Evidence of Very Large, Quiescent Lyman Alpha Forest Clouds

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Session 14 -- Lyman Alpha Clouds
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

[14.01] Evidence of Very Large, Quiescent Lyman Alpha Forest Clouds

C.B. Foltz (MMTO), N. Dinshaw, C.D. Impey (Steward Obs.), R. Weymann (OCIW), S. Morris (DAO)

Ultraviolet spectra of the quasar pair 0107-025A,B (${\rm z}_{\rm em} = 0.956, 0.952$; angular separation $= 1.44^\prime$) taken with the Faint Object Spectrograph on the Hubble Space Telescope reveal a number of absorption lines common to both spectra, 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. These observations provide a direct and model-independent lower limit on the cloud diameters of $320-360{\rm h}_{100}^{-1}$ kpc (${\rm q}_0=0.5$) in the redshift range $0.5 < {\rm z} < 0.9$, larger by nearly an order of magnitude than the best lower limits for higher redshift absorbers determined from the ground. Maximum likelihood estimates of the characteristic cloud diameters in the context of spherical, constant column density clouds, give a most probable diameter of $700{\rm h}_{100}^{-1}$ kpc with 95\% confidence that the characteristic diameter lies in the range $540 < D < 1720{\rm h}_{100}^{-1}$ kpc. The rms velocity difference between the two lines of sight for the common features is only about $100 \ {\rm km} \ {\rm s}^{-1}$. Taken together, these observations lead to a picture of absorbing clouds that are both larger in extent and more quiescent than can be explained by current theoretical models. We suggest that the data imply that the absorbers are coherent systems of low dimensionality, such as sheets, filaments or flattened disks.

We present ground-based spectra that support our assumption that the Ly $\alpha$ lines that we are observing are not members of metal-containing systems. We also point out the existence of a third quasar, seen projected $2.00^\prime$ from 0107-025B and $2.93^\prime$ from A. Spectroscopy of this third quasar will allow sampling of the absorbers over two dimensions on scales approaching a megaparsec.

This study was supported by grant GO-5320.01-93A from the Space Telescope Science Institute, AURA, Inc.

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