Mathematics – Logic
Scientific paper
Jun 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt........13j&link_type=abstract
PhD Thesis, University of Hamburg, 2002
Mathematics
Logic
10
Quasars, Agn Host Galaxies, Spectroscopy, Photometry, Methods
Scientific paper
We present two new methods to extract spectral information on the host galaxies of quasars enabling us to study their colours, stellar composition, and interstellar gas content. The first method uses images in several optical and near-infrared bands for which host galaxy fluxes are extracted via multi-component modelling of nucleus and host. The second directly extracts the host galaxy component from on-nuclear quasar spectra via modelling of the different spatial light distributions of the nuclear and host galaxy component. This is made possible for the first time.
A comparison of the two methods for objects with both broad-band colours as well as spectra available, shows that both methods deliver reliable results as soon as minimal signal-to-noise requirements, and maximal nucleus-to-host ratio are fulfilled.
These methods are applied to date the stellar components of three samples of intermediately luminous low-redshift quasars. The first sample is a complete sample of 19 quasars with z<0.2, observed in seven optical and near-infrared bands with ESO telescopes. For the two other samples deep on-nuclear spectra were obtained, with the ESO 3.6m telescope and the VLT.
We find both disk and elliptical host galaxies. While the disks show largely normal morphologies, colours and stellar population compared to inactive intermediate type disks, the ellipticals are morphologically normal, but show unusually blue colours. Rotating gas disks are found for the majority of our ellipticals. Their stellar populations are much younger than expected from their inactive counterpart, in general compatible with continuous star formation or ages of about 2 Gyrs.
We propose and discuss an explanation within the framework of hierarchical galaxy formation and evolution. Our elliptical quasar hosts would be the result of recent (<0.5 Gyrs) merger events of luminous spiral galaxies. In the merger the spheroidal morphology is created, but the stellar populations present are largely those of the precursor galaxies. The results support a direct connection between the evolution of elliptical galaxies, AGN activity and the evolution of central black holes in galaxies.
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