The growing number of observational details questions the standard picture of the evolution of dwarf ellipticals and argues for a complex evolution dominateded by environmental effects. Here we compile some standard and most recent observational issues and discuss the results of simple models in comparison with observations. The necessity of further model developments are justified and prospected.

I will review the progress of VLT spectroscopy of large numbers of individual stars in nearby dwarf spheroidal galaxies. This spectroscopy has allowed us to obtain detailed insights into the chemical and dynamical properties of the resolved stellar population in these nearby systems.

Observations suggest systematic differences between chemical abundances of stars in satellite galaxies and those in the Milky Way halo. Specifically, for the same [Fe/H] values, stars in surviving satellite galaxies display significantly lower [α/Fe] ratios than stars in the stellar halo.

Here we investigate whether the observed differences can be explained in the framework of hierarchical structure formation. We model the chemical enrichment of a typical Milky Way galaxy in a ΛCDM Universe using, in combination, i) a semi-analytical code and numerical simulations that model the accretion and disruption of halo substructure and ii) a chemical evolution model that takes into account each satellite's star formation, metal enrichment and stellar feedback. Our results suggest that the observed chemical abundance patterns are a natural outcome in the process of hierarchical assembly of the Galaxy. We find that the stellar halo is built up from satellite galaxies accreted early on (more than 8–9 Gyr ago) and enriched in α-elements produced in Type II supernovae (average [α/Fe] values between 0.2–0.5). In contrast, satellites which survive today were typically accreted late (within the last 4–5 Gyr) and had at the time of accretion nearly solar [α/Fe] values as a result of the longer contribution of Type Ia supernovae.

Dwarf galaxies tend to have a large spread in metal abundances and overall lower metallicites than large Spiral or Elliptical ones. Here we propose that if all stars are formed from an invariant canonical IMF within each star cluster which are distributed according to an embedded cluster mass function (ECMF), then the integrated stellar IMF over the whole galaxy (the integrated galaxial IMF, IGIMF) must be steeper than the canonical IMFs within the clusters, and additionally they must depended on the stellar mass of the galaxy. In particular, dwarf galaxies and low-surface brightness galaxies show steeper galaxial IMFs which vary strongly with the star-formation rate. The resulting number of Supernovae II per low-mass star, and the chemical enrichment history of these galaxies therefore vary substantially depending on the galaxy assembly history.

We present the observed metallicity distribution for 437 red giants in the Carina dwarf spheroidal (dSph) galaxy that was derived from FLAMES medium-resolution spectroscopy of the near-infrared Ca triplet (CaT). We find a mean [Fe/H] of −1.7, dex and a full spread in metallicities of at least one dex. Most of this width is due to the occurrence of three populations at different [Fe/H], which are likely to be related to the three star forming (SF) episodes that governed Carina's evolution and also correlate with peaks in the age-distribution. Both the lack of any correlation between stellar colour and metallicity and a comparison of the metallicities with chemical evolution models, which suggest a major role of infalling gas, contribute to the detailed understanding of Carina's evolutionary history. Moreover, we find a mild dependence of metallicity on radial location, reflected in a shift of the distribution's peak towards the metal poor end in the outer regions.

With the new discoveries that some dEs are rotationally supported and that dIs may show a large variety in star formation histories, the perceived relationships between these two families of galaxies are changing. There are at least three viable channels for the origin of dwarf elliptical galaxies with strong observational evidence that support their reality. I will discuss the observational evidence for each of these channels and the likely physical processes which determine each channel.

We consider the feedback effects of the early reionization on the formation of small galaxies. For this purpose, we perform 3D radiation hydrodynamic simulations with incorporating the radiative transfer for ionizing photons. As a result, it is found that the early reionization is so devastating for low mass systems with $M_{\rm vir} \lesssim 10^8 M_\odot$ or $v_{\rm circ} \lesssim 20 {\rm km/s}$, and almost all gas is photo-evaporated in more than 95% of low mass systems. These results indicate that the low mass dwarf galaxies are not formed directly from isolated CDM density perturbations.

M32 and other compact dwarf galaxies present interesting evolutionary puzzles. Why are most dwarf galaxies low density systems while a very few, like M32, have the highest stellar densities of any galaxies? We suggest that gas capture should be added to the tidal stirring mix as a way to make high density M32-type dwarfs starting with relatively ordinary satellite galaxies. This paper presents some of the basic features and issue associated with gas capture models.

If dwarf-elliptical galaxies formed their stars very rapdily (on timescales of less than 1 Gyr), they may in principle be detectable out to high redshift. Prior to the discovery of cosmic acceleration, it appeared that rapid and late formation dwarf elliptical galaxies might be required to explain the number counts of faint galaxies. A plausible hypothesis emerged: that photoionization by the UV background prevents gas cooling in low-mass halos until $z \lesssim 1.5$. The discovery of cosmic acceleration eased the tension between predicted galaxy number counts and galaxy-evolution models. Nevertheless, there is some evidence for relatively late star formation in nearby dE's, and the photoionization delay mechanism still appears to have some merit. It is thus of interest to look back in time to see if we can find starbursting dwarf galaxies at moderate redshift. We review the connection between faint-blue galaxies and bursting-dwarf galaxies and discuss some attempts to identify progenitors to dE galaxies in the Hubble Ultra Deep Field (HUDF) observations. We find roughly 85 galaxies in the HUDF with redshifts $0.6<z<1.1$ that appear to have formed most of their stars at $z<1.5$. Of these, 70% have half-light radii less than 1.5 kpc. These are thus “smoking gun” candidates for dwarf galaxies that are either collapsing for the first time at moderate redshifts or have otherwise been unable to form stars for more than 1/3$ of a Hubble time.

The epochs when many dwarf ellipticals were forming their stellar populations have recently become accessible to direct observation, as surveys at $z = 1$–5 have now uncovered thousands of galaxies. I briefly review the properties of various categories of high redshift galaxies in search of clues for possible links to today's galaxy populations. While much of the emphasis up to now has been on massive elliptical galaxies, with several groups claiming to have identified their progenitors with a variety of different techniques, dwarf galaxies have generally been overlooked—no doubt because the observational difficulties at high redshift are still considerable. Based on available data, it is possible that at least some of the ‘Ly$\alpha$ emitters’ and the damped Ly$\alpha$ systems may be on evolutionary paths which will transform them into the galaxies which are the subject of this meeting.

We present evidence that FCC 046, a dE,N galaxy with an offset nucleus, suffered a counter-streaming instability. This instability may explain the presence of lopsided nuclei in other dE,Ns if these galaxies are weakly rotating. The counter-streaming needed for the instability may result from the destruction of box orbits during the assembly of the nucleus.

We present a study of the evolution of late-type field dwarfs over the last 1.5 Gyr, based on HST ACS observations carried out as part of the GEMS survey. This study is amongst the first to probe the evolution of dwarfs over such a large timescale. The comparison of structural properties, particularly size and scale length, indicates that the dwarfs in the redshift range $z \sim$ 0.01 to 0.15 (look-back times up to 1.8 Gyr) are more extended than local dwarfs. We argue that this difference is due to the star formation activity becoming more centrally concentrated in late-type dwarfs over the last ${\sim} 1.5$ Gyr. We discuss several possible causes for this evolution. We also find a lack of blue compact dwarfs in the GEMS sample and interpret this as indication of the fact that strong, centrally concentrated star formation is a feature of evolved dwarfs that are entering their final stages of evolution.

We present a discovery of a giant stellar halo in NGC 6822, a dwarf irregular galaxy in the Local Group. This halo is mostly made of old red giants, showing striking features: 1) it is several times larger than the main body of the galaxy seen in the optical images, and 2) it is elongated in the direction almost perpendicular to the HI disk of NGC 6822. The structure of this stellar halo looks similar to the shape of dwarf elliptical galaxies, indicating that the halos of dwarf irregular galaxies share the same origin with those of the dwarf elliptical galaxies.

If the CDM paradigm is correct, then dwarf galaxies are far rarer than dark matter haloes of comparable circular velocity at the present day. Some process or processes must suppress galaxy formation almost completely on these small scales. Several plausible mechanisms may explain the scarcity of dwarf galaxies in the local universe, but it has been hard to distinguish between the different models observationally. The spatial distribution and kinematics of dwarf galaxies contain additional information about the age and formation history of these systems. Interpreted in the CDM framework, they provide powerful new constraints on the physics of dwarf suppression, ruling out at high significance the simplest models, in which the halo occupation probability is constant above some fixed mass or velocity threshold. We describe recent tests of small-scale galaxy formation models using the satellites of the local group and other nearby groups, and discuss the implications of these results for studies of the stellar halo, sub-galactic dark matter and the reionization of the universe at high redshift.

Many past attempts to kill MOND have only strengthened the theory. Better data on galaxy velocity curves clearly favor MOND (without fine-tuning) over cold dark matter. The usual critism on the incompleteness of classical MOND has spurred a Modified Relativity (MR) by Bekenstein. After outlining cosmology and lensing in MOND, we review MOND on small scales. We point out some potential problems of MOND in two-body relaxation and tidal truncation. We argue that the tidal field in any MOND-like gravity theory predicts that the Roche lobe sizes of a binary system are simply proportional to the binary baryonic mass ratio to the power 1/3. An immediate application of this result is that the tidal field and tidal truncation radii of million-star globular clusters and million-star dwarf galaxies (e.g., the Milky Way satellites NGC2419 and Carina) would be very similar because of the one-to-one relation between gravity and baryon distribution. This prediction appears, however, inconsistent with the fact that all globulars are truncated to much smaller sizes than all dwarf galaxies. Whether tide is uniquely determined by baryons can also be used to falsify any MOND-like gravity theory, whether classical or relativistic.

We present preliminary results of 3-D hydro simulations of the interstellar medium evolution in dwarf spheroidal galaxies undergoing star formation for the first time. The star formation is assumed to occur in a sequence of instantaneus bursts separated by quiescent periods. Different models are made changing the number and the intensity of the bursts in such a way that the final mass of the formed stars remains the same. We followed the enrichment of the ISM taking into account the contribution of both type Ia and II supernovae. The aim of our models is to find a star formation history compatible with the observed spread of stellar age and metallicity in such galaxies and to reproduce the observed mass-metallicity relation.

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 present a series of N-body simulations examining the tidal impact of satellite dwarfs on galactic disks, in particular on the abilities of satellites to excite warps like the one seen in the disk of the Milky Way. We have examined satellites with masses spanning 1.5 orders of magnitude, ranging from the Sgr dSph to the LMC. Orbits similar to the eccentric polar orbit of Sgr were examined, along with orbits of intermediate inclination and of lower eccentricity. The fundamental physical processes which govern the transfer of angular momentum between the satellite and disk, and within the disk, are identified to be precession of the satellite orbit, shrinking of the satellite orbit due to dynamical friction, warping of the disk within a slewing potential, and differential precession. We find that although very large satellites can tidally excite warps shortly after each perigalactic passage, the effects of such satellites are quite destructive and the resulting disks have smaller warps and appear more disturbed than the disks of the Milky Way and other strongly-warped galaxies. Smaller satellites, however, may be capable of producing the minor warps typical of many disk galaxies.

A cold dark matter halo big enough to host the Milky Way contains hundreds of subhalos massive enough to host dwarf galaxies. The difference to the much smaller observed number of satellite galaxies seems to be a problem for CDM. The galaxy number density profile and disk like configuration are also different form the total subhalo populations in CDM simulations. A number of different models of dwarf galaxy formation which are able to reproduce the right number of luminous subhalos have been proposed. Some of them also give the right radial distributions and make disk like configurations more probable. Additional information about the typical formation times and sites of dwarf galaxies can be found in the stellar halo of the Milky Way, i.e. from the stellar debris of tidally disrupted dwarfs. A stellar halo with a realistic concentration is obtained when most dwarfs form early (before redshift 10) in small halos (virial mass above $10^8$ solar masses). This mass scale found from simulations of dark matter structure formation coincides with the virial temperature of $10^4 K$ which is needed for efficient atomic cooling.

We discuss the role of environmental mechanisms in the evolution of dwarf galaxy satellites using high-resolution N-Body+SPH simulations that include simultaneously tidal forces, ram pressure and heating from ionizing radiation fields. Tidally induced bar-buckling instabilities can transform a rotating disky dwarf into pressure supported spheroidals. Efficient gas removal requires instead a combination of tidal mass loss and ram pressure stripping in a diffuse gaseous corona around the primary system. The efficiency of ram pressure depends strongly on how extended the gas remains during the evolution. Bar driven inflows that tend to drive the gas to the bottom of the potential well can be opposed by the heating from external radiation fields. We show that even fairly massive dwarfs ($V_{peak} >$ 30 km/s) would be stripped of their gas over a few Gyr if they enter the Milky Way halo at $z > 2$ thanks to the effect of the cosmic UV background. Gas mass loss can be much faster, occurring in less than 1 Gyr, if dwarf satellites have their first close approach to the primary at the epoch of bulge formation. Indeed at that time the primary galaxy should have a FUV luminosity comparable to that of major present-day starbursts, resulting in a local UV field even more intense than the cosmic background.