Hα imaging with Hubble Space Telescope-NICMOS of an elusive damped Lyα cloud at z=0.6

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 Ly alpha system along this line of sight has eluded detection. We present a deep narrowband H alpha image of the field of this z(abs) = 0.656 damped Ly alpha absorber, obtained through the F108N filter of NICMOS1 on board the HST. The goal of this project was to detect any H alpha emission 10 times closer than previous studies to unveil the damped absorber. We do not detect H alpha emission between 0.05 and 6 (0.24 and 30 h(-1) kpc) from the QSO, with a 3 sigma flux limit of 3.70 x 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 sigma 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 Ly alpha absorbers that are not associated with the central regions of Milky Way-like disks. Damped Ly alpha 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.