The work contained within this thesis
Is made up primarily of two pieces
Both address active galactic nuclei
And the galaxies that live nearby
The obscured fraction of the population
Is the topic of one publication
And the type-II fraction in the optical regime
In chapter four this is the theme
I research the vicinity overdensity
Around radio galaxies in chapter three, you’ll see
I reduce some spectra at redshift one
But not all observations in the end got done
With the spectra I have I do what I can
As if all target observations had actually ran
In the end I conclude with results and the theme
of research to be done further downstream.
The works contained herein addresses two major topics in extragalactic
astrophysics, namely the Type-II AGN fraction and the Overdensity-Radio power
relation.
Quantifying the Type-II AGN fraction has been attempted by many works
in many different observational regimes, finding rather contrasting results.
Accretion onto supermassive black holes contributes between 5 per cent and 20
per cent of the luminosity of the Universe, and seems to be closely linked to
star formation processes. The large uncertainty on this value is due to the ill-determined
contribution from obscured accretion, namely the Type-II fraction.
In Chapters 3 and 4 I address this issue from a theoretical standpoint in the X-ray
regime and an observational standpoint in the optical regime respectively.
In Chapter 3 I show how crude X-ray spectroscopy of partially obscured AGN
can lead to catastrophic underestimations of the intrinsic X-ray luminosity of
these sources. Acting over an entire population, these partial obscurers can
produce an obscured AGN fraction which decreases as a function of observed
luminosity. The results are consistent with observations in the X-ray vs. IR
luminosity of AGN classes.
In Chapter 4 I select a statistically significant sample of AGN from an unbiased
250μm galaxy sample. After spectroscopic classification I find the optical Type-
II AGN fraction to be consistent across several decades in [OIII] luminosity, a
common proxy for intrinsic AGN luminosity. I also investigate the relation of
AGN activity to host galaxy mass, as well as star formation activity and star
formation history.
Probing the environments of protoclusters will help to constrain the models of
structure formation in the Universe. Until now, no dataset has been big enough
to probe the environments of high redshift radio galaxies at a statistical level;
While many believe that the feedback processes of high luminosity radio jets will
have a direct impact on star formation in the surrounding medium it has not
been tested. In Chapter 2 I investigate this on an statistical level, finding no
meaningful correlation between radio galaxy radio power and source overdensity
in the vicinities of these sources.
In Chapter 5 I discuss the reduction of a 24μm sample at redshift z ∼ 1 for
direct comparison with a local 12μm sample. With only a fraction of the target
sample being observed, no statistically significant results could be derived, but the
objects are spectroscopically classified and spectroscopic redshifts are measured
where possible. Correlations in the data set are investigated and the limitations
of the sample selection strategy are discussed