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

Abstract

Using a simple color selection based on B-, z- and K-band photometry, -0.2$>BzK = (z−K)AB−(B−z)AB>−0.2, we picked out 52 star-forming galaxies at 1.4z2.5 (sBzKs) from a K-band selected sample (KVega<22.0) in an area of ~5.5 arcmin2 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 1.4z2.5 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−1 and a typical stellar mass of ~1010M⊙. 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.