Star-Forming Galaxies at z~2 in the Hubble Ultra Deep Field
  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<z<2.5
(sBzKs) from a K-band selected sample (K_Vega<22.0) in an area of ~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<z<2.5 with K_Vega<22.0.
  Down to K_Vega<22.0, the number counts of sBzKs increase linearly with the
K-band 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_sun/yr and a typical
stellar mass of 10^10 M_sun. 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.
