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Session 95 - Invited.
Oral session, Thursday, January 16
Frontenac Ballroom,

[95.01] Quasar Absorption Lines: The Evolution of Galactic Gas Over Cosmic Time

J. Charlton (Penn State)

A view of the formation and evolution of galaxies and structure over the whole history of the Universe requires observations both of stars and of gas. From the stars in galaxies, now observed in deep images back in time to less than a billion years past the Big Bang, we can study the evolution of galaxy morphology and of star formation rates. Direct observation of gas in the Universe at all epochs is also possible, using absorption spectra of quasars as a probe of intervening material. This absorption arises not only from the gas in developed galaxies and in their environments, but also from the clumps of gas that will eventually combine to form galaxies, and from the gas spread through the Universe that is gradually flowing into the galaxies. This study of gas through quasar absorption lines has opened the possibility of observing directly the formation of galaxies through the assembly of their gas over time. Furthermore, with high resolution spectroscopy, the substructures observed in absorption profiles provide information about the internal workings of galaxies.

This talk will present an overview of progress toward a comprehensive picture of the formation and evolution of galaxies through quasar absorption line studies. The absorption profiles that are observed due to the passage of the quasar light through a given structure are a convolution of several properties of the gas, including its spatial and kinematic distribution, its chemical composition, and its state of ionization. Illustrative models will be utilized to show how these various factors affect the appearance of synthetic spectra. Beginning with the philosophy ``what you see is what you get'', the kinematic spectral signatures of higher redshift absorbers will be modeled by familiar components of nearby galaxies: a rotating disk, an isothermal halo, and gas in radial inflow. A combination of these basic models goes a long way toward producing a variety of complex absorption profiles which are in fact similar to those observed by Keck/HIRES for the MgII doublet for absorption by galaxies at intermediate redshift (0.4 < z < 1.7). A set of photoionization models will be presented in order to demonstrate the relationships between chemical species with different ionization potentials, such as OVI, NV, CIV, SiIV, MgII, FeII, CaII, NaI, and HI. In some observed absorption systems the high and low ionization gas profiles have similar shapes, indicating that they trace the same structures. However, in other systems the high ionization gas is more spread out in velocity, indicating separate high and low ionization components -- the latter shielded from ionizing radiation coming from outside the structure.

Ultraviolet, visible, and near--IR spectroscopy combine to provide information on the variety of chemical and ionization species from the present back to the epochs of the most distant quasars. A summary will be presented of the latest observations of the gross properties and detailed absorption profile properties of the various species of high and low ionization gas, and of how these evolve over time. Interpretation of this information provides tantalizing suggestions that we are viewing the gradual formation of galaxies and the settling of their components over the course of cosmic time.

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Program listing for Thursday