We present a blind time-delay cosmographic analysis for the lens system DES J0408-5354. This system is extraordinary for the presence of two sets of multiple images at different redshifts, which provide the opportunity to obtain more information at the cost of increased modelling complexity with respect to previously analysed systems. We perform detailed modelling of the mass distribution for this lens system using three band Hubble Space Telescope imaging. We combine themeasured time delays, line-of-sight central velocity dispersion of the deflector, and statistically constrained external convergence with our lens models to estimate two cosmological distances. We measure the 'effective' time-delay distance corresponding to the redshifts of the deflector and the lensed quasar D-Delta t(eff) = 3382(-115)(+146) Mpc and the angular diameter distance to the deflector D-d = 1711(-280)(+376) Mpc, with covariance between the two distances. From these constraints on the cosmological distances, we infer the Hubble constant H-0 = 74.2(-3.0)(+2.7) km s(-1) Mpc(-1) assuming a flat Lambda CDM cosmology and a uniform prior for Omega(m) as Omega(m) similar to U(0.05, 0.5). This measurement gives the most precise constraint on H-0 to date from a single lens. Our measurement is consistent with that obtained from the previous sample of six lenses analysed by the H-0 Lenses in COSMOGRAIL's Wellspring (H0LiCOW) collaboration. It is also consistent with measurements of H-0 based on the local distance ladder, reinforcing the tension with the inference from early Universe probes, for example, with 2.2 sigma discrepancy from the cosmic microwave background measurement.

Group V elements in crystal structure isostructural to black phosphorus with unique puckered two-dimensional layers exhibit exciting physical and chemical phenomena. However, as the first element of group V, nitrogen has never been found in the black phosphorus structure. Here, we report the synthesis of the black phosphorus-structured nitrogen at 146 GPa and 2200 K. Metastable black phosphorus-structured nitrogen was retained after quenching it to room temperature under compression and characterized in situ during decompression to 48 GPa, using synchrotron x-ray diffraction and Raman spectroscopy. We show that the original molecular nitrogen is transformed into extended single-bonded structure through gauche and trans conformations. Raman spectroscopy shows that black phosphorus-structured nitrogen is strongly anisotropic and exhibits high Raman intensities in two A(g) normal modes. Synthesis of black phosphorus-structured nitrogen provides a firm base for exploring new type of high-energy-density nitrogen and a new direction of two-dimensional nitrogen.

We report the discovery of TOI 694 b and TIC 220568520 b, two low-mass stellar companions in eccentric orbits around metal-rich Sun-like stars, first detected by the Transiting Exoplanet Survey Satellite (TESS). TOI 694 b has an orbital period of 48.05131 +/- 0.00019 days and eccentricity of 0.51946 +/- 0.00081, and we derive a mass of 89.0 +/- 5.3 M-Jup (0.0849 +/- 0.0051 M-circle dot) and radius of 1.111 +/- 0.017 R-Jup (0.1142 +/- 0.0017 R-circle dot). TIC 220568520 b has an orbital period of 18.55769 +/- 0.00039 days and eccentricity of 0.0964 +/- 0.0032, and we derive a mass of 107.2 +/- 5.2 M-Jup (0.1023 +/- 0.0050 M-circle dot) and radius of 1.248 +/- 0.018 R-Jup (0.1282 +/- 0.0019 R-circle dot). Both binary companions lie close to and above the hydrogen-burning mass threshold that separates brown dwarfs and the lowest-mass stars, with TOI 694 b being 2s above the canonical mass threshold of 0.075 M-circle dot. The relatively long periods of the systems mean that the magnetic fields of the low-mass companions are not expected to inhibit convection and inflate the radius, which according to one leading theory is common in similar objects residing in short-period tidally synchronized binary systems. Indeed we do not find radius inflation for these two objects when compared to theoretical isochrones. These two new objects add to the short but growing list of low-mass stars with well-measured masses and radii, and highlight the potential of the TESS mission for detecting such rare objects orbiting bright stars.

We present the second public data release of the Dark Energy Survey, DES DR2, based on optical/near-infrared imaging by the Dark Energy Camera mounted on the 4 m Blanco telescope at Cerro Tololo Inter-American Observatory in Chile. DES DR2 consists of reduced single-epoch and coadded images, a source catalog derived from coadded images, and associated data products assembled from 6 yr of DES science operations. This release includes data from the DES wide-area survey covering similar to 5000 deg(2) of the southern Galactic cap in five broad photometric bands, grizY. DES DR2 has a median delivered point-spread function FWHM of g = 1.11 '', r = 0.95 '', i = 0.88 '', z = 0.83 '', and Y = 0.'' 90, photometric uniformity with a standard deviation of < 3 mmag with respect to Gaia DR2 G band, a photometric accuracy of similar to 11 mmag, and a median internal astrometric precision of similar to 27 mas. The median coadded catalog depth for a 1.'' 95 diameter aperture at signal-to-noise ratio = 10 is g = 24.7, r = 24.4, i = 23.8, z = 23.1, and Y = 21.7 mag. DES DR2 includes similar to 691 million distinct astronomical objects detected in 10,169 coadded image tiles of size 0.534 deg(2) produced from 76,217 single-epoch images. After a basic quality selection, benchmark galaxy and stellar samples contain 543 million and 145 million objects, respectively. These data are accessible through several interfaces, including interactive image visualization tools, web-based query clients, image cutout servers, and Jupyter notebooks. DES DR2 constitutes the largest photometric data set to date at the achieved depth and photometric precision.

We describe the Dark Energy Survey (DES) photometric data set assembled from the first three years of science operations to support DES Year 3 cosmologic analyses, and provide usage notes aimed at the broad astrophysics community. Y3 GOLD improves on previous releases from DES, Y1 GOLD, and Data Release 1 (DES DR1), presenting an expanded and curated data set that incorporates algorithmic developments in image detrending and processing, photometric calibration, and object classification. Y3 GOLD comprises nearly 5000 deg(2) of grizY imaging in the south Galactic cap, including nearly 390 million objects, with depth reaching a signal-to-noise ratio similar to 10 for extended objects up to i (AB) similar to 23.0, and top-of-the-atmosphere photometric uniformity <3 mmag. Compared to DR1, photometric residuals with respect to Gaia are reduced by 50%, and per-object chromatic corrections are introduced. Y3 GOLD augments DES DR1 with simultaneous fits to multi-epoch photometry for more robust galactic color measurements and corresponding photometric redshift estimates. Y3 GOLD features improved morphological star-galaxy classification with efficiency >98% and purity >99% for galaxies with 19 < i (AB) < 22.5. Additionally, it includes per-object quality information, and accompanying maps of the footprint coverage, masked regions, imaging depth, survey conditions, and astrophysical foregrounds that are used to select the cosmologic analysis samples.

Luminous quasars at z > 5.6 can be studied in detail with the current generation of telescopes and provide us with unique information on the first gigayear of the universe. Thus far, these studies have been statistically limited by the number of quasars known at these redshifts. Such quasars are rare, and therefore, wide-field surveys are required to identify them, and multiwavelength data are required to separate them efficiently from their main contaminants, the far more numerous cool dwarfs. In this paper, we update and extend the selection for the z similar to 6 quasars presented in Banados et al. (2014) using the Pan- STARRS1 (PS1) survey. We present the PS1 distant quasar sample, which currently consists of 124 quasars in the redshift range 5.6 less than or similar to z less than or similar to 6.7 that satisfy our selection criteria. Of these quasars, 77 have been discovered with PS1, and 63 of them are newly identified in this paper. We present the composite spectra of the PS1 distant quasar sample. This sample spans a factor of similar to 20 in luminosity and shows a variety of emission line properties. The number of quasars at z > 5.6 presented in this work almost doubles the previously known quasars at these redshifts, marking a transition phase from studies of individual sources to statistical studies of the high-redshift quasar population, which was impossible with earlier, smaller samples.

We present statistics from a survey of intervening Mg II absorption toward 100 quasars with emission redshifts between z - 3.55 and z - 7.09. Using infrared spectra from Magellan/FIRE, we detect 280 cosmological Mg II absorbers, and confirm that the comoving line density of W-r > 0.3 angstrom Mg II absorbers does not evolve measurably between z = 0.25 and z = 7. This is consistent with our detection of seven Mg II systems at z > 6, redshifts not covered in prior searches. Restricting to systems with W-r > 1 angstrom, there is significant evidence for redshift evolution. These systems roughly double in density between z = 0 and z = 2-3, but decline by an order of magnitude from this peak by z similar to 6. This evolution mirrors that of the global star formation rate density, potentially reflecting a connection between star formation feedback and the strong Mg II absorbers. We compared our results to the Illustris cosmological simulation at z = 2-4 by assigning absorption to cataloged dark matter halos and by direct extraction of spectra from the simulation volume. Reproducing our results using the former requires circumgalactic Mg II envelopes within halos of progressively smaller mass at earlier times. This occurs naturally if we define the lower integration cutoff using SFR rather than mass. Spectra calculated directly from Illustris yield too few strong Mg II absorbers. This may arise from unresolved phase space structure of circumgalactic gas, particularly from spatially unresolved turbulent or bulk motions. The presence of circumgalactic magnesium at z > 6 suggests that enrichment of intra-halo gas may have begun before the presumed host galaxies' stellar populations were mature and dynamically relaxed.

Ratios of different ions of the same element encode ionization information independently from relative abundances in quasar absorption line systems, crucial for understanding the multiphase nature and origin of absorbing gas, particularly at z > 6 where H I cannot be observed. Observational considerations have limited such studies to a small number of sightlines, with most surveys at z > 6 focused upon the statistical properties of individual ions such as Mg II or C IV. Here we compare high- and low-ionization absorption within 69 intervening systems at z > 5, including 16 systems at z > 6, from Magellan/FIRE spectra of 47 quasars together with a Keck/High Resolution Echelle Spectrometer (HIRES) spectrum of the "ultraluminous" z = 6.3 quasar SDSSJ010013.02 +280225.8. The highest redshift absorbers increasingly exhibit low-ionization species alone, consistent with previous single-ion surveys that show the frequency of Mg II is unchanging with redshift, while C IV absorption drops markedly toward z = 6. We detect no C IV or Si IV in half of all metal-line absorbers at z > 5.7, with stacks not revealing any slightly weaker C IV just below our detection threshold, and most of the other half have N-C II > N-C (IV). In contrast, only 20% of absorbers at 5.0-5.7 lack high-ionization gas, and a search of 25 HIRES sightlines at z similar to 3 yielded zero such examples. We infer that these low-ionization high-redshift absorption systems may be analogous to metal-poor damped Ly alpha systems (similar to 1% of the absorber population at z similar to 3), based on incidence rates and absolute and relative column densities. Simple photoionization models suggest that circumgalactic matter at redshift six has systematically lower chemical abundances and experiences a softer ionizing background relative to redshift three.

We present the discovery of PSO J083.8371+11.8482, a weak emission line quasar with extreme star formation rate at z = 6.3401. This quasar was selected from Pan-STARRS1, UHS, and unWISE photometric data. Gemini/GNIRS spectroscopy follow-up indicates a Mg ii-based black hole mass of M-BH = (2.0-(+0.7)(0.4)) x 10(9) M-circle dot and an Eddington ratio of = L-bol/E-dd =0.5(-0.2)(+0.1), in line with an actively accreting supermassive black hole (SMBH) at z greater than or similar to 6. Hubble Space Telescope imaging sets strong constraint on lens boosting, showing no relevant effect on the apparent emission. The quasar is also observed as a pure point source with no additional emission component. The broad-line region (BLR) emission is intrinsically weak and not likely caused by an intervening absorber. We found rest-frame equivalent widths of EW (Mg II)(rest) = 8.7 +/- 0.7 angstrom. A small proximity zone size (R-p = 1.2 +/- 0.4 pMpc) indicates a lifetime of only t(Q) = 10(3.4 +/- 0.7) years from the last quasar phase ignition. ALMA shows extended [C II] emission with a mild velocity gradient. The inferred far-infrared luminosity (L-FIR = 1.2 +/- 0.1) x 10(13) L-circle dot) is one of the highest among all known quasar hosts at z greater than or similar to 6. Dust and [C II] emissions put a constraint on the star formation rate of SFR = 900-4900 M-circle dot yr(-1), similar to that of a hyperluminous infrared galaxy. Considering the observed quasar lifetime and BLR formation timescale, the weak-line profile in the quasar spectrum is most likely caused by a BLR that is not yet fully formed rather than by continuum boosting by gravitational lensing or a soft continuum due to super-Eddington accretion.

Neutron star mergers offer unique conditions for the creation of the heavy elements, and additionally provide a testbed for our understanding of this synthesis known as the r-process. We have performed dynamical nucleosynthesis calculations and identified a single isotope, Cf-254, which has a particularly high impact on the brightness of electromagnetic transients associated with mergers on the order of 15 to 250 days. This is due to the anomalously long half-life of this isotope and the efficiency of fission thermalization compared to other nuclear channels. We estimate the fission fragment yield of this nucleus and outline the astrophysical conditions under which Cf-254. has the greatest impact to the light curve. Future observations in the mid-infrared, which are bright during this regime, could indicate the production of actinide nucleosynthesis.