ON NEAR-INFRARED H-ALPHA SEARCHES FOR HIGH-REDSHIFT GALAXIES

The lack of success of Ly alpha searches for high-redshift z > 2 held 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(-1), lambda = 2.177 mu m) 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 arcmin(2) (0.28h(-2) Mpc(2)) to a 4 sigma limiting narrow-band flux f = 2.7 x 10(-16) erg cm(-2) s(-1), a factor of several deeper than previously s 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 18h(-2) M(circle dot) 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.