Groups of dwarf galaxies in the Local supercluster

We present a project on study of groups composed of dwarf galaxies only. We selected such structures using HyperLEDA and NED databases with visual inspection on SDSS images and on digital copy of POSS. The groups are characterized by size of few tens of kpc and line-of-sight velocity dispersion about 18 km/s. Our groups similar to associations of nearby dwarfs from Tully et al. (2006). This specific population of multiple dwarf galaxies such as IZw18 may contain significant amount of dark matter. It is very likely that we see them at the stage just before merging of its components.Comment: 6 pages, 4 figures; the proceedings of the conference "A Universe of dwarf galaxies" (Lyon, June 14-18, 2010

Local galaxy flows within 5 Mpc

We present Hubble Space Telescope/WFPC2 images of sixteen dwarf galaxies as part of our snapshot survey of nearby galaxy candidates. We derive their distances from the luminosity of the tip of the red giant branch stars with a typical accuracy of ~12%. The resulting distances are 4.26 Mpc (KKH 5), 4.74 Mpc (KK 16), 4.72 Mpc (KK 17), 4.66 Mpc (ESO 115-021), 4.43 Mpc (KKH 18), 3.98 Mpc (KK 27), 4.61 Mpc (KKH 34), 4.99 Mpc (KK 54), 4.23 Mpc (ESO 490-017), 4.90 Mpc (FG 202), 5.22 Mpc (UGC 3755), 5.18 Mpc (UGC 3974), 4.51 Mpc (KK 65), 5.49 Mpc (UGC 4115), 3.78 Mpc (NGC 2915), and 5.27 Mpc (NGC 6503). Based on distances and radial velocities of 156 nearby galaxies, we plot the local velocity-distance relation, which has a slope of H_0 = 73 km/(c * Mpc) and a radial velocity dispersion of 85 km/s. When members of the M81 and CenA groups are removed, and distance errors are taken into account, the radial velocity dispersion drops to sigma_v=41 km/s. The local Hubble flow within 5 Mpc exibits a significant anisotropy, with two infall peculiar velocity regions directed towards the Supergalactic poles. However, two observed regions of outflow peculiar velocity, situated on the Supergalactic equator, are far away (~50 degr.) from the Virgo/anti-Virgo direction, which disagrees with a spherically symmetric Virgo-centric flow. About 63% of galaxies within 5 Mpc belong to known compact and loose groups. Apart from them, we found six new probable groups, consisting entirely of dwarf galaxies.Comment: 22 pages, 7 figures. A&A Accepted. High resolution Figures 1 and 2 (9616k) are available at http://www.sao.ru/~sme/figs.tar.g

The Local Void is Really Empty

Are voids in the distribution of galaxies only places with reduced matter density and low star formation efficiency or are they empty of matter? There is now compelling evidence of expansion away from the Local Void at very high velocities. The motion is most reasonably interpreted as an evacuation of the void, which requires that the void be very large and very empty.Comment: Proceedings IAU Symp. 244: Dark Galaxies and Lost Baryons. Cardiff, 25-29 June, 2007. 6 pages, 5 figure

Ionized gas outflow in the isolated S0 galaxy NGC 4460

We use integral-field and long-slit spectroscopy to study the bright extended nebulosity discovered in the isolated lenticular galaxy NGC 4460 during a recent H-alpha survey of nearby galaxies. An analysis of archival SDSS, GALEX, and HST images indicates that current star formation is entirely concentrated in the central kiloparsec of the galaxy disc. The observed ionized gas parameters (morphology, kinematics and ionization state) can be explained by a gas outflow above the plane of the galaxy caused by a star formation in the circumnuclear region. Galactic wind parameters in NGC 4460: outflow velocity, total kinetic energy - are several times smaller comparing with the known galactic wind in NGC 253, which is explained substantially lower total star formation rate. We discuss the cause of the star formation processes in NGC 4460 and in two other known isolated S0 and E galaxies of the Local volume: NGC 404 and NGC 855. We provide evidence suggesting that feeding of isolated galaxies by intergalactic gas on a cosmological time scale is a steady process without significant variations.Comment: 11 pages, 7 figures; accepted by MNRA

The observed infall of galaxies towards the Virgo cluster

We examine the velocity field of galaxies around the Virgo cluster induced by its overdensity. A sample of 1792 galaxies with distances from the Tip of the Red Giant Branch, the Cepheid luminosity, the SNIa luminosity, the surface brightness fluctuation method, and the Tully-Fisher relation has been used to study the velocity-distance relation in the Virgocentric coordinates. Attention was paid to some observational biases affected the Hubble flow around Virgo. We estimate the radius of the zero-velocity surface for the Virgo cluster to be within (5.0 - 7.5) Mpc corresponding to (17 - 26)^\circ at the mean cluster distance of 17.0 Mpc. In the case of spherical symmetry with cosmological parameter \Omega_m=0.24 and the age of the Universe T_0= 13.7 Gyr, it yields the total mass of the Virgo cluster to be within M_T=(2.7 - 8.9) * 10^{14} M_\sun in reasonable agreement with the existing virial mass estimates for the cluster.Comment: 22 pages, 11 figures, 2 tables. Accepted for publication in MNRA

Star formation history and environment of the dwarf galaxy UGCA 92

We present a quantitative star formation history of the nearby dwarf galaxy UGCA 92. This irregular dwarf is situated in the vicinity of the Local Group of galaxies in a zone of strong Galactic extinction (IC 342 group of galaxies). The galaxy was resolved into stars with HST/ACS including old red giant branch. We have constructed a model of the resolved stellar populations and measured the star formation rate and metallicity as function of time. The main star formation activity period occurred about 8 - 14 Gyr ago. These stars are mostly metal-poor, with a mean metallicity [Fe/H] ~ -1.5 -- -2.0 dex. About 84 per cent of the total stellar mass was formed during this event. There are also indications of recent star formation starting about 1.5 Gyr ago and continuing to the present. The star formation in this event shows moderate enhancement from ~ 200 Myr to 300 Myr ago. It is very likely that the ongoing star formation period has higher metallicity of about -0.6 -- -0.3 dex. UGCA 92 is often considered to be the companion to the starburst galaxy NGC 1569. Comparing our star formation history of UGCA 92 with that of NGC 1569 reveals no causal or temporal connection between recent star formation events in these two galaxies. We suggest that the starburst phenomenon in NGC 1569 is not related to the galaxy's closest dwarf neighbours and does not affect their star formation history.Comment: 11 pages, 9 figures, accepted to MNRA

The Hubble flow around the Local Group

We use updated data on distances and velocities of galaxies in the proximity of the Local Group (LG) in order to establish properties of the local Hubble flow. For 30 neighbouring galaxies with distances 0.7 < D_LG < 3.0 Mpc, the Local flow is characterized by the Hubble parameter H_loc = (78+/-2) km/(s*Mpc), the mean-square peculiar velocity sigma_v = 25 km/s, corrected for errors of radial velocity measurements (~4 km/s) and distance measurements (~10 km/s), as well as the radius of the zero-velocity surface R_0 = (0.96+/-0.03) Mpc. The minimum value for sigma_v is achieved when the barycenter of the LG is located at the distance D_c = (0.55+/-0.05) D_M31 towards M31 corresponding to the Milky Way-to-M31 mass ratio M_MW / M_M31 ~ 4/5. In the reference frame of the 30 galaxies at 0.7 - 3.0 Mpc, the LG barycenter has a small peculiar velocity ~(24+/-4) km/s towards the Sculptor constellation. The derived value of R_0 corresponds to the total mass M_T(LG) = (1.9+/-0.2) 10^12 M_sun with Omega_m = 0.24 and a topologically flat universe, a value in good agreement with the sum of virial mass estimates for the Milky Way and M31.Comment: 14 pages, 6 figures, 1 table. Accepted for publication in MNRA

The very local Hubble flow

We present Hubble Space Telescope/WFPC2 images of eighteen galaxies situated in the vicinity of the Local Group (LG) as part of an ongoing snapshot survey of nearby galaxies. Their distances derived from the magnitude of the tip of the red giant branch are 1.92+-0.10 Mpc (ESO 294-010), 3.06+-0.37 (NGC404), 3.15+-0.32 (UGCA105), 1.36+-0.07 (SexB), 1.33+-0.08 (NGC3109), 2.64+-0.21 (UGC6817), 2.86$+-0.14 (KDG90), 2.27+-0.19 (IC3104), 2.54+-0.17 (UGC7577), 2.56$+-0.15 (UGC8508), 3.01+-0.29 (UGC8651), 2.61+-0.16 (KKH86), 2.79$+-0.26 (UGC9240), 1.11+-0.07 (SagDIG), 0.94+-0.04 (DDO210), 2.07$+-0.18 (IC5152), 2.23+-0.15 (UGCA438), and 2.45+-0.13 (KKH98). Based on the velocity-distance data for 36 nearest galaxies around the LG, we find the radius of the zero-velocity surface of the LG to be R_0 = (0.94+-0.10) Mpc, which yields a total mass M_{LG} = (1.3+-0.3)*10^{12} M_{\sun}. The galaxy distribution around the LG reveals a Local Minivoid which does not contain any galaxy brighter than M_V=-10 mag within a volume of ~100 Mpc^3. The local Hubble flow seems to be very cold, having a one-dimensional mean random motion of ~30 km/s. The best-fit value of the local Hubble parameter is 73+-15 km/s Mpc^{-1}. The luminosity function for the nearby field galaxies is far less steep than one for members of the nearest groups.Comment: 13 pages, 6 figures, accepted for publication in the A&A. Version has low resolution figures and does not include Figure 2. All high resolution figures (8366k) are available at http://luna.sao.ru/~sme/figures.tar.g

Galaxy Flow in the Canes Venatici I Cloud

We present an analysis of Hubble Space Telescope/WFPC2 images of eighteen galaxies in the Canes Venatici I cloud. We derive their distances from the luminosity of the tip of the red giant branch stars with a typical accuracy of ~12 %. The resulting distances are 3.9 Mpc (UGC 6541), 4.9 Mpc (NGC 3738), 3.0 Mpc (NGC 3741), 4.5 Mpc (KK 109), >6.3 Mpc (NGC 4150), 4.2 Mpc (UGC 7298), 4.5 Mpc (NGC 4244), 4.6 Mpc (NGC 4395), 4.9 Mpc (UGC 7559), 4.2 Mpc (NGC 4449), 4.4 Mpc (UGC 7605), 4.6 Mpc (IC 3687), 4.7 Mpc (KK 166), 4.7 Mpc (NGC 4736), 4.2 Mpc (UGC 8308), 4.3 Mpc (UGC 8320), 4.6 Mpc (NGC 5204), and 3.2 Mpc (UGC 8833). The CVn I cloud has a mean radial velocity of 286 +- 9 km/s, a mean distance of 4.1 +- 0.2 Mpc, a radial velocity dispersion of 50 km/s, a mean projected radius of 760 kpc, and a total blue luminosity of 2.2 * 10^{10} L_{\sun}. Assuming virial or closed orbital motions for the galaxies, we estimated their virial and their orbital mass-to-luminosity ratio to be 176 and 88 M_{\sun}/L_{\sun}, respectively. However, the CVn I cloud is characterized by a crossing time of 15 Gyr, and is thus far from a state of dynamical equilibrium. The large crossing time for the cloud, its low content of dSph galaxies ($< 6$ %), and the almost ``primordial'' shape of its luminosity function show that the CVn I complex is in a transient dynamical state, driven rather by the free Hubble expansion than by galaxy interactions.Comment: 23 pages, 6 figures, A&A in preparation. The version does not include Figure 2. High resolution figures 1 and 2 (11311k) are available at http://luna.sao.ru/~sme/figsCVn.tar.g

A homogeneous sample of binary galaxies: Basic observational properties

A survey of optical characteristics for 585 binary systems, satisfying a condition of apparent isolation on the sky, is presented. Influences of various selection effects distorting the average parameters of the sample are noted. The pair components display mutual similarity over all the global properties: luminosity, diameter, morphological type, mass-to-luminosity ratio, angular momentum etc., which is not due only to selection effects. The observed correlations must be caused by common origin of pair members. Some features (nuclear activity, color index) could acquire similarity during synchronous evolution of double galaxies. Despite the observed isolation, the sample of double systems is seriously contaminated by accidental pairs, and also by members of groups and clusters. After removing false pairs estimates of orbital mass-to-luminosity ratio range from 0 to 30 f(solar), with the mean value (7.8 plus or minus 0.7) f(solar). Binary galaxies possess nearly circular orbits with a typical eccentrity e = 0.25, probably resulting from evolutionary selection driven by component mergers under dynamical friction. The double-galaxy population with space abundance 0.12 plus or minus 0.02 and characteristic merger timescale 0.2 H(exp -1) may significantly influence the rate of dynamical evolution of galaxies