The future carbon balance of boreal ecosystems under increasing temperatures is highly uncertain. In particular, the net effects of a longer growing season versus enhanced respiration are poorly understood. Here, we use a geostatistical inverse model from 2012 to 2014 to determine temperature sensitivity in Alaskan biomes throughout the growing season, in order to identify the relative effects of these competing phenomena. We find that temperature explains a large portion of the disparities in autumn carbon flux between 2013 and 2014. Boreal forests experienced a growing season extension during the warm October of 2013 that offset increased respiration into autumn in years with high temperatures. In contrast, increased temperatures in the tundra and shrublands led to a large respiration signal during October 2013, producing a greater net carbon release. These results suggest a greater vulnerability of Alaskan tundra and shrubland carbon stocks compared to boreal forest carbon stocks under warming.

Agricultural intensification in India has increased nitrogen pollution, leading to water quality impairments. The fate of reactive nitrogen applied to the land is largely unknown, however. Long-term records of riverine nitrogen fluxes are nonexistent and drivers of variability remain unexamined, limiting the development of nitrogen management strategies. Here, we leverage dissolved inorganic nitrogen (DIN) and discharge data to characterize the seasonal, annual, and regional variability of DIN fluxes and their drivers for seven major river basins from 1981 to 2014. We find large seasonal and interannual variability in nitrogen runoff, with 68% to 94% of DIN fluxes occurring in June through October and with the coefficient of variation across years ranging from 44% to 93% for individual basins. This variability is primarily explained by variability in precipitation, with year- and basin-specific annual precipitation explaining 52% of the combined regional and interannual variability. We find little correlation with rising fertilizer application rates in five of the seven basins, implying that agricultural intensification has thus far primarily impacted groundwater and atmospheric emissions rather than riverine runoff. These findings suggest that riverine nitrogen runoff in India is highly sensitive to projected future increases in precipitation and intensification of the seasonal monsoon, while the impact of projected continued land use intensification is highly uncertain.

We have detected Ly alpha emission from a damped Ly alpha system (DLA) that lies near the bright quasar HS1549+1919. The DLA has the same redshift as HS1549+1919 and was discovered in the spectrum of a faint QSO that lies 4900 away (380 proper kpc). The emission line's luminosity, double-peaked profile, and small spatial separation from the DLA suggest that it may be fluorescent Ly alpha emission from gas that is absorbing the nearby QSO's radiation. If this is the case, our observations show that the DLA has a size of at least 1."5 and that the QSO's luminosity 1 million years ago was similar to its luminosity today. A survey for similar systems within similar to 1' of bright QSOs would put interesting limits on the mean quasar lifetime.

To date, all of the reported hypervelocity stars (HVSs), which are believed to be ejected from the Galactic center, are blue and therefore almost certainly young. Old-population HVSs could be much more numerous than the young ones that have been discovered, but still have escaped detection because they are hidden in a much denser background of Galactic halo stars. Discovery of these stars would shed light on star formation at the Galactic center, would constrain the mechanism by which they are ejected from it, and, if they prove numerous, would enable detailed studies of the structure of the dark halo. We analyze the problem of finding these stars and show that the search should be concentrated around the main-sequence turnoff (0: 3 < g - i < 1.1) at relatively faint magnitudes (19: 5 < g < 21: 5). If the ratio of turnoff stars to B stars is the same for HVSs as it is in the local disk, such a search would yield about 1 old-population HVS per 45 deg(2). A telescope similar to the Sloan 2.5 m could search about 20 deg2 per night, implying that such a population, should it exist, would show up in interesting numbers in short order.

We present a method for obtaining accurate black hole (BH) mass estimates from the Mg II emission line in active galactic nuclei (AGNs). Employing the large database of AGN measurements from the Sloan Digital Sky Survey (SDSS) presented by Shen et al., we find that AGNs in the redshift range 0.3-0.9, for which a given object can have both H beta and Mg II line widths measured, display a modest but correctable discrepancy in Mg II-based masses that correlates with the Eddington ratio. We use the SDSS database to estimate the probability distribution of the true (i.e., H beta-based) mass given a measured Mg II line width. These probability distributions are then applied to the SDSS measurements from Shen et al. across the entire Mg II-accessible redshift range (0.3-2.2). We find that accounting for this residual correlation generally increases the dispersion of Eddington ratios by a small factor (similar to 0.09 dex for the redshift and luminosity bins we consider). We continue to find that the intrinsic distribution of Eddington ratios for luminous AGNs is extremely narrow, 0.3-0.4 dex, as demonstrated by Kollmeier et al. Using the method we describe, Mg II emission lines can be used with confidence to obtain BH mass estimates.

We report on the discovery of a bright Ly alpha blob associated with the z = 3 quasar SDSS J124020.91+145535.6 which is also coincident with strong damped Ly alpha absorption from a foreground galaxy (a so-called proximate damped Ly alpha (PDLA) system). The one-dimensional spectrum acquired by the Sloan Digital Sky Survey (SDSS) shows a broad Ly alpha emission line with a FWHM similar or equal to 500 km s(-1) and a luminosity of L-Ly alpha = 3.9 x 10(43) erg s(-1) superposed on the trough of the PDLA. Follow-up observations using the Keck/LRIS spectrometer confirm that this source has a Ly alpha nebula with spatial extent exceeding 5 '', corresponding to a proper size > 39 kpc. Mechanisms for powering the large Ly alpha luminosity in this nebula are discussed. We use a Monte Carlo radiative transfer simulation to investigate the possibility that the line emission is fluorescent recombination radiation from a kpc-scale PDLA galaxy powered by the ionizing flux of the quasar, but find that the predicted Ly alpha flux is several orders of magnitude lower than observed. We conclude that the Ly alpha emission is not associated with the PDLA galaxy at all, but instead is intrinsic to the quasar's host and similar to the extended Ly alpha "fuzz" which is detected around many active galactic nuclei. PDLAs are natural coronagraphs that block their background quasar at Ly alpha and we discuss how systems similar to SDSS J124020.91+145535.6 might be used to image the neutral hydrogen in the PDLA galaxy in silhouette against the screen of extended Ly alpha emission from the background quasar.

We present limits on the ejection of solar-metallicity ("metal-rich") old-population hypervelocity stars (HVS) from a sample of over 290,000 stars selected from the Sloan Digital Sky Survey. We derive the speed at the solar circle from the measured positions and radial velocities (RVs) by assuming a radial orbit and adopting a simple isothermal model of the Galactic halo, which enables us to identify candidate bound and unbound ejectees. We examine the kinematics and metallicity distribution of this sample and find no metal-rich ejectees, from which our limits are derived. However, while tuned for metal-rich stars, our experiment is also sensitive to metal-poor ejectees. We find four candidate bound metal-poor F-stars from this sample, all with negative Galactocentric RV (i.e., returning toward the Galactic center ( GC)). We additionally find two candidate metal-poor unbound stars (one F and one G). However, existing proper-motion measurements of these two stars make them unlikely to be emerging from the GC. The metal-rich nondetection places a limit on the rate of ejection of old-population stars from the GC of <35 Myr(-1). Comparing to the rate for more massive B-star ejectees of similar to 0.6 Myr(-1), our limit on the rate of ejection of old-population HVS shows that the mass function at the GC is not bottom-heavy and is consistent with being normal to top-heavy. Future targeted surveys of old-population HVS could determine if it is indeed top-heavy.

We present spectroscopic confirmation of the "Pisces Overdensity," also known as "Structure J," a photometric overdensity of RR Lyrae stars discovered by the Sloan Digital Sky Survey at an estimated photometric distance of similar to 85 kpc. We measure radial velocities for eight RR Lyrae stars within Pisces. We find that five of the eight stars have heliocentric radial velocities within a narrow range of -87 km s(-1) < v(r) < -67 km s(-1), suggesting that the photometric overdensity is mainly due to a physically associated system, probably a dwarf galaxy or a disrupted galaxy. Two of the remaining three stars differ from one another by only 9 km s(-1), but it would be premature to identify them as a second system.

We present predictions for the fluorescent Ly alpha emission signature arising from photoionized, optically thick structures in smoothed particle hydrodynamic cosmological simulations of a Lambda CDM universe using a Monte Carlo Ly alpha radiative transfer code. We calculate the expected Ly alpha image and two-dimensional spectra for gas exposed to a uniform ultraviolet ionizing background as well as gas exposed additionally to the photoionizing radiation from a local quasar, after correcting for the self-shielding of hydrogen. As a test of our numerical methods and for application to current observations, we examine simplified analytic structures that are uniformly or anisotropically illuminated. We compare these results with recent observations. We discuss future observing campaigns on large telescopes and realistic strategies for detecting fluorescence owing to the ambient metagalactic ionization and in regions close to bright quasars. While it will take hundreds of hours on the current generation of telescopes to detect fluorescence caused by the ultraviolet background alone, our calculations suggest that on the order of 10 sources of quasar-induced fluorescent Ly alpha emission should be detectable after a 10 hr exposure in a 10 arcmin(2) field around a bright quasar. These observations will help probe the physical conditions in the densest regions of the intergalactic medium as well as the temporal light curves and isotropy of quasar radiation.

We analyse cosmological hydrodynamic simulations that include theoretically and observationally motivated prescriptions for galactic outflows. If these simulated winds accurately represent winds in the real Universe, then material previously ejected in winds provides the dominant source of gas infall for new star formation at redshifts z < 1. This recycled wind accretion, or wind mode, provides a third physically distinct accretion channel in addition to the 'hot' and 'cold' modes emphasized in recent theoretical studies. The recycling time of wind material (t(rec)) is shorter in higher mass systems owing to the interaction between outflows and the increasingly higher gas densities in and around higher mass haloes. This differential recycling plays a central role in shaping the present-day galaxy stellar mass function (GSMF), because declining t(rec) leads to increasing wind mode galaxy growth in more massive haloes. For the three feedback models explored, the wind mode dominates above a threshold mass that primarily depends on wind velocity; the shape of the GSMF therefore can be directly traced back to the feedback prescription used. If we remove all particles that were ever ejected in a wind, then the predicted GSMFs are much steeper than observed. In this case, galaxy masses are suppressed both by the ejection of gas from galaxies and by the hydrodynamic heating of their surroundings, which reduces subsequent infall. With wind recycling included, the simulation that incorporates our favoured momentum-driven wind scalings reproduces the observed GSMF for stellar masses 109 M(circle dot) < M < 5 x 1010 M(circle dot). At higher masses, wind recycling leads to excessive galaxy masses and star formation rates relative to observations. In these massive systems, some quenching mechanism must suppress not only the direct accretion from the primordial intergalactic medium but the re-accretion of gas ejected from star-forming galaxies. In short, as has long been anticipated, the form of the GSMF is governed by outflows; the unexpected twist here for our simulated winds is that it is not primarily the ejection of material but how the ejected material is re-accreted that governs the GSMF.