The Fundamental Plane of E Galaxies in Compact Groups

We present a study focusing on the nature of compact groups through the study of their elliptical galaxies. We determine central velocity dispersions (σ_o) for 18 bright elliptical galaxies located in the core of Hickson compact groups and a control sample of 12 brightbona fide ellipticals located in the field or very loose groups. Several tests are carried out to avoid systematic effects in σ measurements. We use these velocity dispersions to compare the position of 11 compact group galaxies in the Fundamental Plane to that of a large and homogeneous sample of elliptical galaxies (Burstein et al., 1987).We find that little or no significant difference exists, as far as the Fundamental Plane is concerned, between ellipticals in compact groups and their counterparts in other environments. This revised version was published online in July 2006 with corrections to the Cover Date.     

A study of neutral hydrogen in five small galaxy groups

High-resolution H  i imaging observations of a heterogeneous sample of small galaxy groups are presented. The five galaxy groups studied show a broad range of individual H  i properties: e.g. loose groups surrounding LGG 138 and the genuinely compact LGG 455 are identified; a massive ring of neutral gas is discovered encircling two luminous galaxies in the LGG 138 group; a galaxy-sized mass of H  i is found in LGG 455 confined to an extragalactic cloud which exceeds the threshold density for star formation, yet is optically invisible; and the CCG 1 group is argued to be a chance alignment of Centaurus cluster galaxies. Global results of the study are that the deficit of H  i flux in synthesis imaging data compared with single-dish data is put forward as a quantitative measure of the diffuseness of neutral gas in galaxy groups; several groups contain gas-poor galaxies that ordinarily would contain detectable quantities of H  i – this is interpreted as being caused by an increased chance of gas-sweeping collisions in the group environment; and some evidence is found to support previous findings that compact groups preferentially occur in loose systems.     

Spectral and photometric study of compact groups of compact galaxies

Results are presented from a spectral, photometric, and morphological study of two compact groups of compact galaxies (CGCG) from Shahbazian’s list. The observations were made on the 1.5-m ESO telescope and the 2.6-m telescope at the Byurakan Astrophysical Observatory (BAO) during 1999–2000. These data are compared with data for the Shahbazian compact group 4. A high spatial density of galaxies, ranging from 10000 to 100000 galaxies per Mpc³, is observed in all the groups that were studied. The discovery of a Seyfert Sy1 galaxy in the Shahbazian 355 compact group is especially noteworthy. The direct photographs of the CGCGs that were studied contain no signs of merging or tidal interactions among the members of these groups. This suggests that the galaxies in these dense systems have a common origin. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 347–353 (August 2007).     

On the origin of the faint-end of the red sequence in high-density environments

With the advent of the new generation wide-field cameras it became possible to survey in an unbiased mode galaxies spanning a variety of local densities, from the core of rich clusters, to compact and loose groups, down to filaments and voids. The sensitivity reached by these instruments allowed to extend the observation to dwarf galaxies, the most “fragile” objects in the universe. At the same time models and simulations have been tailored to quantify the different effects of the environment on the evolution of galaxies. Simulations, models, and observations consistently indicate that star-forming dwarf galaxies entering high-density environments for the first time can be rapidly stripped from their interstellar medium. The lack of gas quenches the activity of star formation, producing on timescales of \({\sim }\)1 Gyr quiescent galaxies with spectro-photometric, chemical, structural, and kinematical properties similar to those observed in dwarf early-type galaxies inhabiting rich clusters and loose groups. Simulations and observations consistently identify ram pressure stripping as the major effect responsible for the quenching of the star-formation activity in rich clusters. Gravitational interactions (galaxy harassment) can also be important in groups or in clusters whenever galaxies have been members since early epochs. The observation of clusters at different redshifts combined with the present high infalling rate of galaxies onto clusters indicate that the quenching of the star-formation activity in dwarf systems and the formation of the faint end of the red sequence is a very recent phenomenon.     

Compact Groups of Galaxies: Evolution of the Stellar Population

We present a study focusing on the nature of compact groups (CGs) through the study of their elliptical (E) galaxies. Parameters describing the internal dynamics and the stellar populations of the galaxies are compared with their counterparts in other environments. Our sample has24 bright E galaxies located in the core of Hickson CGs and 11 bright ‘bona fide’ Es, located in the field or very loose groups. Their spectra were obtained with the 2.1 m Telescope at KPNO and have a 4.1 Å resolution over the wavelength range 3500–7000 Å. We have found that, from the dynamical point of view, E galaxies in CGs are essentially similar to those in dense clusters. Moreover, the stellar populations of Es in CGs seem to be older and less metal rich than those in the field, behaviour which has also been observed by Rose et al. (1994).     

H i content in galaxies in loose groups

Gas deficiency in cluster spirals is well known and ram-pressure stripping is considered the main gas removal mechanism. In some compact groups too gas deficiency is reported. However, gas deficiency in loose groups is not yet well established. Lower dispersion of the member velocities and the lower density of the intragroup medium in small loose groups favour tidal stripping as the main gas removal process in them. Recent releases of data from the H  i Parkes All-Sky Survey (HIPASS) and catalogues of nearby loose groups with associated diffuse X-ray emission have allowed us to test this notion. In this paper, we address the following questions: (i) do galaxies in groups with diffuse X-ray emission statistically have lower gas content compared to the ones in groups without diffuse X-ray emission? (ii) does H  i deficiency vary with the X-ray luminosity, L _X, of the loose group in a systematic way? We find that (i) galaxies in groups with diffuse X-ray emission, on average, are H  i deficient, and have lost more gas compared to those in groups without X-ray emission; the latter are found not to have significant H  i deficiency; (ii) no systematic dependence of the H  i deficiency with L _X is found. Ram-pressure-assisted tidal stripping and evaporation by thermal conduction are the two possible mechanisms to account for this excess gas loss.     

H i imaging of galaxies in X-ray bright groups

Environment plays an important role in the evolution of the gas contents of galaxies. Gas deficiency of cluster spirals and the role of the hot intracluster medium in stripping gas from these galaxies is a well-studied subject. Loose groups with diffuse X-ray emission from the intragroup medium (IGM) offer an intermediate environment between clusters and groups without a hot IGM. These X-ray bright groups have smaller velocity dispersion and lower temperature than clusters, but higher IGM density than loose groups without diffuse X-ray emission. A single-dish comparative study of loose groups with and without diffuse X-ray emission from the IGM, showed that the galaxies in X-ray bright groups have lost more gas on average than the galaxies in non X-ray bright groups. In this paper we present GMRT H  i observations of 13 galaxies from four X-ray bright groups: NGC 5044, 720, 1550 and IC1459. The aim of this work is to study the morphology of H  i in these galaxies and to see if the hot IGM has in any way affected their H  i content or distribution. In addition to disturbed H  i morphology, we find that most galaxies have shrunken H  i discs compared to the field spirals. This indicates that IGM-assisted stripping processes like ram pressure may have stripped gas from the outer edges of the galaxies.     

Isolated Shakhbazian compact groups

It is more appropriate to study the dynamics and evolution of compact groups using a sample of isolated compact groups in the nearby vicinity of which there are no accordant redshift galaxies. To look for isolated compact groups we inspected the environment of 78 Shakhbazian compact groups, with known redshifts. We found that 26 of nearby groups with V < 40000 km s⁻¹ are isolated compact groups in the vicinity of which up to a projected distance of 1 Mpc there are no accordant redshift galaxies. For four of them, the redshift of only two members are known, so their being groups is not certain. In the vicinities of eleven distant groups (V > 40000 km s⁻¹) no accordant redshift galaxies are detected as well. The reason for this may be the faintness of galaxies there. These groups may possibly be isolated.     

Galaxy evolution in a new Southern Compact Groups sample

Compact groups are association of 4-8 galaxies with high spatial densities (Δρ/ρ∼10–20), but low velocity dispersion. Galaxy-galaxy interactions (e.g. close tidal encounters or mergers) are therefore likely to dominate any environmentally-dependent evolution of galaxies in these compact, highlocal-density, groups, unlike in the field, where galaxy densities are too low, and in clusters, where relative velocities are too high. We present here the results from spectroscopic and photometric observations of a new sample of compact groups in the Southern Sky, selected by an automatic detection algorithm applied to a digitized galaxy database. This revised version was published online in July 2006 with corrections to the Cover Date.     

The color properties of compact groups of main galaxies from the SDSS Data Release 4

In order to compare color statistical properties of member galaxies in compact groups with those of field galaxies, we use a compact group (CG) sample which contains 4217 CGs extracted from the MAIN galaxy sample of the SDSS Data Release 4 (SDSS4), and construct a random group sample which has the same distributions of redshift and number of member galaxies as those of the CG sample. It turns out that the mean colors of galaxies in CGs are redder than those of galaxies in random groups. Additionally, we also find that member galaxies in real CGs have a smaller dispersion of colors than those in random groups-at least for g-r, r-i, and i-z colors. Published in Astrofizika, Vol. 50, No. 1, pp. 29–37 (February 2007).     

Early versus late type galaxies in compact groups

We find a strong correlation between the effective radius of the largest early-type galaxies in compact groups of galaxies and the velocity dispersion of the groups. The lack of a similar correlation for late type galaxies is supportive of the so called second generation merging scenario which predicts that ellipticals should dominate the internal dynamics of the groups, while late-type galaxies are mainly recent interlopers which are still in an early stage of interaction with the group potential.     

On the cometary shape of compact groups of compact galaxies

It is shown that the brightest members of elongated, compact groups of compact galaxies (CGCGs) are located not in the central part of a group but at its edge. This suggests that compact groups of compact galaxies (n>8) have a cometary shape, with their opening angle being less than 60°, for the most part.Translated from Astrofizika, Vol. 39, No. 4, pp. 585–592, October–December, 1996.     

Are X-ray properties of loose groups different from those of compact groups?

We compare the X-ray properties of loose and compact galaxy groups, using a combined sample of 42 groups. We find that we are unable to separate loose and compact groups on the luminosity–temperature relation, the luminosity–velocity dispersion relation or the velocity dispersion–temperature relation using equally weighted errors. This suggests that the distinction between compact and loose groups is not a fundamental one, and we argue that a more useful distinction is that between X-ray bright and X-ray faint systems.
Given their similarity in X-ray properties, we combine the loose and compact subsamples to derive relations based on the full sample. This provides the highest statistical quality results to date on the way in which the correlations in X-ray properties of low‐mass systems depart from those seen in rich clusters.     

A fresh look at discordant redshift galaxies in compact groups

We reexamine the statistics of discordant redshift galaxies in compact groups. We find that 43 out of 100 groups in the Hickson catalog contain at least one discordant redshift galaxy. We show that, despite the prevailing impression, all previous attempts have failed to explain this large number of discordant redshift galaxies. The order of magnitude excess survives all of our attempts to refine the sample.     

Nonthermal particles and photons in starburst regions and superbubbles

Starforming factories in galaxies produce compact clusters and loose associations of young massive stars. Fast radiation-driven winds and supernovae input their huge kinetic power into the interstellar medium in the form of highly supersonic and superalfvenic outflows. Apart from gas heating, collisionless relaxation of fast plasma outflows results in fluctuating magnetic fields and energetic particles. The energetic particles comprise a long-lived component which may contain a sizeable fraction of the kinetic energy released by the winds and supernova ejecta and thus modify the magnetohydrodynamic flows in the systems. We present a concise review of observational data and models of nonthermal emission from starburst galaxies, superbubbles, and compact clusters of massive stars. Efficient mechanisms of particle acceleration and amplification of fluctuating magnetic fields with a wide dynamical range in starburst regions are discussed. Sources of cosmic rays, neutrinos and multi-wavelength nonthermal emission associated with starburst regions including potential galactic “PeVatrons” are reviewed in the global galactic ecology context.     

On structures and number-density distributions in clusters of galaxies

The structures of compact groups of galaxies are compared with the structures of rich clusters of galaxies. It was established that there are structural similarities between these two types of clusters of galaxies. It is imaginable that these structures are typical for all such complexes of galaxies and that the different structures are due to different initial conditions at the beginning of their evolution. For a final answer on this question it is necessary to accumulate more observational material with respect to the distribution of galaxies in such clusters. As part of a larger programme the present paper contains the derived number-density distributions and core radii for ten clusters.     

Red giants in the blue compact galaxy IZw18

Archival Hubble Space Telescope (HST) data have been used for the photometry of stars in blue compact dwarf (BCD) galaxies of the IZw18 system. Applying the spatial selection of stars, we have detected red giants, stars older than 1–2 Gyr, in the galaxies. These red giants have allowed the distance to IZw18 to be reliably determined for the first time: D = 13.9 ± 1.2 Mpc. The presence of old stars in the galaxies of the IZw18 system refutes the hypothesis about the observed primary star formation in these galaxies.     

Compact groups in theory and practice – II. Comparing the observed and predicted nature of galaxies in compact groups

We examine the properties of galaxies in compact groups (CGs) identified in a mock galaxy catalogue based upon the Millennium Run simulation. The overall properties of groups identified in projection are in general agreement with the best available observational constraints. However, only ∼30 per cent of these simulated groups are found to be truly compact in three dimensions, suggesting that interlopers strongly affect our observed understanding of the properties of galaxies in CGs. These simulations predict that genuine CG galaxies are an extremely homogeneous population, confined nearly exclusively to the red sequence: they are best described as 'red and dead' ellipticals. When interlopers are included, the population becomes much more heterogeneous, due to bluer, star-forming, gas-rich, late-type galaxies incorrectly identified as CG members. These models suggest that selection of members by redshift, such that the line-of-sight velocity dispersion of the group is less than 1000 km s⁻¹, significantly reduces contamination to the 30 per cent level. Selection of members by galaxy colour, a technique used frequently for galaxy clusters, is also predicted to dramatically reduce contamination rates for CG studies.     

Redshifts of compact galaxies in systems of galaxies

82 compact galaxies with measured redshift present in systems containing partly normal galaxies have been found, using Zwicky's Catalogues and Morphological Catalogue. For them the mean residual redshift has been obtained. The chance probability for this result is 0.015. A high frequency of compact galaxies among those with large discrepant redshift is remarked. Relatively compact normal galaxies also reveal a positive mean value of . Redshift is correlated with magnitude in groups and pairs of compact galaxies, this depending in pairs on the linear separation and colours of the components. If a colour difference is present, the bluer member has the larger redshift in general. In contrast with the large luminosity deduced from the redshift for many field compacts, compact galaxies in systems are faint. The majority of the proposed associations between quasars and systems of normal galaxies are shown to be probably physical.     

A list of groups of dwarf galaxies in the local supercluster

We report a list of groups consisting of dwarf galaxies only. The sample contains 126 objects, mainly combined in pairs. The most populated group contains six dwarf galaxies. The majority of systems considered reside in the low-density regions and evolve unaffected by massive galaxies. The characteristic sizes and velocity dispersions of groups are 30 kpc and 11 km/s, respectively. They resemble the associations of dwarf galaxies, but are more compact. On the whole, groups and associations form a continuous sequence. Alike the associations, our groups possess high mass-to-luminosity ratios, what is indicative of a large amount of dark matter present in these systems.