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Wylezalek, Dominika (2014): The large-scale environments of radio-loud active galactic nuclei and their evolution across cosmic time. Dissertation, LMU München: Faculty of Physics



Emerging from the cosmic web, galaxy clusters are the most massive gravitationally bound structures in the universe. Thought to have begun their assembly at 2 < z < 3, i.e. 10 to 11 billion years ago, clusters provide insights into the growth of large-scale structure as well as the physics that drives galaxy evolution. The redshift range 1 < z < 3 is a key epoch in their evolution. At z ∼ 1.5, elliptical galaxies start to become the dominant population in cluster cores, and star formation in spiral galaxies is being quenched. But there is also evidence for a progressive increase in the amount of star formation that occurs in galaxy cluster cores at z ≳ 1.5. To understand the dependence of the formation mechanisms of massive galaxies with environment, we must focus on clusters at relatively unexplored redshifts z > 1.5 where major assembly is in progress. The search for galaxy clusters at high redshift, so far, has been mildly successful and only a handful of clusters at z > 1.5 have been confirmed. Because this redshift range was essentially unreachable with previous instrumentation, it was dubbed a ‘redshift desert’. The work presented in this thesis has made a major contribution to this field. The Clusters Around Radio- Loud AGN (CARLA) survey, a 400 hr targeted Warm Spitzer program, observed 420 radio-loud AGN (active galactic nuclei) at 1.3 < z < 3.2 across the full sky. Extensive literature over several decades shows that powerful radio-loud AGN preferentially reside in overdense environments. From this survey, we have identified a sample of ∼ 200 galaxy cluster candidates by selecting strong overdensities of color-selected sources. By studying the luminosity function of the CARLA cluster candidates, we showed that quenching is happening much earlier in clusters around radio-loud AGN than in field galaxy samples. This suggests that our targets may well be the most massive and evolved structures known to date at z > 1.5. We also showed that radio-loud AGN reside in denser environments than similarly massive galaxies. This makes high-redshift clusters around radio-loud AGN particularly interesting as they can reveal how galaxies in the most massive dark matter halos assembled. A complementary project, HERGE (Herschel Radio Galaxy Evolution Project) observed a sample of 71 radio galaxies at 1 < z < 5 at far-IR wavelengths with the Herschel Space Observatory. Supporting data in the mid-IR, partially in the near-IR and at sub-mm wave- lengths allow to study cluster fields in more detail. A pilot project on a single field showed that we can identify cluster members and constrain their star-formation properties. These projects laid the foundation for future work, which will make a significant impact on understanding the formation of the most massive structures over several billion years.