Polarization observations in radio continuum have revealed basic properties of interstellar magnetic fields in a few dozen spiral galaxies (Beck et al. 1996; Beck 2000). Large-scale regular fields form spiral patterns with pitch angles similar to those of the optical spiral arms. The strongest regular fields usually occur between the optical arms, sometimes concentrated in "magnetic arms'' (Beck & Hoernes 1996). The total (= polarized + unpolarized) nonthermal (synchrotron) radio emission is a tracer of the total field which comprises both regular and random field components. It generally peaks on the optical arms because the random field is strongest there. This distinction implies that the regular and random magnetic fields are maintained and affected by different physical processes.

**Table 1:** The VLA sample of barred galaxies.
NGC	Hubble	Lum.	RC3	RA	Dec	d₂₅	q₂₅	$v_{\rm GSR}$	D	i	PA	b/a	2a/	$S_{60~\mu \rm m}$	$S^{\rm tot}_{20~\rm cm}$
	type	class	class	(2000)	(2000)	[ $^\prime$ ]		[km s^-1]	[M	[ $^\circ$ ]	[ $^\circ$ ]		d₂₅	[Jy]	[mJy]
	(1)	(1)	(2)	[h m s]	[ $^\circ$ $^\prime$ $^{\prime \prime }$ ]	(2)	(2)	(2)	pc]					(3)	(4)
1097	SBbc(rs)	I-II	SBS3	02 46 19.0	-30 16 21	9.3	1.48	1193	16	45	135	[0.4]	0.37	45.9	415
1300	SBb(s)	I.2	SBT4	03 19 40.9	-19 24 41	6.2	1.51	1496	20	35	86	[0.3]	0.41	2.4	35
1365	SBb(s)	I	SBS3	03 33 36.7	-36 08 17	11.2	1.82	1541	19	40	40	0.51	0.47	78.2	530
2336	SBbc(r)	I	SXR4	07 27 04.4	+80 10 41	7.1	1.82	2345	31	59	178	0.41	0.17	1.0	18
3359	SBc(s)	I.8	SBT5	10 46 37.8	+63 13 22	7.2	1.66	1104	15	55	170	0.32	0.25	4.1	50
3953	SBbc(r)	I-II	SBR4	11 53 49.6	+52 19 39	6.9	2.00	1122	15	61	13	0.89	0.17	2.9	41
3992	SBb(rs)	I	SBT4	11 57 36.3	+53 22 31	7.6	1.62	1121	15	59	67	0.58	0.27	$\simeq$ 3	21
4535	SBc(s)	I.3	SXS5	12 34 20.4	+08 11 53	7.1	1.41	1892	16	26	28	[0.6]	[0.1]	6.5	65
5068	SBc(s)	II-III	SXT6	13 18 55.4	-21 02 21	7.2	1.15	550	7	29	110	0.44	0.16	2.3	39
7479	SBbc(s)	I-II	SBS5	23 04 57.2	+12 19 18	4.1	1.32	2544	34	45	25	0.41	0.46	12.1	109

References: (1) Sandage & Tammann (1981); (2) de Vaucouleurs et al. (1991); (3) Fullmer & Lonsdale (1989); (4) Condon (1987).

Spiral patterns of the regular magnetic field are believed to be generated by dynamo action in a differentially rotating disc (Beck et al. 1996). The dynamo reacts or interacts with non-axisymmetric disturbances like density waves (Mestel & Subramanian 1991; Rohde et al. 1999), but little is known about the effects of bar-like distortions. Chiba & Lesch (1994) suggested that a bar may excite higher dynamo modes, while Moss et al. (1998) found from their models a mixture of modes with rapidly changing appearance.

Radio observations of barred galaxies are rare. The angular resolution of the maps in Condon's (1987) atlas was insufficient to distinguish emission from the bar, the spiral arms and the halo. Another survey of barred galaxies in radio continuum by García-Barreto et al. (1993) had even lower angular resolution; neither survey included polarization. The first high-resolution radio map of a barred galaxy, NGC 1097 (Ondrechen & van der Hulst 1983), showed narrow ridges in total intensity coinciding with the dust lanes, which are tracers of compression regions along the leading (with respect to the sense of rotation) edge of the bar. A similar result was obtained for M 83 (Ondrechen 1985) which hosts a bar of smaller size than NGC 1097. No polarization could be detected in NGC 1097 by Ondrechen & van der Hulst (1983). Radio observations of NGC 1365 at $\lambda\lambda20,\ 6$ and $2~{\rm cm}$ , restricted to a central region, have revealed similar features (Jörsäter & van Moorsel 1995). The first detection of polarized radio emission from the bar region was reported by Ondrechen (1985) for M 83, with a mean fractional polarization at $\lambda6$ cm of 25%. Neininger et al. (1991) mapped the polarized emission from M 83 at $\lambda2.8$ cm. They showed that the regular magnetic field in the bar region is aligned with the bar's major axis. Observed with higher resolution, the regular field is strongest at the leading edges of the bar of M 83 (Beck 2000).

Another barred galaxy which has been studied in detail in radio polarization is NGC 3627 (Soida et al. 2001). The regular field in the bar region is again aligned parallel to the bar's major axis, being strongest at the leading edges of the bar. However, east of the bar the field behaves anomalously, forming a "magnetic arm'' crossing the gaseous arm.

The first high-resolution polarization observations of a galaxy with a massive bar, NGC 1097, were presented by Beck et al. (1999). The magnetic field lines in and around the bar appear to follow the velocity field of the gas expected from a generic gas dynamic model (Athanassoula 1992). The regular magnetic field outside the bar region has a spiral pattern similar to that seen optically. A narrow ridge of greatly reduced polarized intensity indicates the deflection of the field lines in a shear shock (the dust lane), but the magnetic field lines turn more smoothly than the gas streamlines (Moss et al. 2001, hereafter Paper II). Velocity fields are available from HI observations only for the outer parts of NGC 1097 (Ondrechen et al. 1989) and from CO observations only for the circumnuclear ring (Gerin et al. 1988).

NGC 1097 is one of the objects in our sample of barred galaxies observed with the VLA and the ATCA. In this paper we present the full set of radio maps of our survey, smoothed to a common resolution of 30 $^{\prime \prime }$ , and give an overview of their salient properties. Higher-resolution maps of NGC 1097, 1365 and 7479 will be presented and discussed in subsequent papers. New dynamo models for barred galaxies are discussed in Paper II. Further details on the magnetic fields in NGC 1672, 2442 and 7552 will be given by Harnett et al. (2002, hereafter Paper III).

Table 2: The ATCA sample of barred galaxies.
NGC Hubble Lum. RC3 RA Dec d₂₅ q₂₅ $v_{\rm GSR}$ D i PA b/a 2a/ $S_{60~\mu \rm m}$ $S^{\rm tot}_{6~\rm cm}$

type class class (2000) (2000) [ $^\prime$ ] [km s^-1] [M [ $^\circ$ ] [ $^\circ$ ] d₂₅ [Jy] [mJy]

(1) (1) (2) [h m s] [ $^\circ$ $^\prime$ $^{\prime \prime }$ ] (2) (2) (2) pc] (3) (4)

986 SBb(rs) I-II SBT2 02 33 34.3 -39 02 43 3.9 1.32 1907 25 ? ? [0.5] 0.46 23.1 40

1313 SBc(s) III-IV SBS7 03 18 15.5 -66 29 51 9.1 1.32 292 4 38 170 0.63 0.31 10.4 59

1433 SBb(s) I-II PSBR2 03 42 01.4 -47 13 17 6.5 1.10 920 12 27 17 0.33 0.36 3.3 -

1493 SBc(rs) III SBR6 03 57 27.9 -46 12 38 3.5 1.07 900 12 30 ? 0.32 0.18 2.2 -

1559 SBc(s) II.8 SBS6 04 17 37.4 -62 47 04 3.5 1.74 1115 15 55 65 [0.3] [0.2] 23.8 120

1672 SBb(rs) II SBS3 04 45 42.2 -59 14 57 6.6 1.20 1155 15 39 170 0.41 0.68 34.8 100

2442 SBbc(rs) II SXS4P 07 36 23.9 -69 31 50 5.5 1.12 1236 16 24 40 [0.5] 0.42 $\simeq$ 22 70

3059 SBc(s) III SBT4 09 50 08.1 -73 55 17 3.6 1.12 1056 14 ? ? [0.3] [0.2] 9.6 -

5643 SBc(s) II-III SXT5 14 32 41.5 -44 10 24 4.6 1.15 1066 14 ? ? [0.4] [0.35] 18.7 64

7552 SBbc(s) I-II PSBS2 23 16 11.0 -42 35 01 3.4 1.26 1568 21 31 1 0.29 0.59 72.9 140

References: (1)-(3) see Table 1; (4) Whiteoak (1970).

1 Introduction