Early-type dwarf (dE) galaxies exhibit a puzzling variety of observable properties. Are they at all a single class of objects? Could some of them be “primordial” galaxies, while others were created by environmental processes? Here I provide a brief overview of early-type dwarfs, focusing on galaxy clusters, where dEs are the dominant galaxy population by number.

The dwarf elliptical galaxies (dEs) are more luminous than dwarf spheroidals and fainter than “normal” ellipticals. In this paper I investigate the interconnection between these three families of early-type galaxies by studying their stellar populations properties. Some dwarf ellipticals have strong metallicity and age gradients, up to − 0.5 dex and 0.2 dex per decade of radius, respectively. The dispersion of the gradients is similar to the one observed in the dwarf spheroidals and normal ellipticals and there is no gradient-luminosity relation. The star formation history of dEs is characterised by a mass-dominant old population, generally prolonged to intermediate ages. This pattern is apparently independent of the environment (scarce groups or dense clusters), as confirmed by the observations of the Local Group dwarf spheroidals. At higher masses, the S0s have a similar star formation history, while the tail of late star formation is rarer and shorter in elliptical galaxies (MB ~ −19). I conclude that the transition between different classes is smooth, without any abrupt changes in their stellar population properties.

We present new medium resolution kinematic data for a sample of 21 dwarf early-type galaxies (dEs) mainly in the Virgo cluster. These data are used to study the origin of the dE population inhabiting rich clusters. Within them we detect two different subpopulations: half of the sample (52%) are rotationally supported and the other half are pressure supported. We also find that the rotationally supported dEs are located in the outer parts of the cluster, present disky morphological shapes and are younger than those pressure supported that are concentrated in the core of the cluster without any underlying structures. Our analysis reveals that the rotationally supported objects have rotation curves similarly shaped to those of star forming galaxies of similar luminosities and follow the Tully-Fisher relation. This is expected if dE galaxies are the descendant of low luminosity star forming systems which recently entered the cluster and lost their gas due to a ram pressure stripping event, quenching their star formation activity and transforming them into quiescent systems, but conserving their angular momentum.

We present the results of a Hubble Space Telescope (HST) study of dwarf galaxies in the nearby Perseus Cluster, down to MV = −12, spanning the core and outer regions of this cluster. We examine how the colour magnitude relation, structure and morphology are affected by environment for the lowest mass galaxies. On average, dwarfs in the outer regions of Perseus are more disturbed than those in the cluster core, with higher asymmetries and clumpier light distributions. We measure the (V − I)0 colours of the dEs, and find that dwarfs in the cluster core and outskirts lie on the same colour magnitude relation. We suggest that the disturbed dwarfs in the cluster outskirts are likely “transition dwarfs”, with their colours transforming before their structures. Finally, we infer from the smoothness of the cluster core population that these dwarfs must be highly dark matter dominated to prevent their disruption by the cluster potential.

We analyze the structural parameters and stellar population characteristics of early types in the Virgo cluster. In both cases, we find noticeable deviations from a simple joint behavior of dwarfs and giants; nevertheless, by comparison to a semianalytic model of galaxy formation, we conclude the relations might be reproduced by processes that form dwarfs and giants altogether. The work presented here is based on Janz & Lisker (2008, 2009).

In the local universe dwarf galaxies represent 80% of the overall galaxy population, while their type-distribution (passive, early- vs. star-forming, late-type dwarfs) depends on galaxy density (i.e. morphology-density relation). However, the details of the processes governing these transformations are still not well understood. This project aims to unveil these processes using surface photometry of a large sample of late-type galaxies from SDSS in combination with evolutionary synthesis models. We derive structural properties and compare them with other data and model predictions.

Besides giant elliptical galaxies, a number of low-mass stellar systems inhabit the cores of galaxy clusters, such as dwarf elliptical galaxies (dEs/dSphs), ultra-compact dwarf galaxies (UCDs), and globular clusters. The detailed morphological examination of faint dwarf galaxies has, until recently, been limited to the Local Group (LG) and the two very nearby galaxy clusters Virgo and Fornax. Here, we compare the structural parameters of a large number of dEs/dSphs in the more distant clusters Hydra I and Centaurus to other dynamically hot stellar systems.

Some results of 3D spectroscopy for a small sample of dwarf elliptical galaxies are presented. By measuring Lick indices and investigating radial profiles of the SSP-equivalent ages and metallicities of the stellar populations, I have found evolutionary distinct cores in all of them. The ages of these cores are 2–4 Gyr, and the metallicities are higher than the solar one. Outside the cores, the stellar populations are typically old, T > 8Gyr, and the metallicities are subsolar.

Dwarf elliptical galaxies are the most common galaxy type in nearby galaxy clusters, yet they remain relatively poorly studied objects and many of their basic properties have yet to be quantified. In this contribution we present the preliminary results of a study of 4 Virgo and 1 field galaxy obtained with the SAURON integral field unit on the William Herschel Telescope (La Palma). While traditional long-slit observations are likely to miss more complicated kinematic features, with SAURON we are able to study both kinematics and stellar populations in two dimensions, obtaining a much more detailed view of the mass distribution and star formation histories.

The Phoenix dwarf galaxy is the nearest and most typical representative of the class of the so-called transition dwarf galaxies (from spheroidal to irregular). We present here precise velocity measurements obtained from high-resolution spectroscopy of more than one hundred member stars, allowing us to study the stellar kinematics and estimate the internal velocity dispersion and dark matter content.

Ultra-compact dwarf galaxies (UCDs) are predominatly found in the cores of nearby galaxy clusters. Besides the Fornax and Virgo cluster, UCDs have also been confirmed in the twice as distant Hydra I and Centaurus clusters. Having (nearly) complete samples of UCDs in some of these clusters allows the study of the bulk properties with respect to the environment they are living in. Moreover, the relation of UCDs to other stellar systems in galaxy clusters, like globular clusters and dwarf ellipticals, can be investigated in detail with the present data sets. The general finding is that UCDs seem to be a heterogenous class of objects. Their spatial distribution within the clusters is in between those of globular clusters and dwarf ellipticals. In the colour-magnitude diagram, blue/metal-poor UCDs coincide with the sequence of nuclear star clusters, whereas red/metal-rich UCDs reach to higher masses and might have originated from the amalgamation of massive star cluster complexes in merger or starburst galaxies.

As part of the HST/ACS Coma Cluster Treasury Survey, we have undertaken a Keck/LRIS spectroscopic campaign to determine membership for faint dwarf galaxies. In the process, we discovered a population of Ultra Compact Dwarf galaxies (UCDs) in the core region of the Coma Cluster. At the distance of Coma, UCDs are expected to have angular sizes 0.01 < Re < 0.2 arcsec. With ACS imaging, we can resolve all but the smallest ones with careful fitting. Candidate UCDs were chosen based on magnitude, color, and degree of resolution. We spectroscopically confirm 27 objects as bona fide UCD members of the Coma Cluster, a 60% success rate for objects targeted with MR < −12. We attribute the high success rate in part to the high resolution of HST data and to an apparent large population of UCDs in Coma. We find that the UCDs tend to be strongly clustered around giant galaxies, at least in the core region of the cluster, and have a distribution and colors that are similar to globular clusters. These findings suggest that UCDs are not independent galaxies, but rather have a star cluster origin.

Dwarf elliptical galaxies are frequently excluded from bright galaxy samples because they do not follow the same linear relations in diagrams involving effective half light radii, Re, or mean effective surface brightnesses, ⟨μ⟩e. However, using two linear relations which unite dwarf and bright elliptical galaxies we explain how these lead to curved relations when one introduces either Re or ⟨μ⟩e. In particular, the curved ⟨μ⟩e − Re relation is derived here. This and other previously misunderstood curved relations, once heralded as evidence for a discontinuity between faint and bright elliptical galaxies at MB ≈ −18 mag, actually support the unification of such galaxies as a single population whose structure (i.e. stellar concentration) varies continuously with stellar luminosity and mass.

We analyse evolutionary parameters of globular clusters in dwarf galaxies obtained in our medium-resolution spectroscopic observations, and their connection with the properties of the host galaxies. We find that 1) the metallicity spread in globular cluster systems is wider for larger galaxies; 2) metal-rich clusters are young and preferentially found in galaxies more massive than ∼109M⊙; 3) intermediate-age globular clusters in early-type dwarf galaxies are richer in metals than star clusters representing dynamically cold gas-rich environments in dwarf irregulars; 4) the age-metallicity relation is special for each galaxy, and depends not only on its mass, but also on some other factors, probably environmental conditions.

In the last decade we witness an advent of new types of dwarf stellar systems including ultra-compact dwarfs, ultra-faint dwarf spheroidals, and exotic globular clusters, breaking the old simple paradigm for dwarf galaxies and globular clusters. These objects be- come more intriguing, and understanding of these new findings becomes more challenging. Recently we discovered a new type of large scale structure in the Virgo cluster of galaxies: it is composed of globular clusters. Globular clusters in Virgo are found wandering between galaxies (intracluster globular clusters) as well as in galaxies. These intracluster globular clusters fill a significant fraction in the area of the Virgo cluster and they are dominated by blue globular clusters. These intracluster globular clusters may be closely related with the first dwarf galaxies in the universe.

We analyze the stellar population parameters of seven ultra-compact dwarfs (UCDs) in the Virgo cluster. We find that the stellar population properties of UCDs are similar to those nuclei of early-type dwarf galaxies (dEs) that lie in the dense region of the cluster. This evidence suggests that the Virgo UCDs may have formed through the stripping of dE nuclei.

It has recently been suggested that the peculiar cluster Pal 1 is associated to the Canis Major dwarf galaxy, which existence is still at the center of a debate. Our first measurement of the cluster’s chemical abundance ratios allows us to examine this association and to advance further in the clarification of Pal 1 possible origin.

In this contribution, we discuss the chemical properties of the complex stellar population of the Galactic globular cluster Omega Centauri.

We present two newly discovered cE galaxies with tidal tails, the “smoking gun” of the stripping scenario for cE formation.

Two competing theories posit that Ultra Compact Dwarfs (UCDs) form either as the stripped nuclei of dwarf galaxies or as giant globular clusters (GGCs) associated with the largest globular cluster (GC) systems. By focussing on the field and group environments where young UCDs may be most common, we have discovered the first UCD that is clearly the result of recent (< 4Gyr ago) stripping of a companion galaxy during a minor galaxy merger. However, we have also found a definitive case of a multiple-UCD system created via GC formation processes, which are likely associated with major galaxy mergers.