NICMOS imaging search for high-redshift damped Ly alpha galaxies

We are engaged in a programme of imaging with the STIS and NICMOS (NIC2) in struments aboard the Hubble Space Telescope (HST), to search for the galaxy counterparts of 18 high-redshift z > 1.75 damped Ly alpha absorption lines and five Lyman-Emit systems seen in the spectra of 16 target quasars. This paper presents the results of the imaging campaign with the NIC2 camera. W e describe the steps followed in reducing the data and combining in mosaics , and the methods used for subtracting the image of the quasar in each fiel d, and for constructing error frames that include the systematic errors ass ociated with the PSF subtraction. To identify candidate counterparts, which are either compact or diffuse, we convolved the image and variance frames with circular top-hat filters of diameter 0.45 and 0.90 arcsec respectively , to create frames of summed SIN within the aperture. For each target quasa r we provide catalogues listing positions and aperture magnitudes of all so urces within a square of side 7.5 arcsec centred on the quasar, detected at S/N > 6. We find a total of 41 candidates, of which three have already bee n confirmed spectroscopically as the counterparts. We provide the aperture magnitude detection limits as a function of impact parameter, for both dete ction filters, for each field. The average detection limit for compact (dif fuse) sources is H-AB = 25.0 (24.4) at an angular separation of 0.56 (0.79) arcsec from the quasar, improving to H-AB = 25.5 (24.8) at large angular s eparations. For the brighter sources we have measured the half-light radius and the n parameter of the best-fitting deconvolved Sersic-law surface bri ghtness profile, and the ellipticity and orientation.