Absorption Lines In Quasar Spectra Research Articles

Evolution of Lyman α Clouds at Low Redshifts

Numerous and narrow absorption lines in quasar spectra, the so-called Lyman α forest, can be a unique probe to explore the early universe. In addition, several detections of an absorption trough in the spectra have placed stringent constraints on the physical properties of the intergalactic medium at high redshifts. They would play an important role in relation to the evolution of the UV background radiation field. Focusing on the mutual relations between the quasar absorption system and the UV background radiation field, we examine the evolution of minihalos, gas clouds confined by the gravity of cold dark matter, and explore reasonable possibilities for minihalos to reproduce the redshift distribution of Lyα clouds at 0 ≲ z ≲ 4. We can find that the redshift distribution of minihalos shows good agreement with the observations over the whole range of redshifts 0 ≲ z ≲ 4 under two alternative cases: first, the UV background radiation has a peak intensity at z ∼ 1; second, the Lyα forest has a comoving number density that decreases exponentially with a characteristic timescale, τ ∼ a few Gyr.

Heavy-element enrichment in low-density regions of the intergalactic medium.

Models for the composition of the diffuse intergalactic medium predict that low-density intergalactic gas at high redshift should be very poor in heavy elements. This is because locations of early star formation (and thus of heavy-element synthesis) and of gas delivery from such stars are located preferentially within higher-density regions of the intergalactic gas. Here we present a method for analysing carbon and oxygen absorption lines in quasar spectra that allows us to probe the heavy-element abundances at a redshift of three within low-density regions of intergalactic gas. We find that the ratio of triply ionized carbon to neutral hydrogen is roughly constant over a wide range of densities, and that, even as the density approaches zero, the ratio remains high. This unexpected enrichment of low-density gas in heavy elements suggests that early generations of small galaxies might be much more efficient at ejecting heavy elements into the intergalactic medium than has previously been thought.

GENETIC ALGORITHMS, PULSAR PLANETS, AND IONIZED INTERSTELLAR MICROTURBULENCE

ABSTRACT We probe the intense microturbulence in the Galactic center and the radio-wave scattering it generates by analyzing observations of extragalactic sources, OH and H2O masers, and free-free emission. The region responsible for the enhanced, anisotropic angular broadening of Sgr A* and nearby OH masers is within 150 pc of the Galactic center and has an angular radius ≈ 1°. The enhanced scattering probably occurs in the interface regions between 107 K gas and molecular clouds and is a manifestation of the energetic processes occurring in the Galactic center. Radio scattering measurements are also used to probe turbulent gas toward the Galactic anticenter. Ionized gas at Galactocentric distances ∼ 50 kpc is suggested by absorption lines in quasar spectra, the appearance of the H 1 disks of nearby galaxies, and models for low-redshift quasar absorption systems and Galactic ''fountains.'' We conducted multifrequency, Very Long Baseline Array (VLBA) observations on twelve extragalactic sources in order to measure their scattering diameters. Seven sources are at |b| < 1° and their lines of sight potentially probe path lengths >rsim 50 kpc through the disk. We find that the ionized disk is unwarped, has an extent of ≈ 20 kpc, and traces the extent of massive star formation in the outer Galaxy. Planetary companions to neutron stars are challenging to recognize amid the several processes that contribute to pulsar arrival time data. We use a genetic algorithm to search for planetary companions to pulsars. Genetic algorithms are an optimization method that uses biological-like concepts such as survival of the fittest, mutation, and chromosome exchange. The algorithm searches parameter space in the same way that life finds optimal niches in the biological environment---incremental rewarding of successful variations. Fitting for Keplerian orbits requires a search through four non-linear parameters per planet and is especially difficult if there is a large range of planetary masses and orbital periods. We find that the GA is more efficient and more accurate than the downhill simplex and simulated annealing algorithms. We confirm the presence of a second planetary companion to PSR B0329+54 and identify possible companions to B1911-04 and B1929+10.

Evolution of the Intergalactic Medium and UV Background Radiation

The absence of an HI Gunn-Peterson trough extending to the blue side of the Lyman α emission in the spectra of high-redshift quasars has placed a severe limit on the density of neutral hydrogen in the intergalactic medium. In addition, a probable detection of a HeII Gunn-Peterson trough has been reported, though it is controversial. On the other hand, numerous and narrow absorption lines in quasar spectra have been interpreted as the so-called Lyman α forest. They would play an important role in relation to the evolution of the UV background radiation at high redshifts. We calculate the evolution of the UV background radiation, and explore the ionization states of intergalactic gas and clouds while trying to solve any mutual relations among them consistently. We examine two such models for the Lyman α clouds as pressure-confined and for the clouds confined by cold dark matter, the minihalos. For the former, the evolution is not consistent with other observational constraints, because these clouds with low HI column densities evolve too rapidly to be missed in detection at low redshifts, z ≲ 2, contrary to the HST observations that more numerous Lyman α absorption lines exist than predicted by extrapolation from high redshift. Otherwise, the minihalos cannot reproduce the evolution of the Lyα forest at high redshifts if assumed that the evolution of the UV background is proportional to the number of QSOs. Consequently, we propose a two-component model: pressure-confined clouds at high redshifts and minihalos at low redshifts, while satisfying all of the observational constraints marginally within resonable parameters.

NARROW QUASAR ABSORPTION LINES AND THE HISTORY OF THE UNIVERSE

In order to get an estimation of the parameters of the cosmological model the statistics of narrow absorption lines in quasar spectra is evaluated. To this end a phenomenological model of the evolution of the corresponding absorbers in density, size, number and dimension is presented and compared with the observed evolution in the spectral density of the lines and their column density seen in the equivalent width. In spite of the wide range of possible models, the Einstein-deSitter model is shown to be unlikely because of the implied fast evolution in mass.

The redshift distribution of QSO Lyman-alpha absorbption systems

view Abstract Citations (83) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Redshift Distribution of QSO Lyman-Alpha Absorption Systems Tytler, David Abstract The redshift distribution of Ly-alpha absorption lines in quasar spectra is shown to be more complex than is generally believed. In addition to the systematic increase in the density of lines with z(abs), line density also increases by a similar amount as z(em) increases or equivalently as the rest-frame wavelength of the absorbed radiation decreases. There is also a significant and apparently separate lack of Ly-alpha absorption lines in QSO Ly-alpha emission lines at z(abs) = about z(em). There are several plausible causes for these effects, including serious inhomogeneities in published line samples, unidentified H2 absorption lines, photoionization by the individual observed QSOs, the failure of some Ly-alpha absorbing clouds to cover QSO emission-line regions, QSO ejecta, and large-scale (70/h100 Mpc) inhomogeneities in the universe at z = about 2.5. Previously reported estimates of the rate of evolution of the Ly-alpha systems should be considered highly suspect. It is strongly recommended that future observations of Ly-alpha lines be made only at high resolutions of about 0.15 A (10 km/s), since the lower resolutions of 1 A normally employed seem to be totally inadequate for the reliable determination of the Ly-alpha system redshift distribution. Publication: The Astrophysical Journal Pub Date: October 1987 DOI: 10.1086/165616 Bibcode: 1987ApJ...321...69T Keywords: Absorption Spectra; Line Spectra; Lyman Alpha Radiation; Quasars; Red Shift; Spectral Energy Distribution; Computational Astrophysics; Cosmology; Intergalactic Media; Astrophysics; COSMOLOGY; GALAXIES: INTERGALACTIC MEDIUM; LINE IDENTIFICATIONS; QUASARS full text sources ADS | data products NED (21) SIMBAD (20)

Lyman absorption lines in quasar spectra: evidence for gravitationally-confined gas in dark minihaloes

Lyman absorption lines in quasar spectra: evidence for gravitationally-confined gas in dark minihaloes

Lyman-Alpha Absorption Lines Caused by Superclusters

We have reexamined the La absorption lines in quasar spectra caused by intervening superciusters. A superciuster is assumed to be a sheet-like or a cylindrical shape and to follow the Hubble expansion parallel to the sheet or to the axis of the cylinder. If the gas density within superciusters is 104 ~ 103 cm3 , the equivalent width of absorption lines is well re­ produced. However, the space density of superciusters is too low to explain all absorption lines. From the present analysis, it is conciuded that about several percent of absorption lines may be originated in super-ciusters and that they show a strong dependence upon the redshift.

Analysis of 'noise' in the rich absorption-line spectra of quasars. I - Method of scrambled standard lines

view Abstract Citations (5) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Analysis of "noise" in the rich absorption-line spectra of quasars. I. Methods of scrambled standard lines. Joss, P. C. ; Ruffa, G. J. Abstract A method is presented for estimating the number of accidental redshift systems in quasar absorption-line spectra. The method is based on the use of simulated sets of standard spectral lines in which correlations among the actual line wavelengths of specific ions are neglected. This method is applied to the observed spectra of Ton 1530, PHL 938, and PHL 957. Higher estimates of the number of accidental systems are derived than those obtained by some previous investigators. The original method devised by Bahcall (1968) gives a reasonable lower limit to the number of accidental systems, while the present method gives a reasonable upper limit to the number of such systems. Publication: The Astrophysical Journal Pub Date: December 1977 DOI: 10.1086/155688 Bibcode: 1977ApJ...218..347J Keywords: Absorption Spectra; Line Spectra; Quasars; Random Noise; Red Shift; Spectrum Analysis; Random Processes; Statistical Analysis; Tables (Data); Astrophysics full text sources ADS | data products NED (3)

On the Origin of the Absorption Lines in Quasar Spectra

view Abstract Citations (7) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Origin of the Absorption Lines in Quasar Spectra Roeder, R. C. Abstract The application of Bahcall and Peebles's statistical test for the origin of the absorption lines found in quasar spectra has been extended. The test has been carried out by using several model universes and different values of z , the cutoff redshift. Any conclusion about the validity of the cosmological origin of the absorption lines is seen to depend, in the case of multiple redshifts, upon z and the cosmological model. Publication: The Astrophysical Journal Pub Date: February 1972 DOI: 10.1086/151297 Bibcode: 1972ApJ...171..451R full text sources ADS | data products NED (44)