Galaxy formation and evolution in the Millennium Simulation

This Thesis addresses the topic of galaxy formation and evolution
in the universe. In collaboration with D. Croton, G. de Lucia, V.
Springel, and S.D.M. White, I made use of the Millennium
simulation, a very large N-body simulation of dark-matter evolution
in a cosmological volume carried out at the MPA in 2005 by Springel
2005, to explore the predictions made by the most recent generation
of semi-analytic models for galaxy formation. These models are
incorporating a new mode of feedback from active galactic nuclei
(AGN), which have their origins in super-massive black holes
accreting mass and turning it into energy. Because of its
observational signature in the radio regime this feedback is called
"radio mode" and it counteracts the cooling flows of cold gas in
undisturbed dark-matter haloes hosting galaxy clusters, which would
otherwise show much higher star-formation of their central object
than is observed. Previous work by Croton 2006 and De Lucia 2006
has shown that with the new semi-analytic model the population of
local galaxies can be reproduced quite accurately. In order to
study the evolution of the population out to higher redshifts, the
semi-analytic predictions have been compared to a number of
observations in various filter bands, in particular to two recent
efforts to get a comprehensive multi-wavelength dataset of high
redshift galaxies carried out by the DEEP2 (Davis 2001) and COSMOS
(Scoville 2006) collaborations. The approach taken was to perform
as broad a comparison as possible to gain firm constraints on the
assumed physics in our model. Therefore a multitude of
observational properties was contrasted with the model predictions
such as clustering, luminosity functions, stellar mass functions,
number counts per area and redshift to a certain magnitude limit.
In order to facilitate the comparison between simulations and
recent intermediate and high-redshift surveys, it is very useful to
have a number of independent mock observations of the simulated
galaxies, which provide good enough statistics to get a handle on
cosmic variance. To this end I have devised a computer program that
calculates the simulated galaxies lying on the backward light cone
of a hypothetical observer out to arbitrarily high redshifts,
taking advantage of the periodicity of the simulation box but
avoiding replications. The output provides accurately interpolated
redshifts, positions, observer frame and rest-frame magnitudes,
dust extinction, as well as all the intrinsic galaxy properties
like stellar mass and star formation rate. Utilising this tool it
is also possible to make predictions for future galaxy surveys,
deeper in magnitude and redshift than current ones. Presently the
mock catalogues are used by the DEEP2 and COSMOS teams as a
comparison sample in general and as a means to assess their
selection effects and improve their data reduction in particular.
First comparisons of counts in apparent magnitude and redshift gave
promising results, showing good agreement in the low and
intermediate range. The same holds for the angular clustering
analysis except for the faintest magnitudes. Thus we conclude that
our current understanding of the processes governing galaxy
formation and evolution from the very first objects to the present
day population is realistic but still incomplete. In particular the
treatment of the interplay between star formation and negative
feedback and the various processes influencing satellite galaxies
in big galaxy clusters have potential for improvement. In the
following I will give a brief outline of the thesis. After setting
the stage for any kind of model in Chapter 1 by defining the
geometry of the universe and the cosmological parameters that
determine it, I will describe our semi-analytical model of galaxy
formation in Chapter 2, where it will be also explained how to
construct realistic mock observations of the simulated galaxies.
First in Chapter 3 it will be verified that a simple model which
assumes that galaxies are conserved but evolve in luminosity due to
their star formation histories cannot account for the observed
evolution of the galaxy population in the universe. This fact can
be understood in the context of hierarchical models where massive
and luminous galaxies assembled from smaller objects. Chapter 4
proceeds with exploring the predictions from the considerably more
sophisticated semi-analytic model based on an N-body simulation of
the hierarchical growth of dark matter structures. For this
analysis a set of mock light-cones was constructed for direct
comparison with the data which shows reasonably good agreement
between model and observations at low redshift and for bright
apparent magnitudes. These light-cones represent one of the largest
samples of realistic mock observations currently available. They
can be used for testing data analysis techniques usually applied to
real observations on a well defined sample of artificial galaxies
to verify how well the derivation of galaxy properties from the
data works. In Chapter 5 we will demonstrate how one can measure
the evolution of the galaxy merger rate from observing close
projected galaxy pairs. Interestingly we find that the calibration
needed for the conversion is significantly different from what has
typically been assumed in previous studies. Additionally we will
demonstrate that galaxy merger rates and dark-matter merger rates
show considerably different evolution with redshift. Consequently
we conclude that merger rate studies are less suitable as a probe
of cosmic structure formation than initially assumed, but
nonetheless they can be of great help to understand the formation
and evolution of galaxies in a hierarchical universe. Finally these
results will be summarised and discussed in Chapter 6 where I will
also give a brief outlook on the future of this work, a short
glimpse of which is already presented in the Appendix.