THE SURFACE BRIGHTNESS FLUCTUATION SURVEY OF GALAXY DISTANCES. II. LOCAL AND LARGE-SCALE FLOWS

摘要

We present results from the Surface Brightness Fluctuation (SBF) Survey for the distances to 300 early-type galaxies, of which approximately half are ellipticals. A modest change in the zero point of the SBF relation, derived by using Cepheid distances to spirals with SBF measurements, yields a Hubble constant Ho = 77 ± 4 ± 7 km s~-1 Mpc~-1, somewhat larger than the HST Key Project result. We discuss how this difference arises from a different choice of zero point, a larger sample of galaxies, and a different model for large-scale flows. Our result is 4/100 larger than found in a recent comparison of the SBF Survey peculiar velocities with predictions derived from the galaxy density field measured by red- shift surveys (Blakeslee et al. 1999b). The zero point of the SBF relation is the largest source of uncer- tainty, and our value for H_o is subject to all the systematic uncertainties of the Key Project zero point, including a 5/100 decrease if a metallicity correction for the Cepheids is adopted. To analyze local and large-scale flows-departures from smooth Hubble flow-we use a parametric model for the distribution function of mean velocity and velocity dispersion at each point in space. These models include a uniform thermal velocity dispersion and spherical attractors whose position, amplitude, and radial shape are free to vary. Our modeling procedure performs a maximum likelihood fit of the model to the observations. Our models rule out a uniform Hubble flow as an acceptable fit to the data. Inclusion of two attractors, one of which having a best-fit location coincident with the Virgo cluster and the other having a fit loca- tion slightly beyond the Centaurus clusters (which we refer to by convention as the Great Attractor), reduces X~2/N from 2.1 to 1.1. The fits to these attractors both have radial profiles such that υ ≈ r~-1 (i.e., isothermal) over a range of overdensity between about 10 and 1, but fall off more steeply at larger radius. The best-fit value for the small-scale, cosmic thermal velocity is 180 ±14 km s~-1. The quality of the fit can be further improved by the addition of a quadrupole correction to the Hubble flow. The dipole velocity offset from the CMB frame for the volume we survey (amplitude ～150 km s~-1) and the quadrupole may be genuine (though weak) manifestations of more distant density fluctuations, but we find evidence that they are more likely due to the inadequacy of spherical models to describe the density profile of the attractors. The residual dipole we find is comparable to the systematic error in these simple, parametrized models; in other words, our survey volume of R < 3000 km s~-1 is, in a mass aver- aged sense, essentially at rest with respect to the CMB. This contradicts claims of large amplitude flows in much larger volumes that include our sample. Our best-fitting model, which uses attenuated power- law mass distributions for the two attractors, has enclosed mass overdensities at the Local Group of 7 × 10~14 M⊙ for the Virgo Attractor and 9 × 10~15 M⊙ for the Great Attractor. Without recourse to information about the overdensities of these attractors with respect to the cosmic mean we cannot provide a good constraint on Ω_M, but our data do give us accurate measurements in terms of δ, the overdensities of the enclosed masses with respect to the background : δΩ_M~2/3 = 0.33 for the Virgo Attrac- tor and δ Ω_M~2/3 = 0.27 for the Great Attractor.