Anatomy of Ursa majoris

Abstract

A nearby friable cloud in Ursa Majoris contains 270 galaxies with radial velocities 500 < VLG < 1500 km s-1 inside the area of RA = [11h. 0, 13h. 0] and Dec. = [ + 40°, +60°]. At present, 97 galaxies of them have individual distance estimates. We use these data to clarify the structure and kinematics of the UMa complex. According to Makarov & Karachentsev, most of the UMa galaxies belong to seven bound groups, which have the following median parameters: velocity dispersion of 58 km s-1, harmonic projected radius of 300 kpc, virial mass of 2 × 1012M⊙ and virial mass-to-K-band luminosity ratio of 27M⊙/L⊙. Almost a half of the UMa cloud population are gas-rich dwarfs (Ir, Im, BCD) with active star formation seen in the GALEX UV-survey. The UMa groups reside within 15-19 Mpc from us, being just at the same distance as the Virgo cluster. The total virial mass of the UMa groups is 4 × 1013M⊙, yielding the average density of dark matter in the UMa cloud to be ωm = 0.08, i.e. a factor of 3 lower than the cosmic average. This is despite the fact that the UMa cloud resides in a region of the Universe that is an apparent overdensity. A possible explanation for this is that most mass in the Universe lies in the empty space between clusters. Herewith, the mean distances and velocities of the UMa groups follow nearly undisturbed Hubble flow without a sign of the 'Z-wave' effect caused by infall towards a massive attractor. This constrains the total amount of dark matter between the UMa groups within the cloud volume. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.