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 MUSE (Multi-Unit 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 al 2007).

• 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).

• Gravitational Lenses:

  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