Taking advantage of the impressive sensitivity of Spitzer to detect massive galaxies at high redshift, we study the mid-infrared environments of powerful, high-redshift radio galaxies at 1.2 < z < 3. Galaxy cluster member candidates were isolated using a single Spitzer/IRAC mid-infrared color criterion, [3.6] − [4.5] > −0.1 (AB), in the fields of 48 radio galaxies at 1.2 < z < 3. Using a counts-in-cell analysis, we identify a field as overdense when 15 or more red IRAC sources are found within 1 ′ (i.e., 0.5 Mpc at 1.2 < z < 3) of the radio galaxy to the 5σ flux density limits of our IRAC data (f 4.5 = 13.4µJy). We find that radio galaxies lie preferentially in medium to dense regions, with 73% of the targeted fields denser than average. Our (shallow) 120s data permit the rediscovery of previously known clusters and protoclusters associated with radio galaxies as well as the discovery of new promising galaxy cluster candidates at z > 1.2.

We use time‐dependent, one‐dimensional disc models to investigate the evolution of protostellar discs that form through the collapse of molecular cloud cores and in which the primary transport mechanism is self‐gravity. We assume that these discs settle into a state of thermal equilibrium with Q= 2 and that the strength of the angular momentum transport is set by the cooling rate of the disc. The results suggest that these discs will attain a quasi‐steady state that persists for a number of free‐fall times and in which most of the mass within 100 au is located inside 10–20 au. This pile‐up of mass in the inner disc could result in temperatures that are high enough for the growth of magnetohydrodynamic turbulence which could rapidly drain the inner disc and lead to FU Orionis‐like outbursts. In all our simulations, the inner regions of the discs (r < 40 au) were stable against fragmentation, while fragmentation was possible in the outer regions (r > 40 au) of discs that formed from cores that had enough initial angular momentum to deposit sufficient mass in these outer regions. The large amount of mass in these outer regions, however, suggests that fragmentation will lead to the formation of sub‐stellar and stellar mass companions, rather than planetary mass objects. Although mass accretion rates were largely consistent with observations, the large disc masses suggest that an additional transport mechanism (such as magnetorotational instability occurring in the upper layers of the disc) must operate in order to drain the remaining disc material within observed disc lifetimes.

We present a statistical study of the environments of 63 high-redshift radio galaxies (HzRGs) between redshifts 1 ≤ z ≤ 5.2, using the 24 μm waveband of the MIPS instrument aboard the Spitzer Space Telescope. Using a counts-in-cell analysis, a statistically significant source overdensity is found in 1.75 radius circular cells centred on the HzRGs when compared to reference fields. We report an average overdensity of δ (=N targets /N reference ) = 2.2 ± 1.2 at a flux density cut of f 24 μm = 0.3 mJy. This result implies that HzRGs are likely to lie in protoclusters of active and star-forming galaxies at high redshift. Over 95% of our targeted HzRGs lie in higher than average density fields. Further, 20 (32%) of our selected fields are found to be overdense to at least a 3σ significance, of which 9 are newly identified protocluster candidates. We observe a weak correlation between redshift and 24 μm source density, and discuss the populations being probed at different redshifts. In our uniformly selected sample, which was designed to cover two orders of magnitude in radio luminosity throughout z = 1−4, we find that the 24 μm source density does not depend on radio luminosity. We also compare this result with recent work describing IRAC source overdensities around the same HzRGs and find correlations between the results.