Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992pasp..104..976c&link_type=abstract
Publications of the Astronomical Society of the Pacific, v.104, p.976
Astronomy and Astrophysics
Astronomy
2
Scientific paper
This dissertation consists of a study of large-scale velocity deviations in a uniformly expanding universe. At the onset of this work, galaxy motions in the northern hemisphere, contrary to the impressive sky coverage in the south, were still very poorly mapped. To improve on this situation, we identified a well-defined sample of 380 northern Sb-Sc UGC spirals with a magnitude cutoff of m_B=15.5. Distances are estimated using the Tully-Fisher relation but with rotation velocities measured from long-slit spectra in lieu of 21 cm delta-V's. Surface-brightness profiles and total magnitudes are extracted for 820 r-band images, and (H-alpha +[N II]) rotation curves, fluxes, and rotation profiles are measured from 450 red spectra. The galaxy magnitudes are accurate internally to +-0.022 mag and the delta-V's agree internally to +- 12 km s^-1. A tight linear correlation of optical versus radio velocity widths with a scatter of 24 km s^-1 is also derived. The redshift distribution peaks at 5400 km s^-1, with a tail extending out to 15,000 km s^-1. We find evidence for large-scale motions near the concentration of Perseus-Pisces (PP), but identify a vast region of unperturbed, quiet Hubble flow (QHF) perpendicular to PP. The peculiar velocity (PV) data are corrected for uniform-density Malmquist bias when they are analyzed in estimated distance space; an empirical correction for selection effects mainly due to magnitude cutoff is derived from the QHF region and applied to the PV's in redshift space. Both analyses are consistent in showing the PP supercluster as streaming in bulk towards the Local Group at 392 +- 79 km s^-1 (with respect to the microwave background rest frame). The cause for such motion remains undetermined, but it is most certainly not due to one single object. Gravitational collapse within PP is detected, but the statistical error is large. While we have not calculated a formal bulk motion for the whole volume of space bounded by the Great Attractor and PP, it appears, from simple considerations, that a reasonable streaming solution will have a net amplitude of 450 +- 100 km s^-1 in the general direction of l=310 degrees, b=+20 degrees. We show that the Rubin-Ford effect (using Schechter's non-diameter-corrected solution) is perfectly consistent with our estimated rough bulk streaming. However, comparison of our measured PV's with radial-velocity reconstructions using IRAS galaxies shows conflicting results at PP which remain, at present, unresolved. An emerging picture of large-scale velocity fields shows the main infall regions to be correlated with the observed clumpiness of luminous matter. This provides support in favor of models based on the gravitational instability picture. Our data, combined with published PV surveys, provide strong support for the existence of low-amplitude perturbations in the universe on scales not yet probed by current samples (> 120 h^-1 Mpc). The implications of cosmic bulk motions with such large coherence lengths pose a serious challenge for models of generation of structure in the universe. (SECTION: Dissertation Abstracts)
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