Hα Imaging withHubble Space Telescope‐NICMOS of an Elusive Damped Lyα Cloud atz = 0.6

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Abstract
Despite previous intensive ground-based imaging and spectroscopic campaigns and wideband Hubble Space Telescope (HST) imaging of the z = 0.927 QSO 3C 336 field, the galaxy that hosts the damped Lyα system along this line of sight has eluded detection. We present a deep narrowband Hα image of the field of this zabs = 0.656 damped Lyα absorber, obtained through the F108N filter of NICMOS1 on board the HST. The goal of this project was to detect any Hα emission 10 times closer than previous studies to unveil the damped absorber. We do not detect Hα emission between 005 and 6'' (0.24 and 30 h-1 kpc) from the QSO, with a 3 σ flux limit of 3.70 × 10-17 h-2 ergs s-1 cm-2 for an unresolved source, corresponding to a star formation rate (SFR) of 0.3 h-2 M☉ yr-1. This leads to a 3 σ upper limit of 0.15 M☉ yr-1 kpc-2 on the SFR density, or a maximum SFR of 1.87 M☉ yr-1 assuming a disk of 4 kpc in diameter. This result adds to the number of low-redshift damped Lyα absorbers that are not associated with the central regions of Milky Way-like disks. Damped Lyα absorption can arise from high-density concentrations in a variety of galactic environments including some that, despite their high local H I densities, are not conducive to widespread star formation.