Research:

Early-type dwarf galaxies in clusters

Left: Brighter shading = more dEs have a nucleus.
Right: dEs with disk features (image & unsharp mask)
(click for large image)

Early-type dwarf (dE) galaxies are small, low surface brightness objects that are characterized by their smooth appearance and typically have no ongoing star formation and almost no gas or dust. Their masses range from ten million to one billion solar masses, and their half-light radii lie between 500 and 3,000 parsecs. They are the most numerous galaxies in clusters – more than a thousand dEs populate the nearby Virgo cluster – and are thus ideal probes to study the physical processes that govern galaxy formation and evolution in such environments.

Proposed formation scenarios, like the structural transformation of infalling spiral galaxies through tidal forces (e.g., Moore et al. 1996, Nature 379, 613), need to explain the existence of several different dE subpopulations. Among the brighter Virgo cluster dEs, those with weak spiral arms or bars (Lisker et al. 2006a), those with blue centers caused by young stars (Lisker et al. 2006b), and those without a compact stellar nucleus are all shaped like thick disks and tend to reside in regions of lower density. In contrast, nucleated dEs without disk features are spheroidal and are concentrated towards the higher-density cluster center (Lisker et al. 2007). This correlation between dE structure and distribution suggests that the dE subclasses either formed through different mechanisms or are observed at different evolutionary stages. Indeed, from a multicolour analysis we found that the non-nucleated dEs have somewhat younger stellar populations than the nucleated dEs, as deduced from population synthesis models (Lisker et al., in preparation).

In the further course of our studies, we aim at disentangling the environmental influence on the star formation history of the Virgo dEs, and try to answer the question about the actual significance of the various suggested formation channels. Our analyses are based on the wealth of publicly available data from the Sloan Digital Sky Survey (SDSS), a multicolour imaging and spectroscopic survey covering more than 8,000 square degrees of the sky.

Papers of our series on Virgo cluster dEs:

T. Lisker, E. K. Grebel, and B. Binggeli.
Virgo cluster early-type dwarf galaxies with the SDSS.
IV. The color-magnitude relation.
2008, The Astronomical Journal, 135: 380-399
( PDF )

T. Lisker, E. K. Grebel, B. Binggeli, and K. Glatt.
Virgo cluster early-type dwarf galaxies with the SDSS.
III. Subpopulations: distributions, shapes, origins.
2007, The Astrophysical Journal, 660: 1186-1197
( PDF )

T. Lisker, K. Glatt, P. Westera, and E. K. Grebel.
Virgo cluster early-type dwarf galaxies with the SDSS.
II. Early-type dwarfs with central star formation.
2006b, The Astronomical Journal, 132: 2432-2452
( PDF )

T. Lisker, E. K. Grebel, and B. Binggeli.
Virgo cluster early-type dwarf galaxies with the SDSS.
I. On the possible disk nature of bright early-type dwarfs.
2006a, The Astronomical Journal, 132: 497-513
( PDF )