The Local Group and other neighboring galaxy groups

Abstract

Over the last few years, rapid progress has been made in distance measurements for nearby galaxies based on the magnitude of the tip of red giant branch stars. Current CCD surveys with HST and large ground- based telescopes bring $\sim$10%-accurate distances for roughly a hundred galaxies within 5 Mpc. The new data on distances to galaxies situated in (and around) the nearest groups: the Local Group, M81 group, CenA/M83 group, IC342/Maffei group, Sculptor filament, and Canes Venatici cloud allowed us to determine their total mass from the radius of the zero- velocity surface, $R_0$, which separates a group as bound against the homogeneous cosmic expansion. The values of $R_0$ for the virialized groups turn out to be close each other, in the range of 0.9 -- 1.3 Mpc. As a result, the total masses of the groups are close to each other, too, yielding total mass-to-blue luminosity ratios of 10 -- 40 $M_{\sun}/L_{\sun}$. The new total mass estimates are 3 -- 5 times lower than old virial mass estimates of these groups. Because about half of galaxies in the Local Volume belong to such loose groups, the revision of the amount of dark matter (DM) leads to a low local density of matter, $Ω_m \simeq 0.04$, which is comparable with the global baryonic fraction $Ω_b$, but much lower than the global density of matter, $Ω_m = 0.27$. To remove the discrepancy between the global and local quantities of $Ω_m$, we assume the existence of two different DM components: 1) compact dark halos around individual galaxies and 2) a non-baryonic dark matter ``ocean'' with $Ω_{dm1} \simeq 0.07$ and $Ω_{dm2} \simeq 0.20$, respectively.