The lack of success of Ly \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 ^ { \prime } ) 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 } ( 0.28 h ^ { -2 } Mpc ^ { 2 } ) to a 4 \sigma limiting narrow-band flux f = 2.7 \times 10 ^ { -16 } erg cm ^ { -2 } s ^ { -1 } , 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 } ~ { } { M } _ { \odot } { yr } ^ { -1 } .
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 .