On near-infrared H searches for high-redshift galaxies

The lack of success of Lyα searches for high-redshift z>2 field galaxies may be due to extinction by dust, suggesting that surveys based on lines of longer wavelength, particularly Hα, may be more effective. To test the dust hypothesis we have undertaken deep broad- (K') and narrow-band (5000 km s^-1, λ=2.177μm) imaging of the field towards the quasar PHL957, in an attempt to detect Hα emission from a known galaxy of redshift z=2.313. We cover an area of 4.9 arcmin² (0.28h^-2Mpc²) to a 4σ limiting narrow-band flux $$f=2.7\times10^{-16}\enspace \text{erg}\enspace \text {cm}^{-2}\text s^{-1}$$, a factor of several deeper than previously published surveys. We detect the Hα+[NII] emission line in this galaxy at the 3.3σ level, inferring a star formation rate of 18 h^-2 M_⊙yr^-1. This is a factor of only a few times larger than the rate seen in some Sc galaxies today. The faint flux level reached in this work demonstrates the promise of narrow-band imaging in the near-infrared as a technique for finding normal galaxies at high redshifts.