Formation and Evolution of Early-Type Dwarf Galaxies
Igor CHILINGARIAN
Observatoire de Paris
Résumé :
I will review and compare evolutionary scenarios for two different classes
of early-type dwarf galaxies: diffuse ellipticals (dE's) and compact
ellipticals (cE's).
dE's are numerically dominant population in clusters of galaxies, but their
origin and evolution is a matter of debate. The main question is ``how and
why they lose their gas and stopped star formation?'' Several scenarios of
gas removal from dE's exist: galactic winds, ram pressure stripping,
gravitational harassment. A new method of estimating internal kinematics and
stellar populations parameters based of fitting spectroscopic observations
by synthetic spectra has been developed. This technique has been applied to
3D-spectroscopy (Russian 6-m telescope) of dE galaxies in Virgo cluster and
nearby groups and multi-object spectroscopy (ESO VLT) of a sample of dE's in
the Abell496 cluster. Our discovery of young nuclei and embedded stellar
discs in dE's strengthens the evolutionary link between early and late type
dwarfs (dIrr's) and supports the scenario of ram pressure stripping of ISM
by the intergalactic medium as the main channel of dIrr->dE transformation.
Contrary to dE's, cE (or M32-like) galaxies, having similar luminosities and
stellar masses to dE's are extremely rare objects, found only in vicinities
of massive galaxies (giant spirals or cluster cD's). I will present our
recent discovery of the 6-th known cE galaxy in the Universe. Its
properties: stellar content, kinematics, chemical element abundances
uniquely argue for its origin from a more massive S0 or Sa galaxy, that must
have lost up-to 90 percent of its stellar mass due to tidal interaction with
the cD. We have conducted a search for cE galaxies in the central regions of
nearby clusters using unprecedented power of the International Virtual
Observatory (VO). I will present some preliminary results of this study.
