Recent work has shown that the star formation-density relation -- in which galaxies with low star formation rates are preferentially found in dense environments -- is still in place at z~1, but the situation becomes less clear at higher redshifts. We
use mass-selected samples drawn from the UKIDSS Ultra-Deep Survey to show that galaxies with quenched star formation tend to reside in dense environments out to at least z~1.8. Over most of this redshift range we are able to demonstrate that this star formation-density relation holds even at fixed stellar mass. The environmental quenching of star formation appears to operate with similar efficiency on all galaxies regardless of stellar mass. Nevertheless, the environment plays a greater role in the build-up of the red sequence at lower masses, whereas other quenching processes dominate at higher masses. In addition to a statistical analysis of environmental densities, we investigate a cluster at z=1.6, and show that the central region has an elevated fraction of quiescent objects relative to the field. Although the uncertainties are large, the environmental quenching efficiency in this cluster is consistent with that of galaxy groups and clusters at z~0. In this work we rely on photometric redshifts, and describe some of the pitfalls that large redshift errors can present.
Much of the science that is made possible by multiwavelength redshift surveys requires the use of photometric redshifts. But as these surveys become more ambitious, and as we seek to perform increasingly accurate measurements, it becomes crucial to t
ake proper account of the photometric redshift uncertainties. Ideally the uncertainties can be directly measured using a comparison to spectroscopic redshifts, but this may yield misleading results since spectroscopic samples are frequently small and not representative of the parent photometric samples. We present a simple and powerful empirical method to constrain photometric redshift uncertainties in the absence of spectroscopic redshifts. Close pairs of galaxies on the sky have a significant probability of being physically associated, and therefore of lying at nearly the same redshift. The difference in photometric redshifts in close pairs is therefore a measure of the redshift uncertainty. Some observed close pairs will arise from chance projections along the line of sight, but it is straightforward to perform a statistical correction for this effect. We demonstrate the technique using both simulated data and actual observations, and discuss how its usefulness can be limited by the presence of systematic photometric redshift errors. Finally, we use this technique to show how photometric redshift accuracy can depend on galaxy type.
Recent studies have shown that distant red galaxies (DRGs), which dominate the high-mass end of the galaxy population at z~2.5, are more strongly clustered than the population of blue star-forming galaxies at similar redshifts. However these studies
have been severely hampered by the small sizes of fields having deep near-infrared imaging. Here we use the large UKIDSS Ultra Deep Survey to study the clustering of DRGs. The size and depth of this survey allows for an unprecedented measurement of the angular clustering of DRGs at 2<z_phot<3 and K<21. The correlation function shows the expected power law behavior, but with an apparent upturn at theta<~10. We deproject the angular clustering to infer the spatial correlation length, finding 10.6+-1.6 h^-1 Mpc. We use the halo occupation distribution framework to demonstrate that the observed strong clustering of DRGs is not consistent with standard models of galaxy clustering, confirming previous suggestions that were based on smaller samples. Inaccurate photometric redshifts could artificially enhance the observed clustering, however significant systematic redshift errors would be required to bring the measurements into agreement with the models. Another possibility is that the underlying assumption that galaxies interact with their large-scale environment only through halo mass is not valid, and that other factors drive the evolution of the oldest, most massive galaxies at z~2.
