Young, metal-enriched cores in early-type dwarf galaxies in the Virgo cluster based on colour gradients

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
e-mail: linda@dwarfgalaxies.net
² Centre for Astrophysics and Supercomputing, Swinburne University, Hawthorn, VIC 3122, Australia
³ Division of Astronomy, Department of Physics, University of Oulu, PO Box 3000, 90014 Oulun Yliopisto, Finland
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Aix-Marseille Université, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13388 Marseille, France
⁶ Department of Astronomy & Center for Galaxy Evolution Research, Yonsei University, 03722 Seoul, Korea
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁸ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁹ Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
¹⁰ Department of Astrophysics, University of Vienna, Tuerkenschanzstr. 17, 1180 Vienna, Austria
¹¹ University of the Pacific, Department of Physics, 3601 Pacific Avenue, Stockton, CA 95211, USA
¹² Instituto de Astrofísica de Canarias, C/ Via Láctea s/n, 38200 La Laguna, Canary Islands, Spain
¹³ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹⁴ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK

Received: 30 March 2017
Accepted: 4 July 2017

Abstract

Early-type dwarf galaxies are not simply featureless, old objects, but were found to be much more diverse, hosting substructures and a variety of stellar population properties. To explore the stellar content of faint early-type galaxies, and to investigate in particular those with recent central star formation, we study colours and colour gradients within one effective radius in optical (g − r) and near-infrared (i − H) bands for 120 Virgo cluster early-type galaxies with − 19 mag <M_r< − 16 mag. Twelve galaxies turn out to have blue cores, when defined as g − r colour gradients larger than 0.10 mag/R_eff, which represents the positive tail of the gradient distribution. For these galaxies, we find that they have the strongest age gradients, and that even outside the blue core, their mean stellar population is younger than the mean of ordinary faint early-type galaxies. The metallicity gradients of these blue-cored early-type dwarf galaxies are, however, in the range of most normal faint early-type galaxies, which we find to have non-zero gradients with higher central metallicity. The blue central regions are consistent with star formation activity within the last few 100 Myr. We discuss whether these galaxies could be explained by environmental quenching of star formation in the outer galaxy regions while the inner star formation activity continued.