Quasar Quartet Embedded in Giant Nebula Reveals Rare Massive Structure in Distant Universe

All galaxies once passed through a hyperluminous quasar phase powered by accretion onto a supermassive black hole. But because these episodes are brief, quasars are rare objects typically separated by cosmological distances. In a survey for Lyman-alpha emission at redshift z ~ 2, we discovered a physical association of four quasars embedded in a giant nebula. Located within a substantial overdensity of galaxies, this system is probably the progenitor of a massive galaxy cluster. The chance probability of finding a quadruple quasar is estimated to be ~10^-7, implying a physical connection between Lyman-alpha nebulae and the locations of rare protoclusters. Our findings imply that the most massive structures in the distant universe have a tremendous supply (~ 10^11 solar masses) of cool dense (volume density ~1 cm^-3) gas, which is in conflict with current cosmological simulations.

Deep HeII and CIV Spectroscopy of a Giant Lyman alpha Nebula: Dense Compact Gas Clumps in the Circumgalactic Medium of a z~2 Quasar

The recent discovery by Cantalupo et al. (2014) of the largest (~500 kpc) and luminous Ly-alpha nebula associated with the quasar UM287 (z=2.279) poses a great challenge to our current understanding of the astrophysics of the halos hosting massive z~2 galaxies. Either an enormous reservoir of cool gas is required $Msimeq10^{12}$ $M_{odot}$, exceeding the expected baryonic mass available, or one must invoke extreme gas clumping factors not present in high-resolution cosmological simulations. However, observations of Ly-alpha emission alone cannot distinguish between these two scenarios. We have obtained the deepest ever spectroscopic integrations in the HeII and CIV lines with the goal of detecting extended line emission, but detect neither line to a 3$sigma$ limiting SB $simeq10^{-18}$ erg/s/cm$^2$/arcsec$^2$. We construct models of the expected emission spectrum in the highly probable scenario that the nebula is powered by photoionization from the central hyper-luminous quasar. The non-detection of HeII implies that the nebular emission arises from a mass $M_{rm c}lesssim6.4times10^{10}$ $M_{odot}$ of cool gas on ~200 kpc scales, distributed in a population of remarkably dense ($n_{rm H}gtrsim3$ cm$^{-3}$) and compact ($Rlesssim20$ pc) clouds, which would clearly be unresolved by current cosmological simulations. Given the large gas motions suggested by the Ly-alpha line ($vsimeq$ 500 km/s), it is unclear how these clouds survive without being disrupted by hydrodynamic instabilities. Our study serves as a benchmark for future deep integrations with current and planned wide-field IFU such as MUSE, KCWI, and KMOS. Our work suggest that a $simeq$ 10 hr exposure would likely detect ~10 rest-frame UV/optical emission lines, opening up the possibility of conducting detailed photoionization modeling to infer the physical state of gas in the CGM.

Lyman-alpha Forest Tomography from Background Galaxies: The First Megaparsec-Resolution Large-Scale Structure Map at z>2

We present the first observations of foreground Lyman-$alpha$ forest absorption from high-redshift galaxies, targeting 24 star-forming galaxies (SFGs) with $zsim 2.3-2.8$ within a $5 times 15$ region of the COSMOS field. The transverse sightline separation is $sim 2,h^{-1}mathrm{Mpc}$ comoving, allowing us to create a tomographic reconstruction of the 3D Ly$alpha$ forest absorption field over the redshift range $2.20leq zleq 2.45$. The resulting map covers $6,h^{-1}mathrm{Mpc} times 14,h^{-1}mathrm{Mpc}$ in the transverse plane and $230,h^{-1}mathrm{Mpc}$ along the line-of-sight with a spatial resolution of $approx 3.5,h^{-1}mathrm{Mpc}$, and is the first high-fidelity map of large-scale structure on $simmathrm{Mpc}$ scales at $z>2$. Our map reveals significant structures with $gtrsim 10,h^{-1}mathrm{Mpc}$ extent, including several spanning the entire transverse breadth, providing qualitative evidence for the filamentary structures predicted to exist in the high-redshift cosmic web. Simulated reconstructions with the same sightline sampling, spectral resolution, and signal-to-noise ratio recover the salient structures present in the underlying 3D absorption fields. Using data from other surveys, we identified 18 galaxies with known redshifts coeval with our map volume enabling a direct comparison to our tomographic map. This shows that galaxies preferentially occupy high-density regions, in qualitative agreement with the same comparison applied to simulations. Our results establishes the feasibility of the CLAMATO survey, which aims to obtain Ly$alpha$ forest spectra for $sim 1000$ SFGs over $sim 1 ,mathrm{deg}^2$ of the COSMOS field, in order to map out IGM large-scale structure at $langle z rangle sim 2.3$ over a large volume $(100,h^{-1}mathrm{Mpc})^3$.

Metal-enriched, sub-kiloparsec gas clumps in the circumgalactic medium of a faint z = 2.5 galaxy

We report the serendipitous detection of a 0.2 L$^*$, Lyman-$alpha$ emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright background QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption system ($N_mathrm{HI}=10^{16.94pm0.10}$ cm$^{-2}$) with many associated metal absorption lines at the same redshift as the foreground galaxy. Using photoionization models that carefully treat measurement errors and marginalise over uncertainties in the shape and normalisation of the ionizing radiation spectrum, we derive the total hydrogen column density $N_mathrm{H}=10^{19.4pm0.3}$ cm$^{-2}$, and show that all the absorbing clouds are metal enriched, with $Z=0.1$-$0.6 Z_odot$. These metallicities and the systems large velocity width ($436$ km$,$s$^{-1}$) suggest the gas is produced by an outflowing wind. Using an expanding shell model we estimate a mass outflow rate of $sim5 M_odot,$yr$^{-1}$. Our photoionization model yields extremely small sizes ($<$100-500 pc) for the absorbing clouds, which we argue are typical of high column density absorbers in the circumgalactic medium (CGM). Given these small sizes and extreme kinematics, it is unclear how the clumps survive in the CGM without being destroyed by hydrodynamic instabilities. The small cloud sizes imply that even state-of-the-art cosmological simulations require more than a $1000$-fold improvement in mass resolution to resolve the hydrodynamics relevant for cool gas in the CGM.

Quasars Probing Quasars VI. Excess HI Absorption Within One Proper Mpc of z~2 Quasars

With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasars environment in absorption. We use a sample of 650 projected quasar pairs to study the HI Lya absorption transverse to luminous, z~2 quasars at proper separations of 30kpc < R < 1Mpc. In contrast to measurements along the line-of-sight, regions transverse to quasars exhibit enhanced HI Lya absorption and a larger variance than the ambient intergalactic medium, with increasing absorption and variance toward smaller scales. Analysis of composite spectra reveals excess absorption characterized by a Lya equivalent width profile W = 2.3A (R/100kpc)^-0.46. We also observe a high (~60%) covering factor of strong, optically thick HI absorbers (HI column log NHI > 17.3) at separations R<200kpc, which decreases to ~20% at R~1Mpc, but still represents a significant excess over the cosmic average. This excess of optically thick absorption can be described by a quasar-absorber cross-correlation function xi_QA(r) = (r/r_0)^gamma with a large correlation length r_0 = 12.5+2.7-1.4 Mpc/h (comoving) and gamma = 1.68+0.14-0.30. The HI absorption measured around quasars exceeds that of any previously studied population, consistent with quasars being hosted by massive dark matter halos Mhalo~10^12.5 Msun at z~2.5. The environments of these massive halos are highly biased towards producing optically thick gas, and may even dominate the cosmic abundance of Lyman limit systems and hence the intergalactic opacity to ionizing photons at z~2.5. The anisotropic absorption around quasars implies the transverse direction is much less likely to be illuminated by ionizing radiation than the line-of-sight, which we interpret in terms of the same obscuration effects frequently invoked in unified models of active galactic nuclei.

Metal-Poor, Cool Gas in the Circumgalactic Medium of a z = 2.4 Star-Forming Galaxy: Direct Evidence for Cold Accretion?

In our current galaxy formation paradigm, high-redshift galaxies are predominantly fuelled by accretion of cool, metal-poor gas from the intergalactic medium. Hydrodynamical simulations predict that this material should be observable in absorption against background sightlines within a galaxys virial radius, as optically thick Lyman-limit systems (LLSs) with low metallicities. Here we report the discovery of exactly such a strong metal-poor absorber at an impact parameter R_perp = 58 kpc from a star-forming galaxy at z = 2.44. Besides strong neutral hydrogen [N(HI) = 10^(19.50 +/- 0.16) cm^-2] we detect neutral deuterium and oxygen, allowing a precise measurement of the metallicity: log10(Z / Zsolar) = -2.0 +/- 0.17, or (7-15) x 10^-3 solar. Furthermore, the narrow deuterium linewidth requires a cool temperature < 20,000 K. Given the striking similarities between this system and the predictions of simulations, we argue that it represents the direct detection of a high redshift cold-accretion stream. The low-metallicity gas cloud is a single component of an absorption system exhibiting a complex velocity, ionization, and enrichment structure. Two other components have metallicities > 0.1 solar, ten times larger than the metal-poor component. We conclude that the photoionized circumgalactic medium (CGM) of this galaxy is highly inhomogeneous: the majority of the gas is in a cool, metal-poor and predominantly neutral phase, but the majority of the metals are in a highly-ionized phase exhibiting weak neutral hydrogen absorption but strong metal absorption. If such inhomogeneity is common, then high-resolution spectra and detailed ionization modelling are critical to accurately appraise the distribution of metals in the high-redshift CGM.

Quasars Probing Quasars IV: Joint Constraints on the Circumgalactic Medium from Absorption and Emission

We have constructed a sample of 29 close projected quasar pairs where the background quasar spectrum reveals absorption from optically thick HI gas associated with the foreground quasar. These unique sightlines allow us to study the quasar circumgalactic medium (CGM) in absorption and emission simultaneously, because the background quasar pinpoints large concentrations of gas where Ly-a emission, resulting from quasar-powered fluorescence, resonant Ly-a scattering, and/or cooling radiation, is expected. A sensitive slit-spectroscopic search (1-sigma limits of SB_Lya ~= 3e-18 erg/s/cm^2/arcsec^2) for diffuse Ly-a emission in the environments of the foreground quasars is conducted. We fail to detect large-scale ~ 100 kpc Ly-a emission, either at the location of the optically thick absorbers or in the foreground quasar halos, in all cases except a single system. We interpret these non-detections as evidence that the gas detected in absorption is shadowed from the quasar UV radiation due to obscuration effects, which are frequently invoked in unified models of AGN. Small-scale R_perp <~ 50 kpc extended Ly-a nebulosities are detected in 34% of our sample, which are likely the high-redshift analogs of the extended emission-line regions commonly observed around low-redshift (z < 0.5) quasars. We also detect a compact high rest-frame equivalent width (W_Lya > 50 A) Ly-alpha-emitter with luminosity L_Lya =2.1+-0.32e41 erg/s at small impact parameter R_perp=134 kpc from one foreground quasar, and argue that it is more likely to result from quasar-powered fluorescence, than simply be a star-forming galaxy clustered around the quasar. Our observations imply that much deeper integrations with upcoming integral-field spectrometers such as MUSE and KCWI will be able to routinely detect a diffuse Ly-a glow around bright quasars on scales R ~ 100 kpc and thus directly image the CGM. [abridged]

Think Outside the Color Box: Probabilistic Target Selection and the SDSS-XDQSO Quasar Targeting Catalog

We present the SDSS-XDQSO quasar targeting catalog for efficient flux-based quasar target selection down to the faint limit of the Sloan Digital Sky Survey (SDSS) catalog, even at medium redshifts (2.5 <~ z <~ 3) where the stellar contamination is significant. We build models of the distributions of stars and quasars in flux space down to the flux limit by applying the extreme-deconvolution method to estimate the underlying density. We convolve this density with the flux uncertainties when evaluating the probability that an object is a quasar. This approach results in a targeting algorithm that is more principled, more efficient, and faster than other similar methods. We apply the algorithm to derive low-redshift (z < 2.2), medium-redshift (2.2 <= z <= 3.5), and high-redshift (z > 3.5) quasar probabilities for all 160,904,060 point sources with dereddened i-band magnitude between 17.75 and 22.45 mag in the 14,555 deg^2 of imaging from SDSS Data Release 8. The catalog can be used to define a uniformly selected and efficient low- or medium-redshift quasar survey, such as that needed for the SDSS-IIIs Baryon Oscillation Spectroscopic Survey project. We show that the XDQSO technique performs as well as the current best photometric quasar-selection technique at low redshift, and outperforms all other flux-based methods for selecting the medium-redshift quasars of our primary interest. We make code to reproduce the XDQSO quasar target selection publicly available.

MASE: A New Data--Reduction Pipeline for the Magellan Echellette Spectrograph

We introduce a data reduction package written in Interactive Data Language (IDL) for the Magellan Echellete Spectrograph (MAGE). MAGE is a medium-resolution (R ~4100), cross-dispersed, optical spectrograph, with coverage from ~3000-10000 Angstroms. The MAGE Spectral Extractor (MASE) incorporates the entire image reduction and calibration process, including bias subtraction, flat fielding, wavelength calibration, sky subtraction, object extraction and flux calibration of point sources. We include examples of the user interface and reduced spectra. We show that the wavelength calibration is sufficient to achieve ~5 km/s RMS accuracy and relative flux calibrations better than 10%. A light-weight version of the full reduction pipeline has been included for real-time source extraction and signal-to-noise estimation at the telescope.

Quasars Probing Quasars III: New Clues to Feedback, Quenching, and the Physics of Massive Galaxy Formation

Galaxies hosting z~2 quasars are the high-$z$ progenitors of todays massive `red-and-dead galaxies. With close pairs of quasars at different redshifts, a background quasar can be used to study a foreground quasars halo gas in absorption, providing a wealth of information about feedback, quenching, and the physics of massive galaxy formation. We present a Keck/HIRES spectrum of the bright background quasar in a projected pair with angular separation 13.3 corresponding to 108kpc at the redshift of the foreground quasar z_fg=2.4360 +/- 0.0005, precisely determined from Gemini/GNIRS near-IR spectroscopy. Our echelle spectrum reveals optically thick gas (NHI~10^19.7), coincident with the foreground quasar redshift. The ionic transitions of associated metal-lines reveal the following properties of the foreground quasars halo: (1) the kinematics are extreme with absorption extending to +780km/s relative to z_fg; (2) the metallicity is nearly solar; (3) the temperature of the predominantly ionized gas is T<~20,000K; (4) the electron density is n_e~1 cm^-3 indicating a characteristic size ~10 - 100pc for the absorbing `clouds; (7) there is a negligible amount of warm gas 10^5K < T < 10^6K; (8) the gas is unlikely illuminated by the foreground quasar, implying anisotropic or intermittent emission. The mass of cold T~10^4K gas implied by our observations is significant, amounting to a few percent of the total expected baryonic mass density of the foreground quasars dark halo at r~100kpc. The origin of this material is still unclear, and we discuss several possibilities in the context of current models of feedback and massive galaxy formation.