The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 2012 June 13, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 to 79 keV, extending the sensitivity of focusing far beyond the ∼10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than 100-fold improvement in sensitivity over the collimated or coded mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity and spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives: (1) probe obscured active galactic nucleus (AGN) activity out to the

We use SDSS+GALEX+Galaxy Zoo data to study the quenching of star formation in lowredshift galaxies. We show that the green valley between the blue cloud of star-forming galaxies and the red sequence of quiescent galaxies in the colour-mass diagram is not a single transitional state through which most blue galaxies evolve into red galaxies. Rather, an analysis that takes morphology into account makes clear that only a small population of blue earlytype galaxies move rapidly across the green valley after the morphologies are transformed from disk to spheroid and star formation is quenched rapidly. In contrast, the majority of blue star-forming galaxies have significant disks, and they retain their late-type morphologies as their star formation rates decline very slowly. We summarize a range of observations that lead to these conclusions, including UV-optical colours and halo masses, which both show a striking dependence on morphological type. We interpret these results in terms of the evolution of cosmic gas supply and gas reservoirs. We conclude that late-type galaxies are consistent with a scenario where the cosmic supply of gas is shut off, perhaps at a critical halo mass, followed by a slow exhaustion of the remaining gas over several Gyr, driven by secular and/or environmental processes. In contrast, early-type galaxies require a scenario where the gas supply and gas reservoir are destroyed virtually instantaneously, with rapid quenching accompanied by a morphological transformation from disk to spheroid. This gas reservoir destruction could be the consequence of a major merger, which in most cases transforms galaxies from disk to elliptical morphology, and mergers could play a role in inducing black hole accretion and possibly AGN feedback.

Hard X-ray (10 keV) observations of active galactic nuclei (AGNs) can shed light on some of the most obscured episodes of accretion onto supermassive black holes. The 70-month Swift/BAT all-sky survey, which probes the 14-195 keV energy range, has currently detected 838 AGNs. We report here on the broadband X-ray (0.3-150 keV) characteristics of these AGNs, obtained by combining XMM-Newton, Swift/XRT, ASCA, Chandra, and Suzaku observations in the soft X-ray band ( 10  keV) with 70-month averaged Swift/BAT data. The nonblazar AGNs of our sample are almost equally divided into unobscured (N 10 cm H 22 2 < -) and obscured (N 10 cm H 22 2  -) AGNs, and their Swift/BAT continuum is systematically steeper than the 0.3-10 keV emission, which suggests that the presence of a high-energy cutoff is almost ubiquitous. We discuss the main X-ray spectral parameters obtained, such as the photon index, the reflection parameter, the energy of the cutoff, neutral and ionized absorbers, and the soft excess for both obscured and unobscured AGNs.

We describe the results of an extremely deep, 0.28 deg 2 survey for z ¼ 3:1 Ly emission-line galaxies in the Extended Chandra Deep FieldYSouth. By using a narrowband 5000 8 filter and complementary broadband photometry from the MUSYC survey, we identify a statistically complete sample of 162 galaxies with monochromatic fluxes brighter than 1:5 ; 10 À17 ergs cm À2 s À1 and observer's frame equivalent widths greater than 80 8. We show that the equivalent width distribution of these objects follows an exponential with a rest-frame scale length of w 0 ¼ 76In addition, we show that in the emission line, the luminosity function of Ly galaxies has a faint-end power-law slope of ¼ À1:49 þ0:45 À0:34 , a bright-end cutoff of log L Ã ¼ 42:64 þ0:26 À0:15 , and a space density above our detection thresholds of (1:46 AE 0:12) ; 10 À3 h 3 70 galaxies Mpc À3 . Finally, by comparing the emission-line and continuum properties of the Ly emitters, we show that the star formation rates derived from Ly are $3 times lower than those inferred from the rest-frame UV continuum. We use this offset to deduce the existence of a small amount of internal extinction within the host galaxies. This extinction, coupled with the lack of extremely high equivalent width emitters, argues that these galaxies are not primordial Population III objects, although they are young and relatively chemically unevolved.

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We studied the clustering properties and multiwavelength spectral energy distributions of a complete sample of 162 Ly-emitting (LAE) galaxies at z ' 3:1 discovered in deep narrowband MUSYC imaging of the Extended Chandra Deep Field-South. LAEs were selected to have observed frame equivalent widths >80 8 and emission line fluxes >1:5 ; 10 À17 ergs cm À2 s À1. Only 1% of our LAE sample appears to host AGNs. The LAEs exhibit a moderate spatial correlation length of r 0 ¼ 3:6 þ0:8 À1:0 Mpc, corresponding to a bias factor b ¼ 1:7 þ0:3 À0:4 , which implies median dark matter halo masses of log 10 M med ¼ 10:9 þ0:5 À0:9 M . Comparing the number density of LAEs, 1:5 AE 0:3 ; 10 À3 Mpc À3, with the number density of these halos finds a mean halo occupation $1%Y10%. The evolution of galaxy bias with redshift implies that most z ¼ 3:1 LAEs evolve into present-day galaxies with L < 2:5L Ã , whereas other z > 3 galaxy populations typically evolve into more massive galaxies. Halo merger trees show that z ¼ 0 descendants occupy halos with a wide range of masses, with a median descendant mass close to that of L Ã . Only 30% of LAEs have sufficient stellar mass (>$3 ;

We investigate the X-ray number counts in the 1-2 Ms Chandra Deep Fields (CDFs) to determine the contributions of faint X-ray source populations to the extragalactic X-ray background (XRB). X-ray sources were separated into Active Galactic Nuclei (AGN), star-forming galaxies, and Galactic stars based primarily on X-ray-to-optical flux ratios, optical spectral classifications, X-ray spectra, and intrinsic X-ray luminosities. Number-count slopes and normalizations below 2 × 10 −15 erg cm −2 s −1 were calculated in each band for all source types assuming a single power-law model. We find that AGN continue to dominate the number counts in the 0.5-2.0 keV and 2-8 keV bands. At flux limits of ≈ 2.5 × 10 −17 erg cm −2 s −1 (0.5-2.0 keV) and ≈ 1.4 × 10 −16 erg cm −2 s −1 (2-8 keV), the overall AGN source densities are 7166 +304 −292 sources deg −2 and 4558 +216 −207 sources deg −2 , respectively; these are factors of ∼ 10-20 higher than found in the deepest optical spectroscopic surveys. While still a minority, the number counts of star-forming galaxies climb steeply such that they eventually achieve source densities of 1727 +187 −169 sources deg −2 (0.5-2.0 keV) and 711 +270 −202 sources deg −2 (2-8 keV) at the CDF flux limits. The number of star-forming galaxies will likely overtake the number of AGN at ∼ 1 × 10 −17 erg cm −2 s −1 (0.5-2.0 keV) and dominate the overall number counts thereafter. Adopting XRB flux densities of (7.52 ± 0.35) × 10 −12 erg cm −2 s −1 deg −2 (0.5-2.0 keV) and (2.24 ± 0.11) × 10 −11 erg cm −2 s −1 deg −2 (2-8 keV), the CDFs resolve a total of 89.5 +5.9 −5.7 % and 86.9 +6.6 −6.3 % of the extragalactic 0.5-2.0 keV and 2-8 keV XRBs, respectively. AGN as a whole contribute ≈83% and ≈95% to the these resolved XRB fractions, respectively, while star-forming galaxies comprise only ≈3% and ≈2%, respectively, and Galactic stars comprise the remainder. Extrapolation of the number-count slopes can easily account for the entire 0.5-2.0 keV and 2-8 keV XRBs to within statistical errors. We additionally examine the X-ray number counts as functions of intrinsic Xray luminosity and absorption, finding that sources with L 0.5−8 keV > 10 43.5 erg s −1 and N H < 10 22 cm −2 are the dominant contributors to the 0.5-2.0 keV XRB flux density, while sources with L 0.5−8 keV = 10 42.5 -10 44.5 erg s −1 and a broad range of absorption column densities primarily contribute to the 2-8 keV XRB flux density. This trend suggests that even less intrinsically luminous, more highly obscured AGN may dominate the number counts at higher energies where the XRB intensity peaks. Finally, we revisit the reported differences between the CDF-North and CDF-South number counts, finding that the two fields are consistent with each other except for 2-8 keV detected sources below F 2−8 keV ≈ 1 × 10 −15 erg cm −2 s −1 , where deviations gradually increase to ≈ 3.9σ.

We study the incidence of nuclear obscuration on a complete sample of 1310 AGN selected on the basis of their rest-frame 2-10 keV X-ray flux from the XMM-COSMOS survey, in the redshift range 0.3 < z < 3.5. We classify the AGN as obscured or un-obscured on the basis of either the optical spectral properties and the overall SED or the shape of the X-ray spectrum. The two classifications agree in about 70% of the objects, and the remaining 30% can be further subdivided into two distinct classes: at low luminosities X-ray un-obscured AGN do not always show signs of broad lines or blue/UV continuum emission in their optical spectra, most likely due to galaxy dilution effects; at high luminosities broad line AGN may have absorbed X-ray spectra, which hints at an increased incidence of small-scale (subparsec) dust-free obscuration. We confirm that the fraction of obscured AGN is a decreasing function of the intrinsic X-ray luminosity, while the incidence of absorption shows significant evolution only for the most luminous AGN, which appear to be more commonly obscured at higher redshift. We find no significant difference between the mean stellar masses and star formation rates of obscured and un-obscured AGN hosts. We conclude that the physical state of the medium responsible for obscuration in AGN is complex, and mainly determined by the radiation environment (nuclear luminosity) in a small region enclosed within the gravitational sphere of influence of the central black hole, but is largely insensitive to the wider scale galactic conditions.