1,720,982 research outputs found
THE LAST EIGHT-BILLION YEARS OF INTERGALACTIC C IV EVOLUTION
Full text linkWe surveyed the Hubble Space Telescope UV spectra of 49 low-redshift quasars for z < 1 C IV candidates, relying solely on the characteristic wavelength separation of the doublet. After consideration of the defining traits of C IV doublets (e.g., consistent line profiles, other associated transitions, etc.), we defined a sample of 38 definite (group G = 1) and five likely (G = 2) doublets with rest equivalent widths Wr for both lines detected at . We conducted Monte Carlo completeness tests to measure the unblocked redshift (Δz) and co-moving path length (ΔX) over which we were sensitive to C IV doublets of a range of equivalent widths and column densities. The absorber line density of (G = 1+2) doublets is for log N(C+3) ≥ 13.2, and has not evolved significantly since z = 5. The best-fit power law to the G = 1 frequency distribution of column densities has coefficient k = 0.67+0.18 –0.16 × 10–14 cm2 and exponent α N = –1.50+0.17 –0.19, where N 0 = 1014 cm–2. Using the power-law model of f(N(C+3)), we measured the C+3 mass density relative to the critical density: for 13 ≤ log N(C+3) ≤ 15. This value is a 2.8 ± 0.7 increase in compared to the error-weighted mean from several 1 < z < 5 surveys for C IV absorbers. A simple linear regression to over the age of the universe indicates that has slowly but steadily increased from z = 5 → 0, with .Space Telescope Science Institute (U.S.) (HST archival grant 10679)National Science Foundation (U.S.) (NSF CAREER grant AST 05_48180)United States. National Aeronautics and Space Administration (NASA contract NAS 5-2655
PRECIOUS METALS IN SDSS QUASAR SPECTRA. II. TRACKING THE EVOLUTION OF STRONG, 0.4 < z < 2.3 Mg II ABSORBERS WITH THOUSANDS OF SYSTEMS
Full text linkWe have performed an analysis of over 34,000 Mg II doublets at 0.36 < z < 2.29 in Sloan Digital Sky Survey (SDSS) Data Release 7 quasar spectra; the catalog, advanced data products, and tools for analysis are publicly available. The catalog was divided into 14 small redshift bins with roughly 2500 doublets in each and from Monte Carlo simulations, we estimate 50% completeness at rest equivalent width W [subscript r] ≈ 0.8 Å. The equivalent width frequency distribution is described well by an exponential model at all redshifts, and the distribution becomes flatter with increasing redshift, i.e., there are more strong systems relative to weak ones. Direct comparison with previous SDSS Mg II surveys reveals that we recover at least 70% of the doublets in these other catalogs, in addition to detecting thousands of new systems. We discuss how these surveys came by their different results, which qualitatively agree but because of the very small uncertainties, differ by a statistically significant amount. The estimated physical cross section of Mg II-absorbing galaxy halos increased approximately threefold from z = 0.4 to z = 2.3, while the W [subscript r] ≥ 1 Å absorber line density, dN[subscript Mg II/d X, grew by roughly 45%. Finally, we explore the different evolution of various absorber populations—damped Lyα absorbers, Lyman limit systems, strong C IV absorbers, and strong and weaker Mg II systems—across cosmic time (0 < z < 6).National Science Foundation (U.S.) (Grant AST-0908920)National Science Foundation (U.S.) (Grant AST-1109915
PROBING THE INTERGALACTIC MEDIUM/GALAXY CONNECTION. V. ON THE ORIGIN OF Lyα AND O VI ABSORPTION AT z < 0.2
No full textWe analyze the association of galaxies with Lyα and O VI absorption, the most commonly detected transitions of the low-z intergalactic medium (IGM), in the fields of 14 quasars with z[subscript em] = 0.06–0.57. Confirming previous studies, we observe a high covering fraction for Lyα absorption to impact parameter ρ = 300 h[superscript −1][subscript 72] kpc: 33/37 of our L > 0.01L* galaxies show Lyα equivalent width W[superscript Lyα] 50 [bar under >] 50 mÅ. Galaxies of all luminosity L > 0.01L* and spectral type are surrounded by a diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is estimated at ∼10[superscript 10.5±0.3]M [subscript ☉] for a constant N[subscript H] = 10[superscript 19] cm[superscript −2]. The virialized halos and extended CGM of present-day galaxies are responsible for most strong Lyα absorbers (W[superscript Lyα] > 300 mÅ) but cannot reproduce the majority of observed lines in the Lyα forest. We conclude that the majority of Lyα absorption with W[superscript Lyα] = 30–300 mÅ occurs in the cosmic web predicted by cosmological simulations and estimate a characteristic width for these filaments of ≈400 h[superscript −1][subscript 72] kpc. Regarding O VI, we observe a near unity covering fraction to ρ = 200 h[superscript −1][subscript 72] kpc for L > 0.1L* galaxies and to ρ = 300 h[superscript −1][subscript 72] kpc for sub-L*(0.1L* 70mÅ) arise in the virialized halos of L > 0.1L* galaxies. Unlike Lyα, the weaker O VI systems (W[superscript 1031] ≈ 30 mÅ) arise in the extended CGM of sub-L* galaxies. The majority of O [VI] gas observed in the low-z IGM is associated with a diffuse medium surrounding individual galaxies with L ≈ 0.3L* and rarely originates in the so-called warm-hot IGM (predicted by cosmological simulations
PROBING THE IGM/GALAXY CONNECTION. IV. THE LCO/WFCCD GALAXY SURVEY OF 20 FIELDS SURROUNDING UV-BRIGHT QUASARS
No full textWe publish the survey for galaxies in 20 fields containing ultraviolet bright quasars (with z [subscript em] ≈ 0.1-0.5) that can be used to study the association between galaxies and absorption systems from the low-z intergalactic medium (IGM). The survey is magnitude limited (R ≈ 19.5 mag) and highly complete out to 10' from the quasar in each field. It was designed to detect dwarf galaxies (L ≈ 0.1L*) at an impact parameter ρ ≈ 1 Mpc (z = 0.1) from a quasar. The complete sample (all 20 fields) includes R-band photometry for 84,718 sources and confirmed redshifts for 2800 sources. This includes 1198 galaxies with 0.005 < z < (z [subscript em] – 0.01) at a median redshift of 0.18, which may associated with IGM absorption lines. All of the imaging was acquired with cameras on the Swope 40'' telescope and the spectra were obtained via slit mask observations using the WFCCD spectrograph on the Dupont 100'' telescope at Las Campanas Observatory. This paper describes the data reduction, imaging analysis, photometry, and spectral analysis of the survey. We tabulate the principal measurements for all sources in each field and provide the spectroscopic data set online
Precious Metals in SDSS Quasar Spectra. I. Tracking the Evolution of Strong, 1.5 < z < 4.5 C IV Absorbers with Thousands of Systems
Full text linkWe have vastly increased the C IV statistics at intermediate redshift by surveying the thousands of quasars in the Sloan Digital Sky Survey (SDSS) Data-Release 7. We visually verified over 16,000 C IV systems with 1.46 5 (infrared) surveys, we see an approximately 10-fold increase in dNCIV/dX over z ≈ 6 → 0, for W r ≥ 0.6 Å. This suggests a monotonic and significant increase in the enrichment of gas outside galaxies over the 12 Gyr lifetime of the universe.National Science Foundation (U.S.) (Astronomy & Astrophysics Postdoctoral Fellowship (AST-1003139))Alfred P. Sloan Foundation (Research Fellowship)Massachusetts Institute of Technology. Dept. of Physic
Mg II ABSORPTION AT 2 < z < 6 WITH MAGELLAN/FIRE. II. A LONGITUDINAL STUDY OF HI, METALS, AND IONIZATION IN GALACTIC HALOS
Full text linkWe present a detailed study of H I and metals for 110 Mg II absorption systems discovered at 1.98 12 Gyr). We observe a significant strengthening in the characteristic N(H I) for fixed Mg II equivalent width as one moves toward higher redshift. Indeed, at our sample's mean [bar over z] = 3.402, all Mg II systems are either damped Lyα absorbers (DLAs) or sub-DLAs, with 40.7% of systems exceeding the DLA threshold (compared to 16.7% at [bar over z] = 0.927). We set lower limits on the metallicity of the Mg II systems where we can measure H I; these results are consistent with the full DLA population. The classical Mg II systems (W[λ2796 over 0] = 0.3-1.0 A), which preferentially associate with sub-DLAs, are quite metal rich at ~0.1 solar. We applied quantitative classification metrics to our absorbers to compare with low-redshift populations, finding that weak systems are similar to classic Mg II absorbers at low redshift. The strong systems either have very large Mg II and Fe II velocity spreads implying non-virialized dynamics or are more quiescent DLAs. There is tentative evidence that the kinetically complex systems evolve in similar fashion to the global star formation rate. We speculate that if weaker Mg II systems represent accreting gas as suggested by recent studies of galaxy-absorber inclinations, then their high metal abundance suggests re-accretion of recently ejected material rather than first-time infall from the metal-poor intergalactic medium, even at early times.National Science Foundation (U.S.) (Grant AST-0908920)National Science Foundation (U.S.) (Grant AST-1109115)Massachusetts Institute of Technology. Undergraduate Research Opportunities Progra
THE INCIDENCE OF LOW-METALLICITY LYMAN-LIMIT SYSTEMS AT z ~ 3.5: IMPLICATIONS FOR THE COLD-FLOW HYPOTHESIS OF BARYONIC ACCRETION
Full text linkCold accretion is a primary growth mechanism of simulated galaxies, yet observational evidence of "cold flows" at redshifts where they should be most efficient (z = 2–4) is scarce. In simulations, cold streams manifest as Lyman-limit absorption systems (LLSs) with low heavy-element abundances similar to those of the diffuse intergalactic medium (IGM). Here we report on an abundance survey of 17 H i-selected LLSs at z = 3.2–4.4 which exhibits no metal absorption in Sloan Digital Sky Survey spectra. Using medium-resolution spectra obtained at Magellan, we derive ionization-corrected metallicities (or limits) with a Markov-chain Monte Carlo sampling that accounts for the large uncertainty in N[subscript H i] measurements typical of LLSs. The metal-poor LLS sample overlaps with the IGM in metallicity and can be described by a model where 71[+13 over -11]% are drawn from the IGM chemical abundance distribution. These represent roughly half of all LLSs at these redshifts, suggesting that 28%–40% of the general LLS population at z ~ 3.7 could trace accreting gas. An ancillary sample of ten LLSs without any a priori metal-line selection is fit by a model having 48[+14 over -12]% of metallicities drawn from the IGM. We compare these results with regions of a moving-mesh simulation. The observed and simulated LLS metallicity distributions are in good agreement, after accounting for known uncertainties in both, with the fraction of simulated baryons in IGM-metallicity LLSs within a factor of two of the observed value. A statistically significant fraction of all LLSs have low metallicity and therefore represent candidates for accreting gas; large-volume simulations can establish what fraction of these candidates actually lie near galaxies and the observational prospects for detecting the presumed hosts in emission
THE LAST EIGHT-BILLION YEARS OF INTERGALACTIC Si IV EVOLUTION
No full textWe identified 24 Si iv 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 Wr for both lines detected at ≥3σWr. The absorber line density of the G = 1doublets was dNSiiv/dX=1.4+0.4−0.3for 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)×10−14cm2 and slope α N=−16+0.3−0.3. Using the power-law model off(N(Si+3)), we measured the Si+3 mass density relative to the critical density: Si+3=(3.7+2.8−1.7)×10−8for 13logN(Si+3)15. From Monte Carlo sampling of the distributions, we estimated our value to be a factor of 4.8+3.0−1.9higher than the 2z4.5Si+3. From a simple linear fit to Si+3 over the age of the universe, we estimated a slow and steady increase from z=5.5→0 with dSi+3/d Ω age =(0.61±0.23)×10−8Gyr−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 medianN(Si+3)/N(C+3) =0.16.United States. National Aeronautics and Space Administration (NASA contract NAS5-32985)Space Telescope Science Institute (U.S.) (NASA contract NAS 5-26555)Space Telescope Science Institute (U.S.) (HST archival grant 10679)National Science Foundation (U.S.) (NSF CAREER grant AST 05_48180)Massachusetts Institute of Technology. Department of Physic
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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