Hubble space telescope faint object spectrograph observations of a unique grouping of five QSOs: the sizes and shapes of low-z Lyα forest absorbers.
MONIER E.M., TURNSHEK D.A. and HAZARD C.
Abstract (from CDS):
Observations of QSO pairs suggest that the structures responsible for Lyα 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 σ Lyα 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 σ 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<R<218 h–1 kpc for spheres, and a most likely radius of 196 h–1 kpc and a 95% confidence interval of 157<R<289 h–1 kpc for disks. We also model filamentary structures as cylinders and compare the simulated behavior of these cylinders with the observed occurrence of pairs in the three smallest separation pairs. We find that cylinders of R~130-200 h–1 kpc and few Mpc long could be expected to produce the observed behavior of the Lyα lines in these QSO pairs and note that these dimensions are in good agreement with the sizes of filamentary structures (or segments thereof) predicted by numerical simulations of large-scale structure at low redshifts. 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.
Galaxies: Intergalactic Medium - Galaxies: Quasars: Absorption Lines - Galaxies: Quasars: General