Lutz Wisotzki: Research
My research activities in astrophysics can be subsumed under the
general heading of "observational cosmology" - that is, the study of
the origin and evolution of cosmic structures with astronomical
telescopes. Below I give a brief summary of my past and present
research in this field.
I am now also involved in two major spectroscopic survey projects.
Since 2007 I am the Instrument Scientist for
Spectroscopic Explorer), currently being built for the ESO-VLT and
scheduled for first light in 2012. I am also a member of the
HETDEX project (Hobby-Eberly
Telescope Dark Energy Experiment), which will as well start taking
data in 2012.
Further information about the activities in our research group
"Galaxies and Quasars" can be found in our group web pages.
Past and current research topics by Lutz Wisotzki
The Hamburg/ESO Survey
Since 1993 I have lead the Hamburg/ESO Survey (HES), a digital
objective-prism survey of the entire southern extragalactic sky
(Wisotzki et al. 1996, 2000). The
prime aim of this survey was to generate the largest possible sample of the
optically brightest quasars, for statistical studies as well as to
provide targets for detailed follow-up studies. In this, the HES was
highly complementary to all other recent fainter AGN surveys
including SDSS and 2dF. The survey is now completed and has not only
fulfilled its original aim, but also yielded a lot of discoveries in
other fields, most notably the three most metal-poor stars known in
the Milky Way (led by Norbert Christlieb).
More about the HES can be found here .
Demographics of Active Galactic Nuclei
We have used the HES quasar sample to study the AGN luminosity
function, with particular emphasis on the local luminosity function
(Koehler et al 1997; Schulze et al 2009). The HES also provided
significant constraints on the bright end of the AGN LF,
demonstrating the shape change of the AGN luminosity function (now
often called `AGN downsizing') independently of the X-ray surveys
(Wisotzki 2000). For the faint end of the AGN population, the
COMBO-17 survey (led by Chris Wolf and
Klaus Meisenheimer) was ground-breaking as the deepest large AGN sample
for several years (Wolf et al. 2003).
We are currently applying new approaches of AGN demographics to
the HES quasar samples, exploiting spectroscopically estimated black
hole masses and accretion rates.
AGN Host Galaxies:
Our team has been among the first to systematically pursue colour
measurements of quasar host galaxies. We have imaged a low-redshift
sample of QSOs in several optical/NIR bands, and we found a
significant excess of light from young stars especially in E-type
host galaxies (Jahnke et al. 2004a). We extended this approach to
higher redshifts using HST and VLT, for intermediate (Sanchez et
al. 2004) and high z (Jahnke et al. 2004b; Schramm et al.
2008). Our current aim is to homogeneously estimate
stellar bulge masses for AGN hosts, to be combined with virial black
hole mass estimates in order to obtain insight into the cosmic
evolution of the black hole / bulge mass ratio.
We are presently using the relatively new observing technique of Integral Field
Spectroscopy to study the spatial distribution of ionised gas in AGN
host galaxies and quasars. We have demonstrated that the presence of an
extended emission-line region in a quasar host is closely correlated
with nuclear spectral properties (Husemann et al. 2008). In an investigation of
high-redshift quasars we found that even among radio-quiet quasars,
extended Lyman-alpha regions appear to be ubiquitous (Christensen et
Quasar Absorption Lines and the Intergalactic Medium:
Follow-up spectroscopic studies of HES-selected bright high-redshift
quasars have provided significant constraints on e.g. absorber
sizes and structure (Smette et al. 1995, Lopez et al. 2005) and
revealed evidence for inhomogeneous He II reionisation (Reimers et
al. 1997). More recently we started a systematic exploration of the
radiative impact of quasars onto the IGM. We are studying the `line
of sight' proximity effect in individual quasar spectra and use this
to estimate the intensity of the cosmic UV background and its
evolution above redshift z > 2 (Dall'Aglio et al 2008a, b). From a
dedicated survey `Quasars near Quasars' we discovered the
`transverse proximity effect in spectral hardness' (Worseck &
Wisotzki 2006, Worseck et al. 2007).
Based on the HES quasar sample and the extreme "magnification bias"
affecting it, we have been among the most successful discoverers of
multiply imaged quasars. Altogether, 9 such systems were found in
our survey. We also discovered the now well-accepted phenomenon of
`spectroscopic signatures of microlensing' manifest in many lensed
quasars (e.g., Wisotzki et al. 1993, 2003).
Last change 30-Oct-2009