SBF Distances to Dwarf Elliptical Galaxies in the Sculptor Group

As part of an ongoing search for dwarf elliptical galaxies (dE) in the vicinity of the Local Group, we acquired deep B and R-band images for five dE candidates identified in the Sculptor (Scl) group region. We carried out a surface brightness fluctuation (SBF) analysis on the R-band images to measure the apparent fluctuation magnitude \bar{m}_R for each dE. Using predictions from stellar population synthesis models the galaxy distances were determined. All of these dE candidates turned out to be satellites of Scl group major members. A redshift measurement of the dE candidate ESO294-010 yielded an independent confirmation of its group membership: the [OIII] and H$_\alpha$ emission lines from a small HII region gave a heliocentric velocity of 117(\pm 5) km s-1, in close agreement with the velocity of its parent galaxy NGC 55 (v_\odot=125 km s-1). The precision of the SBF distances (5 to 10%) contributes to delineating the cigar-like distribution of the Scl group members, which extend over distances from 1.7 to 4.4 Mpc and are concentrated in three, possibly four subclumps. The Hubble diagram for nine Scl galaxies, including two of our dEs, exhibits a tight linear velocity--distance relation with a steep slope of 119 km s-1 Mpc-1. The results indicate that gravitational interaction among the Scl group members plays only a minor role in the dynamics of the group. However, the Hubble flow of the entire system appears strongly disturbed by the large masses of our Galaxy and M31 leading to the observed shearing motion. From the distances and velocities of 49 galaxies located in the Local Group and towards the Scl group, we illustrate the continuity of the galaxy distribution which strongly supports the view that the two groups form a single supergalactic structure.Comment: To appear in The Astronomical Journal, December 1998; 28 pages with 22 figure

The stellar correlation function from SDSS - A statistical search for wide binary stars

We study the statistical properties of the wide binary population in the Galaxy field with projected separations larger than 200 AU by constructing the stellar angular two-point correlation function (2PCF) from a homogeneous sample of nearly 670'000 main sequence stars. The selected stars lie within a rectangular region around the Northern Galactic Pole and have apparent r-band magnitudes between 15 and 20.5 mag and spectral classes later than G5 (g-r > 0.5 mag). The data were taken from the Sixth Data Release of the Sloan Digital Sky Survey. We model the 2PCF by means of the Wasserman-Weinberg technique including several assumptions on the distribution of the binaries' orbital parameters, luminosity function, and density distribution in the Galaxy. In particular, we assume that the semi-major axis distribution is described by a single powerlaw. The free model parameters - the local wide binary number density and the power-law index of the semi-major axis distribution - are inferred simultaneously by least-square fitting. We find the separation distribution to follow Oepik's law up to the Galactic tidal limit, without any break and a local density of 5 wide binaries per 1'000 cubic parsec with both components having spectral type later than G5. This implies that about 10% of all stars in the solar neighbourhood are members of such a late-type wide binary system. With a relative statistical (2 sigma) error of about 10%, our findings are in general agreement with previous studies of wide binaries. The data suggest that about 800 very wide pairs with projected separations larger than 0.1 pc exist in our sample, whereas none are found beyond 0.8 pc.Comment: 18 pages, 14 figures, 7 tables; added reference for section

Structure and stellar content of dwarf galaxies. III: B and R photometry of dwarf galaxies in the M101 group and the nearby field

We have carried out CCD photometry in the Cousins B and R bands of 21 dwarf galaxies in and around the M101 group. Eleven are members and suspected members of the group and ten are field galaxies in the projected vicinity of the group. We have derived total magnitudes, effective radii, effective surface brightnesses, as well as galaxy diameters at various isophotal levels in both colours. Best-fitting exponential parameters and colour gradients are also given for these galaxies. Some of the galaxies show a pronounced luminosity excess above the best-fitting exponential at large radii, or surface brightnesses fainter than approx 26 mag/sq_arcsec in R. This feature, while non-significant for a single case and technically difficult to interpret, might be an indication of the existence of an extended old stellar halo in dwarf irregulars. The photometric parameters of the galaxies presented here will be combined with previously published data for nearby dwarf galaxies and statistically analysed in a forthcoming paper.Comment: 15 pages, 4 tables and 13 figures. For a full resolution version see http://www.astro.unibas.ch/galaxies/papers.html To appear in A&A

Structure and stellar content of dwarf galaxies IV. B and R photometry of dwarf galaxies in the CVnI cloud

We have carried out CCD photometry in the Cousins B and R bands of 15 galaxies in the Canes Venatici I cloud. Total magnitudes, effective radii, effective surface brightnesses, as well as galaxy radii at various isophotal levels in both colors were determined. Best-fitting exponential parameters and color gradients are also given for these galaxies. The photometric parameters presented here will analyzed in a forthcoming paper, together with previously published data for nearby dwarf galaxies.Comment: 10 pages, submitted to A&AS. For a full resolution version see ftp://merkur.astro.unibas.ch/pub/bremnes/canesv.ps.g

More evidence for hidden spiral and bar features in bright early-type dwarf galaxies

Following the discovery of spiral structure in IC3328 (Jerjen et al.~2000), we present further evidence that a sizable fraction of bright early-type dwarfs in the Virgo cluster are genuine disk galaxies, or are hosting a disk component. Among a sample of 23 nucleated dwarf ellipticals and dS0s observed with the Very Large Telescope in $B$ and $R$, we found another four systems exhibiting non-axisymmetric structures, such as a bar and/or spiral arms, indicative of a disk (IC0783, IC3349, NGC4431, IC3468). Particularly remarkable are the two-armed spiral pattern in IC0783 and the bar and trailing arms in NGC4431. For both galaxies the disk nature has recently been confirmed by a rotation velocity measurement (Simien & Prugniel 2002). Our photometric search is based on a Fourier decomposition method and a specific version of unsharp masking. Some ``early-type'' dwarfs in the Virgo cluster seem to be former late-type galaxies which were transformed to early-type morphology, e.g. by ``harassment'', during their infall to the cluster, while maintaining part of their disk structure.Comment: A&A accepte

The Formation of Low-Mass Cluster Galaxies and the Universal Initial Galaxy Mass Function

Clusters of galaxies have an observed over-density of low-luminosity systems in comparison to the field, although it is not yet agreed whether this effect is the result of initial galaxy mass functions that vary with environment or galaxy evolutionary effects. In this letter we argue that this over-density is the result of low-mass systems with red colors that are over-populating the faint-end of the observed luminosity function in the nearby rich cluster Abell 0426. We show that the luminosity function of Abell 0426 becomes steeper, from the field value alpha = -1.25+/-0.05 to alpha=-1.44+/-0.04, due to a recently identified population of red low-mass cluster galaxies that are possibly the remnants of dynamical stripped high-mass systems. We further demonstrate, through simple models of stripping effects, how cluster luminosity functions can become artificially steep over time from the production of these low-mass cluster galaxies.Comment: Accepted to ApJ letter

Consequences of gravitational radiation recoil

Coalescing binary black holes experience an impulsive kick due to anisotropic emission of gravitational waves. We discuss the dynamical consequences of the recoil accompanying massive black hole mergers. Recoil velocities are sufficient to eject most coalescing black holes from dwarf galaxies and globular clusters, which may explain the apparent absence of massive black holes in these systems. Ejection from giant elliptical galaxies would be rare, but coalescing black holes are displaced from the center and fall back on a time scale of order the half-mass crossing time. Displacement of the black holes transfers energy to the stars in the nucleus and can convert a steep density cusp into a core. Radiation recoil calls into question models that grow supermassive black holes from hierarchical mergers of stellar-mass precursors.Comment: 5 pages, 4 figures, emulateapj style; minor changes made; accepted to ApJ Letter

Dissipative transformation of non-nucleated dwarf galaxies into nucleated systems

Recent photometric observations by the {\it Hubble Space Telescope (HST)} have revealed the physical properties of stellar galactic nuclei in nucleated dwarf galaxies in the Virgo cluster of galaxies. In order to elucidate the formation processes of nucleated dwarfs, we numerically investigate gas dynamics, star formation, and chemical evolution within the central 1 kpc of gas disks embedded within the galactic stellar components of non-nucleated dwarfs. We find that high density, compact stellar systems can be formed in the central regions of dwarfs as a result of dissipative, repeated merging of massive stellar and gaseous clumps developed from nuclear gaseous spiral arms as a result of local gravitational instability. The central stellar components are found to have stellar masses which are typically $~$5% of their host dwarfs and show very flattened shapes, rotational kinematics, and central velocity dispersions significantly smaller than those of their host dwarfs. We also find that more massive dwarfs can develop more massive, more metal-rich, and higher density stellar systems in their central regions, because star formation and chemical enrichment proceed more efficiently owing to the less dramatic suppression of star formation by supernovae feedback effects in more massive dwarfs. Based on these results, we suggest that gas-rich, non-nucleated dwarfs can be transformed into nucleated ones as a result of dissipative gas dynamics in their central regions. We discuss the origin of the observed correlations between physical properties of stellar galactic nuclei and those of their host galaxies.Comment: 13 pages, 4 figures (1 color), ApJL in pres

Deep griz GMOS Imaging of the Dwarf Irregular Galaxy Kar 50

Images obtained with the Gemini Multi-Object Spectrograph (GMOS) are used to investigate the stellar content and distance of the dwarf irregular galaxy Kar 50. The brightest object is an HII region, and the bright stellar content is dominated by stars with g'-r' < 0. The tips of the main sequence and the red giant branch are tentatively identified near r' = 24.9 and i' = 25.5, respectively. The galaxy has a blue integrated color with no significant color gradient, and we conclude that Kar 50 has experienced a recent galaxy-wide episode of star formation. The distance estimated from the brightest blue stars indicates that Kar 50 is behind the M81 group, and this is consistent with the tentative RGB-tip brightness. Kar 50 has a remarkably flat central surface brightness profile, even at wavelengths approaching 1um, although there is no evidence of a bar. In the absence of another large star-forming episode, Kar 50 will evolve into a very low surface brightness galaxy.Comment: 17 pages of text and 8 postscript figures. Accepted for publication in the PAS