The Lyman a Forest in QSOs: A Window on Intergalactic Clouds at High Redshift

Abstract

It is now well established that the number density of Lya forest absorption lines evolves strongly with redshift. When combined with the observed exponential distribution of equivalent widths, this points to a steep dependence of H I column density on z. New information on the nature of the Lya clouds has recently been obtained from high-resolution AAT echelle observations (6.5 km S-l FWHM) of the QSO 2206-199N. Profile fits to unblended Lya lines in the interval Zabs = 2.103 - 2.587 have established a clear trend between the Doppler velocity dispersion b and column density N, suggesting that b is measuring large-scale motions within the Lya clouds rather than their temperatures. If correct, cloud temperatures (set by the lowest b values) are probably < 5000 K instead of the 30000 K usually assumed, implying that the clouds are predominantly neutral. Previous data at lower resolution are shown to be roughly consistent with the new b:N trend, after allowing for the evolutionary dependence of N on z. The new observations are difficult to accommodate within the conventional framework of pressure-confined, low-density, ionised clouds; instead, we suggest that the Lya clouds may be either in the form of dense, thin sheets or may be gravitationally confined.