Constraints upon the Cosmological Density Parameter from Tully-Fisher Observations of IRAS Galaxies

Abstract

Mapping galaxy peculiar velocities, or departures from the uniform expansion of the universe, on megaparsec scales promises to indicate whether number density fluctuations in galaxy counts trace density fluctuations in the underlying matter (to within some factor b), and, if so, to constrain the cosmological density parameter Ω, which dictates whether space will expand forever or one day collapse. To sample the peculiar velocity field to distances approaching that of the Hydra-Centaurus complex (but in all directions) we have extracted an all sky, quasi volume limited sample to 4000 km sec-1 from the 1.9 Jy 60 micron IRAS redshift survey of Michael Strauss and collaborators. These objects enjoy distance predictions as functions of β ≡ Ω0.6/b derived from iterative correction of their redshifts for mutual peculiar gravity, which scales with β. Our volume limited sample most efficiently probes the cosmic flowfield and enjoys objective and uniform selection criteria whose effects upon inferred quantities can be probed with simulations. We have sought to measure relative distances to the 251 objects in this sample judged a priori suitable for use in the Tully-Fisher (luminosity - linewidth) relation. The requisite neutral hydrogen radio spectra and near infrared CCD photometry have been obtained for about one half of these objects. Methods of reducing the radio and CCD data are explained in detail. Isophotal I band magnitudes are reproduced by independent observations to ~ 0.05 magnitudes precision. Errors and biases in neutral hydrogen linewidths at low signal to noise ratio are quantified. The I band magnitude and 21 cm. linewidth data imply a Tully-Fisher (TF) relation at each value of β from 0.05 to 2.00 used by Strauss et al. to iterate the IRAS redshift catalog. The β value producing minimum apparent TF scatter is investigated as a diagnostic with extensive Monte Carlo simulations. Such simulations suggest that, if the β knob setting producing minimum apparent TF scatter were found to equal unity, ~ 100 galaxy distance moduli to ~ 0.5 magnitudes precision would constrain the actual value of β to lie within the range 0.7 ≲ β ≲ 1.3 at a ~ 95% confidence level. 'β response profiles' (apparent TF scatter versus β) observed in various subsets of our provisional database are presented. Our provisional database yields minimum apparent TF scatter at a β value of ~ 0.9. Excising the apparently problematic Virgo cluster region yields a response profile favoring β values of ~ 0.6 ± 0.2, or Ω ~ 0.4 ± 0.3. Excising data points disfavored by statistical estimates of leverage upon β likewise favors low values of β ~ 0.5. Limitations to our current data analysis which preclude our asserting strong constraints upon the density parameter at present are discussed in detail.