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On the Evolution of the Clustering of Galaxies

Charles William Shepherd

Doctor of Philosophy 2002
Graduate Department of Astronomy and Astrophysics, University of Toronto

Explaining the origins and evolution of the structure seen in the local universe is one of the main challenges of modern astronomy. Observations of the evolution of galaxy clustering are a primary constraint on any model of structure formation. How this evolution depends on galaxy properties, however, has not yet been comprehensively investigated.

In this work, the dependence of galaxy clustering on spectral energy distribution (SED), evolution-compensated RC-band absolute magnitude M0RC, and redshift z is investigated within a sample of ~3000 galaxies from the CNOC2 redshift survey. Techniques for estimating the galaxy correlation function and its uncertainty from survey data spanning a wide range of redshifts are developed.

Galaxies with early-type SEDs, M0RC<-20, and 0.12<z<0.47 are ~2.4 times more strongly clustered and have a steeper correlation function slope than late-type galaxies in the same magnitude and redshift range. The relative bias between the two samples is consistent with that observed for nearby galaxies.

The comoving correlation amplitude is moderately sensitive to galaxy luminosity, with brighter galaxies being more strongly clustered, independently of SED. The luminosity dependence of the correlation amplitude for galaxies with 0.12<z<0.40 is similar to that seen in local surveys.

The correlation amplitude for galaxies of all SED types and M0RC<-20 is independent of redshift for 0.12<z<0.47. This is consistent both with the predictions of the biased structure formation scenario and with earlier observations. These results would seem to indicate that the clustering of galaxies has changed little during the past 3 Gyr$.

The correlation amplitudes of the early- and late-type samples evolve remarkably rapidly, however, with both samples becoming less clustered with time. Galaxy mergers, luminosity evolution, and SED evolution are considered as possible causes of the clustering evolution. While the apparent evolution of the clustering in the SED-selected samples is probably caused by evolution of the properties of individual galaxies, it is clear that the situation is somewhat more complex than is indicated by the results described above.

Reproduced with permission by Marlene Cummins library@astro.utoronto.ca
September 24 2002