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

Abstract

A simple two-color selection based on B-, z-, and K-band photometry is proposed for culling galaxies at 1:4 P z P 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 ¼ (z À K ) AB À (B À z) AB > À0:2 allows us to select actively starforming galaxies at z k1:4, independently of their dust reddening. On the other hand, objects with BzK < À0:2 and (z À K ) AB > 2:5 colors include passively evolving galaxies at z k1:4, 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 $10 11 M and sky and volume densities of $1 arcmin À2 and $10 À4 Mpc À3 , respectively. Based on their UV (reddeningcorrected ), X-ray, and radio luminosities, the BzK-selected star-forming galaxies with K < 20 turn out to have average SFR % 200 M yr À1 and median reddening E(B À 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 ¼ 2 star-forming galaxies may be among the precursors of z ¼ 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 ¼ 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 P z P 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.

Morphological properties of massive galaxies at high−z from GOODS

“…39, which corresponds to ∼ 3.3 kpc. The smaller sizes for the passive BzKs compared to those that are actively star-forming was also seen in the smaller samples analysed by Daddi et al (2004). Figure 2.…”

“…This redshift range has been relatively less explored due to the difficulty in obtaining robust samples. However, with the recent deep, nearinfrared imaging and spectroscopic surveys, various schemes have been proposed for color selection of galaxies with 1.4 < z < 2.5, for example, the distant red galaxies (DRGs) selection (Franx et al 2003), and the BzK selection (Daddi et al 2004).…”

Estimating Photometric Redshifts Using Genetic Algorithms

“…This was surprising because of the relationship between redshift and SFR in galaxies shown in [Daddi et al, 2004]. In our future work we will continue BzK tests when we have additional data from other surveys.…”

“…The differences between colours is also described as BzK. BzK photometry is a method used for selecting actively star forming galaxies, independently of dust reddening [Daddi et al, 2004;. Star formation rate (SFR) of galaxies is a challenging area within astronomy due to dust absorption and re-radiation which increases the wavelength (reddening).…”

“…B-V < R-I), which will extend upon our use of BzK relationships. Relational conditions are used in astronomy, for example in finding star forming galaxies [Daddi et al, 2004]. A third research direction is to perform template matching using HYPER-Z [Bolnzella et al 2000] and to compare the results with the GA.…”