Annu. Rev. Astron. Astrophys. 1996. 34: 155-206 Copyright © 1996 by Annual Reviews. All rights reserved

3.6. Magnetic Fields in Galactic Halos

Vertical dust lanes are often seen in edge-on galaxies, which may indicate vertical magnetic field lines (Sofue 1987). Their initial detection via polarized radio emission in NGC 4631 by Hummel et al (1988a) prompted a systematic search in several nearby edge-on galaxies. Radio halos were detected also in NGC 253 (Carilli et al 1992) and in NGC 4666 (M Dahlem et al, in preparation). A survey of 181 edge-on galaxies observed with the Effelsberg and VLA radio telescopes (Hummel et al 1991b) disclosed no other cases with pronounced halos.

In contrast, NGC 891 (Hummel et al 1991a), NGC 3628 (Reuter et al 1991), NGC 5775 (Golla & Beck 1990), and most other edge-on galaxies (Hummel 1990) do not possess extended radio halos. These galaxies have thick disks with typical synchrotron scale heights of 1 kpc. In most of these galaxies the observed field orientations are approximately parallel to the disk (Dumke et al 1995; see also Table 3). The same is true for NGC 4945 (Harnett et al 1989) and NGC 1808 (Dahlem et al 1990), but there the polarized emission is restricted to two localized regions, one on each side of the plane. In the disks themselves, the polarized emission at 6 cm is weak due to Faraday depolarization.

The other extremes are NGC 4565 (Sukumar & Allen 1991) and M31. The radio emission from any thick disk of M31 is not detectable and must be at least 200 times weaker than for NGC 891 (Berkhuijsen et al 1991). Either the low star-formation rates in M31 and NGC 4565 are below the threshold for the chimney-type outflows (Dahlem et al 1995) or the dynamo does not operate in the halos of these galaxies.

The increase of the degree of polarization with height above the disk of NGC 891 has been analyzed by Hummel et al (1991a). The data can be well modeled by Faraday depolarization in a thermal gas of scale height 1 kpc together with a turbulent magnetic field of scale height 4 kpc. The scale height of the thermal gas as derived from the radio data agrees well with that observed in H (Rand et al 1990, Dettmar 1990). The scale height of the turbulent halo field is consistent with equipartition between the field and cosmic-ray energy densities, where z_b = 2 z_CR = (3 + _s) z_syn 3.6 kpc for a synchrotron scale height of z_syn 0.9 kpc and _s 1.0 (Hummel et al 1991a).

NGC 253 is the edge-on galaxy with the brightest and largest halo observed so far (Carilli et al 1992), extending to at least 9 kpc above the plane. It also has the brightest X-ray halo (Pietsch 1994), so that a strong outflow from the disk or the nucleus driven by the high star-formation rate seems probable. Gas outflow from the nucleus has indeed been found (Dickey et al 1992). Nevertheless, the regular magnetic field is predominantly parallel to the plane in the disk and in the halo (Beck et al 1994b; see Figure 4), possibly due to strong differential rotation even near the center.

Figure 4. Total radio intensity (contours) and magnetic field orientation of NGC 253 (obtained by rotating E-vectors by 90°), observed at 2.8 cm with the Effelsberg telescope (disk field) and at 20 cm wavelength with the VLA (halo field). The resolution is 70 arcsec. (From (Beck et al 1994b.)

NGC 4631 is another rare case of an extended radio halo, possibly driven by a strong galactic wind. The synchrotron scale height of 2 kpc is twice as large as for the bulk of edge-on galaxies (Hummel 1990). The magnetic field lines are roughly perpendicular to the inner disk, which is almost rigidly rotating (Hummel et al 1991a, Golla & Hummel 1994). In this respect, NGC 4631 is exceptional compared with most edge-on galaxies. NGC 4631 also shows signs of gravitational interaction. A few regions with field orientations parallel to the disk are visible in the (differentially rotating) outer disk.

A striking case of a strong galactic wind is M82, which has quasi-radial field lines (Reuter et al 1994). Even a field of 50 µG strength (Klein et al 1988) cannot resist the flow with a velocity in excess of 1000 km s^-1.

Vertical magnetic fields may be a result of disk-halo interactions. The Parker instability produces alternating vertical magnetic fields. A vertical galactic wind of speed V_z could also drag the field from the disk. However, an azimuthal gradient of V_z is required to produce B_z from B and a radial gradient of V_z to obtain B_z from B_r (see Section 7).

Rotation measures in galactic halos are important for revealing the direction of the field and thus its parity with respect to the midplane. Golla & Hummel (1994) could not find a clear RM pattern from their data of NGC 4631. Beck et al (1994b) determined rotation measures at a few positions in the lower halo of NGC 253 and found weak evidence for RMs of the same sign at 5 kpc above and below the plane, as expected for an even-parity mode.

In face-on galaxies "coronal holes" have been observed as regions of high rotation measure (with ensuing higher depolarization) with neither enhanced plasma density (H or X-ray emission) nor enhanced total field strength (total synchrotron emission). In these regions magnetic lines probably open into the halo. The RM maps of IC 342 (Krause et al 1989a) and NGC 6946 (Beck 1991) seem to show such phenomena. The rotation measures in NGC 6946, determined between 18 cm and 20 cm, are small and almost constant, except in the southwest quadrant, where both high and low values occur in a region of 10 kpc in extent (Beck 1991). The spiral arms in the southwest quadrant of NGC 6946 are more diffuse and the X-ray emission is weaker (Schlegel 1994) than in the remainder of the galaxy. Thus galactic coronal holes may occur in regions of low star-forming activity.