Detecting clusters of galaxies in the Sloan Digital Sky Survey.

摘要

Clusters of galaxies are the most massive gravitationally bound systems known in this universe, and have been invaluable tools for numerous cosmological studies. The Sloan Digital Sky Survey (SDSS) which will in due course map a quarter of the entire sky, will not only produce redshifts of the nearby 106 galaxies, but also five-color images that go as deep as z > 0.5. Our goal is to produce a catalog of SDSS galaxy clusters to z ∼ 0.5, and this thesis is that first step: a solid groundwork for detecting SDSS clusters with various automated cluster detection algorithms. We present a comparison of three different automated cluster finding algorithms using a Monte Carlo simulation technique: The Matched Filter (MF; Postman et al. 1996), the Adaptive Matched Filter (AMF; Kepner et al. 1999) and the Voronoi Tessellation Technique (VTT). We propose a Hybrid method that combines the two Matched Filters (HMF) for optimal results. Although the HMF and VTT exhibit slight differences with respect to the background fluctuations, we find that they yield similar selection functions, when optimal thresholds are chosen. We then present a catalog of ∼1200 clusters detected with the above methods in 150 deg2 of commissioning data, which is already the largest cluster catalog to date reaching uniformly to z = 0.5. We have visually inspected all candidates and find the detection algorithms to have an 80% efficiency on average, and 90% when detected by two or more methods simultaneously. We present cluster redshifts available for a third of the sample, and compare various cluster characteristics according to which method has detected them; we find that the clusters detected by the VTT have considerably shallower density profiles, which is due to the different constraints each method imposes for detection. Finally, we investigate the alignment effect of Brightest Cluster Galaxies (BCG) with their host cluster. We find that the BCGs that are dominant within the cluster exhibit a strong alignment signal, whereas less dominant BCGs clearly do not, suggesting the alignment mechanism to be strongly tied with the process which makes the BCG dominant, possibly through direct galaxy mergers. We conclude with presenting several ongoing and future projects related to SDSS clusters.