7 IR imaging of the gravitational lens system 1600+434

One of the main objectives of the WENSS survey was to increase the small number of known gravitational lenses by mapping flat-spectrum radio sources, in order to provide for the first time a sample large enough to do reasonable statistics. In addition, optical/IR follow-up studies of confirmed lens systems allow the determination of the redshifts of the lensed objects and provide information about the lensing galaxy type. In our run of 1995 August, we performed IR imaging of the object 1600+434, a lensed quasar at redshift 1.6, which consists of two images, separated by 1.4 arcsec, both in the radio and in the optical (Jackson et al. 1995). The lens has been detected in the K- and J-bands but not resolved, although the seeing was good, probably because our pixel was 0.55 arcsec and undersampled the point spread function (PSF). In order to estimate the IR magnitudes and colours of the lensing galaxy, we have tried to carefully subtract the two quasar images using the astrometry of these two point sources from Jackson et al. (1995) and the astrometry of the lensing galaxy obtained from HST images (Jackson, private communication). We computed the pixel positions of the two quasar components assuming that the position of the lensing galaxy coincided with the light peak of the image. The quasars subtraction procedure was carried out constructing two empirical two-dimensional PSFs. To this aim, after selecting some isolated stars in the field, we calculated the FWHM for all these objects and used its mean value to construct a gaussian PSF. The intensity of the PSF to be subtracted for the North Western image of the quasar has been obtained with a trial and error procedure which best eliminates the extension to the North West of our blended J- and K-band images. The intensity of the PSF corresponding to the other quasar image was then fixed by the flux ratio in the radio (1.25) (Jackson, private communication). Figure 2 shows the J-band image of the lens system before and after subtraction of the two quasar images. This procedure allowed us to estimate the magnitude of the lensing galaxy in the two bands, which is listed in Table 6 together with the observing parameters.
Figure 2: The J-band image (North is to the top and East to the left) of the gravitational lens system 1600+434, before a) and after b) the subtraction of the two quasar images. Of the two bright objects in the field the one to the north-west is the partial superposition of the two quasar images and of the lensing galaxy, while the one to the south-east is a nearby galaxy

We would like to stress that the photometric information on the lensing galaxy which we extract from our data is based on the VLA and HST astrometry and on the radio flux ratio for the two quasar images and is only partially affected by our relatively poor detector sampling. The stated accuracy in the lensing galaxy magnitude takes into account the uncertainty in the registration of the positions of the quasar images on the pixel grid.