Damped and sub-damped Lyman-α absorbers in z > 4 QSOs

Astronomy and Astrophysics (Impact Factor: 5.08). 12/2009; DOI: 10.1051/0004-6361/200811541
Source: OAI

ABSTRACT We present the results of a survey of damped (DLA, log N(H I) > 20.3) and sub-damped Lyman-α systems (19.5 < log N(H I) < 20.3) at z > 2.55 along the lines-of-sight to 77 quasars with emission redshifts in the range 4 < z_(em) < 6.3. Intermediate resolution (R ~ 4300) spectra were obtained with the Echellette Spectrograph and Imager (ESI) mounted on the Keck telescope. A total of 100 systems with log N(H I) > 19.5 were detected of which 40 systems are damped Lyman-α systems for an absorption length of ΔX = 378. About half of the lines of sight of this homogeneous survey have never been investigated for DLAs. We study the evolution with redshift of the cosmological density of the neutral gas and find, consistent with previous studies at similar resolution, that Ω_(DLA,HI) decreases at z > 3.5. The overall cosmological evolution of Ω_(HI) shows a peak around this redshift. The H I column density distribution for log N(H I) ≥ 20.3 is fitted, consistent with previous surveys, with a single power-law of index α ~ −1.8 ± 0.25. This power-law overpredicts data at the high-end and a second, much steeper, power-law (or a gamma function) is needed. There is a flattening of the function at lower H I column densities with an index of α ~ −1.4 for the column density range log N(H I) = 19.5−21. The fraction of H I mass in sub-DLAs is of the order of 30%. The H I column density distribution does not evolve strongly from z ~ 2.5 to z ~ 4.5.

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    ABSTRACT: We present chemical abundance measurements from medium resolution observations of 8 sub-damped Lyman-alpha absorber and 2 strong Lyman-limit systems at z < 1.5 observed with the MIKE spectrograph on the 6.5m Magellan II Clay telescope. These observations were taken as part of an ongoing project to determine abundances in z < 1.5 quasar absorption line systems (QSOALS) focusing on sub-DLA systems. These observations increase the sample of Zn measurements in z < 1.5 sub-DLAs by ~50%. Lines of Mg I, Mg II, Al II, Al III, Ca II, Mn II, Fe II, and Zn II were detected and column densities were determined. Zn II, a relatively undepleted element and tracer of the gas phase metallicity is detected in two of these systems, with [Zn/H]=-0.05\pm0.12 and [Zn/H]>+0.86. The latter system is however a weak system with N(H I)<18.8, and therefore may need significant ionisation corrections to the abundances. Fe II lines were detected in all systems, with an average Fe abundance of <[Fe/H]>=-0.68, higher than typical Fe abundances for DLA systems at these redshifts. This high mean [Fe/H] could be due to less depletion of Fe onto dust grains, or to higher abundances in these systems. We also discuss the relative abundances in these absorbers. The systems with high metallicity show high ratios of [Mn/Fe] and [Zn/Fe], as seen previously in another sub-DLA. These higher values of [Mn/Fe] could be a result of heavy depletion of Fe onto grains, unmixed gas, or an intrinsically non-solar abundance pattern. Based on Cloudy modeling, we do not expect ionisation effects to cause this phenomenon. Comment: 20 pages, 21 figures, accepted for publication to MNRAS
    Monthly Notices of the Royal Astronomical Society 12/2007; · 5.52 Impact Factor
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    ABSTRACT: We present high signal-to-noise, ~5 Å resolution (FWHM) spectra of 66 z 4 bright quasars obtained with the 4 m Cerro Tololo Inter-American Observatory and 4.2 m William Hershel telescopes. The primary goal of these observations was to undertake a new survey for intervening absorption systems detected in the spectra of background quasars. We look for both Lyman-limit systems (column densities N ≥ 1.6 × 1017 atoms cm-2) and damped Lyα systems (column densities N ≥ 2 × 1020 atoms cm-2). This work resulted in the discovery of 49 Lyman-limit systems, 15 of which are within 3000 km s-1 of the quasar emission and thus might be associated with the quasar itself, 26 new damped Lyα absorption candidates, 15 of which have z > 3.5, and numerous metal absorption systems. In addition, 10 of the quasars presented here exhibit intrinsic broad absorption lines.
    The Astronomical Journal 12/2007; 121(4):1799. · 4.97 Impact Factor
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    ABSTRACT: In the last few years, improvements in high-resolution observations of quasi-stellar objects (QSOs) are enabling the use of absorption line systems as probes of the metallicity at early epochs, in particular through the observation of damped Lyman alpha systems. However, the determination of chemical abundances from the ionic column densities obtained from the observed absorption lines depends on unobserved ions. In this paper, it is shown that the ionization correction factors for OI, NI, CII, MgII and SiII, relative to HI, may be significant only for systems with hydrogen column densities N(HI)<10^20 cm^-2 for a soft radiation field. For a harder radiation field, the correction factors for the singly ionized ions are significant for systems with column densities up to 10^21 cm^-2.
    Monthly Notices of the Royal Astronomical Society 08/1995; 276:268-272. · 5.52 Impact Factor

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