We present the results of the first, deep ALMA imaging covering the full 4.5 arcmin 2 of the HUDF imaged with WFC3/IR on HST. Using a 45-pointing mosaic, we have obtained a homogeneous 1.3-mm image reaching σ 1.335 µJy, at a resolution of 0.7 arcsec. From an initial list of 50 > 3.5σ peaks, a rigorous analysis confirms 16 sources with S 1.3 > 120 µJy. All of these have secure galaxy counterparts with robust redshifts ( z = 2.15). Due to the unparalleled supporting data, the physical properties of the ALMA sources are well constrained, including their stellar masses (M * ) and UV+FIR star-formation rates (SFR). Our results show that stellar mass is the best predictor of SFR in the high-redshift Universe; indeed at z ≥ 2 our ALMA sample contains 7 of the 9 galaxies in the HUDF with M * ≥ 2 × 10 10 M , and we detect only one galaxy at z > 3.5, reflecting the rapid drop-off of high-mass galaxies with increasing redshift. The detections, coupled with stacking, allow us to probe the redshift/mass distribution of the 1.3-mm background down to S 1.3 10 µJy. We find strong evidence for a steep star-forming 'main sequence' at z 2, with SFR ∝ M * and a mean specific SFR 2.2 Gyr −1 . Moreover, we find that 85% of total star formation at z 2 is enshrouded in dust, with 65% of all star formation at this epoch occurring in high-mass galaxies (M * > 2 × 10 10 M ), for which the average obscured:unobscured SF ratio is 200. Finally, we revisit the cosmic evolution of SFR density; we find this peaks at z 2.5, and that the star-forming Universe transits from primarily unobscured to primarily obscured at z 4.

We used the Karl G. Jansky Very Large Array (VLA) to image one primary beam area at 3 GHz with 8 FWHM resolution and 1.0 µJy beam −1 rms noise near the pointing center. The P (D) distribution from the central 10 arcmin of this confusion-limited image constrains the count of discrete sources in the 1 < S(µJy) < 10 range. At this level the brightness-weighted differential count S 2 n(S) is converging rapidly, as predicted by evolutionary models in which the faintest radio sources are starforming galaxies; and ≈ 96% of the background originating in galaxies has been resolved into discrete sources. About 63% of the radio background is produced by AGNs, and the remaining 37% comes from star-forming galaxies that obey the far-infrared (FIR) / radio correlation and account for most of the FIR background at λ ≈ 160 µm. Our new data confirm that radio sources powered by AGNs and star formation evolve at about the same rate, a result consistent with AGN feedback and the rough The confusion at centimeter wavelengths is low enough that neither the planned SKA nor its pathfinder ASKAP EMU survey should be confusion limited, and the ultimate source detection limit imposed by "natural" confusion is ≤ 0.01 µJy at ν = 1.4 GHz. If discrete sources dominate the bright extragalactic background reported by ARCADE 2 at 3.3 GHz, they cannot be located in or near galaxies and most are ≤ 0.03 µJy at 1.4 GHz.

Radio source counts constrain galaxy populations and evolution, as well as the global star formation history. However, there is considerable disagreement among the published 1.4-GHz source counts below 100 µJy. Here we present a statistical method for estimating the µJy and even sub-µJy source count using new deep wide-band 3-GHz data in the Lockman Hole from the Karl G. Jansky Very Large Array (VLA). We analyzed the confusion amplitude distribution P(D), which provides a fresh approach in the form of a more robust model, with a comprehensive error analysis. We tested this method on a large-scale simulation, incorporating clustering and finite source sizes. We discuss in detail our statistical methods for fitting using Monte Carlo Markov chains, handling correlations, and systematic errors from the use of wide-band radio interferometric data. We demonstrated that the source count can be constrained down to 50 nJy, a factor of 20 below the rms confusion. We found the differential source count near 10 µJy to have a slope of −1.7, decreasing to about −1.4 at fainter flux densities. At 3 GHz the rms confusion in an 8 arcsec FWHM beam is ∼ 1.2 µJy beam −1 , and a radio background temperature ∼ 14 mK. Our counts are broadly consistent with published evolutionary models. With these results we were also able to constrain the peak of the Euclidean normalized differential source count of any possible new radio populations that would contribute to the cosmic radio background down to 50 nJy.

Faraday rotation measures (RMs) of extragalactic radio sources provide information on line-of-sight magnetic fields, including contributions from our Galaxy, source environments, and the intergalactic medium (IGM). Looking at differences in RMs, ∆RM, between adjacent sources on the sky can help isolate these different components. In this work, we classify adjacent polarized sources in the NVSS as random or physical pairs. We recompute and correct the uncertainties in the NVSS RM catalog, since these were significantly overestimated. Our sample contains 317 physical and 5111 random pairs, all with Galactic latitudes |b| ≥ 20 • , polarization fractions ≥ 2%, and angular separations between 1.5 and 20 . We find an rms ∆RM of 14.9 ± 0.4 rad m −2 and 4.6 ± 1.1 rad m −2 for random and physical pairs, respectively. This means polarized extragalactic sources that are close on the sky, but at different redshifts, have larger differences in RM than two components of one source. This difference of ∼ 10 rad m −2 is significant at 5σ, and persists in different data subsamples. While there have been other statistical studies of ∆RM between adjacent polarized sources, this is the first unambiguous demonstration that some of this RM difference must be extragalactic, thereby providing a firm upper limit on the RM contribution of the IGM. If the ∆RMs originate local to the sources, then the local magnetic field difference between random sources is a factor of two larger than between components of one source. Alternatively, attributing the difference in ∆RMs to the intervening IGM yields an upper limit on the IGM magnetic field strength of 40 nG.