We present results from photometric and spectroscopic survey of giant stars in the Leo I dwarf spheroidal galaxy. We find compelling evidence that this galaxy has been tidally disrupted.

We have studied the distribution of dwarf-galaxy satellite systems around our Milky Way (MW) and the Andromeda (M31) galaxy. The anisotropy is quantified and the form of their distribution is found to be incompatible with that expected if they were cold-dark-matter sub-structures. The origin of these satellites therefore can not be cosmological. Rather the Milky Way and Andromeda satellites probably stem from a local evolutionary mechanism.

We study the faint end of the HI mass function (HIMF) in order to test the predictions of the CDM theory on the number density of objects with small (dark) masses. The neutral hydrogen is much better tracer of the underlying mass distribution compared to the luminous matter and can be used to test the existence of a population of small galaxies in which the star formation has been partially or completely suppressed during cosmic evolution. Due to technical limitations, the existing HI surveys are not very sensitive on masses below 10$^8$ HI $M_{\odot}$. We designed a blind HI survey to be sensitive for objects with small HI masses. The surveyed area is in the Canis Venatici groups of galaxies and covers in total $\approx$86 deg$^2$ of sky, with the observed velocities in the range $-350 < cz < 1400$ km s$^{-1}$. We detected 69 objects, 22 of them for the first time in HI. All new HI detections fall in the lower part of the mass-histogram, confirming our ability to detect galaxies with small HI masses. The calculated HIMF is flat in the faint end regime ($\alpha {\sim}{-}$1), different from the steep rise predicted by CDM models. Possible effects of the environment on the estimated HIMF parameters are discussed.

An all-sky list of 88 nearby dwarf spheroidal (dSph) galaxies with distances $D < 10$ Mpc is considered. Most of the objects have recently been found by Karachentseva & Karachentsev based on POSSII/ESO-SERC survey. A hundred more dSph galaxies are expected in this volume, being missed so far because of their low luminosity and low surface brightness. Apart from 22 dSph members of the Local Group, there are 33 dSphs in other nearby groups that have been resolved recently into stars with HST. Only 5% of the local dSphs are situated outside the known groups.

We discuss observational correlation between basic parameters of the dwarf spheroidal galaxies,in particular, absolute magnitude, surface brightness, metallicity, and so-called “tidal index”. The observed number of dSphs in group increases with luminosity of its brightest galaxy. In a “synthetic” nearby group, dwarf spheroidal galaxies are distributed in depth quite symmetrically about the principal galaxy, having an rms distance scatter of 200 kpc. Projected radial distribution of dSphs in the synthetic group follows the profile $N(R) ~ exp(-R/200$ kpc).

A CCD survey of selected areas in the Virgo cluster was undertaken with the goal of finding dwarf ellipticals fainter in surface brightness than known in the Local Group. This was successful, and galaxies as much as 2 magnitudes fainter than e.g., Umi, have been found. These tend to be larger in radius, due to the detection methods, and thus more luminous than the LG as well. Two fields containing such galaxies were imaged with ACS/HST, with the result that individual RGB stars have been resolved in these galaxies, which are 15 Mpc distant.

The observational evidence for kinematic substructure in Local Group dSphs is reviewed. The properties of these substructures are consistent with their being disrupted star clusters. The persistence of cold substructure argues strongly against the presence of dark matter cusps in the haloes of dSphs. A formation scenario for dSphs is described involving the merger of star clusters in the potential well of a low-mass dark matter halo.

We compare the properties of dwarf galaxies in the Local Group with the simulated galaxies formed before reionization in a cosmological simulation of unprecedented spatial and mass resolution including radiative feedback effects. We find that a subset of the Local Group dwarfs, the dwarf Spheroidals, are remarkably similar to the simulated dwarf galaxies in all their properties already before reionization. Simulated and observed dwarf Spheroidal galaxies not only have similar properties but also follow the same scaling relations. Based on this similarity and on the observed ages of their dominant stellar populations we propose the hypothesis that Local Group dwarfs form in two different ways: (i) most dwarf Spheroidals are pristine fossils of the pre-reionization era and (ii) dwarf irregulars are more massive galaxies that formed most of their stars later, after reionization. There is also a group of “polluted fossils” with properties that are intermediate between these two main groups. We predict the existence of many more dwarf Spheroidals, fainter and with lower surface brightness than the observed population.

We review some properties of ultra compact dwarf galaxies (UCDs) in Fornax and of their brighter counterparts in Abell 1689, among which are two M32 twins, and discuss various possible UCD formation scenarios. We note that it is indispensable to carefully take into account the bright end of the globular cluster luminosity function when estimating the number density of UCDs. It is suggested that the search for more luminous UCDs in dense and rich galaxy clusters is the best way towards establishing UCDs as a new class of galaxies.

We determine star formation rates for a sample of 50 DLAs from properties of the absorbing gas alone. Assuming thermal balance, we determine the grain photoelectric heating rate of the neutral gas from the [C II] $158\,\mu$ m cooling rate per H atom, $\ell_{c}$, inferred from C II$^{*}$ 1335.7 and damped Ly$\alpha$ absorption lines. We deduce the star formation rate per unit area and the FUV luminosity per unit co-moving volume, ${\cal L}_{\nu}^{DLA}$. Comparison of ${\cal L}_{\nu}^{DLA}$ with the luminosity density of Lyman Break Galaxies, ${\cal L}_{\nu}^{LBG}$, shows that ${\cal L}_{\nu}^{DLA}>>{\cal L}_{\nu}^{LBG}$ for most models. These models are ruled out if our assumption that LBGs dominate the total FUV luminosity density of the Universe is correct. The only feasible models are those in which ${\cal L}_{\nu}^{DLA}\approx {\cal L}_{\nu}^{LBG}$. We conclude that DLAs in which $\ell_{c}>10^{-27.1}$ ergs s$^{-1}$H$^{-1}$ contain centrally located LBGs, while the gas in DLAs with $\ell_{c}<10^{-27.1}$ ergs s$^{-1}$H$^{-1}$ is heated by background radiation alone.

Recent observations demonstrate that dwarf galaxies in clusters, despite their faintness, may be a critical galaxy type for understanding the physical processes behind galaxy formation. Dwarfs are the most common type of galaxy and are particularly abundant in rich galaxy clusters. The dominate model for explaining the formation of these systems thorough Cold Dark Matter models is that the bulk of their stellar mass formed early, and within their present environments. Recent results however suggest that some dwarfs appear in clusters after the bulk of its members form, a scenario not predicted in standard hierarchical structure formation models. Many of these systems appear to be younger and more metal rich than dwarfs in lower density areas, suggesting they are possibly created by a tidal process induced by the cluster. Several general galaxy cluster observations, including steep luminosity functions and the origin of intracluster light, are natural outcomes of these processes.

We present the results of near-infrared imaging of the dwarf spheroidal galaxies Fornax and Leo I as part of a photometric survey of stellar populations in Local Group dwarf galaxies. Wide-field observations in the J, H, and $K_s$ bands have been obtained with the SOFI camera at the ESO NTT at La Silla, Chile. The aim of this project is to study the evolution of Local Group dwarf spheroidal galaxies, with special regard to star formation at intermediate epochs. The near-infrared data, together with optical catalogs from the Padova Local Group wide-field survey, provide a very large color baseline ideal to study the properties of red giant branch (RGB) and asymptotic giant branch (AGB) stars.

What is the origin of the numerous population of diffuse elliptical galaxies (dE)? These galaxies formed their stars several billion years ago and lost their gas. Though the stellar winds resulting from star formation and the interactions with the environment undoubtedly play a role, their respective role and details of this evolution are still debated.

In this presentation we will review the first 3D spectroscopic observations of a handful of dE galaxies. These data reveal complex kinematical structures, with embedded discs and counter rotating cores, and they open extremely promising perspectives for studying the history of the stellar population throughout these various features.

The presence of disks, which was already known from detailed image analysis, their complex kinematics, and the new constraints on the stellar population enforce the hypothesis of the evolutionary connection between dEs and disk galaxies.

We report on the recent developments of our long-term investigation of the near-IR luminosity-metallicity relation for dwarf irregular galaxies in nearby groups. A very well-defined relation is emerging from our observational database, and a preliminary discussion of its implications is given.

We report on the use of high-resolution spectra to obtain a detailed description of the Sagittarius dwarf spheroidal internal dynamics, its Mass and Mass to Light ratio (M/L). Our direct measure of the central velocity dispersion of SGR give $\sigma_{SGR}=8.1\pm0.4$ km/s which translates in a total mass estimate of $M_{SGR} =1.6\times10^8 M_{\odot}$ and corresponding $(M/L)_{SGR}=9.1 (M/L)_{\odot}$. We also report on a possible detection of rotation in the core of SGR.

The starlight coming from the intergalactic space in galaxy clusters and groups witnesses the violent tidal interactions that galaxies experience in these dense environments. Such interactions may be (at least partly) responsible for the transformation of normal star-forming galaxies into passive dwarf ellipticals (dEs).

In this contribution we present the first systematic study of the IntraCluster Light (ICL) for a statistically representative sample (Zibetti et al. 2005), which comprises 683 clusters selected between $z=0.2$ and 0.3 from ${\sim}1500 \deg^2$ in the SDSS. Their ICL is studied by stacking the images in the $g$-, $r$-, and $i$-band after masking out all galaxies and polluting sources. In this way a very uniform background illumination is obtained, that allows us to measure surface brightnesses as faint as 31 mag arcsec$^{-2}$ and to trace the ICL out to 700 kpc from the central galaxy. We find that the local fraction of light contributed by intracluster stars rapidly decreases as a function of the clustercentric distance, from $\sim$40% at 100 kpc to $\sim$5% at 500 kpc. By comparing the distribution and colours of the ICL and of the clusters galaxies, we find indication that the main source of ICL are the stars stripped from galaxies that plunge deeply into the cluster potential well along radial orbits. Thus, if dEs are the remnants of these stripped progenitors we should expect similar radial orbital anisotropies and correlations between the dE luminosity function and the amount of ICL in different clusters.

The diffuse emission we measure is contaminated by faint unresolved galaxies: this makes our flux estimate depend to some extent on the assumed luminosity function, but, on the other hand, allows us to constrain the number of faint galaxies. Our present results disfavour steep ($\alpha<-1.35$) faint-end powerlaw slopes.

In this contribution, we report on the discovery of dwarf spheroidals (dSphs) in the core of the Fornax cluster. Their photometric properties – like magnitude, colour, surface brightness – are very similar to those of Local Group dSphs. However, at a given total magnitude, dSphs in Fornax seem to be more extended than their Local Group counterparts. The membership of several dwarf galaxy candidates in Fornax has been confirmed by surface brightness fluctuation measurements on deep wide-field images taken with the Magellan telescope. The analysis of these images also confirms the flat faint end slope of the luminosity function for dSphs in Fornax which contradicts the expected large number of small dark matter halos connected to dwarf galaxies in $\Lambda$CDM theory. Dwarf spheroidals have also been detected in the Hydra I and Centaurus cluster. A preliminary analysis of their photometric properties shows that they obey similar scaling relations as their counterparts in Fornax and the Local Group.

I discuss here the main physical constraints on Dark Matter particles that can be obtained by the study of the astrophysical signals produced by their annihilation within Dwarf Galaxies. The present results are obtained in the framework of neutralino Dark Matter scenarios and under the assumption that Dwarf Galaxies are dominated by Dark Matter.

The satellite system of M31 is the next closest satellite system to our own, and is well within reach of current generations of ground-based instrumentation. We have conducted a survey of this system with the Isaac Newton Telescope Wide Field Camera, particularly focussing on the seven dwarf spheroidal companions, and derived homogeneous, fundamental information for them. This reveals the presence of severe tidal distortion and multiple structural components in some systems. In addition, a comparison of the M31 dwarf spheroidals to their Galactic counterparts shows that the M31 dwarf spheroidals have scale radii generally twice as large as the Galactic dwarfs, suggesting that the tidal field of M31 may be substantially weaker than that for the Galaxy. A Keck/DEIMOS study of the newly discovered satellite And IX suggests that this galaxy has an anomalously large metallicity for its luminosity, and shows it to be strongly dark matter dominated.

In this paper, we present a statistical study of the X-ray properties of 11 isolated early-type galaxies (IEGs) observed with the Chandra ACIS-S3, and then discuss the implications of these properties.

We present the internal velocity dispersions and spectral line indices for six Virgo ultra-compact dwarf (UCD) galaxies obtained from spectroscopy with the Keck II 10-m telescope. We use these results: 1) to compare the Virgo UCDS with the Fornax UCD population; 2) to compare UCDs with globular clusters (GCs) to determine if UCDs and the most luminous GCs are the same or distinct systems; 3) to further test the nucleated dwarf elliptical (dE,N) stripping hypothesis for UCD formation.