|Annu. Rev. Astron. Astrophys. 1996. 34:
Copyright © 1996 by . All rights reserved
3.7. Magnetic Fields in High-Redshift Galaxies
It is likely that spiral galaxies have possessed their large-scale magnetic fields at least 6 × 109 yr ago (corresponding to a redshift z 0.5) (Kronberg 1994, Perry 1994). The most convincing evidence is the detection of Faraday rotation attributed to a galaxy at z = 0.395 (Kronberg et al 1992). The inferred large-scale magnetic field strength is 1-4 µG and its direction reverses on a scale of 3 kpc. Kronberg et al (1992) argue for a bisymmetric magnetic structure, but this may equally well be an axisymmetric field with reversals (Poezd et al 1993).
Statistical studies of quasar samples (Kronberg & Perry 1982, Welter et al 1984, Perry et al 1993) indicate that excess Faraday rotation correlates with the presence of intervening absorbers. The size of the absorbers has been estimated as 45 kpc, with their global magnetic fields of 2-10 µG; these are probably galactic disks and/or halos. Wolfe et al (1992), Oren & Wolfe (1995) have argued that damped Ly systems [i.e. putative young galactic disks (Wolfe 1988, Wolfe et al 1993)] possess µG-strength global magnetic fields at z 1-2 when they are only 1-3 Gyr old. (However, statistical analyses of this kind are extremely difficult, in particular because of poor statistics, different selection effects, complications in isolating contributions of other intervenors such as our Galaxy, galaxy clusters, etc (Perry et al 1993, Perry 1994). The earliest time at which galaxies possess their large-scale magnetic fields still has to be established. Theoretical models of magnetic fields in young galaxies are discussed in Section 5.3.
A straightforward implication of these studies is a lower limit on the seed magnetic field required for galactic dynamos. If an = 1 cosmology is assumed, then this limit is 2 × 10-18 G (Kronberg et al 1992), or even possibly 10-9-10-11 G if a tentative identification of excess RM in the quasar 1331 + 170 with an absorber at z = 1.775 is confirmed (Perry 1994) (see also Section 5).