Robert A. E. Fosbury

Formation epochs, star formation histories and sizes of massive early-type galaxies in cluster and field environments at z=1.2: insights from the rest-frame UV

We derive stellar masses, ages and star formation histories of massive early-type galaxies in the z=1.237 RDCS1252.9-2927 cluster and compare them with those measured in a similarly mass-selected sample of field contemporaries drawn from the GOODS South Field. Robust estimates of these parameters are obtained by comparing a large grid of composite stellar population models with 8-9 band photometry in the rest-frame NUV, optical and IR, thus sampling the entire relevant domain of emission of the di!erent stellar populations. Additionall y, we present new, deep U-band photometry of both fields, giving access to the critical FUV rest-frame, in order to constrain empirically the dependence on the environment of the most recent star formation processes. We also analyze the morphological properties of both samples to examine the dependence of their scaling relations on their mass and environment. We find that early-type galaxies, both in the cluster and in the field, show analogous optical morphologies, follow comparable mass vs. size relation, have congruent average surface stellar mass densities and lie on the same Kormendy relation. We also that a fraction of early-type galaxies in the field employ longer timescales, ! , to assemble their mass than their cluster contemporaries. Hence we conclude that, while the formation epoch of early-type only depends on their mass, the environment does regulate the timescales of their star formation histories. Our deep U-band imaging strongly supports this conclusions. I shows that cluster galaxies are at least 0.5 mag fainter than their field contemporaries of similar mass and optical-to-infrared colors, implying that the last episode of star formation must have happened more recently in the field than in the cluster. Subject headings: galaxies: clusters: individual: RDCS1252.9-2927 — galaxies: high-redshift — galaxies: fundamental parameters — galaxies: evolution — galaxies: formation — galaxies: elliptical — cosmology: observations

Giant Lyα nebulae around z > 2 radio galaxies: evidence for infall

We present an investigation into the possible relationship between side-to-side asymmetries of powerful radio galaxies at high redshift, with the goal of understanding the geometry, orientation and gas dynamics of these sources. Our sample consists of 11 radio galaxies at 2.3 ≤z≤ 3.6 previously known to have giant, kinematically quiescent nebulae. We identify several correlated asymmetries: on the side of the brightest radio jet and hotspot (i) the redshift of the kinematically quiescent nebula is highest, (ii) Lyα is brighter relative to the other lines and continuum, (iii) the radio spectrum is flattest and (iv) the radio structure has its highest polarization. These asymmetries are not found to be correlated with either the radio arm length asymmetry or the brightness asymmetry of the UV–optical emitting material. The correlation between the radio brightness asymmetry and the radial velocity of the quiescent gas also appears to be present in powerful radio galaxies with 0 ≲z≲ 1. Collectively, these asymmetries are most naturally explained as an effect of orientation, with the quiescent nebulae in infall: this is the first study to distinguish between the rotation, infall, outflow and chaotic motion scenarios for the kinematically quiescent emission-line nebulae around powerful active galactic nuclei.