Computer Science
Scientific paper
Sep 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt........76p&link_type=abstract
PhD thesis, University of Cambridge
Computer Science
1
Cosmology, Neutral Gas, Evolution, Galaxy, Quasar, Absorption, Damped, Sub-Damped, Lyman, Alpha
Scientific paper
In recent years, an extremely successful method to observationally study early stages of galaxy formation has been provided by the study of quasar absorbers. Quasar absorption lines are systems intercepting our line-of-sight to a given quasar and thus produce a feature in the quasar spectrum. Damped Lyman-α systems (hereafter DLAs) have N (H I) > 2 × 1020 atoms cm-2, and were originally thought to be the precursors of present day disk galaxies but there is evidence that they may be dominated by gas-rich proto-dwarf galaxies representing the basic building blocks of hierarchical growth of structure. Since their detection is independent of their size, shape, and covering factor, they provide a unbiased method with which to study early galaxies. DLAs are a subset of Lyman-limit Systems (hereafter LLS) which have hydrogen column densities N (H I) > 1.6 × 1017 atoms cm-2. At z < 1, they are probably associated with galactic halos. Finally, the Lyman-α forest is composed of many small column density systems ranging from N (H I) =1012 to 1.6 × 1017 atoms cm-2.
This thesis presents a sample of 66 bright z ⪆ 4 quasars observed with the 4 m Cerro Tololo Inter-American Observatory telescope and the 4.2 m William Hershel telescope. The first part of the study concentrates on the quasars themselves via the fitting of quasar continua and the measurement of continuum depression parameters characterising the mean absorption across the Lyman-α forest. The quasar spectra are then analysed to investigate the absorption systems they contain. This led to the discovery of 26 new DLAs, 34 LLS and many associated metal lines which enables the analysis of the evolution of the column density distribution, f(N,z), and the total mass in high-column density neutral hydrogen quasar absorbers. The observed number of LLS per unit redshift is used to constrain f(N,z) below the DLA limit in the range N(HI) = 1.6 × 1017 to 2× 1020 atoms cm-2. The joint analysis shows unambiguously that f(N,z) deviates significantly from a single power law and that a Γ-law distribution of the form f(N,z) = (f*/N*)(N/N*)-β exp(-N/N*) provides a better description of the observations. These results are further used to determine the amount of neutral gas contained in both DLAs and in systems with N(HI) ≥ 2 × 1019 atoms cm-2 (``sub-DLAs''). In the redshift range 2 -- 3, 85% of the neutral H I + He II mass density is in DLAs, however we find that at z>3.5 this fraction drops to 55% and that the remaining neutral gas mass lies in sub-DLAs. After correction of the observed mass in H I for this ``missing'' neutral gas the comoving mass density no longer shows any evidence for a decrease over the range z= 2 -- 5. The change in the column density distribution supports a picture, where at z>3.5, we may be directly observing the formation of high column density neutral hydrogen systems from lower column density units. Finally, predictions on the redshift evolution of the sub-DLAs number density are presented. Preliminary results from measuring their incidence from archival UVES echelle data seem in good agreement with our predictions.
No associations
LandOfFree
Properties of Lyman-alpha Absorbers at High-Redshift does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Properties of Lyman-alpha Absorbers at High-Redshift, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Properties of Lyman-alpha Absorbers at High-Redshift will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1162347