
TY  - JOUR
A1  - W Hunstead, Richard 
A1  - Pettini, Max 
T1  - The Lyman a Forest in QSOs: A Window on Intergalactic Clouds at High Redshift
JF  - Australian Journal of Physics
JO  - Aust. J.  Phys.
Y1  - 1990
VL  - 43
IS  - 2
SP  - 211
EP  - 226
KW  - 
UR  - https://doi.org/10.1071/PH900211 
N2  - 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. 
ER  -

