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On Near-Infrared H-Alpha Searches for High-Redshift Galaxies

The lack of success of Lyman-alpha 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 H-alpha, may be more effective. To test the dust hypothesis we have undertaken deep broad- (K') and narrow-band (5000 km/s, 2.177 micron) imaging of the field towards the quasar PHL957, in an attempt to detect H-alpha emission from a known galaxy of redshift z=2.313. We cover an area of 4.9 square arcmin (0.28 h^{-2}Mpc^2) to a 4-sigma limiting narrow-band flux f=2.7x10^{-16}erg/s/cm^2, a factor of several deeper than previously published surveys. We detect the H-alpha+[N II] emission line in this galaxy at the 3.3-sigma level, inferring a star formation rate of 18 h^{-2} solar masses per year. This is a factor 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.