A distance scale from the infrared magnitude/H I velocity-width relation. III - The expansion rate outside the local supercluster

Details A distance scale from the infrared magnitude/H I velocity-width relation. III - The expansion rate outside the local supercluster A distance scale from the infrared magnitude/H I velocity-width relation. III - The expansion rate outside the local supercluster

Jul 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980apj...239...12a&link_type=abstract

Astrophysical Journal, Part 1, vol. 239, July 1, 1980, p. 12-37. Research supported by the University of Washington

Astronomy and Astrophysics

Astronomy

156

Cosmology, Decimeter Waves, Galactic Clusters, Hubble Constant, Infrared Astronomy, Microwave Spectra, Radial Velocity, Astronomical Photometry, Distance, Elliptical Galaxies, Error Analysis, Hubble Diagram, Hydrogen Atoms, Red Shift, Relic Radiation, Spectral Line Width, Spiral Galaxies, Tables (Data)

Scientific paper

Infrared magnitudes and 21 cm H I velocity widths are presented for galaxies in the Pegasus I cluster (V ≍ 4000 km s-1), the Cancer cluster (V ≍ 4500 km s-1), cluster Zwicky 1400.4 ± 0949 (Z74-23) (V ≍ 6000 km s-1), and the Perseus supercluster (V ≍ 5500 km s-1). The data are used to determine redshift-independent distances from which values of the Hubble ratio can be derived. With a zero point based solely on the Sandage-Tammann distances to M3 1 and M33, the following results are obtained (zero-point error excluded):
Pegasus I.--r = 42 ± 4 Mpc, V/r = 91 ± 8 km s-1 Mpc-1;
Cancer.--r = = 49 ± 6 Mpc, V/r = 89 ± 11 km s-1 Mpc-1;
Z74-23.--r = 6l ± 4 Mpc, V/r = 96 ± 7 km s-1 Mpc-1;
Perseus supercluster.--r = 53 ± 2 Mpc, V/r = 104 ± 6 km s-1 Mpc-1;
The closely similar value of the Hubble ratio found in the four independent samples suggests that the zero-point calibration in the IR/H I technique does not depend on environment. The difference between the mean of these Hubble ratios, V/r = 95 ± 4 km s-1 Mpc -1, and that measured for Virgo in Paper II, V/r = 65 ±4 km s-1 Mpc-1, is significant at a formal level of 5 σ.
The simplest explanation of the discrepancy is to postulate a Local Group component of motion in the direction of Virgo. The resulting velocity perturbation is ΔV = 480 ± 75 km s-1. This value agrees well with recent observations of a dipole term in the 3 K microwave background, the only other anisotropy test for which a detection significance of 5 σ or more is claimed. We are thus led to a preliminary estimate for the value of the Hubble constant of H0 = 95 ± 4 km s-1 Mpc-1. If a zero point based on de Vaucouleurs's distances to M31 and M33 is adopted instead, all distances decrease by , and the Hubble constant increases by a similar amount.
A variety of possible systematic errors which might affect the present conclusions are investigated, but we can find none that are relevant. In particular, because the galaxy samples are chosen from a cluster population which is generally all at the same distance, Malmquist bias does not occur. In fact, two of the clusters (Pegasus I and Z74-23) are sampled in both magnitude and velocity width to a level as deep as Virgo itself.
Other observational data related to the value of H0 are examined, as are a number of previously used anisotropy tests, including color-luminosity relations, brightest cluster member(s), central surface brightnesses, and supernovae. We find that some of these tests support the present results, while contrary evidence is currently weak.
A model in which Virgo gravitationally retards the Hubble flow of galaxies within the Local Supercluster provides a natural interpretation of our findings. A range of 1.5-3 in local density contrast then leads to a value of the density parameter Ω ≍ 0.7-0.2. The deceleration parameter q0 is then 0.35-0.1 for a simple Friedmann-type expanding universe.

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