Using a simple color selection based on B - , z - and K -band photometry , BzK = ( z - K ) _ { AB } - ( B - z ) _ { AB } > -0.2 , we picked out 52 star-forming galaxies at 1.4 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } z \lower 2.15 pt \hbox { $ % \buildrel < \over { \sim } $ } 2.5 ( sBzKs ) from a K -band selected sample ( \hbox { $K _ { Vega } $ } < 22.0 ) in an area of \sim 5.5 arcmin ^ { 2 } of the Hubble Ultra Deep Field ( UDF ) .
We develop a new photometric redshift method , and the error in our photometric redshifts is less than 0.02 ( 1 + z ) .
From the photometric redshift distribution , we find the BzK color criterion can be used to select star-forming galaxies at 1.4 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } z \lower 2.15 pt \hbox { $ % \buildrel < \over { \sim } $ } 2.5 with \hbox { $K _ { Vega } $ } < 22.0 .
Down to \hbox { $K _ { Vega } $ } < 22.0 , the number counts of sBzKs increase linearly with the K magnitude ; the sBzKs are strongly clustered , and most of them have irregular morphologies on the ACS images .
They have a median reddening of E ( B - V ) \sim 0.28 , an average star formation rate of \sim 36 M _ { \odot } yr ^ { -1 } and a typical stellar mass of \sim 10 ^ { 10 } M _ { \odot } .
The UV criterion for the galaxies at z \sim 2 can select most of the faint sBzKs in the UDF , but it does not work well for bright , massive , highly-reddened , actively star-forming galaxies .