Dwarf LSB galaxies and their environment: The Virgo Cluster, the Ursa Major Cluster, isolated galaxies and voids

The widely varying dwarf galaxy counts in different environments provide a strong challenge to standard hierarchical clustering models. The luminosity function is not universal, but seems to be strongly dependent upon environment. In this paper we describe an automated procedure for detecting and measuring very Low Surface Brightness (LSB) features in deep CCD data. We apply this procedure to large area CCD survey fields of the Virgo cluster and find 105 dwarf LSB galaxies previously uncatalogued over an area of ∼14 sq deg. We show that there are many more faint (14≤M_B≤–10) LSB galaxies than would be predicted from extrapolation of the Virgo cluster catalogue luminosity function. Over our limited range of measurement the faint end slope of the luminosity function becomesα=–1.6. Although these galaxies contribute a small fraction of the total stellar light of the cluster, they may contribute significantly to the mass in galaxies if they have large mass-to-light ratios similar to those recently measured for Local Group dwarf galaxies. By a comparison with different environments, the Virgo Cluster seems to be highly `dwarf rich' if considering the dwarf to giant ratio or it's luminosity function faint end slope. This revised version was published online in July 2006 with corrections to the Cover Date.     

Faint field galaxies: an explanation of the faint blue excessusing number evolution models

Pure luminosity evolution models for galaxies provide an unacceptable fit to the redshifts and colors of faint galaxies. In this paper we demonstrate, using HST morphological number counts derived both from the I ₈₁₄-band of WFPC2 in the Medium Deep Survey (MDS) and the Hubble Deep Field (HDF) and from the H _1.6-band of NICMOS, and ground-based spectroscopic data of the Hawaii Deep Field and the Canada-France Redshift Survey, that number evolution is necessary for galaxies, regardless of whether the cosmic geometry is flat, open, or Λ-dominated. Furthermore, we show that the number evolution is small at redshifts of z<1, but large at z>1, and that this conclusion is valid for all the three cosmological models under consideration. If the universe is open or Λ-dominated, the models, which are subject to the constraint of the conservation of the comoving mass density of galaxies, naturally predict a population of star-forming galaxies with the redshift distribution peaking at z=2∼ 3, which seems to be consistent with the recent findings from Lyman-break photometric selection techniques. If the cosmological model is flat, however, the conservation of the comoving mass density is invalid. Hence, in order to account for the steep slope of B-band number counts at faint magnitudes in the flat universe, such a star-forming galaxy population has to be introduced ad hoc into the modelling alongside the merger assumption. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Evolutionary Status of Early-type Galaxies in Abell 2390

We explore the evolution of the early-type galaxy population in the rich cluster Abell 2390 at z=0.23. For this purpose, we have obtained spectroscopic data of 51 elliptical and lenticular galaxies with MOSCA at the 3.5 m telescope on Calar Alto Observatory. As our investigation spans both a broad range in luminosity (–22.3≤M_B≤–:19.3)and a wide field of view (10′×10′), the environmental dependence of different formation scenarios can be analysed in detail as a function of radius from the cluster center. In this paper, we present first results on the Faber-Jackson relation and, for a subsample of 14 galaxies with morphological and structural parameters from HST, we also investigate the evolution of the Kormendy relation and the Fundamental Plane. We find a mild luminosity evolution of the early-type galaxies in Abell 2390: our objects are on average brighter by m _B∼0.4 mag. This revised version was published online in July 2006 with corrections to the Cover Date.     

Environmental effects on galaxy luminosity functions: the cluster Abell 496

We have derived the galaxy luminosity function in various regions of the cluster of galaxies Abell 496 from a wide field image in the I band. A Schechter function fit in the 17≤ I_AB ≤22(–19.5≤ M_IAB ≤ –14.5) magnitude interval gives a steep power law index, which is somewhat steeper in the outer regions than in the inner zones. This result agrees with previous findings obtained by several teams on the Coma cluster and can be interpreted as due to the fact that faint galaxies are accreted by large ones in the central regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Blue Compact Galaxies at Redshifts Z=0.2-1.3

We study the nature of faint blue compact galaxies (BCGs) at redshifts z ∼ 0.2 - 1.3 using Keck and HST. Despite being very luminous (L_B ∼ L^*), most distant BCGs have masses M ∼ 10¹⁰M_⊙, i.e., they are dwarf stellar systems. The majority of these galaxies have colors, sizes, surface brightnesses, luminosities, velocity widths, excitations, star formation rates (SFR), and mass-to-light ratios characteristic of the most luminous nearby HII galaxies. The more massive BCGs form a more heterogeneous class of evolved starburst, similar to local disk starburst galaxies. Without additional star formation, HII-like BCGs will most likely fade to resemble today's spheroidal galaxies such as NGC 205. This revised version was published online in July 2006 with corrections to the Cover Date.     

Faint Features of Interacting Galaxies Revealed by the Digital Coaddition of 13 Tech-Pan Films of the Virgo Cluster

We have digitally coadded APM scans of 13 Kodak TechPan films of the SE region of the Virgo Cluster of galaxies. The ≈6.2°×6.2°field of the R-band films combined with the resolution of ≈2 arcsec pixel^- results in data-file sizes of about 222 MBytes. The 13 scanned films have been aligned, coadded, corrected for vignetting effects and cleaned of stellar features. To illustrate the astrophysical uses of this technique, we present high-contrast images of a sample of interacting galaxies in the field. Several very faint features (but very clearly seen in the coadded array) – such as the interaction between IC 3481,IC 3481A and IC 3483, and NGC 4410A and B and IC 790 – have been presented with clarity for the first time. This revised version was published online in July 2006 with corrections to the Cover Date.     

The resolved red giant branches of E/S0 galaxies

Formation paradigms for massive galaxies have long centered around two antipodal hypotheses – the monolithic-collapse and the accretion/merger scenarios. Empirical data on the stellar contents of galaxy halos is crucial in order to develop galaxy formation and assembly scenarios which have their root in observations, rather than in numerical simulations. The Hubble Space Telescope (HST) has enabled us to study directly individual stars in the nearby E/S0 galaxies Cen A, NGC 3115, NGC 5102, and NGC 404. We here present and discuss HST single-star photometry in V and I bands. Using color-magnitude diagrams and stellar luminosity functions, we gauge the galaxies' stellar contents. This can be done at more than one position in the halo, but data with deeper limiting magnitudes are desired to quantify the variation of metallicity with galactocentric radius. We here compare the color distributions of red giant stars with stellar isochrones, and we intercompare the galaxies' halo populations, noting that their total absolute V magnitudes cover the range from about –21.5 to –17.5. In the future, we plan to model the stellar metallicity distributions with the aim to constrain chemical enrichment scenarios, a step towards unravelling the evolutionary history of elliptical and lenticular galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analysis of the properties of galaxy clusters in the Leo supercluster region

We analyze the properties of galaxy clusters in the region of the Leo supercluster using observational data from the SDSS and 2MASS catalogs. We have selected 14 galaxy clusters with a total dynamical mass of 1.77 × 10¹⁵ M _⊙ in the supercluster region 130 by 60 Mpc in the plane of the sky (z ≃ 0.037). The composite luminosity function of the supercluster is described by a Schechter function with parameters that, within the error limits, correspond to field galaxies and does not differ from the luminosity function of the richer Ursa Major (UMa) supercluster for the same luminosity range (the bright end). The luminosity functions of early-type and late-type galaxies in Leo at the faint end are characterized by a sharp decrease (α = −0.60±0.08) and a steep increase (α = −1.44± 0.10) in the number of galaxies, respectively. In the virialized cluster regions, the fraction of early-type galaxies selected by the u-r color, bulge contribution, and concentration index among the galaxies brighter than M _K ^* + 1 is, on average, 62%. This fraction is smaller than that in the UMa supercluster at a 2–3σ level. The near-infrared luminosities of galaxy clusters down to a fixed absolute magnitude correlate with their masses almost in the same way as for other samples of galaxy clusters (L _200,K ∝ M ₂₀₀^0.63±0.11)).     

The fate of dwarf galaxies in clusters and the origin of intracluster stars

The main goal of this paper is to compare the relative importance of destruction by tides vs. destruction by mergers, in order to assess if tidal destruction of galaxies in clusters is a viable scenario for explaining the origin of intracluster stars. We have designed a simple algorithm for simulating the evolution of isolated clusters. The distribution of galaxies in the cluster is evolved using a direct gravitational N-body algorithm combined with a subgrid treatment of physical processes such as mergers, tidal disruption, and galaxy harassment. Using this algorithm, we have performed a total of 148 simulations. Our main results are:

The physical parameters of the evolving population of faint galaxies

The excess number of blue galaxies at faint magnitudes is a subject of much controversy. Recent Hubble Space Telescope results have revealed a plethora of galaxies with peculiar morphologies tentatively identified as the evolving population. We report the results of optical spectroscopy and near-infrared photometry of a sample of faint HST galaxies from the Medium Deep Survey to ascertain the physical properties of the faint morphological populations. We find four principal results. First, the population of objects classified as 'peculiar' are intrinsically luminous in the optical ( M _B  ∼ −19). Secondly these systems tend to be strong sources of [O  II ] line luminosity. Thirdly the optical–infrared colours of the faint population (a) confirm the presence of a population of compact   blue galaxies and (b) show the stellar populations of irregular/peculiar galaxies encompass a wide range in age. Finally a surface-brightness comparison with the local galaxy sample of Frei et al. shows that these objects are not of anomalously low surface brightness, rather we find that all morphological classes have evolved to a higher surface brightness at higher redshifts ( z  > 0.3).     

Multiwavelength study of 230 IRAS FSC galaxies

A sample of 230 galaxies has been compiled based on their IRAS FSC fluxes to study their multiwavelength properties and carry out comparative analyses with other similar samples. To understand the nature of these galaxies, a comparison with a sample of bright ULIRGs having fluxes at 60 μm fν > 1 Jy and 14 optically faint IRAS FSC galaxies is presented. This comparison shows that galaxies found by IRAS are not always strong infrared sources and that the objects from the sample of ULIRGs represent a sample of extreme galaxies, which are very powerful infrared sources. We have confirmed the consistency between Star Formation Rates (SFRs) calibrated based on luminosities of the PAH feature at 7.7 μm and radio 1.4 GHz flux densities. We have estimated the extinction for our 230 objects using SFRs calibrated from the PAH feature compared to ultraviolet flux, which shows that only 1% of the ultraviolet continuum typically escapes extinction by dust within a starburst.     

Hα Emission and Dynamics in Groups of Galaxies

H_α luminosities of a sample of galaxies in nearby compactgroups are presented. Our purpose is to study the influence of thegroup environment on the star formation rates (SFRs) of the galaxies in thegroups, provided that the H_α luminosity is a good tracer of theSFR of disc galaxies. Measuring the global L /L _B of the groups – including early-type galaxies – we find that the average value of the H_α emission is not significantly different from thatmeasured for field galaxies, and that most of the groups that show thehighest level of L /L _B, with respect to a set of synthetic groups built out of field galaxies, show tidal features in at least one of their members. Finally, we have exploredthe relationship between the ratio L /L _B and severalrelevant dynamical parameters of the groups (velocity dispersion, crossingtime, radius and mass-to-luminosity ratio) and have found no clearcorrelation. This suggests that the exact dynamical state of a groupdoes not appear to control the SFR of the group as a whole. This revised version was published online in July 2006 with corrections to the Cover Date.     

Morphologies of AGN host galaxies using HST/ACS in the CDFS-GOODS field

Using HST/ACS images in four bands F435W, F606W, F775W and F850LP, we identify optical counterparts to the X-ray sources in the Chandra Deep Field South in the GOODS South field. A detailed study has been made of these sources to study their morphological types. We use methods like decomposition of galaxy luminosity profiles, color maps and visual inspection of 192 galaxies which are identified as possible optical counterparts of Chandra X-ray sources in the CDFS-GOODS field. We find that most moderate luminosity AGN hosts are bulge dominated in the redshift range (z≈0.4–1.3), but not merging/interacting galaxies. This implies probable fueling of the moderate luminosity AGN by mechanisms other than those merger driven.     

Stellar Populations at Large Redshifts

It is now possible to probe stellar populations at large lookback times. The Butcher-Oemler Effect in distant clusters is un-ambiguous evidence for unexpectedly recent evolution of disk galaxies in dense environments, probably through starbursts induced by interactions. Recent applications of the "red envelope" test at z ≲ 1 indicate that some luminous galaxies formed ∼ 10–13 Gyr ago. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of Star Formation Rate Estimates from Hα, FIR and Radio Data

We use three indicators of massive star formation, H_α, FIR and non-thermal radio luminosities, to compare estimates of the star formation rate (SFR) for a sample of 34 spiral galaxies. To adjust the SFR values obtained from these indicators, we considered the slope, α, and/or the upper mass limit M _up of the initial mass function (IMF) as free parameters. The best agreement between the indicators is found for M _up≈ 60-100 M⊙ and α ≈–3.1 at the high-mass end of the IMF (M>10 M⊙.Parallelwith the SFR we also estimated the FIR excess X _FIR, defined as the fraction of the observed FIR not directly related to young massive stars. X _FIR is found to be well correlated with types of spiral galaxies and their colours (B-V): the redder a galaxy, the higher its FIR excess. We conclude that for any parameters of the IMF the observed FIR flux of early-type spiral galaxies needs an additional source of energy apart from massive star radiation. This revised version was published online in July 2006 with corrections to the Cover Date.     

The evolutionary history of early-type galaxies as derived from the fundamental plane

The fundamental plane (FP) scaling relations and their evolution are a powerful tool for studying the global properties of early-type galaxies and their evolutionary history. The form of the FP, as derived by surveys in the local Universe at wavelengths ranging from the U to the K band, cannot be explained by metallicity variations alone among early-type galaxies; systematic variations in age, dark matter content, or homology breaking are required. A large-scale study of early-type galaxies at 0.1 < z < 0.6demonstrates that the SB intercept of the FP, the rest frame (U-V) colour, and the absorption line strengths all evolve passively, thereby implying a high mean formation redshift for the stellar content. The slope of the FP evolves with redshift, which is broadly consistent with systematic age effects occurring along the early-type galaxy sequence. The implication that the least luminous early-type galaxies formed later than the luminous galaxies is discussed in the context of the evolution of thecolour–magnitude relation, the Butcher–Oemler effect and hierarchical galaxy formation models. This revised version was published online in July 2006 with corrections to the Cover Date.     

Galaxy Evolution from Deep Multicolor Surveys

A composite sample of NIR-selected galaxies having extended multicolor coverage has been used to probe the cosmological evolution of the blue luminosity function and of the stellar mass function. The bright fraction of the sample has spectroscopic redshifts, and the remaining fraction well-calibrated photometric redshifts. The resulting blue luminosity function shows an increasing brightening with redshift respect to the local luminosity function. Hierarchical CDM models predictions are in agreement only at low and intermediate redshifts but fail to reproduce the observed brightening at high redshifts (z ∼ 2–3). This brightening marks the epoch where starburst activity triggered by galaxy interactions could be an important physical mechanism for the galaxy evolution. At the same time the NIR galaxy sample has been used to trace the evolution of the cosmological stellar mass density up to ∼3. A clear decrease of the average mass density is apparent with a fraction ∼15% of the local value at z ∼ 3. UV bright star-forming galaxies are substancial contributors to the evolution of the stellar mass density. Although these results are globally consistent with Λ–CDM scenarios, they tend to underestimate the mass density produced by more massive galaxies present at z > 2.     

Search for companions of nearby isolated galaxies

The radial velocities are measured for 45 galaxies located in the neighborhoods of 29 likely isolated galaxies in a new catalog. We find that about 85% of these galaxies actually are well isolated objects. 4% of nearby galaxies with V _LG < 3500 km/s are this kind of cosmic "orphan" Translated from Astrofizika, Vol. 52, No. 2, pp. 203–210 (May 2009).     

Finding Faint Intermediate-Mass Black Holes in the Radio Band

We discuss the prospects for detecting faint intermediate-mass black holes, such as those predicted to exist in the cores of globular clusters and dwarf spheroidal galaxies. We briefly summarize the difficulties of stellar dynamical searches, then show that recently discovered relations between black hole mass, X-ray luminosity and radio luminosity imply that in most cases, these black holes should be more easily detected in the radio than in the X-rays. Finally, we show upper limits from some radio observations of globular clusters, and discuss the possibility that the radio source in the core of the Ursa Minor dwarf spheroidal galaxy might be a ∼10,000–100,000 M⊙ black hole.     

Flattening of galaxies of different morphological types in subclusters of Coma

This paper deals with the observed variation in the flattening of galaxies with the density of galaxies in the subclusters of Coma surrounding NGC 4889, NGC 4874, and NGC 4839 based on data from the Abastumani Combined Catalog of Galaxies. The mean values of the observed ratios of the diameters of the galaxies, as well as histograms of their distributions, indicate that in the central, dense regions of the subclusters within a volume of 0.5h ₇₅ ⁻¹ Mpc³, E and S0 type galaxies are close to spheroidal. A significant reduction in the mean values of the diameters of the galaxies in the subclusters is noted, regardless of their morphology relative to the galaxies in the halo of the Coma cluster. In the subclusters, spiral galaxies are found with a hydrogen deficit that is more than 5 times the hydrogen deficit in spirals within the halo of the cluster. According to their 3-D coordinates, most of the galaxies with a hydrogen deficit are located closer to the south-east edge of the subcluster surrounding NGC 4874 near an extended gas filament in the x-ray region. This may indicate that the subcluster is moving toward a central condensation of faint galaxies in the Coma cluster and a possible merger with it. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 355–368 (August 2007).