NICMOS Imaging of the Damped Lyα Absorber at Z=1.89 toward LBQS 1210+1731: Constraints on Size and Star Formation Rate

Abstract

We report results of a high-resolution imaging search for the galaxy associated with the damped Lyα (DLA) absorber at z=1.892 toward the z_em=2.543 quasar LBQS 1210+1731, using the Hubble Space Telescope (HST) NICMOS. 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 Hα line emission from the DLA absorber. After suitable point-spread function (PSF) subtractions, a feature is seen in both the broadband and narrowband images, at a projected separation of 0.25" from the quasar. This feature may be associated with the DLA absorber, although we cannot completely rule out that it could be a PSF artifact. If associated with the DLA, the object would be ~2-3 h^-1₇₀ kpc in size with a flux of 9.8+/-2.4 μJy in the F160W filter, implying a luminosity at λ_central=5500 Å in the rest frame of 1.5×10¹⁰ h^-2₇₀ L_solar at z=1.89, for q₀=0.5. However, a comparison of the fluxes in the broad and narrow filters indicates that most of the flux in the narrowband filter is continuum emission, rather than redshifted Hα line emission. This suggests that if this object is the absorber, then either it has a low star formation rate (SFR), with a 3 σ upper limit of 4.0 h^-2₇₀ M_solar yr^-1, or dust obscuration is important. It is possible that the Hα emission may be extinguished by dust, but this seems unlikely, given the typically low dust-to-gas ratios observed in DLAs. Alternatively, the object, if real, may be associated with the host galaxy of the quasar rather than with the damped Lyα absorber. H-band images obtained with the NICMOS camera 2 coronagraph show a much fainter structure ~4-5 h^-1₇₀ 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 subtractions and is also likely to be associated with the absorbing galaxy or its 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 detected high-redshift DLA candidates yet to any quasar sight line. If the features 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 sizes and star formation rates of the DLA are even more severe. The size, luminosity, and SFR estimates mentioned above should therefore be conservatively considered as upper limits.