2. PROPERTIES OF THE HEAVY-ELEMENT REDSHIFTS

The typical neutral hydrogen column density is N(HI) 3.0 x 10¹⁸ cm^-2 and the Doppler parameter is in the range 5 < b < 25 km s^-11. Excited fine structure lines of CII* and SiII* are usually not seen, implying an electron density in the absorbing gas n_e 10 cm^-3. There is a wide range of ionization since N(CII) 10¹⁵ cm^-2 and N(CIV) 10¹⁵ cm^-2. From this we can deduce that the gas is probably photoionized by a flat-spectrum source (for example, the meta-galactic QSO flux), that the ionization parameter is = n(H) / n() = 6.5 x 10³ and that a significant fraction of the gas is neutral. The heavy-elements are somewhat underabundant; Z 0.1 Z. Only crude limits can be placed on the sizes of the absorbing clouds. The line depths show that the clouds cover the QSO emission region which is estimated to have a size D 10¹⁹ cm. The Jeans length in the gas is estimated to be 2.4 x 10²¹ cm. The two lines of sight to the gravitationally lensed QSO, Q0957+551 (z_em = 1.40), show the same C IV absorption redshifts at z_em = 1.12, but the detailed absorption profiles are different (Boksenberg and Sargent 1983), implying individual cloud sizes of D < 2.7 x 10²² cm. On the other hand, this observation together with the observation (Shaver and Robertson 1983a; Boksenberg and Sargent 1983) of common absorption redshifts in the QSO pair UM680 / 681 whose separation is 1 arcmin or 300 kpc, implies that the absorption occurs in relatively small clouds which are embedded in much larger structures. The heavy-element redshift systems often exhibit velocity structure scales extending from 20 km s^-1 (the lower limit that has been resolved) up to 600 km s^-1. The value 150 km s^-1 is typical. The number density of typical strong CIV redshift systems is dN / dz 1.5 at z = 2. As we shall see, there is no strong cosmological evolution in the density of absorbers.