A New Photometric Technique for the Joint Selection of Star-forming and Passive Galaxies at 1.4 ≲ z ≲ 2.5^*

A simple two-color selection based on B-, z-, and K-band photometry is proposed for culling galaxies at 1.4 ≲ z ≲ 2.5 in K-selected samples and classifying them as star-forming or passive systems. The method is calibrated on the highly complete spectroscopic redshift database of the K20 survey, verified with simulations and tested on other data sets. Requiring BzK = (z - K)_AB - (B - z)_AB > -0.2 allows us to select actively star-forming galaxies at z ≳ 1.4, independently of their dust reddening. On the other hand, objects with BzK < -0.2 and (z - K)_AB > 2.5 colors include passively evolving galaxies at z ≳ 1.4, often with spheroidal morphologies. Simple recipes to estimate the reddening, star formation rates (SFRs), and masses of BzK-selected galaxies are derived and are calibrated on K < 20 galaxies. These K < 20 galaxies have typical stellar masses of ~10¹¹ M_☉ and sky and volume densities of ~1 arcmin^-2 and ~10^-4 Mpc^-3, respectively. Based on their UV (reddening-corrected), X-ray, and radio luminosities, the BzK-selected star-forming galaxies with K < 20 turn out to have average SFR ≈ 200 M_☉ yr^-1 and median reddening E(B - V) ~ 0.4. This SFR is a factor of 10 higher than that of z ~ 1 dusty extremely red objects, and a factor of 3 higher than found for z ~ 2 UV-selected galaxies, both at similar K limits. Besides missing the passively evolving galaxies, the UV selection appears to miss some relevant fraction of the z ~ 2 star-forming galaxies with K < 20, and hence of the (obscured) SFR density at this redshift. The high SFRs and masses add to other existing evidence that these z = 2 star-forming galaxies may be among the precursors of z = 0 early-type galaxies. A V/V_max test suggests that such a population may be increasing in number density with increasing redshift. Theoretical models cannot reproduce simultaneously the space density of both passively evolving and highly star-forming galaxies at z = 2. In view of Spitzer Space Telescope observations, an analogous technique based on RJL photometry is proposed to complement the BzK selection and to identify massive galaxies at 2.5 ≲ z ≲ 4.0. By selecting passively evolving galaxies as well as actively star-forming galaxies (including strongly dust-reddened ones), these color criteria should help in completing the census of the stellar mass and of the SFR density at high redshift.