The M 81 group of galaxies: New distances, kinematics and structure

We present Hubble Space Telescope/WFPC2 images of the galaxies NGC 2366, NGC 2976, NGC 4236, IC 2574, DDO 53, DDO 82, DDO 165, Holmberg I, Holmberg II, Holmberg IX, K52, K73, BK3N, Garland, and A0952+69 in the M 81 complex. Their true distance moduli, derived from the brightness of the tip of the red giant branch, lie in the range of 27fm 52 (NGC 2366) to 28fm 30 (DDO 165), with a median of 27fm 91, which is typical for other known M 81 group members. Using distances and radial velocities of about 50 galaxies in and around the M 81/NGC 2403 complex, we find the radius of the zero-velocity surface of the M 81 group to be R_0 = (1.05+/-0.07) Mpc, which yields a total mass M(R_0) = (1.6+/-0.3)x 10¹² M_sun and a total mass-to-luminosity ratio M(R_0)/L_B = (38+/-7) M_sun/L_sun. The total mass within R_0 agrees well with the sum of masses estimated via the virial theorem (1.2x 10¹² M_sun) and from orbital motions (2.0x 10¹² M_sun) of companions around M 81 and NGC 2403. We suggest that most of the dark matter in the group is concentrated around the luminous matter, allowing us to explain the observed asymmetry of the peculiar motions of the M 81 companions. M 81 itself has a peculiar velocity of about 130 km s^-1 with respect to the local Hubble flow, but the centroid of the M 81/NGC 2403 complex is almost at rest with respect to Hubble flow (v_pec < 35 km s^-1). 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. Figures 2 to 5 are only available in electronic form at http://www.edpsciences.org