M. Ehle (XMM-Newton Science Operations Centre, ESA, Spain); R. Beck (MPIfR, Germany); V. Shoutenkov (Pushchino Radioastronomy Observatory, Russia); J. Harnett (University of Technology, Sydney); R. F. Haynes (University of Tasmania); A. Shukurov (University of Newcastle, UK); D. D. Sokoloff (Moscow University, Russia); M. Thierbach (MPIfR, Germany)

We have used the Australia Telescope Compact Array and the Very Large Array (VLA) to study the magnetic fields of barred spiral galaxies. The centrally located bars in these galaxies are believed to cause streaming motions in the surrounding gas which lead to enhanced star formation. The galactic magnetic fields are expected to follow the gas flow.

Until recently there have been few detailed radio polarization observations of barred galaxies and little has been known about how the central bars affect the galactic magnetic fields. To study the magnetic fields of barred spiral galaxies, we selected a sample of 20 barred galaxies with an optical size of at least three arcminutes, from the radio surveys of Whiteoak (1970) and Condon et al. (1998). Ten galaxies were observed with the Compact Array at 6, 13 and 20 cm and 10 galaxies were observed with the VLA. For 17 of the 20 galaxies we detected polarized radio emission, indicating the presence of large-scale regular magnetic fields. The radio data will be published as an atlas of magnetic fields in barred galaxies and are available on the Web at http://www.mpifr-bonn.mpg.de/staff/wsherwood/mag-fields.html.

As examples, Figures 1 and 2 show the radio emission at a wavelength of 5.8 cm from the southern galaxies NGC 2442 and NGC 7552, overlaid onto optical images from the Digitized Sky Survey. The contours show the total intensity of the radio emission while the dashes indicate the orientation (assuming little Faraday rotation) and strength of the magnetic field.

NGC 2442 (Figure 1) is a member of the Volans Group of galaxies. The optical image has an asymmetric appearance possibly indicating a tidal interaction with another galaxy. Ionized hydrogen emission from the galaxy shows an unresolved central source and a circumnuclear ring of enhanced star formation which has a radius of approximately 600 parsecs. NGC 2442 exhibits strong radio emission from the nucleus and the ends of the bar. Strongly polarized emission is detected from the northern spiral arm, which has a massive dust lane. The magnetic field lines in this arm are aligned, possibly indicating a compression and/or shearing of the magnetic field.

NGC 7552 (Figure 2) is a member of the Grus Quartet of galaxies. This galaxy also has a starburst circumnuclear ring with a radius of approximately 800 parsecs. The galaxy does not have an active nucleus but has a nuclear bar (observed in radio continuum and near-infrared) aligned perpendicular to the main central bar. NGC 7552 shows strong, highly polarized radio emission from the centre of the galaxy, the bar and the inner parts of the spiral arms. The angular resolution of the Compact Array observations was not high enough to resolve the circumnuclear ring. The polarized radio emission from NGC 7552 is strong upstream of the dust lanes and the magnetic field lines are oriented at large angles to the major axis of the central bar.

From this study we find that the radio surface brightness and the rate of star formation in spiral galaxies are highest for galaxies with a high content of molecular gas and with a high quadrupole moment of the bar's gravitational potential. However, galaxies with strong bars do not always have bright radio emission. For barred galaxies with moderate radio brightness, the magnetic fields, traced by the polarized radio emission, are strongest between the optical spiral arms, or have a diffuse distribution. For radio-bright barred galaxies we find that the magnetic fields are often strongest upstream of the bar's shock front which is delineated by massive dust lanes, in conflict with numerical models. The magnetic fields are oriented at a large angle to the bar and curve smoothly towards the bar without an indication of a shock front. We propose that shear in the velocity field around a large bar enhances the magnetic fields in barred galaxies.

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