5.2. Lyman Limit and Metal Line Systems
The dN/dz of Lyman limit systems is consistent with that of strong Mg II absorbers [with Wr(Mg II) > 0.3 Å] over the redshift range for which both have been observed, 0.4 < z < 2.2. For Wr(Mg II) > 0.3 Å, = 1.0 ± 0.1, consistent with no evolution. For even stronger Mg II systems [Wr(Mg II) > 1 Å], dN/dz increases more dramatically with z, with = 2.3 ± 1.0.
The number of Mg II systems (equivalent width distribution) continues to increase down to the sensitivity of the best surveys, Wr(Mg II) > 0.02 Å, such that dN/dz = 2.7 ± 0.15 at z ~ 1. The ``weak'' Mg II absorbers are therefore more common than the strong systems [Wr(Mg II) > 0.3 Å] known to be associated with luminous galaxies. Unlike the strong Mg II absorbers, the weak Mg II absorbers are sub-Lyman limit systems (they do not have Lyman limit breaks), and no galaxies have been identified at the redshift of absorption. Yet, photoionization models indicate that the metallicities of these weak absorbers are at least 10% of the solar value, and in some cases comparable to solar. They are a varied population: some have relatively strong Fe II while others have no Fe II detected, and some have strong C IV that requires a separate phase while others have no C IV detected. Those with strong Fe II are constrained to be smaller than 10 pc (the ionization parameter must be small and ne large as can be seen in Figure 6). Also, since Fe is produced primarily by Type Ia supernovae they must be enriched by a relatively old stellar population. Those with weaker or undetected Fe II could be larger (kpcs or tens of kpcs) and possibly enriched by Type II supernovae. Candidate environments that could be traced by weak Mg II absorption are: remnants of pre-galactic star clusters formed in mini-halos at z > 10, super star clusters formed in interactions, tidally stripped material, low surface brightness galaxies, and ejected or infalling clouds (analogous to the Milky Way high velocity clouds).
The evolution of dN/dz for C IV absorbers can be studied in the optical for high redshifts. For W(C IV) > 0.4 Å and z > 1.2, the number decreases with increasing z, as = -2.4 ± 0.8. In this same interval, the number of Lyman limit systems is still increasing with redshift, with = 1.5 ± 0.4. This implies that the dramatic evolution in the number of C IV systems is either due to a change in metallicity or a change in ionization state. The dN/dz for C IV systems peaks at intermediate z and declines, consistent with no evolution until the present. Combining optical and UV data, C IV and Mg II have been compared at 0.4 < z < 2.2. The fraction of systems with large Wr(C IV) / Wr(Mg II) decreases rapidly with decreasing redshift; there is a shift toward ``lower ionization systems''.
It is important to consider that the HI, Mg II, and C IV absorption do not always arise in the same phase. It is possible that the C IV in many z ~ 1 Mg II absorption systems arises in a phase similar to the Galactic coronae. If the origin of this phase is related to star-forming processes in the disk, then it might be expected to diminish below z = 1.2 since the peak star formation rate is passed.
Another important trend is the fact that the very strongest Mg II absorbers evolve away from z = 2 until the present. If we study the kinematic structure of these objects, we find that they commonly have a ``double'' structure, with two separate kinematic regions in the Mg II profile. These objects also have strong C IV which also has separate components around the two Mg II regions in the ``double'' structure. The C IV does not arise primarily in the individual Mg II clouds, nor is it in a smooth, ``common halo'' structure that extends in velocity space around the entire Mg II profile. As more data are collected on the kinematic structure of various transitions in these ``double'' systems, it will be interesting to consider the hypothesis that galaxy pairs in the process of merger are responsible. The number of these is thought to have been dramatically larger in the past.