We report the discovery of a remarkable concentration of massive galaxies with extended X-ray emission at z(spec) = 2.506, which contains 11 massive (M-* greater than or similar to 10(11) M-circle dot) galaxies in the central 80 kpc region (11.6 sigma overdensity). We have spectroscopically confirmed 17 member galaxies with 11 from CO and the remaining ones from Ha. The X-ray luminosity, stellar mass content, and velocity dispersion all point to a collapsed, cluster-sized dark matter halo with mass M-200c = 10(13.9 +/- 0.2) M-circle dot, making it the most distant X-ray-detected cluster known to date. Unlike other clusters discovered so far, this structure is dominated by star-forming galaxies (SFGs) in the core with only 2 out of the 11 massive galaxies classified as quiescent. The star formation rate (SFR) in the 80 kpc core reaches similar to 3400 M-circle dot yr(-1) with a. gas depletion time of similar to 200 Myr, suggesting that we caught this cluster in rapid build-up of a dense core. The high SFR is driven by both a high abundance of SFGs and a higher starburst fraction (similar to 25%, compared to 3%-5% in the field). The presence of both a collapsed, cluster-sized halo and a predominant population of massive SFGs suggests that this structure could represent an important transition phase between protoclusters and mature clusters. It provides evidence that the main phase of massive galaxy passivization will take place after galaxies accrete onto the cluster, providing new insights into massive cluster formation at early epochs. The large integrated stellar mass at such high redshift challenges our understanding of massive cluster formation.

We have used the Planck all-sky submillimetre and millimetre maps to search for rare sources distinguished by extreme brightness, a few hundred millijanskies, and their potential for being situated at high redshift. These "cold" Planck sources, selected using the High Frequency Instrument (HFI) directly from the maps and from the Planck Catalogue of Compact Sources (PCCS), all satisfy the criterion of having their rest-frame far-infrared peak redshifted to the frequency range 353-857 GHz. This colour-selection favours galaxies in the redshift range z = 2-4, which we consider as cold peaks in the cosmic infrared background. With a 4.'5 beam at the four highest frequencies, our sample is expected to include overdensities of galaxies in groups or clusters, lensed galaxies, and chance line-of-sight projections. We perform a dedicated Herschel-SPIRE follow-up of 234 such Planck targets, finding a significant excess of red 350 and 500 mu m sources, in comparison to reference SPIRE fields. About 94% of the SPIRE sources in the Planck fields are consistent with being overdensities of galaxies peaking at 350 mu m, with 3% peaking at 500 mu m, and none peaking at 250 mu m. About 3% are candidate lensed systems, all 12 of which have secure spectroscopic confirmations, placing them at redshifts z > 2.2. Only four targets are Galactic cirrus, yielding a success rate in our search strategy for identifying extragalactic sources within the Planck beam of better than 98%. The galaxy overdensities are detected with high significance, half of the sample showing statistical significance above 10 sigma. The SPIRE photometric redshifts of galaxies in overdensities suggest a peak at z similar or equal to 2, assuming a single common dust temperature for the sources of T-d = 35 K. Under this assumption, we derive an infrared (IR) luminosity for each SPIRE source of about 4x10(12) L-circle dot, yielding star formation rates of typically 700 M-circle dot yr(-1). If the observed overdensities are actual gravitationally-bound structures, the total IR luminosity of all their SPIRE-detected sources peaks at 4 x 10(13) L-circle dot, leading to total star formation rates of perhaps 7 x 10(3) M-circle dot yr(-1) per overdensity. Taken together, these sources show the signatures of high-z (z > 2) protoclusters of intensively star-forming galaxies. All these observations confirm the uniqueness of our sample compared to reference samples and demonstrate the ability of the all-sky Planck-HFI cold sources to select populations of cosmological and astrophysical interest for structure formation studies.

We present results of a ground-based near-infrared campaign with Palomar TripleSpec, Keck NIRSPEC, and Gemini GNIRS to target two samples of reddened active galactic nucleus (AGN) candidates from the 31 deg(2) Stripe 82 X-ray survey. One sample, which is similar to 89% complete to K <16 (Vega), consists of eight confirmed AGNs, four of which were identified with our follow-up program, and is selected to have red R - K colors (> 4, Vega). The fainter sample (K > 17, Vega) represents a pilot program to follow-up four sources from a parent sample of 34 that are not detected in the single-epoch SDSS catalog and have WISE quasar colors. All 12 sources are broad-line AGNs (at least one permitted emission line has an FWHM exceeding 1300 km s(-1)) and span a redshift range 0.59 <z <2.5. Half the (R - K)-selected AGNs have features in their spectra suggestive of outflows. When comparing these sources to a matched sample of blue Type 1 AGNs, we find that the reddened AGNs are more distant (z > 0.5), and a greater percentage have high X-ray luminosities (L-X,L- full > 10(44) erg s(-1)). Such outflows and high luminosities may be consistent with the paradigm that reddened broad-line AGNs represent a transitory phase in AGN evolution as described by the major merger model for black hole growth. Results from our pilot program demonstrate proof of concept that our selection technique is successful in discovering reddened quasars at z > 1 missed by optical surveys.