![[0218+357 with the VLA at
15 GHz]](0218_black.gif)
![[1104-1805 HST I band]](he1104Ism.gif)
This bright double QSO is not very well suited for the determination of
cosmological parameters from lens models because of the small number
of observational constraints. It is radio quiet and the only available
data are the positions of the images and the galaxy and the flux ratio
of the emission lines
which is believed to represent the amplification ratio very well.
A large variety of models is consistent with the observations, leading
to significant uncertainties for the time delay, Hubble constant and
cosmological parameters.
The fact that the component closer to the lensing galaxy is also the brighter one was a bit puzzling at first. However, I showed that the very simple model of a singular isothermal sphere with external shear can easily explain this fact. Have a look at one of the lensing galaxy detection papers for details.
The redshift of the lensing galaxy is not known up to now, leading to
very different estimated time delays for this
system. Spectrophotometric monitoring allowed the first estimate of a
time delay for HE 1104-1805. Using canonical values for H0, it is
possible to find a range of possible redshifts of the lensing
galaxy. Redshifts larger than 1 can be ruled out reliably. The
absorption line systems at z=1.32 and 1.66 are therefore not related to
the lensing galaxy.
![[0911+0551 NOT I band and NTT K band]](0911.gif)
Due to the unique image configuration, this is one of the most
interesting lens systems for cosmological applications. The number of
observational constraints for the models is much larger than for
double systems. In contrast to other quadruple systems as the Einstein
cross, the four images are located in different distances to the
lensing galaxy, providing useful constraints for the radial mass
distribution.
A first simple model for 0911+0551 was published in an ApJ letter. This was based on
elliptical potentials which can lead to quite unrealistic mass
densities for large ellipticities.
Later I calculated more realistic and general models for this system. Some aspects of this work were presented in a talk at the Oslo workshop in June 1998. No proceedings of this workshop are available but you may read the abstract of my talk.
I am still working on this system, examining the uncertainties from modeling for the determination of cosmological parameters. Degeneracies of the model parameters prevent unique solutions of the problem. A publication on this topic is in preparation.
As this QSO was shown to be variable, time delay measurements should
be possible. This is an important candidate for the determination of
the Hubble constant.
Still needed is the redshift of the lensing galaxy and the mass and
redshift of a nearby cluster which contributes significantly to the
lensing potential. I have examined the cluster at the 3.5m
telescope on
Calar Alto in february 1999.
Spectroscopy of the galaxy is
best achieved with the new VLT.
![[image of 2149-2745]](2149.gif)
![[image of the lens galaxy]](2149gal.gif)
The lensing galaxy of this system was discovered recently.
After subtraction of the QSO images, our data showed a lensing galaxy
almost exactly halfway between them.
![[raw HST image of 2149-2745]](HE2149H.gif)
![[cleaned HST image of 2149-2745]](HE2149Hcc.gif)
New data from the CASTLE survey provide a very
different position quite close to the fainter of the two QSO images.
Our publication might be interesting
for lens modellers anyway because it is to my knowledge the first time that the
possibility of up to eight images of one source with simple elliptical models plus
external shear was mentioned.
I am now mainly working on B0218+357. This started 1999 when I
stayed for three months at the Jodrell Bank Observatory. This system
is especially interesting for cosmology because a time delay is
available for the two compact components. No reliable position for the
lensing galaxy is yet available, making estimates of H0 difficult at
this moment. With the models just constrained by data on the compact
components, a huge range of values of H0 is allowed. I am now trying
to constrain the models (especially the position of the centre of the
lens) from radio data of the ringlike extended structure of what might
be a jet in the source plane. It seems to be important to take into
account the way the radio maps are produced to judge the goodness of
fit of a particular model. This means to simultaneously find the best
model of the lens and the source with the LensCLEAN
algorithm. Preliminary results show that the lensing galaxy is
not exactly seen face on but appears elliptical.
The structure of the source is not well known
yet. All that we know is that the ring is due to a blob in the jet,
which is no surprise. But there is more to come and a publication on
the subject is in preparation.
The high dynamic range of the structures in 0218+357 made it necessary to improve the LensCLEAN method originally proposed by Kochanek and Narayan. I am at the moment still doing extensive numerical calculations with real and artificial data to test the method and improve the results.
This is the first time I am working with radio data at this level. Find a larger version of the thumbnail image which was one of my first attempts to produce a map from uv data here. Please note the strong residuals near the bright A component due to the bad cleaning and calibration of the data. I can do much better now but won't include the maps here.
This work is done in collaboration with Andy Biggs and Ian Browne from Jodrell Bank. If you want to know all about the strange creatures in sunny Cheshire, photos taken at the 1999 christmas party are a good point to start...
My work on B0218+357 is finally finished and was published in my PhD thesis. A paper is in preparation.
![[raw HST image of
HE0512 I band]](HE0512I.gif)
![[raw HST image of
HE0512 V band]](HE0512V.gif)
I am currently (summer 2002) reducing spectra and
direct images taken with STIS on
board the HST. We hope to determine metallicities in both lines of
sight and estimate the differential microlensing and extinction
effects. We also hope to detect the lensing galaxy in the direct images.