The large-scale environments of radio-loud active galactic nuclei and their evolution across cosmic time

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.