The Origins and Evolution of Weak Low Ionization Quasar Absorption Line Systems

Open Access
Narayanan, Anand
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
December 06, 2007
Committee Members:
  • Jane Camilla Charlton, Committee Chair
  • Eric D Feigelson, Committee Member
  • Derek Brindley Fox, Committee Member
  • Gordon Paul Garmire, Committee Member
  • Milton Walter Cole, Committee Member
  • quasar absorption lines
  • galaxies
  • intergalactic medium
  • spectroscopy
  • absorption lines
  • circumgalactic gas
  • high velocity clouds
This thesis examines the physical nature of the gaseous structures selected by weak MgII quasar absorption line systems. The classic quasar absorption systems such as damped Lyman-$alpha$ absorbers (DLAs), sub-DLAs and other Lyman Limit systems are widely studied because of their established association with galaxies of a wide range of morphology and luminosity. In contrast, weak MgII absorbers represent a population(s) that does not appear to be directly associated with luminous galaxies. Nonetheless, at z ~ 1, they outnumber the strong MgII absorbers by a factor of 3:1, and also account for a large fraction (> 25%) of the high column density regime of the Lyman-alpha forest. Moreover, a significant fraction of the weak MgII gas clouds have metallicities that are 0.5 - 2 dex higher than the average metallicity of DLA absorbers. In spite of such unique properties, the astrophysical structures and processes associated with weak MgII systems remain unidentified. In this thesis, I present results from UV and optical spectroscopic surveys, of ~ 100 quasars, offering a full census of weak MgII absorbers over the last 10 Gyr cosmological history of the universe, corresponding to 0 < z < 2.4. At z < 1, the redshift number density (dN/dz) estimation for weak MgII absorbers conforms with an apparent lack of evolution for a static population, in a LCDM universe. However, in the context of an evolving extragalactic ionization background radiation, which diminishes in intensity by a factor of ~ 8 from z = 1 to z = 0, the estimated dN/dz, at z = 0, for a static population of absorbers is a factor of ~ 2 larger than the observed dN/dz. This suggests that the gaseous structures that produce weak MgII absorbers are evolving from z = 1 to z =0, and are regenerated at a rate that is consistent with the observed dN/dz. Towards,high-z (z > 1), the dN/dz evolves with the number density reaching a peak at z = 1.2, thereafter declining such that there may not be a large separate population of weak absorbers at z > 2. I hypothesize this observed trend is indicative of the weak MgII clouds, at high-z, being kinematically connected to galaxies that produce strong MgII absorption. Photoionization models have suggested the weak MgII absorbing structures are unstable over astronomical timescales, because of pressure imbalance between a high density, low ionization gas phase traced by CIV lines, and a low density high ionization phase traced by MgII. The constraints on the temperature were derived from high resolution data at R = 45,000 or lower, which allowed for T ~ 10,000 K. The inference on pressure imbalance, which influences the stability of the absorber, would change if the true Doppler parameter of the low ionization lines are smaller than those derived at R = 45,000. This possibility is explored using superhigh spectral resolution (R = 120,000, FWHM = 2.5 km/s). Subaru/HDS observation of the quasar PG 1634+706, in which I demonstrate that the low ionization lines in weak absorbers are already sufficiently resolved at $R = 45,000$. The weak MgII absorbers indeed possess a phase structure that creates pressure imbalance which makes them physically transient over astronomical time-scales. I also present results from the analysis of the chemical and ionization conditions in a sample of 100 weak MgII absorbers identified in the VLT/UVES archive of quasar spectra. The analysis shows that the metallicity in a majority of the absorber's low ionization gas clouds is constrained to Z > 0.1Z_solar. It also reveals the presence of two populations, with different chemical enrichment profiles, contributing to the weak MgII absorption: (1) weak absorbers that are iron-rich, with N(FeII) ~ N(MgII), which are more common towards low (z ~ 1) redshift, and (2) weak absorbers that are iron deficient with N(FeII) < N(MgII), detected at both low (z < 1) and high (z ~ 2) redshifts. The thesis concludes with discussions on the physical nature of the gaseous structures selected by weak MgII absorbers. The fraction of iron deficient weak absorbers are chemically enriched gas clouds in the circumgalactic environments where the gas is diffuse and optically thin in HI. The absorber might correspond to interstellar gas expelled from star-forming galaxies, into their extended halo, in correlated supernova events. Some fraction of this population is also likely to arise along sight lines that intercept tidally stripped material residing in circumgalactic environments. This scenario will be more common towards high-z (z ~ 2) where the fraction of interacting galaxies, and proto-galaxies with irregular luminosity profiles are observed to be high. In these respects, weak MgII quasar absorption systems are analogous to high velocity cloud (HVC) structures at the interface of galaxies and their surrounding intergalactic medium. Additionally, the sub-population of iron-rich weak MgII absorbers, that are common only at low-z (z < 1) must be tracing Type Ia SN enriched gas in small, high metallicity pockets in dwarf galaxies, tidal debris, or other intergalactic structures.