NICMOS imaging of the damped Ly alpha absorber at z=1.89 toward LBQS 1210+1731: Constraints on size and star formation rate

We report results of a high-resolution imaging search for the galaxy associ ated with the damped Ly alpha (DLA) absorber at z = 1.892 toward the z(em) = 2.543 quasar LBQS 1210+1731, using the Hubble Space Telescope (HST) NICMO S. The images were obtained in the broad filter F160W and the narrow filter F190N with camera 2 on NICMOS and were aimed at detecting the absorber in the rest-frame optical continuum and in H alpha line emission from the DLA absorber. After suitable point-spread function (PSF) subtractions, a featur e is seen in both the broadband and narrowband images, at a projected separ ation of 0 ".25 from the quasar. This feature may be associated with the DL A absorber, although we cannot completely rule out that it could be a PSF a rtifact. If associated with the DLA, the object would be approximate to 2-3 h(70)(-1) kpc in size with a flux of 9.8 +/- 2.4 mu Jy in the F160W filter , implying a luminosity at lambda(central) = 5500 Angstrom in the rest fram e of 1.5 x 10(10) h(70)(-2) L-circle dot at z = 1.89, for q(0) = 0.5. Howev er, a comparison of the fluxes in the broad and narrow filters indicates th at most of the flux in the narrowband filter is continuum emission, rather than redshifted H alpha line emission. This suggests that if this object is the absorber, then either it has a low star formation rate (SFR), with a 3 sigma upper limit of 4.0 h(70)(-2) M-circle dot yr(-1), or dust obscuratio n is important. It is possible that the H alpha emission may be extinguishe d by dust, but this seems unIikelv given the typically low dust-to-gas rati os observed in DLAs. Alternatively, the object, if real, may be associated with the host galaxy of the quasar rather than with the damped Ly Phi absor ber. H-band images obtained with the NICMOS camera 2 coronagraph show a muc h fainter structure approximate to 4-5 h(70)(-1) kpc in size and containing four knots of continuum emission, located 0 ".7 away from the quasar. This structure is not seen in images of comparison stars after similar PSF subt ractions and is also likely to be associated with the absorbing galaxy or i ts companions, although we do not know its redshift. We have probed regions far closer to the quasar sight line than in most previous studies of high- redshift intervening DLAs. The two objects we report mark the closest detec ted high-redshift DLA candidates yet to any quasar sight line. If the featu res in our images are associated with the DLA, they suggest faint, compact, somewhat clumpy objects rather than large, well-formed protogalactic disks or spheroids. If the features are PSF artifacts, then the constraints on s izes and star formation rates of the DLA are even more severe. The size, lu minosity, and SFR estimates mentioned above should therefore be conservativ ely considered as upper limits.