4 Individual galaxies

4.1 The VLA sample

NGC 1097 (Fig. 5) has one of the intrinsically longest bars in our sample ($\simeq$16 kpc) and is the most interesting galaxy concerning radio polarization. The nucleus (Seyfert 1 type, Storchi-Bergmann et al. 1997) and the circumnuclear starburst ring of 17 $^{\prime \prime }$ ($\simeq$1.3 kpc) radius (see Hummel et al. 1987; Gerin et al. 1988) are prominent in total radio intensity. Polarized radio emission in this region reveals a spiral magnetic field extending from the circumnuclear ring towards the centre (Beck et al. 1999). NGC 1097 also features strong polarized emission upstream of the shock fronts (identified with the dust lanes offset from the bar major axis), where the regular magnetic field makes a large angle with the bar's major axis and then, upstream of the dust lanes, turns to become aligned with the bar. This strong turning of polarization vectors leads to beam depolarization, so that the upstream regions appear as two elongated minima of polarized emission in Fig. 25. The enhancement of total and polarized emission in the dust lanes is only moderate and much weaker than the gas density enhancement (Beck et al. 1999). Extended polarized emission with apparent magnetic field orientation perpendicular to the optical spiral arms (Fig. 25) is visible in the northeast and southwest. The spiral arms outside the bar region exhibit only weak radio emission. Faraday rotation between $\lambda 22$ cm and $\lambda6$ cm is generally weak, with $\vert RM\vert\le 10~{\rm rad~m^{-2}}$, except in the central region where RM varies between $-35~{\rm rad~m^{-2}}$ and $+35~{\rm rad~m^{-2}}$. RM between $\lambda6$ cm and $\lambda3$ cm is generally higher, it varies between $+160~{\rm rad~m^{-2}}$ in the upstream region and $-150~{\rm rad~m^{-2}}$ in the downstream region of the southern bar, and between $+200~{\rm rad~m^{-2}}$ and $-170~{\rm rad~m^{-2}}$near the centre. The increase of RM with decreasing wavelength is typical for spiral galaxies (Sokoloff et al. 1998).

  

Table 5: The flux density $S_{\lambda }$, the flux density $S_{6~\rm cm}^*$ scaled to a distance of 10 Mpc, the average degree of polarization $p_{\lambda }$(based on the maps at 30 $^{\prime \prime }$ resolution), the average total surface brightness $I_{6~\rm cm}$ (in mJy/beam area), the total equipartition field strength $B_{\rm tot}$, and its resolved regular component $B_{\rm reg}$ for the VLA sample. Values in brackets are lower limits where low signal-to-noise ratios or insufficient uv coverage preclude better estimates.

$$\begin{tabular}{lr@{$~\pm~$}lr@{$~\pm~$}lr@{$~\pm~$}lr@{$~\pm... ... &0.83 &11&3 &3&1\\ \noalign{\smallskip } \hline \end{tabular}$$

  

Table 6: As in Table 5, but for the ATCA sample.

$$\begin{tabular}{lr@{$~\pm~$}lr@{$~\pm~$}lr@{$~\pm~$}lr@{$~\pm... ... &2.16 &15&5 &2&1\\ \noalign{\smallskip } \hline \end{tabular}$$

NGC 1300 (Fig. 6) has one of the most pronounced optical bars in our sample. Total radio intensity is maximum in the nuclear region and at the ends of the bar where the spiral arms start (both are sites of strong star formation). No significant radio emission has been detected in the bar. The polarized emission is weak. The northern extensions visible in the maps at $\lambda$22 and 6 cm are not real and result from poor data coverage in the uv plane. The $\lambda 18$ cm and $\lambda3$ cm maps show less emission than those at $\lambda 22$ cm and $\lambda6$ cm because the signal-to-noise ratios are worse.

NGC 1365 (Fig. 7) is the intrinsically largest and radio-brightest galaxy in our sample. Its bar length is $\simeq$29 kpc. It has a Seyfert 1-type nucleus surrounded by a starburst region (see review by Lindblad 1999). Similarly to NGC 1097, it has significant (though weaker) polarized radio emission upstream of the shock fronts and relatively weak emission enhancements on the dust lanes. The turn of magnetic field lines towards the dust lanes near the bar major axis is much smoother than in NGC 1097. The magnetic field orientations near the centre also form a spiral pattern. Sandqvist et al. (1995) describe a circumnuclear elliptical ring of about 1 kpc in radius, visible in radio continuum at $\lambda\lambda20$and $6~{\rm cm}$ at a resolution of $2.3\hbox{$^{\prime\prime}$ }\times1.0\hbox{$^{\prime\prime}$ }$, and note its similarity to that in NGC 1097. Kristen et al. (1997) have revealed, in the optical range, a large number of bright spots arranged along the ring and suggest that the continuous ring structure might be obscured by dust absorption. The ring is not resolved in our observations. Faraday rotation between $\lambda 22$ cm and $\lambda6$ cm is significant only in the central region ( $RM\simeq -20~{\rm rad~m^{-2}}$). RM between $\lambda6$ cm and $\lambda3$ cm jumps between $\simeq$ $+600~{\rm rad~m^{-2}}$ and $\simeq$ $-600~{\rm rad~m^{-2}}$ near the centre. The spiral arms outside the bar region are bright in radio and the regular magnetic field is well aligned with them (different from NGC 1097). A ridge of polarized emission (Fig. 25) is observed on the inner side of the northwestern spiral arm which may indicate field compression by a density wave, a "magnetic arm'', or depolarization along the optical spiral arm. This galaxy has an extended, almost circular, polarized envelope with the regular field aligned with the optical spiral arms.

NGC 2336 (Fig. 8) has a small optical bar. Two extended regions of total radio emission are prominent, but they do not coincide well with star-forming regions. No emission has been detected from the bar and the nucleus. No polarization has been detected. The northern extensions at $\lambda$18 cm and 22 cm are probably not real but result from poor data coverage in the uv plane.

NGC 3359 (Fig. 9) has enhanced total radio emission from the bar where star formation is strong as well. The total emission from the southern spiral arm peaks in star-forming regions, which are best visible at $\lambda6$ cm. Weak polarized emission has been detected in the interarm regions with field lines parallel to the adjacent optical arm. No polarized emission has been detected from the bar, possibly due to insufficient angular resolution and depolarization.

NGC 3953 (Fig. 10) is notable for its extended, diffuse, polarized radio emission from the outer disc. No significant radio emission from the bar and the nucleus has been detected. Significant Faraday rotation between $\lambda$22 cm and $\lambda$6 cm occurs in the southern part of the galaxy ( $RM\simeq +15~{\rm rad~m^{-2}}$).

NGC 3992 (Fig. 11) features strong polarized radio emission from spiral arms with aligned magnetic fields. Weak emission from the nucleus has only been detected at $\lambda$6 cm and 3 cm (the sensitivity was too low at $\lambda$22 cm and 18 cm). No emission from the bar has been detected. Faraday rotation is significant ( $RM\simeq +20~{\rm rad~m^{-2}}$) between $\lambda$22 cm and $\lambda$6 cm.

NGC 4535 (Fig. 12) exhibits apparently unpolarized radio emission from the central region and the bar, which is small and just resolved in our images. The polarized emission mainly comes from spiral arms with aligned magnetic fields and from the northern interarm region (Fig. 25). Faraday rotation is weak between $\lambda$22 cm and $\lambda$6 cm ( $\vert RM\vert\le 10~{\rm rad~m^{-2}}$), but larger between $\lambda$6 cm and $\lambda$3 cm ( $RM\simeq +80~{\rm rad~m^{-2}}$).

NGC 5068 (Fig. 13) has strong radio emission from the small bar and star-forming regions. Weak polarization has been detected at $\lambda$6 cm in the southern part.

NGC 7479 (Fig. 14) has strong polarized radio emission, mainly due to the nuclear "jet'' which was discovered by Laine & Gottesman (1998). The jet is not resolved in the maps presented here (Figs. 14 and 25). Total emission is enhanced in the western spiral arm. The polarized emission and Faraday rotation in this galaxy will be discussed in detail elsewhere.

4.2 The ATCA sample

NGC 986 (Fig. 15) has strong radio emission from the central star-forming region and the bar. Polarized emission is observed in the inner bar.

NGC 1313 (Fig. 16) exhibits strong radio emission from the bar and star-forming regions. No polarization has been detected.

NGC 1433 (Fig. 17) has weak radio emission from the central region which hosts an irregular star-forming ring of 5 $^{\prime \prime }$ or $\simeq$0.3 kpc radius (Maoz et al. 1996). Star formation has not been detected in the bar but is noticeable in the ring-like spiral arms, in particular at the ends of the bar (see H$\alpha$ image by Crocker et al. 1996) where weak radio emission is seen. No polarization has been detected.

NGC 1493 (Fig. 18) shows weak radio emission from the outer spiral arms. Polarization has only been detected in a small region in the southeast.

NGC 1559 (Fig. 19) possesses massive spiral arms with strong star formation. The small bar of about 40 $^{\prime \prime }$ length is oriented almost east-west. Very strong radio emission originates in the bar and the disc. Polarized emission is strongly asymmetric with peaks near the ends of the bar and magnetic field lines at large angles to the bar (Fig. 26). The region where the magnetic field is strongly aligned and almost perpendicular to the bar's major axis is larger than in NGC 1097.

NGC 1672 (Fig. 20) has the second largest bar in our sample ($\simeq$20 kpc). Its radio emission is very strong in the nucleus, the bar and the inner part of the spiral arm region. The nucleus is known to be of Seyfert 2 type and is surrounded by a starburst region (Evans et al. 1996). Polarized emission is strongest in the northeastern region upstream of the dust lanes, with magnetic field lines at large angles to the bar, smoothly turning towards the centre as in NGC 1097 and 1365 (Fig. 26).

NGC 2442 (Fig. 21), a member of the Volans Group, has an asymmetric appearance which may be a result of tidal interaction (Mihos & Bothun 1997) or ram pressure stripping (Ryder et al. 2001). The H$\alpha$ image by Mihos & Bothun (1997) shows an unresolved central source and a circumnuclear star-forming ring of 8 $^{\prime \prime }$ ($\simeq$0.6 kpc) radius. NGC 2442 exhibits strong radio emission from the nucleus and the ends of the bar. Very strong and polarized emission has been detected in the northern arm (hosting a massive dust lane) with aligned field lines, possibly a signature of field compression and/or shearing. Diffuse radio emission is visible in the eastern part (Fig. 21), with a blob of highly polarized emission (Fig. 26). This galaxy will be discussed in Paper III (Harnett et al. 2002).

NGC 3059 (Fig. 22) has a small optical bar. Diffuse, polarized radio emission has been detected in the whole disc, indicating a widespread, spiral, regular magnetic field similar to that of non-barred spiral galaxies. This is not surprising as the bar is small and under-resolved. We cannot exclude that this galaxy has a magnetic field component aligned with the bar.

NGC 5643 (Fig. 23) has strong, diffuse radio emission from the nucleus, the bar and the disc. The central region and outer disc are weakly polarized with some indication of a spiral pattern.

NGC 7552 (Fig. 24), a member of the Grus Quartet, has a starburst circumnuclear ring of 8 $^{\prime \prime }$ ($\simeq$0.8 kpc) radius, observed in radio continuum and near-infrared, a nuclear bar (observed in radio continuum and near-infrared) lying perpendicular to the primary bar, but no active nucleus (Forbes et al. 1994a,b). It is notable for strong, highly polarized radio emission from the centre, the bar and the inner parts of the spiral arms. Our resolution is insufficient to resolve the radio ring. Polarized emission is strong upstream of the dust lanes, with magnetic field lines oriented at large angles to the bar major axis (Fig. 26); these features make this galaxy similar to NGC 1097 and NGC 1672. The outer extensions in Fig. 24 are artifacts due to unsufficient uv coverage. A detailed discussion of this galaxy will be given in Paper III.