Structure and physical properties of ellipticals galaxies

Elliptical galaxies are the most luminous and massive stellar aggregates in the Universe. They are found preferentially in high density environments, such as rich clusters of galaxies, where they constitute the dominant galaxy population. They appear as a uniform class of objects and because of their uniformity they can be identified at great distances, which makes them excellent cosmological probes. Elliptical galaxies stand out as signpost of the dynamically most mature environments in the Universe, where presumably most of the stars have formed long ago. They pose, in spite of their apparent simplicity, a rich variety of astrophysical problems. The present research proposal focalizes on a number of topics which are essential in understanding the nature and evolution of elliptical galaxies: How complex are stellar populations in elliptical galaxies? A stellar population contains the hallmarks of an earlier epoch in the galaxy evolution and is therefore a key to measure not only ages but also age differences as a function of redshift. Furthermore, stellar dynamics constrains depth and shape of the galactic potenital well and is therefore a sensitive tracer of the mass-to-light ratio and its local variations. Assuming that stars of different ages probably have also different kinematical characteristics, the use of representative template stars can help to discriminate different stellar populations within a galaxy. New high-resolution spectroscopic observations over an extended wavelength region will provide the data to discriminate between giant and dwarf populations in ellipticals. The contributions of various stellar populations to the observed spectrum will be studied through a novel approach to analyze the line-of-sight-velocity distributions. The interstellar medium and its relevance in galaxy evolution has been widely neglected for elliptical galaxies. Using combined spectral information over an extended wavelength region (X-rays, UV, optical, IR and radio wavelengths) will reveal information on the related physical processes, such as ionization mechanisms, and give new insights on element abundances in elliptical galaxies. The cores of galaxies are astrophysically unique, reaching extreme stellar densities, occasionally experiencing intense bursts of star formation and possibly hosting a central supermassive black hole. One of the focal points in this context is the interrelation of nuclear to extra-nuclear activity. Accretion of external material together with environmentally induced activity, such as merging events are thought to be directly related to AGN and starburst phenomena. Interactions can also severely alter the properties of a whole galaxy up to modify its original morphological type. However, the detailed processes are not yet fully understood. The proposed project aims also in understanding theses processes. Dwarf galaxies as a class represent a significant part of the luminous mass in the Universe. Dwarf elliptical galaxies are of particular interest since their relation to luminous ellipticals is not well understood. It is unclear whether ellipticals and dwarf ellipticals follow a common sequence in their physical properties. New, deep observations on dwarf ellipticals will shed light on their stellar populations, their dynamical state and, since their gravitational potentials are expected to be highly dark matter dominated, will probe the mass-to-light ratio out to large radial distances.

Elliptical galaxies are the most luminous and massive stellar aggregates in the Universe and appear as a homogenous class of objects. Because of these properties ellipticals can be identified over great distances are therefore excellent cosmological probes. Elliptical galaxies represent the dynamically most evolved stellar systems in the evolution of galaxies. In spite of their apparent simplicity, they pose a number of key astrophysical problems which are addressed by high resolution observations from ground and space together with state-of-the-art modelling. Ellipticals are now seen as complex systems from the morphological, dynamical and chemical point of view. The analysis of an extended spectroscopic data sample of nearby ellipticals yielded a flat age distribution for the stellar population with ages up to 15 Giga-years. Intrinsically young ellipticals (less than 4 Giga-years) are only found in the field, whereas cluster ellipticals are significantly older than 5 Giga-years. The stellar population is not well mixed as expected, but show significant metallicity gradients indicating that the central regions are on the average younger than the outskirts. A significant fraction of the evolution path of a galaxy takes place in the environment of galaxy groups where the presence of ellipticals indicates the final stages of an evolving group. A detailed investigation of the properties of galaxy groups was carried out in order to study such evolutionary phenomena. The early phases of group evolution are dominated by a loose aggregate of disk galaxies similar to our Local Group. Such systems show often considerable sub-structures, which could be the birthplace of so called compact galaxy groups. Galaxy interactions are found to play an important role in the subsequent evolution. The likely end products are elliptical galaxies, being merger remnants, embedded in a hot, diffuse gas medium and surrounded by a population of dwarf galaxies. Galaxy clusters may then accrete such systems. Galaxy clusters of the local Universe are dominated in numbers by dwarf elliptical galaxies, whose properties are found, despite apparent similar morphologies, to be largely detached from that of massive ellipticals. The study of a comprehensive sample of dwarf ellipticals revealed peculiar features in many dwarf ellipticals such as kinematically decoupled cores and warped stellar disks suggesting that galaxy harassment induced by the strong tidal forces of galaxy interactions plays a major role in the evolution of dwarf ellipticals. Otherwise most of the observed properties suggest an evolutionary scenario with an early period of quiescent star formation combined with some post-merger star bursts where the gas is then removed by supernova-driven galactic winds.