Diffuse stellar components in galaxies and galaxy clusters

In currently favoured scenarios, the Universe evolves from a highly
homogeneous phase, emerging from the hot Big Bang, to the
present-day state, which is characterised by a wealth of
hierarchically organised structures, spanning several orders of
magnitudes in size: galaxies, clusters of galaxies, superclusters,
walls and filaments. Structures are formed via gravitational
instability and grow hierarchically: the smallest ``haloes''
collapse first and then grow by accreting mass from other haloes or
by merging with other similar structures. Gravitational and
dynamical interactions, like mergers, accretions, tidal distortions
and disruptions thus play a fundamental role in shaping galaxies
and galaxy clusters. As a natural by-product of these interactions,
stars, originally located within galaxies, are ejected into the
space surrounding galaxies and into intracluster space, giving rise
to diffuse stellar components. The study of these components can
reveal important details of galaxy and cluster formation, and are
therefore of great interest. Observations in this field are
severely hampered by the extremely low surface brightness that has
to be measured, corresponding to less than 1/1,000 of the typical
surface brightness of the sky. So far, this has prevented observing
large statistical samples of stellar haloes and intracluster
stellar populations. The statistical characterisation of stellar
haloes and of the intracluster light is the objective of this
thesis. In order to do this, I have developed a new method in which
a large number (approx. 1,000) of relatively shallow images of
homogeneous objects are stacked to produce an extremely deep
average image. Systematic effects that arise from contaminating
sources and instrumental biases in the observation of individual
objects are cancelled out by taking the average of many different
observations and by adopting a conservative masking of the
polluting sources. The large image database required for this
technique has been provided by the Sloan Digital Sky Survey, the
largest optical survey of the Universe ever undertaken, which will
eventually cover approximately 10,000 square degrees of sky with
5-band photometry and spectroscopy. In the first part of the thesis
I have studied the stellar haloes of disc galaxies. From the
stacking of 1043 galaxies I have been able to infer the almost
ubiquitous presence of this component around discs, thus
demonstrating that haloes are essential ingredients of galaxy
evolution. On average, stellar haloes have power-law profiles,
consistent with those of the Milky Way and M31. Their shape is
moderately flattened. The average halo colours hint at old and
fairly metal-enriched stars. However, a puzzling emission excess in
the redmost bands has been measured that cannot be explained by any
stellar emission, but suggests the presence of ionised gas. These
results have been confirmed by the analysis of a galaxy, which has
been observed by the Hubble Space Telescope in its deepest exposure
(the Ultra Deep Field). The second part of the thesis is devoted to
a study of the intracluster light (ICL) from the stacking of a
sample of 683 clusters of galaxies in the redshift range 0.2--0.3.
The average contribution of the ICL to the total light of a cluster
is 17.5 +- 2.0% within 700 kpc from the cluster centre. The ICL is
significantly more centrally concentrated than the light in
galaxies, consistent with the idea that the ICL is formed via tidal
stripping and disruption of galaxies that plunge deep into the
cluster potential. The colours of the ICL are consistent with those
of the cluster galaxies, thus indicating that the intracluster
stars stem from the same population as the stars in galaxies. The
amount of ICL correlates more strongly with the luminosity of the
central galaxy of the cluster than with cluster richness.
Furthermore, the ICL aligns more strongly with the central galaxy
than with the larger scale galaxy distribution in the cluster. This
strongly suggest that the mechanism of formation of the ICL is
strongly coupled with the process of growth of the cluster central
galaxy.