A Search for the Damped Lyα Absorber atz = 1.86 toward QSO 1244+3443 with NICMOS

Abstract

We have carried out a high-resolution imaging search for the galaxy associated with the damped Lyα absorber (DLA) at z = 1.859 toward the zem = 2.48 quasar QSO 1244+3443, using the Hubble Space Telescope (HST) and the near-infrared camera and multiobject spectrometer (NICMOS). Images were obtained in the broad filter F160W and the narrow filter F187N with camera 2 on NICMOS with the goal of detecting the rest-frame optical continuum and the Hα line emission from the DLA. After point-spread function (PSF) subtraction, two weak features are seen at projected separations of 016-024 from the quasar. Parts of these features may be associated with the DLA, although we cannot completely rule out that they could be artifacts of the PSF. If associated with the DLA, the objects would be ≈1-2 h kpc in size with integrated flux densities of 2.5 and 3.3 μJy in the F160W filter, implying luminosities at λcentral = 5600 A in the DLA rest frame of 4.4-5.9 × 109 h L☉ at z = 1.86, for q0 = 0.5. However, no significant Hα line emission is seen from these objects, suggesting low star formation rates (SFRs). Our 3 σ upper limit on the SFR in the DLA is 1.3 h-270 M☉ yr-1 for q0 = 0.5 (2.4 h-270 M☉ yr-1 for q0 = 0.1). This together with our earlier result for LBQS 1210 + 1731 mark a significant improvement over previous constraints on the star formation rates of DLAs. Dust within the DLA could extinguish Hα emission, but this would require the dust content in the DLA to be much higher than that inferred from previous DLA observations. A combination of low star formation rate and some dust extinction is likely to be responsible for the lack of Hα emission. Alternatively, the objects, if real, may be associated with the host galaxy of the quasar rather than with the DLA. In any case, our observations suggest that the DLA is not a large bright protodisk, but a compact object or a low surface brightness galaxy. If the two features are PSF artifacts or associated with the quasar host, then the constraints on the size and luminosity of the DLA are even more severe.