Star - Forming Galaxies at z ∼ 2 in the Hubble Ultra Deep Field ∗

Using a simple color selection based on B-, zand K-band photometry, BzK = (z −K)AB − (B − z)AB > −0.2, we picked out 52 star-forming galaxies at 1.4 < ∼ z < ∼ 2.5 (sBzKs) from a K-band selected sample (KVega < 22.0) in an area of ∼ 5.5 arcmin 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 < ∼ z < ∼ 2.5 with KVega < 22.0. Down to KVega < 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 ) ∼ 0.28, an average star formation rate of ∼ 36 M⊙ yr and a typical stellar mass of ∼ 10M⊙. The UV criterion for the galaxies at z ∼ 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.