AAS Meeting #193 - Austin, Texas, January 1999
Session 85. Intergalactic Medium
Oral, Friday, January 8, 1999, 10:00-11:30am, Room 10 (B)

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[85.03D] The Sizes and Shapes of Low-Redshift Lyman-alpha Absorbers

E.M. Monier (The Ohio State University)

Groups of QSOs closely spaced on the sky provide a way to measure the transverse sizes, shapes, and characteristic clustering lengths of the structures responsible for absorption lines in the Lyman-alpha forest. Recent observations of QSO pairs have suggested radii of hundreds of kiloparsecs for spherical or disklike absorbers, while cosmological simulations of large-scale structure produce shapes that will only be discerned through observations of groups of more than two QSOs. In my thesis, I used two groupings of four QSOs each (a gravitational lens and a physical grouping) to investigate the sizes and shapes of absorbers at high and low redshift on a variety of scales. I will present results from an analysis of HST-FOS spectra of the physical grouping, a unique configuration of five QSOs with pair separations ranging from 1 to 8 arcmin. The redshifts of the four quasars with detected flux in the observed region of 1700 - 2300 Å\ are z = 1.10, 1.01, 1.01, and 0.86. The separations between the lines of sight range from ~200 kpc to ~2 Mpc at z = 0.7. In the pair with the smallest separation approximately one-third of the lines are in common, meaning that Lyman-alpha lines can be found within 150 km/sec of each other in the opposing lines of sight. For the pairs with larger separations, the number of lines in common is consistent with that expected from chance. The results imply that an upper limit to the absorber sizes is crossed in the interval between the two smallest separation pairs, and I will discuss estimates on the sizes of spherical, disklike, and filamentary absorbers in the context of current models for the origin of the low-z Lyman-alpha forest. The overall results demonstrate that observations of QSO groups can be used to derive information on the sizes and shapes of absorbing material in the intergalactic medium, providing insight into the evolution of gas and galaxies in the Universe.

The author(s) of this abstract have provided an email address for comments about the abstract: monier@astronomy.ohio-state.edu

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