The clustering and abundance of star-forming and passive galaxies at z ∼ 2

We use the UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) Ultra-deep survey (UDS), currently the deepest panoramic near-infrared survey, together with deep Subaru optical imaging to measure the clustering, number counts and luminosity function of galaxies at z similar to 2 selected using the BzK selection technique. We find that both star-forming (sBzK) and passive (pBzK) galaxies, to a magnitude limit of K-AB < 23, are strongly clustered. The passive galaxies are the most strongly clustered population, with scalelengths of r(0) = 15.0(-2.2)(+1.9) h(-1) Mpc compared with r(0) = 6.75(-0.37)(+0.34) h(-1) Mpc for star-forming galaxies. The direct implication is that passive galaxies inhabit the most massive dark matter haloes, and are thus identified as the progenitors of the most massive galaxies at the present day. In addition, the pBzKs exhibit a sharp flattening and potential turnover in their number counts, in agreement with other recent studies. This plateau cannot be explained by the effects of incompleteness. We conclude that only very massive galaxies are undergoing passive evolution at this early epoch, consistent with the downsizing scenario for galaxy evolution. Assuming a purely passive evolution for the pBzKs from their median redshift to the present day, their luminosity function suggests that only similar to 2.5 per cent of present-day massive ellipticals had a pBzK as a main progenitor.