Dwarf spheroidals in the M 81 group – Metallicity distribution functions and population gradients

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstrasse 12-14, 69120 Heidelberg, Germany e-mail: [lianou;grebel]@ari.uni.heidelberg.de
² University of Leicester, Department of Physics and Astronomy, University Road, LE1 7RH Leicester, UK e-mail: ak326@astro.le.ac.uk

Received: 28 September 2009
Accepted: 25 February 2010

Abstract

Aims. We study the dwarf spheroidal galaxies in the nearby M 81 group in order to construct their photometric metallicity distributions and to investigate the potential presence of population gradients. We select all the dwarf spheroidals with available Hubble Space Telescope / Advanced Camera for Surveys archival observations, nine in total.

Methods. We interpolate isochrones so as to assign a photometric metallicity to each star within a selection box in the color-magnitude diagram of each dwarf galaxy. We assume that the dwarf spheroidals contain mainly an old stellar population. In order to search for metallicity gradients, we examine the spatial distribution of two stellar populations that we separate according to their metallicities.

Results. As a result, we present the photometric metallicity distribution functions, the cumulative histograms and smoothed density maps of the metal-poor and metal-rich stars as well as of the intermediate-age stars.

Conclusions. From our photometric data we find that all the dwarf spheroidals show a wide range in metallicities, with mean values that are typical for old and metal-poor systems, with the exception of one dwarf spheroidal, namely IKN. Some of our dwarf spheroidals exhibit characteristics of transition-type dwarfs. Compared to the Local Group transition type dwarfs, the M 81 group ones appear to have mean metallicity values slightly more metal-rich at a given luminosity. All the dwarf spheroidals considered here appear to exhibit either population gradients or spatial variations in the centroids of their metal-poor and metal-rich population. In addition, there are luminous AGB stars detected in all of them with spatial distributions suggesting that they are well mixed with the old stars.