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 stars on the tip of the red giant branch. Current CCD surveys with the Hubble Space Telescope (HST) and large ground-based telescopes bring ∼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, Cen A/M83 Group, IC 342/Maffei Group, Sculptor filament, and Canes Venatici cloud—allowed us to determine their total mass from the radius of the zero-velocity surface, R0, which separates a group as bound against the homogeneous cosmic expansion. The values of R0 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, as well, yielding total mass to blue luminosity ratios of 10–40 M⊙ L. 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, Ωm ≃ 0.04, which is comparable with the global baryonic fraction Ωb but much lower than the global density of matter, Ωm = 0.27. To remove the discrepancy between the global and local quantities of Ωm, we assume the existence of two different DM components: (1) compact dark halos around individual galaxies and (2) a nonbaryonic dark matter "ocean" with ΩDM1 ≃ 0.07 and ΩDM2 ≃ 0.20, respectively.