Bruno Binggeli

Studies of the Virgo cluster. VI - Morphological and kinematical structure of the Virgo cluster

The structure of the Virgo cluster is analyzed on the basis of the positions, Hubble types, and radial velocities of 1277 Virgo cluster galaxies. The surface distribution of galaxies is considered according to type, and is discussed using maps, isopleths, strip counts, and radial-density distributions. It is found that the Virgo cluster shows pronounced double structure. The main concentration has a large velocity dispersion and is made up predominantly of early-type galaxies, while the secondary concentration has a much smaller velocity dispersion and contains late types. There is a strong spatial segregation of the Hubble types, the early-type galaxies being more concentrated toward the cluster center. There is significant substructure in the cluster core. The irregularity of the Virgo cluster in both configuration and velocity space shows that the core and the envelope are still forming, and hence that the cluster is young. 85 references.

Dwarf elliptical galaxies

AbstractDwarf elliptical (dE) galaxies, with blue absolute magnitudes typically fainter thanMB=−16, are the most numerous type of galaxy in the nearby universe. Tremendous advances have been made over the past several years in delineating the properties of both Local Group satellite dEs and the large dE populations of nearby clusters. We review some of these advances, with particular attention to how well currently available data can constrain(a)models for the formation of dEs(b)the physical and evolutionary connections between different types of galaxies (nucleated and nonnucleated dEs, compact Es, irregulars, and blue compact dwarfs) that overlap in the same portion of the mass-spectrum of galaxies(c)the contribution of dEs to the galaxy luminosity functions in clusters and the field(d)the star-forming histories of dEs and their possible contribution to faint galaxy counts, and(e)the clustering properties of dEs. In addressing these issues, we highlight the extent to which selection effects temper these constraints, and outline areas where new data would be particularly valuable.

Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. III. Subpopulations: Distributions, Shapes, Origins

From a quantitative analysis of 413 Virgo Cluster early-type dwarf galaxies (dEs) with SDSS imaging data, we find that the dE class can be divided into multiple subpopulations that differ significantly in their morphology and clustering properties. Three dE subclasses are shaped like thick disks and show no central clustering: (1) dEs with disk features like spiral arms or bars, (2) dEs with central star formation, and (3) ordinary, bright dEs that have no or only a weak nucleus. These populations probably formed from infalling progenitor galaxies. In contrast, ordinary nucleated dEs follow the picture of classical dwarf elliptical galaxies in that they are spheroidal objects and are centrally clustered like E and S0 galaxies, indicating that they have resided in the cluster for a long time or were formed along with it. These results define a morphology-density relation within the dE class. We find that the difference in the clustering properties of nucleated dEs and dEs with no or only a weak nucleus is not caused by selection biases, as opposed to previously reported suggestions. The correlation between surface brightness and observed axial ratio favors oblate shapes for all subclasses, but our derivation of intrinsic axial ratios indicates the presence of at least some triaxiality. We discuss possible interrelations and formation mechanisms (ram pressure stripping, tidally induced star formation, harassment) of these dE subpopulations.

Studies of the Virgo Cluster. I - Photometry of 109 galaxies near the cluster center to serve as standards

Attention is given to the technical aspects of photometric measurements of 109 galaxies near the center of the Virgo Cluster, noting various types of radii and surface brightness for about 50 E and dE galaxies in the sample that range in absolute magnitude from -20 to -12. These data are combined with data from the literature for giant E and dwarf E galaxies in the Local Group to study the systematic properties of E galaxies over a range of one million luminosities. The radial intensity profiles derived are fitted to the manifold of King (1978) models to derive model-dependent central surface brightness, core radii, and cutoff radii.

Surface BR Photometry of Newly Discovered Dwarf Elliptical Galaxies in the Nearby Sculptor and Centaurus A Groups

Prompted by the recent finding of a large number of gas-rich dwarf irregular galaxies in the nearby southern galaxy groups Sculptor (Scl) and Centaurus A (Cen A) we carried out a complementary survey to search for members of the gas-poor dwarf elliptical (dE) galaxy family. In total 18 dE candidates were identified on SRC Sky Survey films covering the group regions. For five dEs in the Scl group and five in the Cen A group, membership information is available from surface brightness fluctuations distance measurements; two dEs in each group also have independent redshift measurements. The remaining eight galaxies have been associated to the groups based on their morphology. We acquired deep B- and R-band CCD images for all galaxies and determined their photometric and structural parameters. According to the magnitude range covered, -14 < MB < -9 (not counting the bright dS0 NGC 59), these objects are local dwarf spheroidal (dSph) analogues. The surface brightness profiles of most dwarfs are strongly nonexponential but are perfectly explained by the three-parametric Sersic law. The best-fitting Sersic parameters are compared with corresponding data for Virgo dEs and dSphs. The present dEs follow the relations between the Sersic parameters and absolute B-band magnitude for Virgo dEs and local dwarfs, thus confirming the membership in the Scl and Cen A groups, respectively. Most of our dwarfs exhibit color gradients in (B-R)0, in the sense of a reddening outward. We also show and discuss the sky distributions and B-band luminosity functions of the Scl and Cen A groups. The completeness magnitude for both populations (from the present survey) is estimated to be M ≈ -13. We find that the composite luminosity function of four nearby galaxy groups is well fitted by a Schechter function down to M = -14 with a faint end slope α = -1.29(±0.10). In the appendices we present best-fitting Sersic profile parameters for the dSph members of the Local Group and introduce two new dwarf irregulars found in the Cen A group.

DISTANCES, METALLICITIES, AND AGES OF DWARF ELLIPTICAL GALAXIES IN THE VIRGO CLUSTER FROM SURFACE BRIGHTNESS FLUCTUATIONS

We have employed FORS1 and 2 at the Very Large Telescope at the European Southern Observatory (ESO) to acquire deep B and R-band CCD images of 16 dwarf elliptical (dE) galaxies in the direction of the Virgo cluster. For each dwarf, we measure the apparent R-band surface brightness fluctuation (SBF) magnitude R and the (B-R)0 color in a number of fields at different galactocentric distances. From the field-to-field variation of the two quantities, we determine the SBF distance by means of the (B-R)0–R relation. The derived distances of the dwarfs range from 14.9 to 21.3 Mpc, with a mean 1 σ uncertainty of 1.4 Mpc or 8% of the distance, which confirms that there is considerable depth in the distance distribution of early-type cluster members. For VCC 1104 (IC 3388), our SBF distance modulus of (m - M)SBF = 31.15 ± 0.19 (17.0 ± 1.5 Mpc) is in good agreement with the Harris et al result of (m - M)TRGB = 30.98 ± 0.19 mag (15.7 ± 1.5 Mpc) based on Hubble Space Telescope (HST) observations and the tip magnitude of the red giant branch. Combining our results with existing distances for giant Virgo ellipticals, we identify two major galaxy concentrations in the distance distribution: a broad primary clump around (M - m) = 31.0 mag (15.8 Mpc) and a narrow secondary clump around 31.33 mag (18.5 Mpc). An adaptive kernel analysis finds the two concentrations to be significant at the 99% (2.5 σ) and 89% (1.6 σ) levels, respectively. While the near-side clump of Virgo early-type galaxies can be associated with the subcluster centered on M87, the second clump is believed to be mainly due to the far side infalling group of galaxies around M86. The ages and metallicities of the dE stellar populations are estimated by combining the observed (B-R)0 colors with Wortheys stellar population synthesis models. It appears that the Virgo dE galaxies cover a wider range in metallicity, from [Fe/H] ≈ -1.4 (VCC 0815) to -0.5 (NGC 4415), than Fornax cluster dEs. The derived metallicities place the Virgo dEs on the extension of the metallicity-luminosity relation defined by the low-luminosity Local Group dEs. The data further suggest an age range from genuinely old (~17 Gyr) stellar systems such as IC 3019 and IC 0783 to intermediate-age (8–12 Gyr) dwarfs such as NGC 4431 and IC 3468.

Surface Brightness Fluctuation Distances to Dwarf Elliptical Galaxies in the Sculptor Group

As part of an ongoing search for dwarf elliptical galaxies (dEs) in the vicinity of the Local Group (Jerjen et al.), we acquired deep B- and R-band images for five dE candidates identified on morphological criteria 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 R for each dE. Using predictions from stellar population synthesis models (Worthey) giving R values in the narrow range between -1.17 and -1.13, 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 ESO 294-010 yielded an independent confirmation of its group membership: the [O III] and Hα emission lines from a small H II region gave a heliocentric velocity of 117 (±5) km s-1, in close agreement with the velocity of its parent galaxy NGC 55 (v⊙ = 125 km s-1). The precision of the SBF distances (5%–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, which leads to the observed shearing motion. From the distances and velocities of 49 galaxies located in the Local Group and toward 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.

VIRGO CLUSTER EARLY-TYPE DWARF GALAXIES WITH THE SLOAN DIGITAL SKY SURVEY. IV. THE COLOR-MAGNITUDE RELATION

We present an analysis of the optical colors of 413 Virgo cluster early-type dwarf galaxies (dEs), based on Sloan Digital Sky Survey imaging data. Our study comprises (1) a comparison of the color-magnitude relation (CMR) of the different dE subclasses that we identified in paper III of this series, (2) a comparison of the shape of the CMR in low- and high-density regions, (3) an analysis of the scatter of the CMR, and (4) an interpretation of the observed colors with ages and metallicities from population synthesis models. We find that the CMRs of nucleated (dE(N)) and non-nucleated (dE(nN)) dEs are significantly different from each other, with similar colors at fainter magnitudes (m r 17 mag), but increasingly redder colors of the dE(N)s at brighter magnitudes. We interpret this with older ages and/or higher metallicities of the brighter dE(N)s. The dEs with disk features have similar colors as the dE(N)s and seem to be only slightly younger and/or less metal-rich on average. Furthermore, we find a small but significant dependence of the CMR on local projected galaxy number density, consistently seen in all of u − r, g − r, and g − i, and weakly i − z. We deduce that a significant intrinsic color scatter of the CMR is present, even when allowing for a distance spread of our galaxies. No increase of the CMR scatter at fainter magnitudes is observed down to m r ≈ 17 mag (M r ≈ −14 mag). The color residuals, i.e. the offsets of the data points from the linear fit to the CMR, are clearly correlated with each other in all colors for the dE(N)s and for the full dE sample, implying that, at a given magnitude, a galaxy with an older stellar population than average typically also exhibits a higher metallicity than average. Given the observational data for Virgo dEs presented here and in the previous papers of this series, we conclude that there must be at least two different formation channels for early-type dwarfs in order to explain the heterogeneity of this class of galaxy.

Testing the Surface Brightness Fluctuations Method for Dwarf Elliptical Galaxies in the Centaurus A Group

We have obtained deep B and R-band CCD photometry for five dwarf elliptical galaxies that were previously identified on Schmidt films covering the region of the Centaurus A (Cen A) group. From a Fourier analysis of the R-band CCD images we determined the surface brightness fluctuation (SBF) magnitude R for each stellar system. All magnitudes are similar, and, given the small color spread, they suggest that these low surface brightness galaxies lie approximately at the same distance, regardless of the assumed SBF zero point. Long-slit spectra have been acquired to derive redshifts for two of the dwarfs, ESO 269-066 and ESO 384-016. The velocities, v? = 784 km s-1 and v? = 561 km s-1, respectively, identify them unambiguously as Cen A group members. An age (H?)-metallicity (C2 ?4668) analysis of the spectra reveals an underlying old and metal-poor stellar population in both cases. Combining photometric and spectroscopic results we find strong evidence that indeed all dwarf galaxies are Cen A group members. Based on Cepheid, TRGB, and PNLF distances published for the two main Cen A group galaxies, NGC 5128 and NGC 5253, we adopted a mean group distance of 3.96 Mpc to calibrate the apparent fluctuation magnitudes. The resulting absolute SBF magnitudes R of the dEs correlate with the dereddened colors (B-R)0 as predicted by Wortheys stellar synthesis models, using the theoretical isochrones of Bertelli and collaborators. This good agreement allows a calibration of the SBF method for dwarf ellipticals in the color range 0.8 1.2, the R-color relation is steep, and accurate colors are needed to achieve SBF distances with an uncertainty of only 10%. One of the dwarfs, ESO 219-010, is located slightly behind the core of the Cen A group at about 4.8 Mpc, while the remaining four recover the mean group distance of 3.96 Mpc that was put into the calibration. The depth of the group is only 0.5 Mpc, which identifies the Cen A group as a spatially well-isolated galaxy aggregate, in contrast to the nearby Sculptor group.

Structure and stellar content of dwarf galaxies - III.

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 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.