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.

We present beta-delayed neutron emission and beta-delayed fission ( beta df) calculations for heavy, neutron-rich nuclei using the coupled Quasi-Particle Random Phase Approximation plus Hauser-Feshbach (QRPA+HF) approach. From the initial population of a compound nucleus after beta-decay, we follow the statistical decay, taking into account competition between neutrons, gamma-rays, and fission. We find a region of the chart of nuclides where the probability of beta df is similar to 100%, which likely prevents the production of superheavy elements in nature. For a subset of nuclei near the neutron dripline, neutron multiplicity and the probability of fission are both large, leading to the intriguing possibility of multi-chance beta df, a decay mode for extremely neutron-rich heavy nuclei. In this decay mode, beta-decay can be followed by multiple neutron emission, leading to subsequent daughter generations that each have a probability to fission. We explore the impact of beta df in rapid neutron-capture process (gamma-process) nucleosynthesis in the tidal ejecta of a neutron star-neutron star merger and show that it is a key fission channel that shapes the final abundances near the second gamma-process peak.

The rapid neutron-capture ("r-") process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far they fail to account for the observed overenhancement of actinides, present in about 30% of r-process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger (NSM) and explore whether varying levels of neutron-richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, beta-decay, and mass model. For most cases, the actinides are overproduced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. We present a simple actinidedilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of an NSM are likely production sites for the formation of actinides, a significant contribution from another site or sites (e.g., the NSM accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars.

The possibility that nucleosynthesis in neutron star mergers may reach fissioning nuclei introduces significant uncertainties in predicting the relative abundances of r-process material from such events. We evaluate the impact of using sets of fission yields given by the 2016 GEF code for spontaneous (sf), neutron-induced ((n, f)), and beta-delayed (beta df) fission processes which take into account the approximate initial excitation energy of the fissioning compound nucleus. We further explore energy-dependent fission dynamics in the r process by considering the sensitivity of our results to the treatment of the energy sharing and de-excitation of the fission fragments using the FREYA code. We show that the asymmetric-to-symmetric yield trends predicted by GEF 2016 can reproduce the high-mass edge of the second r-process peak seen in solar data and examine the sensitivity of this result to the mass model and astrophysical conditions applied. We consider the effect of fission yields and barrier heights on the nuclear heating rates used to predict kilonova light curves. We find that fission barriers influence the contribution of Cf-254 spontaneous fission to the heating at similar to 100 d, such that a light curve observation consistent with such late- time heating would both confirm that actinides were produced in the event and imply the fission barriers are relatively high along the Cf-254 beta-feeding path. We lastly determine the key nuclei responsible for setting the r-process abundance pattern by averaging over thirty trajectories from a 1.2-1.4 M-circle dot neutron star merger simulation. We show it is largely the odd-N nuclei undergoing (Z, N)(n, f) and (Z, N) beta df that control the relative abundances near the second peak. We find the 'hot spots' for beta-delayed and neutron-induced fission given all mass models considered and show most of these nuclei lie between the predicted N = 184 shell closure and the location of currently available experimental decay data.

The astrophysical production site of the heaviest elements in the universe remains a mystery. Incorporating heavy element signatures of metal-poor, r-process enhanced stars into theoretical studies of r-process production can offer crucial constraints on the origin of heavy elements. In this study, we introduce and apply the "Actinide-Dilution with Matching" model to a variety of stellar groups ranging from actinide-deficient to actinide-enhanced to empirically characterize r-process ejecta mass as a function of electron fraction. We find that actinide-boost stars do not indicate the need for a unique and separate r-process progenitor. Rather, small variations of neutron richness within the same type of r-process event can account for all observed levels of actinide enhancements. The very low-Y-e, fission-cycling ejecta of an r-process event need only constitute 10-30% of the total ejecta mass to accommodate most actinide abundances of metal-poor stars. We find that our empirical Y-e distributions of ejecta are similar to those inferred from studies of GW170817 mass ejecta ratios, which is consistent with neutron-star mergers being a source of the heavy elements in metal-poor, r-process enhanced stars.

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title 'The R-Process Alliance: First Release from the Southern Search for r-Process-Enhanced Stars in the Galactic Halo.' (bibcode: 2018ApJ...858...92H) Copyright: Refer to CDS usage

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title 'RAVE J203843.2-002333: the first highly R-process-enhanced star identified in the RAVE survey.' (bibcode: 2017ApJ...844...18P) Copyright: Refer to CDS usage