Hubble Space Telescope Faint Object Spectrograph Observations of a Unique Grouping of Five QSOS: The Sizes and Shapes of Low-z LYalpha Forest Absorbers

Details Hubble Space Telescope Faint Object Spectrograph Observations of a Unique Grouping of Five QSOS: The Sizes and Shapes of Low-z LYalpha Forest Absorbers Hubble Space Telescope Faint Object Spectrograph Observations of a Unique Grouping of Five QSOS: The Sizes and Shapes of Low-z LYalpha Forest Absorbers

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...522..627m&link_type=abstract

The Astrophysical Journal, Volume 522, Issue 2, pp. 627-646.

Astronomy and Astrophysics

Astronomy

8

Galaxies: Intergalactic Medium, Galaxies: Quasars: Absorption Lines, Galaxies: Quasars: General

Scientific paper

Observations of QSO pairs suggest that the structures responsible for Lyalpha forest absorption lines must have radii of hundreds of kiloparsecs if they are spherical or disklike in shape. However, only observations of groups of three or more QSOs closely spaced on the sky provide a way to measure their transverse sizes/characteristic clustering lengths and shapes. Such observations are needed to test the predictions of cosmological simulations of large-scale structure formation. To further this goal, we have obtained Hubble Space Telescope Faint Object Spectrograph G190H spectra of a grouping of five QSOs to investigate the sizes and shapes of absorbers at a redshift of z~0.7. The angular separations of the pairs in this unique configuration range from 1'-8', for which the corresponding lines of sight probe proper separations as small as 200 h^-1 kpc and up to 2 h^-1 Mpc. The redshifts of the four quasars with detected flux in the observed region of 1700-2300 Å are z=0.86, 1.01, 1.01, and 1.10. In the pair with the smallest separation (0.98′), six greater than 4.5 sigma Lyalpha line pairs are found in common (within 150 km s^-1) between the two lines of sight, significant above the random background at the 99.2% level. In the QSO pairs with larger separations, the number of lines in common is consistent with the chance expectation determined from Monte Carlo simulations. The results imply that an upper limit to the absorber sizes is crossed in the interval between the two smallest separation pairs. In addition, we find evidence that the exclusion of weaker lines results in smaller absorber sizes. Using results from a 4.5 sigma sample of lines in the QSO pair with the smallest separation, we estimate the sizes of simple spherical and thin-disk absorbers using a Bayesian analysis and find a most likely radius of 159 h^-1 kpc and a 95% confidence interval of 135

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