
%0 Journal Article
%A W Hunstead, Richard 
%A Pettini, Max 
%T The Lyman a Forest in QSOs: A Window on Intergalactic Clouds at High Redshift
%J Australian Journal of Physics
%D 1990/04/01
%V 43
%N 2
%P 211-226
%K 
%R https://doi.org/10.1071/PH900211
%U https://www.publish.csiro.au/paper/PH900211 
%X 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. 
