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_solar 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_solar 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.

Based on observations collected at the European Southern Observatory, Chile (ESO programs 70.A-0140, 70.A-0548, 168.A-0485, and 170.A-0788), and with the NASA/ESA Hubble Space Telescope, which is operated by AURA, Inc., under NASA contract NAS5-26555.