The spatial clustering of distant, z ∼ 1, early-type galaxies

We examine the spatial clustering of extremely red objects (EROs) found in a relatively large survey of 700 arcmin(2), containing 400 galaxies with R - Ks > 5 to Ks = 19.2. A comoving correlation length r(o) = 12 +/- 3 h(-1) Mpc is derived, under the assumption that the selection function is described by a passively evolving early-type galaxy population, with an effective redshift of z similar to 1.2. This correlation length is very similar to that of local L* elliptical galaxies implying, at face value, no significant clustering evolution in comoving coordinates of early-type galaxies to the limiting depth of our sample, z similar to 1.5. A rapidly evolving clustering bias can be designed to reproduce a null result; however, our data do not show the corresponding strong reduction in the average population density expected for consistency with underlying growth of the mass-function. We discuss our data in the context of recent ideas regarding bias evolution. The uncertainty we quote on r(o) accounts for the spikey redshift distribution expected along relatively narrow sightlines, which we quantify with detailed simulations. This is an improvement over the standard use of Limber's equation which, because of its implicit assumption of a smooth selection function, underestimates the true noise by a factor of approximate to 3 for the parameters of our survey. We propose a general recipe for the analysis of angular clustering, suggesting that any measurement of the angular clustering amplitude, A, has an intrinsic additional uncertainty of sigma (A)/A = root AC, where AC is the appropriate integral constraint.