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Astron. Astrophys. 363, 117-129 (2000)


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Dwarf spheroidal galaxies in the M81 group imaged with WFPC2 *

I.D. Karachentsev 1, V.E. Karachentseva 2, A.E. Dolphin 3, D. Geisler 4, E.K. Grebel **  5,6, P. Guhathakurta ***  7, P.W. Hodge 5, A. Sarajedini 8, P. Seitzer 9 and M.E. Sharina 1

1 Special Astrophysical Observatory, Russian Academy of Sciences, N.Arkhyz, KChR, 357147, Russia
2 Astronomical Observatory of Kiev University, 04053, Observatorna 3, Kiev, Ukraine
3 Kitt Peak National Observatory, National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ 85726, USA
4 Departmento de Fisica, Grupo de Astronomia, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile
5 Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA
6 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
7 UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
8 Astronomy Department, Wesleyan University, Middletown, CT 06459, USA
9 Department of Astronomy, University of Michigan, 830 Dennison Building, Ann Arbor, MI 48109, USA

Received 17 May 2000 / Accepted 22 August 2000

Abstract

We obtained HST/WFPC2 images of the dwarf spheroidal (dSph) galaxies K61, K63, K64, DDO78, BK6N, and kk77 in the M81 group. Our color-magnitude diagrams show red giant branches with tips (TRGB) falling within the range of [FORMULA] mag. The derived true distance moduli (DM) of the 6 dSphs ranging from 27.71 to 27.93 mag are consistent with their membership in the group. Given accurate distances of 5 other group members, which have been derived via TRGB or cepheids, the mean DM of the M81 group is [FORMULA] mag. We find the difference of the mean distances to the M81 and NGC 2403 groups to be [FORMULA] Mpc, which yields a deprojected separation of 0.9 Mpc. With respect to the Local group, M81 and NGC 2403 have radial velocities of 106 and 267 km s-1 respectively, while the velocities of the group centroids are 142 and 281 km s-1. The higher velocity of the closer system may indicate that these groups are moving towards each other, similar to the Milky Way and M31 in the Local group. Several globular cluster candidates have been identified in the galaxies.

Key words: galaxies: dwarf – galaxies: individual: M 81 group – galaxies: photometry

* Based on observations made with the NASA/ESA Hubble Space Telescope. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.
** Hubble Fellow
*** Alfred P. Sloan Research Fellow

Send offprint requests to: I.D. Karachentsev (ikar@luna.sao.ru)

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000

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