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I.D. Karachentsev (SAO), V.E. Karachentseva (U Kiev), A.E. Dolphin (NOAO), D. Geisler (U Concepción), E.K. Grebel (U Wash), P. Guhathakurta (UCSC), P.W. Hodge (U Wash), A. Sarajedini (Wesleyan U), P. Seitzer (U Mich), M.E. Sharina (SAO)
The bright spiral galaxies M81 and NGC 2403 and their dwarf companions are two of the closest galaxy groups to our own Local Group. The Cepheid distance moduli of M81 and NGC 2403 are (27.80±.20) and (27.51±.24) mag (Freedman et al.\ 1994, Freedman & Madore 1988).
We obtained Hubble Space Telescope WFPC2 images of the dwarf spheroidal galaxies K61, K63, K64, DDO 78, BK6N, kk077 in the M81 group and of DDO 44 in the NGC2403 group. The resulting CMDs show the red giant branch with tips in the range of I(TRGB)= [23.5 -- 24.0] mag. The derived true distance moduli of the six M81 dSphs range from 27.55 to 27.95 mag, consistent with their membership in this group. The TRGB distance modulus of DDO 44, (27.52±.15) mag, confirms it as a companion of NGC\,2403. Taking into account the TRGB distances derived for M82 (Sakai & Madore 1999), for the dSphs BK5N and F8D1 (Caldwell et al.\ 1998), and the BCD UGC6456 (Lynds et al.\ 1998), we obtain a mean distance modulus of (27.82 ±.06) mag for the M81 group. The standard deviation of the individual moduli is 0.17 mag.
We find the difference of the TRGB distances to the two the groups to be (0.47±.25) Mpc. {From} a projected separation of M81 and NGC 2403 of 0.83 Mpc follows a deprojected distance of 0.95 Mpc. With respect to the Local Group, M81 and NGC 2403 have radial velocities of 106 km/s and 216 km/s, while the velocities of the group centroids are 216 km/s and 275 km/s, respectively. The higher velocity of the closer group may indicate that the two groups are moving towards each other.
IDK and EKG are supported by the Henri Chrétien International Research Grant administered by the American Astronomical Society.
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