Active Galactic Nuclei Surveys
Science projects

Current Scientific Projects

AGN host galaxies

color distribution of active and non-active 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

simulation of the 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).

Gravitational lensing


Surveys Science projects Publications

Last change 2007 April 10 by Isabelle Gavignaud