Current Scientific Projects
AGN host galaxies
Most if not all galaxies with a significant bulge component harbor a central
supermassive black hole (SMBH). The formation and evolution of SMBHs and
their host galaxies are tightly linked, as can be inferred from, e.g., the
correlation between bulge mass and SMBH mass. We study all types of AGN -
unobscured (type-1), obscured (type-2) and "optically dull" AGN - at various
cosmic times, covering the redshift range 0.2<z<2.
On images of AGN, the bright active nucleus normally outshines the host
galaxy. A very good modelling of the point spread function is hence mandatory
to study the host galaxy properties. We developed a variety of methods for
the host analysis, including full 2-D modelling of the combined host+point
source light profiles. On the basis of the COMBO17⁄GEMS survey, we have
determined rest-frame colors of a significant sample of AGN (approx.
70 objects) at a mean redshift z∼0.7.
We found that the majority of AGN hosts have bulge-dominated (i.e. early-type)
morphologies, but their colors are oftenly much bluer than those of early-type
non-AGN (see figure). This possibly indicates recent or ongoing star formation
in the AGN phase before feedback processes by the central engine lead to "red
and dead" galaxies in the present-day cosmos.
A fraction of the objects in our data set show evidence for tidal interactions
and mergers. However, it is still an open question what role mergers play in
triggering of nuclear activity. We explore this by combining very large
samples of AGN and quiescent galaxies at low and intermediate redshifts
and establishing new, robust descriptors of tidal features and asymmetries.
Growth of supermassive black holes
AGN feedback processes
Quasar Proximity Effect
The intergalactic medium is kept highly photoionised by the intergalactic
UV background generated by the overall population of quasars and
star-forming galaxies. On lines of sight passing near quasars the
intergalactic medium will be statistically more ionised due to the
local enhancement of the UV flux in its vicinity. The higher UV flux
reduces the density of neutral hydrogen in the gas, thereby creating a
statistically higher transmission close to the quasar than far away
from it. This is the so-called proximity effect. The proximity effect
of a quasar on its own line of sight is well known, and can be used to
measure the intensity of the UV background. However, it is difficult to
detect a transverse proximity effect created by foreground quasars near
a background source due to the large inter-sightline distances involved
and several systematic effects.
The figure shows the simulated
proximity effect of a quasar on its own line of sight and the
transverse proximity effect of a nearby foreground quasar (red
spectrum) compared to the output of a hydrodynamical simulation without
a quasar (black spectrum).
Last change 2007 April 10 by Isabelle Gavignaud