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; 13h] and DEC=
[+40deg; +60deg]. 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 (2011), 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 2.10^12 Msol, and virial- mass-to-K-band-luminosity of 27Msol/Lsol.
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 Virgo cluster. The
total virial mass of the UMa groups is 4.10^13 Msol, yielding the average
density of dark matter in the UMa cloud to be Omega_m = 0.08, i.e. a factor
three 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 toward a massive attractor. This constrains
the total amount of dark matter between the UMa groups within the cloud volume.Comment: correction of a typo in the abstract, 18 pages, 2 figures. accepted
for MNRAS, nov 26, 201
