Using radio sources to find galaxy clusters
Gay, Pamela Lynn
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In this work, we studied the feasibility of using radio source overdensities to find moderate redshift galaxy clusters. We define an overdensity as five NVSS radio sources contained in a 6-arcmin × 6-arcmin box, and find a lower-limit cluster finding rate of 21%. The clusters we identified have low to moderate redshifts (z < 0.5), are observationally average when compared to clusters from the literature, have low richnesses, and have an estimated two physically associated radio sources. While creating comparison samples from the literature we noted two significant trends: (1) the radio source R-z relation breaks down with decreasing S1.5 values and (2) the B-O effect appears to be a function of both redshift and cluster richness. We find that the brightest radio sources in the sky, those drawn from the 3CR and 6C catalogues, have a different relationship between redshift and R magnitude than lower flux sources (S1.5 < 200 mJy). Furthermore, sources drawn from the NVSS catalogue show increased scatter in their R-z relationship with decreasing flux, and the high resolution high sensitivity FIRST catalogue shows only a modest R-z relationship. We feel that only sources with S1.5 ≥ 10 mJy from the NVSS catalogue can be used with any reliability to constrain the redshift of a radio-optical source. The observed dependence of the B-O effect on redshift and richness is a reflection of field galaxy infall rates changing with redshift, the ability of the cluster environment to trigger and/or suppress star formation, and cluster sizes. A rich red cluster can only minimally be blue enhanced by field galaxy infall. Additionally, the primary mechanisms for triggering star formation – interactions and tidal effects – are most effective in systems with low velocity dispersions and a diffuse ICM; e.g. in lower richness clusters. Ram pressure stripping, which truncates star formation, is most effective in high mass systems. Thus, star formation is most likely to be triggered and last in lower richness clusters, while in high richness clusters, what star formation is able to occur is quickly extinguished. These effects are not found in groups, which have widely varying blue fractions.