. Two Cold Dark Matter Universes provide a reasonable fit to the observed properties of galaxies, groups and clusters, including luminosities, colours, density and velocity biases. We show how the properties of galaxies and groups on small scales are inextricably connected with the global properties of the Universe.

The numerical action variational method (NAM) is an elegant, non-chaotic technique for calculating the trajectories of gravitating systems in a cosmological context. It has been used extensively for establishing orbits of Local Group galaxies in a series of papers (Peebles 1989, 1990, 1994, 1995) and for the Local Supercluster (Shaya, Peebles, & Tully 1995).

Our repertoir of tools include code that holds present distances constant and predicts possible redshifts, one that holds present redshifts constant and predicts possible distances, and one that varies masses of each mass tracer and simultaneously satisfies both distance and redshift.

Our results indicate a Ω0 = 0.2, a mass-to-light ratio of field galaxies in the range of 100 M⊙/L⊙ in blue light (t0 = 11 Gyr with no cosmological constant), but for the Virgo Cluster, the value is 6 or 7 times higher. The higher values of Ω0 determined from using the IRAS galaxy distributions may be a result of under weighting this important mass component of the universe plus a poor correlation between cluster and field galaxy distributions.