Tidal dwarfs in the M81 group: The second generation? ^*,**

¹ Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz 369167, Karachaevo-Cherkessia, Russia e-mail: lidia@sao.ru, ikar@luna.sao.ru
² Isaac Newton Institute of Chile, SAO Branch, Russia
³ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Kitt Peak National Observatory, National Optical Astronomy Observatories, PO Box 26732, Tucson, AZ, 85726, USA
⁵ Astronomical Observatory of Kiev University, 04053, Observatorna 3, Kiev, Ukraine
⁶ Departamento de Fisica, Grupo de Astronomia, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁷ Herzberg Fellow, Herzberg Institute of Astrophysics, 5071 W. Saanich Road, Victoria, B.C. V9E 2E7, Canada
⁸ Permanent address: UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
⁹ Department of Astronomy, University of Washington, PO Box 351580, Seattle, WA 98195, USA
¹⁰ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
¹¹ Department of Astronomy, University of Michigan, 830 Dennison Building, Ann Arbor, MI 48109, USA

Corresponding author: L. N. Makarova, lidia@sao.ru

Received: 25 July 2002
Accepted: 24 September 2002

Abstract

We derive quantitative star formation histories of the four suspected tidal dwarf galaxies in the M 81 group, Holmberg IX, BK3N, Arp-loop (A0952+69), and Garland, using Hubble Space Telescope/Wide Field Planetary Camera 2 images in F606W and F814W obtained as part of a Snapshot survey of dwarf galaxies in the Local Universe. We consider the spatial distribution and ages of resolved stellar populations in these dwarf irregular galaxies. We use synthetic color-magnitude diagrams to derive the ages of the major star formation episodes, star formation rates, and approximate metallicity ranges. All the galaxies show evidence of continuous star formation between about 20 and 200 Myr ago with star formation rates in the range – yr^-1. The metallicity of the detected stars spans a wide range, and have lower than solar abundance. A possible scenario is that all four dwarf galaxies were formed from material in the metal-poor outer part of the giant spiral galaxy M 81 after the tidal interaction between M 81, M 82, and NGC 3077  Myr ago. While we do not directly detect pronounced old stellar populations, the photometric limits of our data are such that the presence of such a population is not entirely ruled out.