Dwarf galaxies in clusters

 Galaxy populations in rich galaxy clusters differ in many respect from the field. The origin of these differences is not yet understood, although the reason must be fundamental for understanding the physics behind galaxy formation. One difference is that cluster of galaxies host a huge population of dwarf galaxies. The luminosity function of galaxies in nearby clusters revealed a steep rise at the faint end: typically there are 2-3 times as many low-mass galaxies per giant in clusters than in the field. The origin of this population is still a mistery, and a number of questions regarding their nature remain unanswered. Were they formed at early times along with the cluster ? Or are they a field population accreting on the cluster core in a long-lasting process ? How are dwarf elliptical galaxies related to ``normal'' elliptical galaxies ? warf1 Dwarf elliptical galaxies form a distinct family of objects in term of their structural properties (correlations between luminosity and size or surface brightness, internal density profile). Their colors have a large spread from colors typical of local dwarf spheroidals to those of dwarf irregulars. An extensive study of the origin and evolution of the dwarf elliptical galaxy population in the Perseus cluster (Abell 426) has been undertaken using different techniques. Optical colors, scale lengths, and detailed morphology is addresses using imaging observations with the Advanced Camera for Surveys on board HST (Cycles 13 and 15, proposals "The origin of Dwarf Galaxies and steep luminosity functions in clusters", "The role of environment in the formation of dwarf galaxies", P.I. Conselice). Our deep, high-resolution observations of dwarf ellipticals using the F606W and F814W filters are being analysed to probe fine details of their structure and investigate their origin. The HST observations have been complemented by near-infrared imaging and multi-object spectroscopic observations of dwarf galaxies in an inner region of the Perseus cluster at the Telescopio Nazionale Galileo and wide-area low-resolution spectroscopy using WYFFOS ant the WHT. Spectroscopy establishes the cluster membership of galaxies and allows measuring the velocity dispersion of low-mass galaxies in the cluster. For the brighter galaxies, absorption line indices will constrain the transition from intermediate-mass to low-mass galaxies.

People: E.V. Held, D. Bettoni

Collaboration: C. Conselice, S. Penny (Nottingham Univ.), S. De Rijcke (Gent Univ.), J. Gallagher (Wisconsin Univ.), R. O'Connell (Virginia Univ.)

Recent Publications:Penny et al. (2009), MNRAS 393,1054; De Rijcke et al. (2009), MNRAS 393,798