Abstract
Aims. We use accurate data on distances and radial velocities of galaxies around the Local Group, as well as around 14 other massive nearby groups, to estimate their radius of the zero-velocity surface, R0, which separates any group against the global cosmic expansion. Methods. Our R0 estimate was based on fitting the data to the velocity field expected from the spherical infall model, including effects of the cosmological constant. The reported uncertainties were derived by a Monte Carlo simulation. Results. Testing various assumptions about a location of the group barycentre, we found the optimal estimates of the radius to be 0.91 ± 0.05 Mpc for the Local Group, and 0.93 ± 0.02 Mpc for a synthetic group stacked from 14 other groups in the Local Volume. Under the standard Planck model parameters, these quantities correspond to the total mass of the group ~ (1.6 ± 0.2) × 1012M⊙. Thus, we are faced with the paradoxical result that the total mass estimate on the scale of R0 ≈ (3−4)Rvir is only 60% of the virial mass estimate. Anyway, we conclude that wide outskirts of the nearby groups do not contain a large amount of hidden mass outside their virial radius.
Highlights
Any overdense region in the Universe is driven by the competition between its self-gravity and the cosmic expansion, and can be characterized by an idealized zero-velocity surface that separates these zones. de Vaucouleurs (1958, 1964, 1972) presupposed systematic deviations from linearity in the velocity-distance relation and interpreted these deviations as a local phenomenon caused by the Virgo complex
For a typical galaxy of the Local Volume with a distance of ∼6 Mpc, the the red giant branch stars (TRGB) distance error of ∼300 kpc is comparable with a virial radius of the group, its location can be confidently fixed relative to the group centroid and
Galaxies in the infall zone between the virial radius and the R0 are relatively small in number, ∼15%
Summary
Any overdense region in the Universe is driven by the competition between its self-gravity and the cosmic expansion, and can be characterized by an idealized zero-velocity surface that separates these zones. de Vaucouleurs (1958, 1964, 1972) presupposed systematic deviations from linearity in the velocity-distance relation and interpreted these deviations as a local phenomenon caused by the Virgo complex. Lynden-Bell (1981) and Sandage (1986) focussed on the Local Group of galaxies. They showed that, in the simplest case of the spherically symmetric system in the empty Universe with Λ = 0, the radius of the zero-velocity surface R0 and the total mass of the group MT0 are related as MT0 = (π2/8G) × R30 × T0−2,. For a typical galaxy of the Local Volume with a distance of ∼6 Mpc, the TRGB distance error of ∼300 kpc is comparable with a virial radius of the group, its location can be confidently fixed relative to the group centroid and. We use the most complete data on distances and radial velocities of the Local Volume galaxies to estimate the zerovelocity radius around the local massive galaxies
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