Lyman Alpha Forest Cloud Sizes
Session 28 -- QSO Absorption-Line Studies with HST and Keck
Oral presentation, Tuesday, June 13, 1995, 2:00pm - 5:30pm

## [28.04] Lyman Alpha Forest Cloud Sizes

Nadine Dinshaw (Steward Observatory)

We present ultraviolet spectra of the \Lya\ forest of the quasar pair 0107-025A,B (z=0.956, 0.952; angular separation = 1.44 arcmin) taken with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope\/ (HST\/ ). We detect four absorption lines common to both spectra in the redshift range $0.5 \le z \le 0.9$, and six lines which are seen in the spectrum of one quasar but not the other, indicating that the characteristic transverse size of the absorbers is at least as large as the proper separation of the two lines of sight at the position of the absorbers. The directly-measured lower limit on the characteristic diameter of the clouds is between 320 and $360\,h^{-1}$~kpc (where $h^{-1} \equiv H_0/100$~\kms~Mpc$^{-1}$, $q_0 = 0.5$), larger by nearly an order of magnitude than the best lower limits for higher redshift absorbers determined from the ground. The typical velocity difference between the common absorption lines along the two lines of sight is only about 100\,\kms. These direct measurements lead to a picture of absorbing clouds that are both larger in extent and more quiescent than can easily be explained by current theoretical models.

We present HST\/ Goddard High Resolution Spectrograph (GHRS) observations of this pair of quasars extending to lower redshifts, as well as ground-based observations that support our assumption that the \Lya\ lines that we are observing are not members of metal-containing systems. We also point out the existence of a third quasar, seen projected 2.0 arcmin from 0107-025B and 2.9 arcmin from A. We are in the process of obtaining HST\/ FOS observations of this third quasar which will allow sampling of the absorbers on scales approaching a megaparsec.