On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0 × 10 −21 . It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410 These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

We have investigated the mass-metallicity (M-Z ) relation using galaxies at 0:4 < z < 1:0 from the Gemini Deep Deep Survey (GDDS) and Canada-France Redshift Survey (CFRS). Deep K-and z 0 -band photometry allowed us to measure stellar masses for 69 galaxies. From a subsample of 56 galaxies, for which metallicity of the interstellar medium is also measured, we identified a strong correlation between mass and metallicity for the first time in the distant universe. This was possible because of the larger baseline spanned by the sample in terms of metallicity (a factor of 7) and mass (a factor of 400) than in previous works. This correlation is much stronger and tighter than the luminosity-metallicity relation, confirming that stellar mass is a more meaningful physical parameter than luminosity. We find clear evidence for temporal evolution in the M-Z relation in the sense that at a given mass, a galaxy at z $ 0:7 tends to have lower metallicity than a local galaxy of similar mass. We use the z $ 0:1 Sloan Digital Sky Survey M-Z relation and a small sample of z $ 2:3 Lyman break galaxies with known mass and metallicity to propose an empirical redshift-dependent M-Z relation. According to this relation the stellar mass and metallicity in small galaxies evolve for a longer time than they do in massive galaxies. This relation predicts that the generally metal-poor damped Ly galaxies have stellar masses of the order of 10 8:8 M (with a dispersion of 0.7 dex) all the way from z $ 0:2 to 4. The observed redshift evolution of the M-Z relation can be reproduced remarkably well by a simple closed-box model in which the key assumption is an e-folding time for star formation that is higher or, in other words, a period of star formation that lasts longer in less massive galaxies than in more massive galaxies. Such a picture supports the downsizing scenario for galaxy formation.

We report results from a survey of Mg II absorbers in the spectra of background QSOs that are within close angular distances to a foreground galaxy at z < 0.5, using the Magellan Echellette Spectrograph. We have established a spectroscopic sample of 94 galaxies at a median redshift of z = 0.24 in fields around 70 distant background QSOs (z QSO > 0.6), 71 of which are in an 'isolated' environment with no known companions and located at ρ 120 h −1 kpc from the line of sight of a background QSO. The rest-frame absolute B-band magnitudes span a range from M B − 5 log h = −16.4 to M B − 5 log h = −21.4 and rest-frame B AB − R AB colors range from B AB − R AB ≈ 0 to B AB − R AB ≈ 1.5. Of these 'isolated' galaxies, we find that 47 have corresponding Mg II absorbers in the spectra of background QSOs and rest-frame absorption equivalent width W r (2796) = 0.1 − 2.34 Å, and 24 do not give rise to Mg II absorption to sensitive upper limits. Our analysis shows that (1) W r (2796) declines with increasing distance from 'isolated' galaxies but shows no clear trend in 'group' environments;(2) more luminous galaxies possess more extended Mg II absorbing halos with the gaseous radius scaled by B-band luminosity according to R gas = 75 × (L B /L B * ) (0.35±0.03) h −1 kpc; (3) there is little dependence between the observed absorber strength and galaxy intrinsic colors; and (4) within R gas , we find a mean covering fraction of κ 0.3 ≈ 70% for absorbers of W r (2796) ≥ 0.3 Å and κ 0.1 ≈ 80% for absorbers of W r (2796) ≥ 0.1 Å. The results confirm that extended Mg II absorbing halos are a common and generic feature around ordinary galaxies and that the gaseous radius is a fixed fraction of the dark matter halo radius. The lack of correlation between W r (2796) strength and galaxy colors suggests a lack of physical connection between the origin of extended Mg II halos and recent star formation history of the galaxies. Finally, we discuss the total gas mass in galactic halos as traced by Mg II absorbers. We also compare our results with previous studies. Subject headings: cosmology: observations-intergalactic medium-quasars: absorption lines 1 This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.

We analyze the association of galaxies to Lyα and O VI absorption, the most commonly detected transitions in the low-z intergalactic medium (IGM), in the fields of 14 quasars with z em = 0.06-0.57. Confirming previous studies, we observe a high covering fraction for Lyα absorption to impact parameter ρ = 300 h −1 72 kpc: 33/37 of our L > 0.01L * galaxies show Lyα equivalent width W Lyα ≥ 50 mÅ. Galaxies of all luminosity L > 0.01L * and spectral type are surrounded by a diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is estimated at ∼ 10 10.5±0.3 M ⊙ for a constant N H . The virialized halos and extended CGM of present-day galaxies are responsible for most strong Lyα absorbers (W Lyα > 300 mÅ) but cannot reproduce the majority of observed lines in the Lyα forest. We conclude that the majority of Lyα absorption with W Lyα = 30-300 mÅ occurs in the cosmic web predicted by cosmological simulations and estimate a characteristic width for these filaments of ≈ 400 h −1 72 kpc. Regarding O VI, we observe a near unity covering fraction to ρ = 200 h −1 72 kpc for L > 0.1L * galaxies and to ρ = 300 h −1 72 kpc for sub-L * (0.1L * < L < L * ) galaxies. Similar to our Lyα results, stronger O VI systems (W 1031 > 70 mÅ) arise in the virialized halos of L > 0.1L * galaxies. Unlike Lyα, the weaker O VI systems (W 1031 ≈ 30 mÅ) arise in the extended CGM of sub-L * galaxies. The majority of O VI gas observed in the low-z IGM is associated with a diffuse medium surrounding individual galaxies with L ≈ 0.3L * , and rarely originates in the so-called warm-hot IGM (WHIM) predicted by cosmological simulations.

We model the escape of ionizing radiation from high-redshift galaxies using high-resolution Adaptive Mesh Refinement N-body+hydrodynamics simulations. Our simulations include time-dependent and spatiallyresolved transfer of ionizing radiation in three dimensions, including effects of dust absorption. For galaxies of total mass M 10 11 M ⊙ and star formation rates SFR ≈ 1 − 5 M ⊙ yr −1 , we find angular averaged escape fractions of 1 − 3% over the entire redshift interval studied (3 < z < 9). In addition, we find that the escape fraction varies by more than an order of magnitude along different lines-of-sight within individual galaxies, from the largest values near galactic poles to the smallest along the galactic disk. The escape fraction declines steeply at lower masses and SFR. We show that the low values of escape fractions are due to a small fraction of young stars located just outside the edge of HI disk. This fraction, and hence the escape fraction, is progressively smaller in disks of smaller galaxies because their HI disks are thicker and more extended relative to the distribution of young stars compared to massive galaxies. Our results suggest that high-redshift galaxies are inefficient in releasing ionizing radiation produced by young stars into the intergalactic medium. We compare our predicted escape fraction of ionizing photons with previous results, and find a general agreement with both other simulation results and available direct detection measurements at z ∼ 3. We also compare our simulations with a novel method to estimate the escape fraction in galaxies from the observed distribution of neutral hydrogen column densities along the lines of sights to long duration γ-ray bursts. Using this method we find escape fractions of the GRB host galaxies of 2 − 3%, consistent with our theoretical predictions.

We examine the cosmic star formation rate (SFR) and its dependence on galaxy stellar mass over the redshift range using data from the Gemini Deep Deep Survey (GDDS). The SFR in the most massive galaxies 0.8 ! z ! 2 ( ) was 6 times higher at than it is today. It drops steeply from , reaching the present-. In contrast, the SFR density of intermediate-mass galaxies ( ) 10.2 10.8declines more slowly and may peak or plateau at . We use the characteristic growth time z ∼ 1.5 t { SFR to provide evidence of an associated transition in massive galaxies from a burst to a quiescent star r /r M SFR * formation mode at . Intermediate-mass systems transit from burst to quiescent mode at , while the z ∼ 2 z ∼ 1 lowest mass objects undergo bursts throughout our redshift range. Our results show unambiguously that the formation era for galaxies was extended and proceeded from high-to low-mass systems. The most massive galaxies formed most of their stars in the first ∼3 Gyr of cosmic history. Intermediate-mass objects continued to form their dominant stellar mass for an additional ∼2 Gyr, while the lowest mass systems have been forming over the whole cosmic epoch spanned by the GDDS. This view of galaxy formation clearly supports "downsizing" in the SFR where the most massive galaxies form first and galaxy formation proceeds from larger to smaller mass scales.

SUMMARY RNA import into mammalian mitochondria is considered essential for replication, transcription, and translation of the mitochondrial genome but the pathway(s) and factors that control this import are poorly understood. Previously, we localized polynucleotide phosphorylase (PNPASE), a 3′ → 5′ exoribonuclease and poly-A polymerase, in the mitochondrial intermembrane space, a location lacking resident RNAs. Here, we show a new role for PNPASE in regulating the import of nuclear-encoded RNAs into the mitochondrial matrix. PNPASE reduction impaired mitochondrial RNA processing and polycistronic transcripts accumulated. Augmented import of RNase P, 5S rRNA, and MRP RNAs depended on PNPASE expression and PNPASE–imported RNA interactions were identified. PNPASE RNA processing and import activities were separable and a mitochondrial RNA targeting signal was isolated that enabled RNA import in a PNPASE-dependent manner. Combined, these data strongly support an unanticipated role for PNPASE in mediating the translocation of RNAs into mitochondria.

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