My research has mainly been concerned with some combination of classical cosmology, gravitational lensing, and statistics. With Rainer Kayser I examined the potential of using the redshift distribution of gravitational lenses to determine the cosmological parameters. In the Proceedings of the Seventeenth Texas Symposium on Relativistic Astrophysics is a quick and dirty version of this work, which I originally began in my master's thesis. It turns out that selection effects regarding the brightness of the lenses make the method less powerful than one could have hoped. One can also reverse questions and answers and, assuming some cosmological model, predict lens redshifts and magnitudes as I did in a poster for the IAU symposium on gravitational lenses.
A problem in many branches of inquiry connecting observations with theoretical predictions in cosmology is the calculation of various distances from redshifts. With Rainer Kayser and Thomas Schramm I developed a general and practical method for doing this, not only allowing for arbitrary values of the cosmological constant and the density parameter, but also allowing for varying degrees of inhomogeneity, a modification to the classical distance formulae which can be as important as the other parameters. I also developed a set of FORTRAN routines for doing these calculations, together with a user's guide. (For convenience, you can also get all of these at once in a gzipped tar file.)
In following couple of years I continued my research in cosmology and gravitational lenses as part of the CERES project at the Jodrell Bank Observatory (then known as the Nuffield Radio Astronomy Laboratories. Take a look at my CERES pointers.
CERES is one of the European Union Training and Mobility of Researchers (EU-TMR) Research Networks, CERES standing for Consortium for European Research on Extragalactic Surveys. The coordinator of the network is Ian Browne, also here at Jodrell Bank. The network itself is mainly concerned with exploiting large radio surveys for various purposes. Most of the people involved with the network at Jodrell Bank, and some others at other locations in the network, are concerned mainly with the gravitational lensing aspects of these surveys. These are mainly strong lensing (i.e. multiple imaging) of point sources, as opposed to weak lensing or lensing of extended sources, at arcsecond scales. While initial observations are made in the radio, follow-up work includes other wavelengths as well, mainly infrared and optical. Sources and lenses are at high redshift, which makes gravitational lensing useful for us primarily as a cosmogical tool.
My work in the network concerns mainly the theoretical aspects of gravitational lensing. Up to now this has been mainly lensing statistics, though recently I have been involved in time delay analysis and in the future will move into the area of modelling gravitational lens systems, especially in combination with gravitational lensing statistics. I'm also interested in most aspects of cosmology, and the CMB group here at Jodrell Bank provides me the opportunity to learn about a very different cosmological field from the inside, so to speak. I've even managed to be co-author of a paper a paper which is in some respect concerned with the CMB.
CERES is itself a big collaboration and overlaps with other collaborations, so this can lead to publications with lots of authors, like this description of our goals or this description of just a few goals with just a few authors.
CERES hosted a conference at Jodrell Bank, which I used not to talk about my own research but to point out that other aspects have improved so much with regard to lens time delays that one should start to worry about the other cosmological parameters.
One goal of CERES is to vastly increase the amount of lens data, which lets one get a better understanding of puzzling things, such as relationship between the image separation and source redshift for gravitational lenses (which I first mentioned here) or the scarcity of wide-separation gravitational lenses. A good lens sample can be used to test claims based on inhomogeneous samples, such as the above or our reply to Hawkins's claim that there is a large population of dark lenses, which we first mentioned here. Moving away from the analysis of samples to that of single gravitational lens systems, I'm still mainly concerned with statistics, at least at the moment, for example in the time delay analysis for B0218+357.
The current status of our lens statistics stuff is summarised in a poster for the 1998 Texas Symposium. I've since published a reanalysis of optical lens surveys from the literature (Paper I) with Ralf Quast and we've also done an analysis of JVAS (Paper II) which is (almost) a well-defined complete subset of CLASS. Increasingly important are papers discussing joint constraints from more than one cosmological test. This is hinted at in Papers I and II above, but is done in full force in in Paper III and in Paper IV
These last two papers were the first I wrote after moving to another institute in the CERES network, namely the Kapteyn Instituut at the Rijksuniversiteit Groningen
At the gravitational lens conference in Boston in July 1999, I presented a poster which summarises our current results on lensing statistics, alone and in combination with other cosmological tests, and discusses some of the caveats one needs to keep in mind when comparing constraints from the literature. At the same conference, there was a poster summarising the current state of our analysis of the gravitational lens system 0218+357.
I'm usually not on mainly observational papers, though I am on this paper about an intriguing CLASS system, mainly because I contributed to the calculation of mass-to-light ratios, brightness of the lens in example cosmological models etc. In a drive to get CLASS defined as uniformly as possible, I stressed the need for defining everything uniformly both in abstract terms and in terms of actual calibration of the data etc. The CLASS recalibration actually found a new lens system, a quad (throwing some egg on the faces of some pundits who had tried to explain the small ratio of double to quad lens systems in CLASS through some bias against doubles in our search strategy).
At a Moriond conference, I presented a talk on the (then) current status of CLASS, and another one which revisited the very first topic mentioned above, partially in light of new JVAS and CLASS data. At the XXIVth IAU General Assembly, I presented an invited review of cosmological constraints from strong gravitational lensing, at the Symposium "New Cosmological Data and the Values of the Fundamental Parameters". I was also engaged in a project to eliminate a source of systematic error in such constraints.
With CLASS being more or less complete now, the definitive paper on the lens-candidate selection and followup has been published. Before this, a paper on lensing statistics was published, the first such analysis of the complete CLASS sample.
There must be a lot of useful source code written by people who eventually leave astronomy. Some if it is thus lost. It would be nice if there were a practical way to preserve such code for posterity.
With collaborators in Uppsala, I've been involved in quantitatively checking Hawkins's claim that a substantial fraction of the optical variability of QSOs is caused by microlensing. It turns out that it doesn't hold up, but nevertheless one can still use the idea to put an upper limit on the amount of compact-object dark matter.
last modified on Saturday, December 24, 2005 at 10:25:08 PM by helbig@astro.mNuOlStPiAvMa!x.de