Future high-resolution, high-sensitivity observations at high radio frequencies with the Jansky Very Large Array (VLA) and the planned Square Kilometre Array (SKA, see below), combined with high-resolution CO observations with the Atacama Large Millimeter Array (ALMA), will directly map the detailed field structure and the interaction with the molecular gas in external galaxies (Beck et al. 2015). Images of dust polarization of nearby galaxies in the submm range with ALMA will soon become possible. Another exciting prospect is extragalactic starlight and infrared polarimetry with the European Extremely Large Telescope (E-ELT) (Strassmeier & Ilyin 2009).
Construction of the first phase of the SKA is planned to start in 2018. The low-frequency radio telescopes Low Frequency Array (LOFAR) (van Haarlem et al. 2013) and the Murchison Widefield Array (MWA) (Tingay et al. 2013) are suitable instruments to search for extended synchrotron radiation in outer galaxy disks and halos and the transition region to intergalactic space. Several galaxies have already been observed with LOFAR (Beck et al. 2013; Mulcahy et al. 2014). Low frequencies are also suited to search for small Faraday rotation measures from weak fields in the local ISM of the Milky Way, in the ISM of external galaxies and in intergalactic space, if Faraday depolarization is small.
Small-scale and large-scale magnetic fields may exist in S0 and elliptical galaxies without star formation and without active nucleus because turbulence can be generated by the magneto-rotational instability (MRI) (Sellwood & Balbus 1999) or may result from star formation activity in the past. The detection may become possible with the SKA via deep imaging or RM grids of background sources.
The detection of strong radio emission in distant galaxies (which is at least partly of synchrotron origin) demonstrates that strong magnetic fields existed already in young galaxies with strengths of several 100 µG (Murphy 2009). Total synchrotron emission can be detected with the SKA out to even larger redshifts in starburst galaxies, depending on luminosity and magnetic field strength (Fig. 27). As the amplification of turbulent fields is fast and efficient (Sect. 2) and the star-formation rates were high at high redshifts, the total fields can be expected to be very strong already in young galaxies. Then the energy range of the electrons emitting at a fixed observation frequency shifts to low energies (Eq. 3), where ionization and bremsstrahlung losses may become dominant, so that there must be a maximum redshift until synchrotron emission is detectable. Its measurement with the SKA will constrain models of the evolution of magnetic fields in young galaxies (Schleicher & Beck 2013).
Figure 27. Total synchrotron emission at 20 cm wavelength as a function of redshift z and total magnetic field strength B, and the 5σ detection limits for 10 h and 100 h integration time with the full SKA (from Murphy 2009 and priv. comm.).
Nearby galaxies seen edge-on generally show an ordered, disk-parallel field near the disk plane (Dumke et al. 1995). As a result, polarized emission can also be detected from distant, unresolved galaxies if the inclination is larger than about 20∘ (Stil et al. 2009). This opens a new method to search for ordered fields in distant galaxies with help of deep surveys.
If polarized emission from galaxies is too weak to be detected, the method of RM grids towards background QSOs can still be applied. Here, the distance limit is given by the polarized flux of the background QSO, which can be much higher than that of an intervening galaxy that is identified optically by its Mg II absorption system. With this method, significant regular fields of several µG strength were detected in distant galaxies Bernet et al. 2008; Bernet et al. 2013; Farnes et al. 2014). Mean-field solutions of the α − Ω dynamo theory predict patches of regular fields observable as RMs in evolving regular fields already at z ≤ 3, with increasing coherence scale, until fully developed regular fields are formed at z ≃ 1−1.5 (Arshakian et al. 2009; Rodrigues et al. 2015). Note that RM values measured today have been reduced due to redshift by the dilution factor of (1 + z)−2, so that observations at longer wavelengths are needed to detect RMs from regular fields at high redshifts.
Faraday rotation in the direction of polarized lobes of radio galaxies allows to determine the strength and pattern of a regular field in an intervening galaxy (Kronberg et al. 1992; Irwin et al. 2013). RM data indicate that ionized gas may extend to several 10 kpc distance from galaxies (Bernet et al. 2013). A reliable model for the field structure in and around galaxies needs RM values from a large number of polarized background sources, hence large sensitivity and high survey speed.
The SKA “Cosmic Magnetism” Key Science Project plans to observe a polarization survey over the entire accessible sky with the SKA-MID Band 2 (around 20 cm wavelength) (Johnston-Hollitt et al. 2015). Within 1 h integration per field this will allow the detection of about 10 million discrete extragalactic sources and the measurement of their RMs, about 300 RMs per square degree or a mean spacing between sources of about 3′.
Deep polarization observations with the SKA centered on a sample of nearby galaxies will allow studies of the interaction between magnetic fields and the ISM gas (Beck et al. 2015). The detailed reconstruction of the 3D magnetic field structure will become possible with help of Faraday tomography (Heald et al. 2015) or a dense grid of RM values from background sources (Stepanov et al. 2008). More than 1000 RM values can be expected in the area of M 31. Applying this method to more distant galaxies, simple patterns of regular fields can be recognized out to distances of about 100 Mpc (Stepanov et al. 2008).
The author would like to thank all colleagues who have pursued the studies of magnetic fields in the Milky Way and in galaxies over the past 40 years, especially Richard Wielebinski, Elly M. Berkhuijsen, Marita Krause, Patricia Reich, Wolfgang Reich, Sui Ann Mao and Aritra Basu at MPIfR, Andrew Fletcher and Anvar Shukurov at Newcastle University, Chris Chyży and Marek Urbanik at Kraków University, Dmitry Sokoloff at Moscow University, Peter Frick and Rodion Stepanov at ICMM Perm, Ralf-Jürgen Dettmar at Bochum University, Detlef Elstner at AIP Potsdam, Katia Ferrière at Toulouse University and Fatemeh Tabatabaei at IAC La Laguna. – Elly M. Berkhuijsen and Aritra Basu are acknowledged for careful reading of the manuscript and useful comments. – Support from DFG Research Unit FOR1254 is acknowledged.