**The Hubble Flow From Brightest Cluster Galaxies.***
T. R. Lauer & M. Postman.*
(1992), ApJ, 400, L47.
Journal Article

Initial results are presented from a reinvestigation of the Brightest Cluster Galaxy (BGC) distance scale, which includes richer sampling and complete all-sky observations of Abell clusters within 15,000 km/s. Virgo BCG, NGC 4472, is used to calibrate the BCG Hubble diagram directly, independent of the Virgocentric infall pattern. For the observed distance to NGC 4472 of 14.4 Mpc, Ho = 77 +/- 8 km/s Mpc. Alternatively, if NGC 4472 is at the Sandage and Tammann (1990) 21.9 Mpc distance to Virgo, then Ho = 51 +/- 5. The BCG Hubble constant on either the short or long system is consistent with Hubble constants measured on the same system within the local supercluster. Plausible high values of Ho observed within the local supercluster therefore cannot be explained as biased measures of the true Ho due to velocity anomalies induced by large-scale structure.

The distribution of axis ratios presently computed for a sample of brightest cluster galaxies (BCGs) is found to not vary greatly from a similar distribution for a sample of ordinary ellipticals. A significant difference is noted, however, between the distribution of axis ratios for a sample of BCGs and for a sample of second-brightest cluster ellipticals. It is also noted, in partial support for a hypothesized competition of BCGs for cluster galaxies that would otherwise merge with the second-brightest cluster ellipticals, that the shape of a merger remnant can reflect the relative masses of the progenitor as well as the angular momentum of the merging system.

We have measured the velocity of the Local Group with respect to an inertial frame defined by the 119 Abell and Abell, Corwin, & Olowin (ACO) clusters contained within 15,000 km/s. The observations consist of a full-sky peculiar velocity survey with an effective depth ranging from 8000 to 11,000 km/s, depending on how the observations are weighted with redshift. This is the deepest peculiar velocity survey yet conducted. Clusters are selected by heliocentric redshift, and the sample is volume-limited. We use the Hoessel (1980) relationship between the metric luminosities of the brightest clusters galaxies (BCGs) and the slope of their brightness profiles as the distance indicator. The Cousins R-band luminosity within a metric radius of 10/h kpc yields a typical distance error of 16% for a single BCG. We test for convergence of the local flow on scales within 10,000 km/s (i.e., alignment of the Local Group velocity vector with the cosmic microwave background (CMB) dipole), but we find that our sample is not at rest with respect to the CMB. The velocity of the local Group relative to the Abell cluster sample is 561 +/- 284 km/s toward l = 220 deg, b = -28 deg (+/-27 deg), using optimal redshift weighting. This vector is inconsistent with the Local Group absolute space velocity inferred from the CMB dipole anisotropy at 99.99% confidence. An extensive error analysis has been conducted to validate this result. The result is extremely robust and is insensitive to Galactic extinction, velocity biases, sample composition, and geometry. If the CMB dipole is kinematic in origin, then this result implies that the Abell cluster fram itself is moving at 689 +/- 178 km/s toward l = 343 deg, b = +52 deg (+/- 23 deg), and that the CMB dipole is generated largely by mass concentrations beyond 100/h Mpc.

We investigate the use of brightest cluster galaxies (BCGs) as standard candles for measuring galaxy peculiar velocities on large scales. We have obtained precise large-format CCD surface photometry and redshifts for an all-sky, volume-limited (z less than or = 0.05) sample of 199 BCG. We reinvestigate the Hoessel (1980) relationship between the metric luminosity, L(sub m), within the central 10 kpc/h of the BCGs and the logarithmic slope of the surface brightness profile, alpha. The L(sub m)-alpha relationship reduces the cosmic scatter in L(sub m) from 0.327 mag to 0.244 mag, yielding a typical distance accuracy of 17% per BCG. Residuals about the L(sub m)-alpha relationship are independent of BCG luminosity, BCG B - R(sub c) color, BCG location within the host cluster, and richness of the host cluster. The metric luminosity is independent of cluster richness even before correcting for its dependence on alpha, which provides further evidence for the unique nature of the BCG luminosity function. Indeed, the BCG luminosity function, both before and after application of the alpha-correction, is consistent with a single Gaussian distribution. Half the BCGs in the sample show some evidence of small color gradients as a function of radius within their central 50 kpc/h regions but with almost equal numbers becoming redder as becoming bluer. However, with the central 10 kpc/h the colors are remarkably constant -- the mean B - R(sub c) color is 1.51 with a dispersion of only 0.06 mag. The narrow photometric and color distributions of the BCGs, the lack of 'second-parameter' effects, as well as the unique rich cluster environment of BCGs, argue that BCGs are the most homogeneous distance indicators presently available for large-scale structure research.

Lauer and Postman (LP) observed that all Abell clusters with redshifts less than 15,000 km/s appear to be participating in a bulk flow of 689 km/s with respect to the cosmic microwave background. We find this result difficult to reconcile with all popular models for large-scale structure formation that assume Gaussian initial conditions. This conclusion is based on Monte Carlo realizations of the LP data, drawn from large particle-mesh N-body simulations for six different models of the initial power spectrum (standard, tilted, and Omega(sub 0) = 0.3 cold dark matter, and two variants of the primordial baryon isocurvature model). We have taken special care to treat properly the longest-wavelength components of the power spectra. The simulations are sampled, 'observed,' and analyzed as identically as possible to the LP cluster sample. Large-scale bulk flows as measured from clusters in the simulations are in excellent agreement with those measured from the grid: the clusters do not exhibit any strong velocity bias on large scales. Bulk flows with amplitude as large as that reported by LP are not uncommon in the Monte Carlo data stes; the distribution of measured bulk flows before error bias subtraction is rougly Maxwellian, with a peak around 400 km/s. However the chi squared of the observed bulk flow, taking into account the anisotropy of the error ellipsoid, is much more difficult to match in the simulations. The models examined are ruled out at confidence levels between 94% and 98%.

We model the surface brightness profiles of a sample of 119 Abell brightest cluster galaxies (BCGs), finding a generalized de Vaucouleurs R^1/n^ law, where n is a free parameter, to be appropriate. Departures from the R^1/4^ law are shown to be a real feature of galaxy profiles and not due to observational errors or coupling of n with the other model parameters. BCGs typically have values of n greater than 4. The shape parameter n is shown to correlate with effective half-light radius, such that the larger BCGs have larger values of n. This continues a trend noticed amongst ordinary elliptical galaxies and dwarf elliptical galaxies, such that the brighter galaxies have larger values of n.

**Return to:**
**[** Project Warpfire Homepage **]**