In January 1949 large-scale magnetic fields had no empirical evidence. The theoretical situation can be summarized as follows:
the seminal contributions of Alfvén [1] convinced the community that magnetic fields can have a very large life-time in a highly conducting plasma;
using the new discoveries of Alfvén, Fermi [2] postulated the existence of a large-scale magnetic field permeating the galaxy with approximate intensity of µ G and, hence, in equilibrium with the cosmic rays;
rather surprisingly Alfvén [3] did not appreciate the implications of the Fermi idea and was led to conclude (incorrectly) that cosmic rays are in equilibrium with stars disregarding completely the possibility of a galactic magnetic field;
in 1949 Hiltner [4] and, independently, Hall [5] observed polarization of starlight which was later on interpreted by Davis and Greenstein [6] as an effect of galactic magnetic field aligning the dust grains (3).
According to the presented chain of events it is legitimate to conclude that
the discoveries of Alfvén were essential in the Fermi proposal (who was already thinking of the origin of cosmic rays in 1938 before leaving Italy);
the idea that cosmic rays are in equilibrium with the galactic magnetic fields (and hence that the galaxy possess a magnetic field) was essential in the correct interpretation of the first, fragile, optical evidence of galactic magnetization.
The origin of the galactic magnetization, according to [2], had to be somehow primordial. This idea was further stressed in two subsequent investigations of Fermi and Chandrasekar [7, 8] who tried, rather ambitiously, to connect the magnetic field of the galaxy to its angular momentum.
In the fifties various observations on polarization of Crab nebula suggested that the Milky Way is not the only magnetized structure in the sky. The effective new twist in the observations of large-scale magnetic fields was the development (through the fifties and sixties) of radio-astronomical techniques. From these measurements, the first unambiguous evidence of radio-polarization from the Milky Way (MW) was obtained [10].
It was also soon realized that the radio-Zeeman effect (counterpart of the optical Zeeman splitting employed to determine the magnetic field of the sun) could offer accurate determination of (locally very strong) magnetic fields in the galaxy. The observation of Lyne and Smith [11] that pulsars could be used to determine the column density of electrons along the line of sight opened the possibility of using not only synchrotron emission as a diagnostic of the presence of a large-scale magnetic field, but also Faraday rotation. In the seventies all the basic experimental tools for the analysis of galactic and extra-galactic magnetic fields were present. Around this epoch also extensive reviews on the experimental endeavors started appearing and a very nice account could be found, for instance, in the review of Heiles [12].
It became gradually clear in the early eighties, that measurements of large-scale magnetic fields in the MW and in the external galaxies are two complementary aspects of the same problem. While MW studies can provide valuable informations concerning the local structure of the galactic magnetic field, the observation of external galaxies provides the only viable tool for the reconstruction of the global features of the galactic magnetic fields. As it will be clarified in the following Sections, the complementary nature of global and local morphological features of large-scale magnetization may become, sometimes, a source of confusion.
Since the early seventies, some relevant attention has been paid not only to the magnetic fields of the galaxies but also to the magnetic fields of the clusters. A cluster is a gravitationally bound system of galaxies. The local group (i.e. our cluster containing the MW, Andromeda together with other fifty galaxies) is an irregular cluster in the sense that it contains fewer galaxies than typical clusters in the Universe. Other clusters (like Coma, Virgo) are more typical and are then called regular or Abell clusters. As an order of magnitude estimate, Abell clusters can contain 103 galaxies.
In the nineties magnetic fields have been measured in single Abell clusters but around the turn of the century these estimates became more reliable thanks to improved experimental techniques. In order to estimate magnetic fields in clusters, an independent knowledge of the electron density along the line of sight is needed (see Sec. 3). Recently Faraday rotation measurements obtained by radio telescopes (like VLA (4)) have been combined with independent measurements of the electron density in the intra-cluster medium. This was made possible by the maps of the x-ray sky obtained with satellites measurements (in particular ROSAT (5)). This improvement in the experimental capabilities seems to have partially settled the issue confirming the measurements of the early nineties and implying that also clusters are endowed with a magnetic field of µG strength which is not associated with individual galaxies.
The years to come are full of interesting experimental questions to be
answered. These questions, as it will be discussed in the following,
may have a rather important theoretical impact both on the
theory of processes taking place in the local universe (like
ultra-high energy cosmic ray propagation) and on the models
trying to explain the origin of large scale magnetic fields.
Last but not least, these measurements may have an impact
on the physics of CMB anisotropies and, in particular, on the
CMB polarization. In fact, the same mechanism leading to the
Faraday rotation in the radio leads to a Faraday rotation
of the CMB provided the CMB is linearly polarized.
One of the important questions to be answered is, for instance,
the nature and strength of the supercluster magnetic field and
now more careful statistical studies are starting also
along this important direction.
Superclusters are gravitationally bound systems of clusters. An
example is the local supercluster formed by the local group
and by the VIRGO cluster. Together ROSAT, various observations with the
EINSTEIN, EXOSAT, and GINGA satellites showed
the presence of hot diffuse gas in the Local Supercluster.
The estimated magnetic field
in this system is, again, of the order of the µ G but
the observational evidence is still not conclusive.
Another puzzling evidence is the fact that
Lyman-
systems (with red-shifts z ~ 2.5) are endowed with a magnetic field
[13].
From the historical development of the subject a series of questions arises naturally:
what is the origin of large-scale magnetic fields?
are magnetic fields primordial as assumed by Fermi more than 50 years ago?
even assuming that large-scale magnetic fields are primordial, is there a theory for their generation?
3 It should be noticed that the observations of Hiltner [4] and Hall [5] took place from November 1948 to January 1949. The paper of Fermi [2] was submitted in January 1949 but it contains no reference to the work of Hiltner and Hall. This indicates the Fermi was probably not aware of these optical measurements. Back.
4 The Very Large Array Telescope, consists of 27 parabolic antennas spread over a surface of 20 km2 in Socorro (New Mexico) Back.
5 The ROegten SATellite (flying from June 1991 to February 1999) provided maps of the x-ray sky in the range 0.1-2.5 keV. A catalog of x-ray bright Abell clusters was compiled. Back.