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.

The Last Eight-Billion Years of Intergalactic SiIV Evolution

We identified 24 SiIV absorption systems with z <~ 1 from a blind survey of 49 low-redshift quasars with archival Hubble Space Telescope ultraviolet spectra. We relied solely on the characteristic wavelength separation of the doublet to automatically detect candidates. After visual inspection, we defined a sample of 20 definite (group G = 1) and 4 highly-likely (G = 2) doublets with rest equivalent widths W_r for both lines detected at > 3 sigma. The absorber line density of the G = 1 doublets was dN_SiIV/dX = 1.4+0.4/-0.3 for log N(Si+3) > 12.9. The best-fit power law to the G = 1 frequency distribution of column densities f(N(Si+3)) had normalization k = (1.2+0.5/-0.4) x 10^-14 cm2 and slope alpha = -1.6+0.3/-0.3. Using the power-law model of f(N(Si+3)), we measured the Si+3 mass density relative to the critical density: Omega(Si+3) = (3.7+2.8/-1.7) x 10^-8 for 13 < log N(Si+3) < 15. From Monte Carlo sampling of the distributions, we estimated our value to be a factor of 4.8+3.0/-1.9 higher than the 2 < z < 4.5 <Omega(Si+3)>. From a simple linear fit to Omega(Si+3) over the age of the Universe, we estimated a slow and steady increase from z = 5.5 --> 0 with dOmega/dt_age = (0.61+/-0.23) x 10^-8 Gyr^-1. We compared our ionic ratios N(Si+3)/N(C+3) to a 2 < z < 4.5 sample and concluded, from survival analysis, that the two populations are similar, with median <N(Si+3)/N(C+3)> = 0.16.

Metal-Enriched Plasma in Protogalactic Halos: A Survey of N V Absorption in High-z Damped & Sub-Damped Lyman-alpha Systems

We continue our recent work to characterize the plasma content of high-redshift damped and sub-damped Lyman-alpha systems (DLAs/sub-DLAs), which represent multi-phase gaseous (proto)galactic disks and halos seen toward a background source. We survey N V absorption in a sample of 91 DLAs and 18 sub-DLAs in the redshift range 1.67<z<4.28 with unblended coverage of the N V doublet, using data from VLT/UVES, Keck/HIRES, and Keck/ESI. In DLAs, we find eight secure N V detections, four marginal detections, and 79 non-detections. The detection rate of N V in DLAs is therefore 13^{+5}_{-4}%. Two sub-DLA N V detections are found among a sample of 18, at a similar detection rate of 11^{+15}_{-7}%. We show that the N V detection rate is a strong function of metallicity, increasing by a factor of ~4 at [N/H]=[NI/HI]>-2.3. The N V and CIV component b-value distributions in DLAs are statistically similar, but the median b(N V) of 18 km/s is lower than the median b(O VI) of 25 km/s. Some ~20% of the N V components have b<10 km/s and thus arise in warm photoionized plasma at log (T/K)<4.92; local sources of ionizing radiation (as opposed to the extragalactic background) are required to keep the cloud sizes physically reasonable. The nature of the remaining ~80% of (broad) N V components is unclear; models of radiatively-cooling collisionally-ionized plasma at log(T/K)=5.2-5.4 are fairly successful in reproducing the observed integrated high-ion column density ratios and the component line widths, but we cannot rule out photoionization by local sources. Finally, we identify several unusual DLAs with extremely low metallicity (<0.01 solar) but strong high-ion absorption [log N(N V)>14 or log N(O VI)>14.2] that present challenges to either galactic inflow or outflow models.

A z = 3 Lyman Alpha Blob Associated with a Damped Lyman Alpha System Proximate to its Background Quasar

We report on the discovery of a bright Lyman alpha blob associated with the z=3 quasar SDSSJ124020.91+145535.6 which is also coincident with strong damped Lyman alpha absorption from a foreground galaxy (a so-called proximate damped Lyman alpha system; PDLA). The one dimensional spectrum acquired by the Sloan Digital Sky Survey (SDSS) shows a broad Lyman alpha emission line with a FWHM ~ 500 km/s and a luminosity of L_{Lya} = 3.9e43 erg/s superposed on the trough of the PDLA. Mechanisms for powering this large Lyman alpha luminosity are discussed. We argue against emission from HII regions in the PDLA galaxy since this requires an excessive star-formation rate ~ 500 Msun/yr and would correspond to the largest Lyman alpha luminosity ever measured from a damped Lyman alpha system or starburst galaxy. We use a Monte Carlo radiative transfer simulation to investigate the possibility that the line emission is fluorescent recombination radiation from the PDLA galaxy powered by the ionizing flux of the quasar, but find that the predicted Lyman alpha flux is several orders of magnitude lower than observed. We conclude that the Lyman alpha emission is not associated with the PDLA galaxy at all, but instead is intrinsic to the quasars host and similar to the extended Lyman alpha fuzz which is detected around many AGN. PDLAs are natural coronagraphs that block their background quasar at Lyman alpha, and we discuss how systems similar to SDSSJ124020.91+145535.6 might be used to image the neutral hydrogen in the PDLA galaxy in silhouette against the screen of extended Lyman alpha emission from the background quasar.