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.

We present new limits on the ejection of metal-rich old-population hypervelocity stars (HVSs) from the Galactic center (GC) as probed by the SEGUE-2 survey. Our limits are a factor of 3-10 more stringent than previously reported, depending on stellar type. Compared to the known population of B-star ejectees, there can be no more than 30 times more metal-rich old-population F/G stars ejected from the GC. Because B stars comprise a tiny fraction of a normal stellar population, this places significant limits on the combination of the GC mass function and the ejection mechanism for HVSs. In the presence of a normal GC mass function, our results require an ejection mechanism that is about 5.5 times more efficient at ejecting B stars compared to low-mass F/G stars.

The intergalactic medium (IGM) is the dominant reservoir of baryons at all cosmic epochs. In this paper, we investigate the evolution of the IGM from z = 2 -> 0 in (48h(-1) Mpc)(3), 110-million particle cosmological hydrodynamic simulations using three prescriptions for galactic outflows. We focus on the evolution of IGM physical properties, and how such properties are traced by Ly alpha absorption as detectable using Hubble's Cosmic Origins Spectrograph (COS). Our results broadly confirm the canonical picture that most Ly alpha absorbers arise from highly ionized gas tracing filamentary large-scale structure. Growth of structure causes gas to move from the diffuse photoionized IGM into other cosmic phases, namely stars, cold and hot gas within galaxy haloes, and the unbound and shock-heated warm-hot intergalactic medium (WHIM). By today, baryons are comparably divided between bound phases (35 per cent in our favoured outflow model), the diffuse IGM (41 per cent) and the WHIM (24 per cent). Here we (re) define the WHIM as gas with overdensities lower than that in haloes (rho/(rho) over bar less than or similar to 100 today) and temperatures T > 10(5) K, to more closely align it with the 'missing baryons' that are not easily detectable in emission or Ly alpha absorption. Strong galactic outflows can have a noticeable impact on the temperature of the IGM, though with our favoured momentum-driven wind scalings they do not. When we (mildly) tune our assumed photoionizing background to match the observed evolution of the Ly alpha mean flux decrement, we obtain line count evolution statistics that broadly agree with available (pre-COS) observations. We predict a column density distribution slope of f(N(HI)) alpha N(HI)(-1.70) for our favoured wind model, in agreement with recent observational estimates, and it becomes shallower with redshift. Winds have a mostly minimal impact, but they do result in a shallower column density slope and more strong lines. With improved statistics, the frequency of strong lines can be a valuable diagnostic of outflows, and the momentum-driven wind model matches existing data significantly better than the two alternatives we consider. The relationship between column density and physical density broadens mildly from z = 2 -> 0, and evolves as rho alpha N(HI)(0.74)10(-0.37z) for diffuse absorbers, consistent with previous studies. Linewidth distributions are quite sensitive to spectral resolution; COS should yield significantly broader lines than higher resolution data. Thermal contributions to linewidths are typically subdominant, so linewidths only loosely reflect the temperature of the absorbing gas. This will hamper attempts to quantify the WHIM using broad Ly alpha absorbers, though it may still be possible to do so statistically. Together, COS data and simulations such as these will provide key insights into the physical conditions of the dominant reservoir of baryons over the majority of cosmic time.

The Sloan Digital Sky Survey (SDSS) started a new phase in 2008 August, with new instrumentation and new surveys focused on Galactic structure and chemical evolution, measurements of the baryon oscillation feature in the clustering of galaxies and the quasar Ly alpha forest, and a radial velocity search for planets around similar to 8000 stars. This paper describes the first data release of SDSS-III (and the eighth counting from the beginning of the SDSS). The release includes five-band imaging of roughly 5200 deg(2) in the southern Galactic cap, bringing the total footprint of the SDSS imaging to 14,555 deg(2), or over a third of the Celestial Sphere. All the imaging data have been reprocessed with an improved sky-subtraction algorithm and a final, self-consistent photometric recalibration and flat-field determination. This release also includes all data from the second phase of the Sloan Extension for Galactic Understanding and Exploration (SEGUE-2), consisting of spectroscopy of approximately 118,000 stars at both high and low Galactic latitudes. All the more than half a million stellar spectra obtained with the SDSS spectrograph have been reprocessed through an improved stellar parameter pipeline, which has better determination of metallicity for high-metallicity stars.