![]() | Annu. Rev. Astron. Astrophys. 1996. 34:
155-206 Copyright © 1996 by Annual Reviews. 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).