Hα Imaging with Hubble Space Telescope-NICMOS of an Elusive Damped Lyα Cloud at z=0.6

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 z_abs=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 0.05" 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_solar yr^-1. This leads to a 3 σ upper limit of 0.15 M_solar yr^-1 kpc^-2 on the SFR density, or a maximum SFR of 1.87 M_solar 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. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.