A significant fraction of Virgo cluster early-type dwarf galaxies have blue central colours caused by recent or ongoing star formation. A spectral analysis shows that even in their centers, stellar mass is dominated by an old population. These galaxies are an unrelaxed cluster population that possibly formed from morphological transformation of late-type galaxie

Glatt, K.

Grebel, E. K.

Lisker, T.

Westera, P.

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Galaxy Evolution across the Hubble TimeProceedings IAU Symposium No. 235, 2006F. Combes & J. Palousˇ, eds.c© 2007 International Astronomical Uniondoi:10.1017/S174392130600682XThe Last Stages of Star Formation in dEs?T. Lisker, K. Glatt, P. Westera, and E. K. GrebelAstronomical Institute, Dept. of Physics and Astronomy, University of Basel, SwitzerlandAbstract. A signiﬁcant fraction of Virgo cluster early-type dwarf galaxies have blue centralcolours caused by recent or ongoing star formation. A spectral analysis shows that even in theircenters, stellar mass is dominated by an old population. These galaxies are an unrelaxed clusterpopulation that possibly formed from morphological transformation of late-type galaxies.Keywords. Galaxies: dwarf, galaxies: evolution, galaxies: clusters: individual (Virgo)While early-type dwarf (dE) galaxies are typically considered not to have recent orongoing star formation, the well known Local Group dE NGC 205 does exhibit centralstar formation, gas, and dust. We identiﬁed 23 similar dEs (Fig. 1; Lisker et al. 2006)among our dE sample of 476 certain or possible Virgo cluster members that are covered bythe Sloan Digital Sky Survey (SDSS) Data Release 4 (Adelman-McCarthy et al. 2006)and are listed in the Virgo Cluster Catalog (VCC, Binggeli et al. 1985). These dEswith blue centers (“dE(bc)s”) reach a number fraction of > 15% among the brightest(B < 15) dEs. Their gas content is smaller than that of dwarf irregulars (dIrrs), butsomewhat larger than that of “classical” dEs. Since the dE(bc)s are an unrelaxed clusterpopulation, and since even in their centers, young stars make up only a few percent of thestellar mass (Lisker et al. 2006), we may be seeing the ﬁnal stages of the morphologicaltransformation of infalling, formerly gas-rich galaxies, possibly caused by tidally inducedstar formation in dIrrs (Davies & Phillipps 1988) or similar mechanisms.Figure 1. Virgo cluster early-type dwarfs with recent or ongoing central star formation. Foreach galaxy, the combined image from three SDSS bands (g, r, and i) is shown (left), along withan unsharp mask image (middle) and a g − i colour map (right). The galaxies are, from left toright and from top to bottom: VCC 0170, 1512, 0781, 0173, 0281, 0870, 0021, 1488.ReferencesAdelman-McCarthy, J.K., Agu¨eros, M.A., Allam, S.S., et al. 2006, ApJS 162, 38.Binggeli, B., Sandage, A. & Tammann, G.A. 1985, AJ 90, 1681.Davies, J.I. & Phillipps, S. 1988, MNRAS 233, 553.Lisker, T., Glatt, K., Westera, P., & Grebel, E.K. 2006, AJ, in press (arXiv:astro-ph/0608294).317at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S174392130600682XDownloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 17:13:20, subject to the Cambridge Core terms of use, available

The Last Stages of Star Formation in dEs?

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