The local group of galaxies

Summary. Hubble's (1936, p. 125) view that the Local Group (LG) is “a typical, small group of nebulae which is isolated in the general field” is confirmed by modern data. The total number of certain and probable Group members presently stands at 35. The half-mass radius of the Local Group is found to be kpc. The zero-velocity surface, which separates the Local Group from the field that is expanding with the Hubble flow, has a radius Mpc. The total mass of the LG is . Most of this mass appears to be concentrated in the Andromeda and Milky Way subgroups of the LG. The total luminosity of the Local Group is found to be :. This yields a mass-to-light ratio (in solar units) of . The solar motion with respect to the LG is \,km s, directed towards an apex at , and . The velocity dispersion within the LG is km s. The galaxies NGC 3109, Antlia, Sextans A and Sextans B appear to form a distinct grouping with kpc relative to the LG, that is located beyond the LG zero-velocity surface at a distance of 1.7 Mpc from the Local Group centroid. The luminosity distribution of the LG has a slope . This value is significantly less negative than that which is found in rich clusters of galaxies. The luminosity distribution of the dwarf spheroidal galaxies is steeper than that for dwarf irregulars. Furthermore the dSph galaxies are strongly concentrated within the Andromeda and Milky Way subclusters of the Local Group, whereas the majority of dIr galaxies appear to be free-floating members of the LG as a whole. With the possible exception of Leo I and Leo A, most LG members appear to have started forming stars simultaneously Gyr ago. Many of the galaxies, for which evolutionary data are available, appear to have shrunk with time. This result is unexpected because Hubble Space Telescope observations appear to show galaxies at to be smaller than they are at . In the Large Magellanic Cloud the rate of cluster formation was low for a period that extended from Gyr to Gyr ago. The rate of cluster formation may have increased more rapidly 3–5 Gyr ago, than did the rate of star formation. The reason for the sudden burst of cluster formation in the LMC Gyr ago remains obscure. None of the dwarf galaxies in the LG appears to have experienced a starburst strong enough to have produced a “boojum”. Received 14 April 1999     

On the structure of the low surface brightness dwarf galaxies in the M81 group

A detailed photometry of spheroidal dwarf galaxies in the M81 group has been carried out. The integral characteristics and the structural parameters of the spheroidal dwarfs has been determined. Their luminosity profiles are well fitted to a King law. The investigated spheroidal dwarfs together with the prototypes of the Local Group form a common sequence according to their main parameters. The observational data presented show that dSphs are not linked evolutionary with normal E and dE galaxies, but probably form a separate branch together with irregular low surface brightness dwarfs.     

TheHI content of the Eridanus group of galaxies

The HI content of galaxies in the Eridanus group is studied using the GMRT observations and the HIPASS data. A significant HI deficiency up to a factor of 2–3 is observed in galaxies in the high galaxy density regions. The HI deficiency in galaxies is observed to be directly correlated to the local projected galaxy density, and inversely correlated to the line-of-sight radial velocity. Furthermore, galaxies with larger optical diameters are predominantly in the lower galaxy density regions. It is suggested that the HI deficiency in Eridanus is due to tidal interactions. In some galaxies, evidences of tidal interactions are seen. An important implication is that significant evolution of galaxies can take place in the group environment. In the hierarchical way of formation of clusters via mergers of groups, a fraction of the observed HI deficiency in clusters could have originated in groups. The co-existence of S0s and severely HI deficient galaxies in the Eridanus group suggests that tidal interaction is likely to be an effective mechanism for transforming spirals to S0s.     

Star formation histories of local group dwarf galaxies

We present the study of the star formation histories (SFHs) of a sample of Local Group dwarf galaxies (LGDGs), via the analysis and modelling, with the means of an evolutionary stellar population synthesis, of their colour-magnitude (CM) diagrams. It appears that important parameters to describe the SFHs are star formation rate (SFR) and duration of star formation. We find a possible correlation between the mass and the SFRs. The correlation might be the origin of the observed luminosity-metallicity relation in the LGDGs. A well-defined correlation between the durations of star formation and the distance from M31 or the Galaxy indicates that the current early-type dwarf galaxies should have been transformed from late-type by strong tidal force of the massive galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.     

GMRTHI observations of the Eridanus group of galaxies

The GMRTHI 21 cm-line observations of galaxies in the Eridanus group are presented. The Eridanus group, at a distance of ≈ 23 Mpc, is a loose group of ≈200 galaxies. The group extends to more than 10 Mpc in projection. The velocity dispersion of the galaxies in the group is ≈240 km s⁻¹. The galaxies are clustered into different sub-groups. The overall population mix of the group is 30% (E + S0) and 70% (Sp + Irr). The observations of 57 Eridanus galaxies were carried out with the GMRT for ≈ 200 h. HI emission was detected from 31 galaxies. The channel rms of ≈ 1 mJy beam⁻¹ was achieved for most of the image-cubes made with 4 h of data. The corresponding HI column density sensitivity (3σ) is ≈ 1 × 10²⁰ cm⁻² for a velocity-width of ≈ 13.4 km s⁻¹. The 3σ detection limit of HI mass is ≈ 1.2 X 10⁷ Mpd for a line-width of 50 km s⁻¹. Total HI images, HI velocity fields, global HI line profiles, HI mass surface densities, HI disk parameters and HI rotation curves are presented. The velocity fields are analysed separately for the approaching and the receding sides of the galaxies. These data will be used to study the HI and the radio continuum properties, the Tully-Fisher relations, the dark matter halos, and the kinematical and HI lopsidedness in galaxies.     

A kinematic study of M51-type galaxies

Spectroscopic observations of twelve M51-type binary galaxies with the 6-m Special Astrophysical Observatory telescope are presented. We constructed the rotation curves for the primary galaxies of each binary system and determined the line-of-sight velocities of their companions from the Hα and [N II] 6583 Å emission lines.     

The Tully-Fisher relations of the Eridanus group of galaxies

The Tully-Fisher (TF) or the luminosity-linewidth relations of the galaxies in the Eridanus group are constructed using the HI rotation curves and the luminosities in the optical and in the near-infrared bands. The slopes of the TF relations (absolute magnitudevs log2V _flat) are −8.6 ± 1.1, −10.0 ±1.5, −10.7 ±2.1, and −9.7 ±1.3 in the R, J, H, and K bands respectively for galaxies having flat HI rotation curves. These values of the slopes are consistent with those obtained from studies of other groups and clusters. The scatter in the TF relations is in the range 0.5-1.1 mag in different bands. This scatter is considerably larger compared to those observed in other groups and clusters. It is suggested that the larger scatter in the TF relations for the Eridanus group is related to the loose structure of the group. If the TF relations are constructed using the baryonic mass (stellar +HI + Helium mass) instead of the stellar luminosity, nearly identical slopes are obtained in the R and in the near-infrared bands. The baryonic TF (baryonic massvs log2V _flat) slope is in the range 3.5–4.1.     

How special are brightest group and cluster galaxies?

We use the Sloan Digital Sky Survey (SDSS) to construct a sample of 625 brightest group and cluster galaxies (BCGs) together with control samples of non-BCGs matched in stellar mass, redshift and colour. We investigate how the systematic properties of BCGs depend on stellar mass and on their privileged location near the cluster centre. The groups and clusters that we study are drawn from the C4 catalogue of Miller et al. but we have developed improved algorithms for identifying the BCG and for measuring the cluster velocity dispersion. Since the SDSS photometric pipeline tends to underestimate the luminosities of large galaxies in dense environments, we have developed a correction for this effect which can be readily applied to the published catalogue data. We find that BCGs are larger and have higher velocity dispersions than non-BCGs of the same stellar mass, which implies that BCGs contain a larger fraction of dark matter. In contrast to non-BCGs, the dynamical mass-to-light ratio of BCGs does not vary as a function of galaxy luminosity. Hence BCGs lie on a different Fundamental Plane than ordinary elliptical galaxies. BCGs also follow a steeper Faber–Jackson relation than non-BCGs, as suggested by models in which BCGs assemble via dissipationless mergers along preferentially radial orbits. We find tentative evidence that this steepening is stronger in more massive clusters. BCGs have similar mean stellar ages and metallicities to non-BCGs of the same mass, but they have somewhat higher α/Fe ratios, indicating that star formation may have occurred over a shorter time-scale in the BCGs. Finally, we find that BCGs are more likely to host radio-loud active galactic nuclei than other galaxies of the same mass, but are less likely to host an optical active galactic nucleus (AGN). The differences we find are more pronounced for the less massive BCGs, i.e. they are stronger at the galaxy group level.     

Observations of the compact group of galaxies Shahbazian 130

Byurakan Astrophysical Observatory and Special Astrophysical Observatory, Academy of Sciences. Translated fromAstrofizika, Vol. 35, No. 1, pp. 67–75, July–August, 1991.     

The structure and evolution of M 51-type galaxies

We discuss the integrated kinematic parameters of 20 M 51-type binary galaxies. A comparison of the orbital masses of the galaxies with the sum of the individual masses suggests that moderately massive dark halos surround bright spiral galaxies. The relative velocities of the galaxies in binary systems were found to decrease with increasing relative luminosity of the satellite. We obtained evidence that the Tully-Fisher relation for binary members could be flatter than that for local field galaxies. An enhanced star formation rate in the binary members may be responsible for this effect. In most binary systems, the direction of the orbital motion of the satellite coincides with the direction of the rotation of the main galaxy. Seven candidates for distant M 51-type objects were found in the Northern and Southern Hubble Deep Fields. A comparison of this number with the statistics of nearby galaxies provides evidence for the rapid evolution of the space density of M 51-type galaxies with redshift z. We assume that M 51-type binary systems could be formed through the capture of a satellite by a massive spiral galaxy. It is also possible that the main galaxy and its satellite in some of the systems have a common cosmological origin.     

Synchronous evolution of galaxies in groups: NGC524 group

With the means of panoramic spectroscopy at the SAO RAS BTA telescope, we investigated the properties of stellar populations in the central regions of five early-type galaxies—the NGC524 group members. The evolution of the central regions of galaxies looks synchronized: the average age of stars in the bulges of all the five galaxies lies in the range of 3–6Gyr.Four of the five galaxies revealed synchronized bursts of star formation in the nuclei 1–2 Gyr ago. The only galaxy, in which the ages of stellar population in the nucleus and in the bulge coincide (i.e. the nuclear burst of star formation did not take place) isNGC502, the farthest from the center of the group of all the galaxies studied.     

Origin of disc lopsidedness in the Eridanus group of galaxies

The H  i surface density maps for a sample of 18 galaxies in the Eridanus group are Fourier analysed. This analysis gives the radial variation of the lopsidedness in the H  i spatial distribution. The lopsidedness is quantified by the Fourier amplitude A ₁ of the m = 1 component normalized to the average value. It is also shown that in the radial region where the stellar disc and H  i overlap, their A ₁ coefficients are comparable. All the galaxies studied show significant lopsidedness in H  i . The mean value of A ₁ in the inner regions of the galaxies (1.5–2.5 scalelengths) is ≥ 0.2. This value of A ₁ is twice the average value seen in the field galaxies. Also, the lopsidedness is found to be smaller for late-type galaxies; this is opposite to the trend seen in the field galaxies. These two results indicate a different physical origin for disc lopsidedness in galaxies in a group environment compared to the field galaxies. Further, a large fraction (∼30 per cent) shows a higher degree of lopsidedness ( A ₁≥ 0.3). It is also seen that the disc lopsidedness increases with the radius as demonstrated in earlier studies, but over a radial range that is two times larger than done in the previous studies. The average lopsidedness of the halo potential is estimated to be ∼10 per cent, assuming that the lopsidedness in H  i disc is due to its response to the halo asymmetry.     

Central regions of the early-type galaxies in the NGC 3169 group

We have investigated the central regions of the galaxies in the NGC 3169/NGC 3166/NGC 3156 group with the multipupil fiber spectrograph of the 6-m telescope; the first (central) galaxy in the group is a spiral (Sa) one and the other two galaxies are lenticular ones. The group is known to have an extended HI cloud with a size of more than 100 kpc that is associated in its position, orientation, and rotation with the central galaxy NGC 3169. The mean age of the stellar populations in the centers of all three galaxies has been found to be approximately the same, ～1 Gyr. Since the galaxies are early-type ones and since NGC 3166 and NGC 3156 show no global star formation, we are dealing here with a synchronous star formation burst in the centers of all three galaxies.     

Stellar content of the interacting galaxies of the M51 system

Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for more than 0.6 million stars in the interacting galaxies NGC 5194 and NGC 5195 of the M51 system. Stars of various ages have been identified on the constructed Hertzsprung-Russell diagram: blue and red supergiants, AGB stars, and red giants. The distance to M51 has been measured from the position of the tip of the red giant branch, D = 9.9 ± 0.7 Mpc. We have determined the change in the metallicity of red supergiants along the galactic radius in NGC 5194. Despite the gravitational interaction, the distribution of stars in NGC 5194 does not differ from that in isolated galaxies. The asymmetric stellar structures of NGC5195 (the so-called “feathers”) formed through the interaction of two galaxies have been found to consist mostly of AGB stars.     

Photometry of groups of galaxies. II. The NGC 972 group

A nearby group of galaxies, centered at the spiral galaxy NGC 972 and conspicuously rich in faint dwarf spheroidal galaxies, has been investigated photometrically on the B and V plates, obtained with the Tautenburg 2m Schmidt telescope. For ten certain and probable group members the equivalent B and V brightness profiles have been extracted. Integrated magnitudes and mean colours of faint group members have been estimated. Typically, the dwarfs show a “subexponential” (i.e. a convex curvature over the linear radius) surface brightness profile and often a central excess. Their equivalent profiles are reasonably well fitted by modified isothermal (King) models.     

Radio continuum and far-infrared emission from the galaxies in the Eridanus group

The Eridanus galaxies follow the well-known radio—FIR correlation. The majority (70%) of these galaxies have their star formation rates below that of the Milky Way. The galaxies that have a significant excess of radio emission are identified as low luminosity AGNs based on their radio morphologies obtained from the GMRT observations. There are no powerful AGNs (L _20cm > 10²³ W Hz⁻¹) in the group. The two most far-infrared and radio luminous galaxies in the group have optical and HI morphologies suggestive of recent tidal interactions. The Eridanus group also has two far-infrared luminous but radio-deficient galaxies. It is believed that these galaxies are observed within a few Myr of the onset of an intense star formation episode after being quiescent for at least a 100 Myr. The upper end of the radio luminosity distribution of the Eridanus galaxies (L _20cm ∼ 10²² W Hz⁻¹) is consistent with that of the field galaxies, other groups, and late-type galaxies in nearby clusters.