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

Astronomy and Astrophysics (Impact Factor: 4.48). 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.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the properties of damped Ly{\alpha} absorption systems (DLAs) in semi-analytic models of galaxy formation, including partitioning of cold gas in galactic discs into atomic, molecular, and ionized phases with a molecular gas-based star formation recipe. We investigate two approaches for partitioning gas into these constituents: a pressure-based and a metallicity-based recipe. We identify DLAs by passing lines of sight through our simulations to compute HI column densities. We find that models with "standard" gas radial profiles - where the average specific angular momentum of the gas disc is equal to that of the host dark matter halo - fail to reproduce the observed column density distribution of DLAs. These models also fail to reproduce the distribution of velocity widths {\Delta}v, overproducing low {\Delta}v relative to high {\Delta}v systems. Models with "extended" radial gas profiles - corresponding to gas discs with higher specific angular momentum - are able to reproduce quite well the column density distribution of absorbers over the column density range 19 < log NHI < 22.5 in the redshift range 2 < z < 3.5. The model with pressure-based gas partitioning also reproduces the observed line density of DLAs, HI gas density, and {\Delta}v distribution at z < 3 remarkably well. However all of the models investigated here underproduce DLAs and the HI gas density at z > 3. If this is the case, the flatness in the number of DLAs and HI gas density over the redshift interval 0 < z < 5 may be due to a cosmic coincidence where the majority of DLAs at z > 3 arise from intergalactic gas in filaments while those at z < 3 arise predominantly in galactic discs. We further investigate the dependence of DLA metallicity on redshift and {\Delta}v, and find reasonably good agreement with the observations, particularly when including the effects of metallicity gradients (abbrv.).
    Monthly Notices of the Royal Astronomical Society 08/2013; 441(2). · 5.23 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A significant fraction of the sub-damped Lyman-alpha (sub-DLA) absorption systems in quasar spectra appear to be metal-rich, including many with even super-solar element abundances. This raises the question whether some sub-DLAs may harbor active galactic nuclei (AGNs), since supersolar metallicities are observed in AGNs. Here, we investigate this question based on a mini-survey of 21 quasars known to contain sub-DLAs in their spectra. The X-ray observations were performed with the Chandra X-ray Observatory. In cases of no detection, we estimated upper limits for the X-ray luminosities of possible AGNs at the redshifts of the sub-DLAs. In six cases, we find possible X-ray emission within ~1'' of the background quasar, which is consistent with the presence of a nearby X-ray source. If these nearby X-ray sources are at the redshifts of the sub-DLAs, then their estimated 0.2-10 keV luminosities range between 0.8 × 1044h –2 and 4.2 × 1044h –2 erg s–1, thus ruling out a normal late-type galaxy origin, and suggesting that the emission originates in a galactic nucleus near the center of a protogalaxy. The projected distances of these possible nearby X-ray sources from the background quasars lie in the range of 3-7 h –1 kpc, which is consistent with our hypothesis that they represent AGNs centered on the sub-DLAs. Deeper follow-up X-ray and optical observations are required to confirm the marginal detections of X-rays from these sub-DLA galaxies.
    The Astrophysical Journal 09/2013; 775(2):119. · 6.28 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We study the evolution of atomic and molecular gas in galaxies in semi-analytic models of galaxy formation that include new modeling of the partitioning of cold gas in galactic discs into atomic, molecular, and ionised phases. We adopt two scenarios for the formation of molecules: one pressure-based and one metallicity-based recipe. We find that both recipes successfully reproduce the gas fractions and gas-to-stellar mass ratios of HI and H2 in local galaxies, as well as the HI and H2 disc sizes up to z < 2. The pressure-based recipe reproduces the local observed HI mass function over the whole range probed by observations, whereas the metallicity-based recipe overpredicts the low-mass end. Both of our models predict that the high-mass end of the HI mass function remains nearly constant below redshifts of z < 2.0. The metallicity-based recipe yields a much higher cosmic density of cold gas and much lower cosmic H2 fraction over the entire redshift range probed than the pressure based recipes. These strong differences in HI mass function and cosmic density between the two recipes are driven by low mass galaxies (log(M*/Msun) < 7) residing in low mass halos (log(Mvir/Msun) < 10). Both recipes predict that galaxy gas fractions remain fairly high from z ~ 6 - 3 and drop rapidly at lower redshift. The galaxy H2 fractions show a similar trend, but drop even more rapidly. We reach better agreement with CO observations when adopting a CO-to-H2 conversion model based on galaxy physical properties, rather than a fixed value. The results of this work can serve as predictions for upcoming surveys of the atomic and molecular content of galaxies and provide constraints that can test models of the atomic-to-molecular transition and our understanding of the star-formation and feedback mechanisms acting on the gas content of galaxies.
    Monthly Notices of the Royal Astronomical Society 08/2013; 442(3). · 5.23 Impact Factor

Full-text (2 Sources)

Available from
Aug 15, 2014