ASTRONOMY AND ASTROPHYSICS. Magnetic fields in the spiral galaxy NGC J.L. Han 1,2,3, R. Beck 1, M. Ehle 4,5, R.F. Haynes 5, and R.

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1 Astron. Astrophys. 348, (1999) Magnetic fields in the spiral galaxy NGC 2997 ASTRONOMY AND ASTROPHYSICS J.L. Han 1,2,3, R. Beck 1, M. Ehle 4,5, R.F. Haynes 5, and R. Wielebinski 1 1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-531 Bonn, Germany 2 Beijing Astronomical Observatory and National Astronomical Observatories, CAS, Beijing 00, P.R. China 3 Beijing Astrophysical Center, CAS-PKU, Beijing 0871, P.R. China (jhan@vega.bac.pku.edu.cn) 4 Max-Planck-Institut für Extraterrestrische Physik, P.O. Box 1603, D Garching, Germany 5 Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 21, Australia Received 21 April 1999 / Accepted 18 June 1999 Abstract. We made multi-band radio observations of NGC 2997, including total power mapping at λ20 with the Very Large Array (VLA), sensitive polarization observations at λ cm with the Australia Telescope Compact Array (ATCA) and images at λ6 and λ3 cm with the VLA. The detected regular magnetic field extends from the centre to the outer disk. In the central part (galactic radius R 0.5 ), the dominated field has the form of an axisymmetric spiral in the nearby layer above the galactic plane. Such a field might be the result of an α α dynamo. In the inner disk (0.5 <R 1.5 ), the total power of radio emission is stronger in the regions of the spiral arms, but the polarized emission is detected mainly along the inner edge of the arms, as expected from compressed field due to shocks from density waves. The RM data on the northeastern arm indicate that the field there might be coherent and that the field points inwards, opposite to the central field. This is the first case of a field reversal between the central region and the inner disk detected in any external galaxy. In the outer disk (R > 1.5 ), we found two so-called magnetic spiral arms starting near the major bifurcations of the optical arms. They are offset from the optical arms, extending from the inner to the outer disk and are up to 50% polarized. The magnetic arms might result from an interaction between the dynamo and the density wave. Alternatively, we propose that they may be associated with interarm gaseous features generated by perturbations of the spiral pattern. Key words: ISM: magnetic fields galaxies: magnetic fields galaxies: individual: NGC 2997 galaxies: nuclei galaxies: spiral galaxies: structure 1. Introduction It is now well established (Beck et al. 1996) that regular magnetic fields are generally parallel to the spiral arms in galactic disks. The magnetic field is found to spiral inwards everywhere (eg. in IC 342, M. Krause et al. 1989a), in the majority of cases (see F. Krause & Beck 1998) or outwards so that the field has Send offprint requests to: J.L. Han the form of an axis-symmetric spiral (ASS). In other galaxies (eg. M 81, M. Krause et al. 1989b), the field spirals inwards on one side and outwards on the other side, with the form of a bi-symmetric spiral (BSS). The magnetic field configuration in some galaxies is complex and may be a superposition of these two patterns with some local features (eg. M 51, Berkhuijsen et al. 1997). The interpretation of the magnetic fields in galaxies remains controversial (Sofue et al. 1986; Wielebinski & Krause 1993; Kronberg 1994; Zweibel & Heiles 1997). The most popular explanation of the field origin and evolution in α Ω dynamo concepts (eg. Ruzmaikin et al. 1988) has been questioned and any explanation needs to consider realistic physical conditions in galaxies (see Beck et al. 1996). More 3-dimensional simulations may help to clarify some relevant issues (eg. Moss 1997; Rohde & Elstner 1998; von Linden et al. 1998). Meanwhile, detailed considerations and simulations in the regime of a primordial origin (eg. Piddington 1978, 1981) were presented by Battaner et al. (1990, 1991), Kulsrud et al. (1997) and Howard & Kulsrud (1997). If there is no large-scale field in the pregalactic era, obviously, some kind of dynamo has to work at some stages of (or after) galaxy formation to build it up (Parker 1997; Kulsrud 1997; Kronberg et al. 1999). There are several problems when applying magnetic-field models to explain observations. For example, Han et al. (1998) demonstrated that magnetic fields in M 31 exist also in regions where no synchrotron emission can be detected. Furthermore, Beck et al. (1999) showed that in the bar region of the galaxy NGC 97 the magnetic fields are no longer dynamo-shaped spirals, but follow the streaming motions of the gas in the bar potential. Beck & Hoernes (1996) explicitly recognized the so-called magnetic spiral arms in NGC 6946, regions of strong regular fields which are offset from the optical arms and are similar to those observed in IC 342 (Krause 1993). To explain these magnetic spiral arms is still a difficult task. 3-D dynamo models may be able to generate interarm fields if the turbulent velocity is smaller there (Rohde & Elstner 1998; Shukurov 1998). Fan & Lou (1996, 1997) and Lou & Fan (1998) proposed magnetohydrodynamical (MHD) density waves to explain the magnetic

2 406 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC 2997 Table 1. Observational parameters of VLA observations Frequency (GHz) & Bandwidth (MHz) 50 & 25 Observing date: 1998 Nov. Pointing (J2000) 09 h 45 m 38 ṣ Net observing time (min) 186 Amplitude calibrator 3C8 Calibrator flux (Jy) 8.24/7.59 Phase calibrator(j2000) Shortest baseline 175λ Frequency (GHz) & Bandwidth (MHz) 2 50 Observing date: 1996 May 19/20 + Jun 01/02 Pointing (J2000) 09 h 45 m 38 ṣ Net observing time (min) Amplitude calibrator 3C286 Calibrator flux (Jy) 7.51/7.46 Phase calibrator(j2000) Shortest baseline 566λ Frequency (GHz) & Bandwidth (MHz) 2 50 Observing date: 1996 May20/ /29 + Jun09/ Pointing (J2000) 3 pointings: NGC 2997-W 09 h 45 m 29 ṣ NGC 2997-N 09 h 45 m 43 ṣ NGC 2997-S 09 h 45 m 43 ṣ Net observing time (min) (2+2+4)/pointing Amplitude calibrator 3C286 Calibrator flux (Jy) 5.19/5.17 Phase calibrator (J2000) Shortest baseline 986λ spiral arms. Moss (1998) suggested that gas streaming along the arms may produce arm-like magnetic structure between the optical spiral arms. Since little is known about the observational properties of these arms, detection of more magnetic arms is desired. Note that almost all previous observational results and theoretical studies refer to the outer (and middle) part of galactic disks. Low angular resolutions were used to observe nearby galaxies to get better sensitivity, but just sufficient to resolve the (outer) spiral arms. Such observations revealed the magnetic structure in the outer disks. When radial distance between spiral arms gets smaller and differential rotation (Ω effect) gets weaker towards the galactic centres, what happens to the large-scale magnetic field? Very few observations have sufficient angular resolution to reveal the situation in the central part which might give very important hints or constraints for theoretical explanations. The high-resolution observations at λ6 cm of IC 342 (Krause 1993), M 51 (Neininger & Horellou 1996) and NGC 6946 (Beck &Hoernes 1996) showed that the regular magnetic field possibly follows the spiral arms into the centre. In the barred galaxy NGC 97 the magnetic field near the centre also has a spiral shape, whereas gas and stars are concentrated in a ring (Beck et al. 1999). However, in none of these cases is a detailed analysis and RM information yet available. NGC 2997 is a southern Sc galaxy (de Vaucouleurs et al. 1991), centered at α 2000 =09 h 45 m 39 ṣ 4, δ 2000 = Its distance is about Mpc (H 0 =75kms 1 Mpc 1, and the heliocentric velocity corrected for the Virgocentric inflow, see e.g. Sperandio et al. 1995). Along the major axis, 1 corresponds to a linear scale of about 60 pc. The position angle of the major axis is about 90 (±5, see Peterson 1978; Marcelin et al. 1980). This galaxy is a trailing spiral (Puerari & Dottori 1992) and has an inclination angle of i 40 (Milliard & Marcelin 1981). According to velocity measurements (Peterson 1978), its southern edge is nearest to the observer. This galaxy has not been well studied at radio frequencies, and no HI and CO map is available yet. The only published radio continuum observation is a VLA snapshot with a resolution of 50 at 1.49 GHz by Condon (1987), from which the spiral arms were barely resolved. We have made deep radio polarization observations of the nearby spiral galaxy NGC 2997, with a resolution high enough to resolve its central part 1, and hence we were able to trace the magnetic field spiraling out from its centre. We also detected two magnetic spiral arms in the outer part of this galaxy, providing additional clues to understand this newly recognized phenomenon. We will present our observations and results in Sects. 2 and 3, and discuss them in detail in Sect Observations and data reduction NGC 2997 was observed with the Very Large Array (VLA) in its CnD array at λ3 and λ6 cm and its BnC array at λ20 cm, and with the Australia Telescope Compact Array (ATCA) in five antenna configurations at λ cm. The relevant observational parameters are listed in Table 1 and Table 2. Data processing was done with AIPS standard procedures except the polarization calibration of the ATCA data which has to be performed within the MIRIAD (Sault & Killeen 1996) program package. The visibility data were edited, calibrated and then combined. I, Q, and U maps were obtained by imagr from the combined data. The options robust and uvtaper were adjusted to get maps with different resolutions. The final maps were restored with circular Gaussian beams (, 15, 18 and 25 ). At all four frequencies, the option zero-spacing flux, which was initially taken from the cleaned flux of imagr, and its weight were tried to minimize the negative-bowl effect caused by missing shortbaseline visibility data in the UV plane. At λ3 cm, the I, Q and U maps of 3 pointings were combined in AIPS using tasks hgeom and ltess to get final maps covering the whole galaxy. Maps of the linearly polarized intensity (PI) and the polarization angle (PA) were finally obtained from the Q and U maps. At λ3 cm and λ6 cm the total intensity (TP) and PI maps were corrected for primary-beam attenuation. 1 Throughout this paper, the central part means the area less than 0.5 from the centre of NGC 2997; the region within the galacto-radius of major bifurcations ( 1.5 ) of the two main optical arms but outside the central part is called the inner disk ; the region outside this radius is called the outer disk.

3 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC mJy/beam Fig. 1. The total intensity map at λ20 cm with a resolution of 18. Contours: (-3, 3, 6,, 24, 48, 96) σ. The rms σ is 50 µjy per beam area. 3. Results 3.1. General Total intensity from the extended arms is marginally detected at λ20 cm (Fig. 1), but we failed to detect polarization. In Fig. 2, we present the maps of total intensity (TP) and linearly polarized intensity (PI) observed by the ATCA at λ cm. The polarization angle (PA) vectors (E cm +90 ) are overlaid onto the PI map. The TP and PI maps (E 6cm +90 ) observed by the VLA at λ6 cm are presented in Figs. 3a,3b, respectively. The PI map with a smoothed beam of 25 at λ6 cm is shown in grey scale in Fig. 3c. The TP map observed by the VLA at λ3 cm, with PA (E 3cm +90 ) plotted as well, is presented in Fig. 4. The highest polarized intensity at λ6 cm is 170 µjy per 15 beam at the inner edge of the northern spiral arm and 275 µjy per 15 beam south of the centre. At λ20 cm polarized intensities of 650 µjy and 00 µjy per 18 beam, respectively, would be expected. Our non-detection indicates significant Faraday depolarization at λ20 cm. The position of the central source as measured in our TP maps is 09 h 45 m 38 ṣ 7, (J2000), slightly different from the optical centre given by de Vaucouleurs et al. (1991). The TP flux density integrated over the galaxy out to 4 radius is 255 ± 20 mjy at λ20 cm, 160 ± mjy at λ cm, 67 ± mjy Table 2. Observational parameters of ATCA observations Correlator configuration FULL 8 2 Bandwidth (MHz) 2 8 Pointing (J2000) 09 h 45 m 39 ṣ Net observing time 8 hrs per configuration Amplitude calibrator Calibrator flux (Jy).1 at GHz.6 at GHz.5 at GHz.1 at GHz Phase calibrator(j2000) and Shortest baseline 242λ Configuration Observing date: Frequency (GHz) 6A 1 Mar 18, & D May, & D May 18, & A Oct 29, & A Nov 01, & : only 4 working antennas at λ6 cm, and 34 ± 4 mjy at λ3 cm. These values λ6 cm and λ3 cm are lower limits because of missing spacing problems. As seen in Figs. 1, 2a, 3a and 4, the total intensity is relatively

4 408 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC 2997 Fig. 2a. The total intensity map at λ cm with a resolution of 25, superimposed onto an optical image from Digitized Sky Survey. Contours: (3, 6,, 24, 48, 96, 192, 384) σ. The rms σ is 36 µjy per beam area. The beam size is marked in the lower-left corner. Fig. 2b. As Fig. 2a, but for linearly polarized intensity. Contours: (3, 4.24, 6, 8.5, ) σ. The rms σ is 18 µjy per beam area. The vectors represent E cm +90. The beam size (25 )is marked in the lower-left corner.

5 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC Fig. 3a. The total intensity map at λ6 cm with a resolution of 15, superimposed onto an optical image from Digitized Sky Survey. Contours: (3, 6,, 24, 48, 96, 192, 384) σ. The rms σ is 17 µjy per beam area. The beam size is marked in the lower-left corner. Fig. 3b. As Fig. 3a, but for linearly polarized intensity. Contours: (3, 4.24, 6, 8.5, ) σ. The rms σ is 7 µjy per beam area. The vectors represent E 6cm +90. The beam size (15 ) is marked in the lower-left corner.

6 4 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC Fig. 3c. The polarized intensity map at λ6 cm, but smoothed to a resolution of 25. The magnetic spiral arm at the lower(-left) part becomes clearer. The grey scale is from µjy to 200 µjy per beam area. Contours: (3, 4.24, 6, 8.5,, 17) σ, here the rms σ is 9 µjy per beam area. Fig. 4. The total intensity map at λ3 cm with a resolution of 15, superimposed onto the optical image from Digitized Sky Survey. Contours: (-3, 3, 6,, 24, 48) σ. The rms σ is 17 µjy per beam area. The vectors represent E 3cm +90, and their lengths are proportional to the linearly polarized intensity with a cutoff below 3σ (here σ PI is 6 µjy per beam area). Its orientations almost exactly reflect the intrinsic field with a possible error of only a few degrees. The beam size is marked in the lower-left corner.

7 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC strong in the arm regions. There is some contribution from thermal emission at the higher frequencies, as indicated by peaks at bright Hii regions (see Milliard & Marcelin 1981) in the arms. In the inner disk, the polarized intensity at λ6 cm is generally stronger on the inner edge of the arms (see Figs. 3b and 3c), quite similar to M 51 (Neininger & Horellou 1996). The degrees of linear polarization vary between % at the edge of the southern and northeastern arms and 30% northeast of the centre. There are two bifurcations in the optical arms, one in the northeastern arm and one in the southwestern arm. The magnetic field also splits into two branches though not coincident with the optical features (Fig. 3b). It seems possible that the polarized radio emission is morphologically similar to the optical arms, but with a phase delay in galaxy rotation (see Fig. 3b). Quantitative investigations are desired with better quality data. In the southeastern part of the outer disk, there is a magnetic spiral arm, marginally seen from 09 h 45 m 45 s, 31 to 09 h 45 m 30 s, 31 in the maps of polarized intensities (Fig. 2b, Fig. 3b) and partly also in the λ6 cm total intensity map (Fig. 3a). This feature is detected in the smoothed PI map (Fig. 3c) with enhanced signal-to-noise ratio and in the λ20 cm total intensity map (Fig. 1), where the gap at 09 h 45 m 45 s, 31 is almost filled so that the continuation of the magnetic arm from the inner to the outer disk is highly probable. Its detection in polarized and total intensity shows that the magnetic arm is not an effect of reduced depolarization, but a region of enhanced regular and total field, as in NGC 6946 (Beck & Hoernes 1996). The degree of linear polarization at λ6 cm is 50% east of the centre, but lower ( 15%) at λ cm due to Faraday depolarization. This magnetic arm starts near the bifurcation and extends to the outer optical disk in the south. It is not associated with any optical arm or dust lane (Block et al. 1994a,b). Even extreme contrast enhancement of the optical DSS image does not reveal any faint arm. A second magnetic arm is seen in polarized intensity at λ6 cm inside the western optical arm (Figs. 3b-c), near the bifurcation. Atλ cm (Fig. 2b) this magnetic arm crosses the optical arm and then turns to an orientation parallel to the northwestern optical arm. The pitch angles in both magnetic arms are similarly large and vary with radius. NGC 2997, like NGC 6946, possesses a two-armed, roughly symmetric spiral pattern of the regular magnetic field which does not coincide with the optical spiral pattern (see discussion in Sect. 4.3). However, unlike NGC 6946, NGC 2997 also reveals regular fields at the inner edge of the optical spiral arms, probably compressed by density waves (Sect. 4.2). The total magnetic field strength is about 18 µg in the arm regions, mainly random. This value is estimated from the flux density with the standard minimum-energy hypothesis, assuming that the filling factor is 1 and the ratio of energy densities between protons and electrons is 0. At the inner edge of the arms, where strong linear polarization is observed at λ6 cm, the regular field can be as high as 8 µg. At λ3 cm, the total intensity emerges mainly from the region of the optical spiral arms. Strong linearly polarized emission has been found in the central part, but (due to the low signal-to-noise ratio) only a few regions at the inner edges of the optical spiral arms could be detected. Higher resolution ( ) total power and PI maps at λ6 cm, not shown here, look quite similar to the maps at λ3 cm (Fig. 4) Faraday rotation For spiral galaxies, the observed radio emission emerges from the full thickness of a disk with a scale height of about several hundred pc to 2 kpc, as verified from edge-on galaxies (Dumke et al. 1995). Note that the field structure seen directly in high frequency PA maps is not the same as that shown by the RM maps. They belong to two different physical layers, but may be correlated. The former one is mainly in the emitting disk, and the latter is primarily in the nearby layer of the magneto-ionic disk. In case of strong Faraday depolarization, this layer can be much thinner than the disk (Sokoloff et al. 1998). We compared the PA maps at λλ3 and 6 cm, 6 and cm, and obtained two RM maps (Fig. 5a,b) using AIPS task comb. The 180 ambiguity of PAs leads to an ambiguity in RM of ±n 1 30 rad m 2 (n 1 =0,1,2,...) for λλ3 and 6 cm, and ±n rad m 2 (n 2 =0,1,2,...) for λλ6 and cm. Only n 1 = n 2 =0is consistent with our data on the northern spiral arm where we detected polarization at three wavelengths. To obtain a continuous spiral pattern at λ6 and λ, no jumps in n 1 or n 2 in other regions of the galaxy are allowed. The possible RM contribution from our Galaxy in the direction to NGC 2997 (l, b)=(262.6, 16.8 ) probably is less than about a few tens of rad m 2 (Simard-Normandin & Kronberg 1980). In fact, fitting and averaging the RM map in the central part of this galaxy (Fig. 8) gives a foreground RM of about 8 ± 6 rad m 2. This small foreground RM will not affect the qualitative analyses below. The RM maps in Fig. 5 can be used to identify the direction of the large-scale field, with help from Fig. 3 to see which regions the RM values refer to. Unfortunately, due to lack of sensitivity at λ3 cm, significant RM(3/6) (Fig. 5a) could be determined only in a few regions where polarized intensities are above 5σ in PI maps 2 at both frequencies. The map of RM(3/6) (Fig. 5a) reveals a systematic pattern around the galaxy s centre which is discussed in detail in Sect An elongated patch of negative RM(3/6) along the outer edge of the northeastern arm, taking into account the disk s orientation (Sect. 1), indicates that the magnetic field in this arm points away from us. However, it is not clear how this field continues. RM(3/6) is 53±30 rad m 2 in the northeastern patch and 8 ± 30 rad m 2 in the northern patch. Due to the large errors, the large-scale field structure cannot be derived from Fig. 5a. The RM(6/) values (Fig. 5b) are generally lower than RM(3/6) in the few patches in Fig. 5a, mainly due to beam smoothing and partially due to Faraday depolarization at decimeter wavelengths. This effect acts like optical thickness increasing with increasing wavelength so that only some frac- 2 The PA uncertainty δpa =0.5 (180/π) (σ/pi),sotheδpa should be less than 5.7 for PI above 5σ.

8 4 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC Fig. 5a. The RM(3/6) map obtained by combining PA maps at λ3 cm and λ6 cm with a resolution of 15. The areas with PI less than 5σ on either PI maps at these frequencies are blanked. The contours mark the levels (±1, ±3, ±5 and ±7) 20 rad m 2, with dashed curves for negative values rad/m^2 50 Fig. 5b. The RM(6/) map obtained by combining PA maps at λ6 cm and λ cm with a resolution of 25. The areas with PI less than 5σ on either PI maps at these frequencies are blanked. The contours give the total intensity at λ6 cm with the same resolution (levels: 1,2,4,8,... 0 µjy/beam area).

9 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC Fig. 6. Variation of RM(6/) along the azimuthal angle in a ring in the galaxy s plane between 1 and 2.5 radius. The azimuthal angle is counted counterclockwise, starting from the east. tion of the polarized emission can be observed (Sokoloff et al. 1998). The signs of RM are not affected so that the direction of the large-scale field can still be determined from Fig. 5b, which is based on much larger signal-to-noise ratios than Fig. 5a. RM(6/) is negative all along the main spiral arm in the northeast and east (see azimuthal angles 270 to 340 in Fig. 6). This gives first evidence that the regular field there is coherent, ie. it preserves its original sign when compressed by the density wave (see Sect. 4.2). More sensitive observations are required to strengthen this point. As the northern side of the galaxy is the far side (Sect. 1), the negative RMs in the northeastern arm tell us that the magnetic field in this arm spirals inwards towards the centre, while the field in the central region spirals outwards (Sect. 3.3). Therefore, the field direction obviously reverses its sign somewhere between the central region and the inner disk. Such reversals have not yet been found in any other spiral galaxy. In order to further increase the signal-to-noise ratio, average RM(6/) values were determined in sectors of a ring between 1 and 2.5 from the centre (Fig. 6) by computing average values in Stokes Q and U. The error bars are based on standard deviations in each sector. RM(6/) is mainly positive in the southern half of the galaxy (0 to 180 azimuth), but the errors are large, and negative in the northern half where most of the polarized emission emerges from the inner edge of the optical arm (Fig. 3b). Any axis-symmetric (ASS) magnetic field structure should be visible as a singly-periodic azimuthal variation of RM (Krause et al. 1989a). With a spiral pitch angle of +30, RM should reach its maximum or minimum values at 30 from the major axis, ie. at 30 and 2 azimuthal angle. However, Fig. 6 shows only a maximum around 0, but no clear minimum. Hence NGC 2997 may not host a large-scale magnetic field of simple symmetry. The negative RM values in the western magnetic arm (Fig. 5b) indicate that the field runs outwards and could be the continuation of the central field. In the southern magnetic arm, positive RM(6/) values (Fig. 6) tentatively indicate that the Fig. 7. The total intensity map at 3cm and E 3cm +90 -vectors in the central part of NGC 2997, supposed onto an optical image from the HST (Maoz et al. 1996b). The length of the vectors is proportional to the polarized intensity. The angular resolution is, as marked in the lower-right corner. The contours are (3, 6,, 24, 0, 200 and 244) σ, here the rms σ of the map is 15.5 µjy per beam area. magnetic field there may point towards us, ie. runs outwards, too, whereas the field in the northern optical arm is of opposite sense. We speculate that the superposition of two systems of magnetic structures ( magnetic arms and density-wave related arms) may lead to such reversals The central part One outstanding result is that the spiral magnetic field can be traced to the central part of this galaxy (Fig. 3b, Fig. 7), even to less than a few arcsec (ie. only a few hundred pc). This is to say, the well-ordered field exists also in the central rigidly rotating part of this galaxy, not merely in the differentially rotating part with a galacto-radius larger than 25 ( 1.8 kpc, see the rotation curve given by Peterson 1978 and Sperandio et al. 1995). It is a good example where a well-ordered spiral magnetic feature is seen in the centre of a galaxy. In the very central region (say, < 5 ) beam depolarization occurs so that no polarized emission is detected, but we suspect that the magnetic field might still be well ordered there. The total (minimum-energy) field in the central region is about 34 ± µg. The regular field cannot be estimated because it is strongly twisted within the observation beam. The RM(3/6) map with lower resolution (15 as is shown in Fig. 5a) for the central part appears to have a pattern of positive(lower) negative(right) positive(upper), which might be a signature of a BSS mode field (Tosa & Fujimoto 1978; Sofue et

10 4 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC Fig. 8. The RM map for the central part of this galaxy, with a resolution of. The contours mark the levels (±1, ±3, ±5 and ±7) 20 rad m 2, with dashed curves for negative values. 4. Discussion Only a few galaxies, M 51 (Neininger & Horellou 1996), NGC 6946 (Beck & Hoernes 1996), IC 342 (M. Krause 1993), NGC 97 (Beck et al. 1999) and NGC 2997 (this paper), have been observed at high radio frequencies (eg. λ6 cm) with sufficient resolution to well resolve the arm and interarm regions. Strongly aligned fields have been found at the inner edges of the spiral arms of M 51 and NGC 2997, showing that the field structure is related to the optical arms. Compared to M 51 and NGC 6946, NGC 2997 is an intermediate case with respect to rotation curves and radio polarized emission. M 51 is a tighter wound-up spiral, and its rotation curve has a sharp rise (< 0.3 kpc) followed by a flat part (see eg. Kuno & Nakai 1997 and references therein; Rand 1993). NGC 6946 is an open spiral and is almost rigidly rotating out to 4 ( 8 kpc, Carignan et al. 1990). M 51 has radio emission more related to density waves (Neininger & Horellou 1996; see also discussion of van der Kruit & Allen 1976), while NGC 6946 has a regular field concentrated in interarm regions (Beck & Hoernes 1996). It might be possible that the shape of rotation curves is related to the morphology of radio polarized emission. Our observations of NGC 2997 are significant in three aspects. First, we revealed the spiral field structure in the very central part. Second, this is the second case (after M 51) in which we see clearly that the dominating component of the linearly polarization emission is located at the inner edge of the optical spiral arms in the inner disk ( 1.5 ). The (E+90 o ) vectors are well-aligned parallel to the arms. Third, we detected two magnetic spiral arms in the southeastern and northwestern parts of the outer disk, starting near the bifurcations of the northeastern arm. In the following, we discuss the current theoretical explanations for the large-scale magnetic fields in galaxies and compare them with the observed field structure in NGC Dynamo and field in the central part Fig. 9. Variation of RM(3/6) along the azimuthal angle in a ring in the galaxy s plane between 7.5 and 30 radius. The azimuthal angle is counted counterclockwise, starting from the east. al. 1980; M. Krause et al. 1989b). However, The RM(3/6) map with higher resolution ( in Fig. 8) shows positive RM values in the southern part and negative RM in the northeastern part. The azimuthal variation of RM(3/6) in the central region (Fig. 9) has a dominating singly-periodical component. The positive RM at about 09 h 45 m 39 s 39 in Fig. 5a is not presented in Fig. 8 due to large uncertainties in PAs. It caused the deviations in the northeast (around 300 azimuth) which seem to be insignificant (Fig. 9). The maximum and minimum RM occur at about 70 and 160 azimuth. The dominated field in the central part possibly is of ASS symmetry, pointing outwards. The observed magnetic fields of galaxies (in outer disks) have been mainly explained by α Ω dynamos with a weak seed field (Sofue et al. 1986; Wielebinski & Krause 1993; Kronberg 1994; Beck et al. 1996; Zweibel & Heiles 1997). The key factor for a dynamo to amplify the field is the net helicity that generates α (eg. Subramanian 1998), which should overwhelm turbulent diffusion. Magnetic diffusion and differential rotation are essential parts of dynamo action (Ruzmaikin et al. 1988; Beck et al. 1996; Parker 1997). Without diffusion the field would be wound up. Earlier dynamo models, without considering the spiral structure and the magnetic back-reaction, were obviously over-simplified. First of all, the observed field structure is closely related to the spiral arms, and the velocity field is also modulated by spiral arms. The small-scale turbulence in dynamos can only produce saturated small-scale fields (ie. magnetic ropes, see Subramanian 1998). To generate or maintain large-scale fields over the extent of the galaxy implies a large-scale vorticity of the gas motions in spiral galaxies

11 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC should be considered (see Blackman & Chou 1997). Secondly, the large-scale field, wherever it comes from, will largely reduce the magnetic diffusion (Cattaneo & Vainshtein 1991) and hence strongly quench the α effect (Vainshtein & Cattaneo 1992). In fact, large-scale magnetic fields comparable to those in nearby galaxies were observed in the radio lobes of high-redshift radio galaxies. This indicates that the large-scale field should exist in the early epoch of galaxy formation. Nevertheless, the dynamo should work in a magnetized galactic disk and may at least help to maintain the large-scale field in galaxies and their halos. In most numerical simulations for galactic dynamos, the magnetic field in the galactic centre will be smeared out soon even if there is a seed field there, partially because there is almost no differential rotation so that the classical α Ω dynamo does not work. However, in this almost rigidly rotating part, the α α dynamo may work (Krause & Rädler 1980) and result in a magnetic field of A0 dynamo mode, with a spiral toroidal field reversing its direction below and above the galactic plane and a dipole poloidal field near the centre (Elstner et al. 1992). Evidence for an A0 mode has also been found for the inner part of our Galaxy (Han et al. 1997). It is clear from Fig. 3b and Fig. 7 that the spiral field in the central part of NGC 2997 is following the dust lanes. According to the RM map in Fig. 8, the field above the galactic plane spirals outwards. Such a field might be the result of an α α dynamo in the central part of a galaxy, as discussed above. If so, we predict that high-resolution multi-frequency polarization observations should be able to reveal a large RM gradient near the center caused by the dipole-like poloidal field. In the centre of NGC 2997, there is no active nucleus according to high-resolution UV-band observations (0.05 ) made with the Hubble Space Telescope (see Fig. 2 = Plate 6 of Maoz et al. 1996b) although there is a circumnuclear star-forming ring of young massive stars (Maoz et al. 1996a). According to the high resolution optical picture, the dust lanes extend to less than 1 from the nucleus (Fig. 7; Maoz et al. 1996b), but with much smaller pitch angles than those in the outer disk (Marcelin et al. 1980). Radio polarization observations with higher resolution in the central region would further clarify how the field is correlated with optical features Optical spiral structure and field in the inner disk NGC 2997 has two main optical arms. How the spiral structure forms in a galaxy is not fully understood yet. It probably results from density waves (Lin & Shu 1964), but other mechanisms, such as tidal torques and a bar-shaped gravitational field, could also be partially responsible for the arm structure (eg. M 51, see Zaritsky et al. 1993). However, NGC 2997 has no companion, but three satellites which are quite small and at least 3.7 mag fainter (Zaritsky et al. 1997). There seems to be no bar in the centre either (Jungwiert et al. 1997). The rotation curves for the receding and approaching sides of NGC 2997 obtained by Sperandio et al. (1995) show that there are outward streaming motions, as discussed by Yuan & Kuo (1998) for tightly wound spiral arms. If the magnetic field is frozen into the gas, its structure will be largely modified by gas motions, as shown by the 3-dimensional simulations of Otmianowska-Mazur et al. (1997) and von Linden et al. (1998). The most plausible explanation for the linearly polarized emission observed on the inner edge of spiral arms is shock compression as predicted by density wave theory (Lin et al. 1969; Roberts et al. 1975) and revealed by the velocity field observed in spiral galaxies. In computer simulations, taking into account the spiral pattern, one can easily get the compressed magnetic field aligned with the spiral structure, no matter whether the dynamo is involved (eg. Bykov et al. 1997; Rohde & Elstner 1998) or not (Otmianowska-Mazur et al. 1997; von Linden et al. 1998). This boldly indicates that the density wave which is currently used to explain the spirals is at least partially responsible for the large-scale structure of magnetic field in galaxies. Here we wish to clarify one fundamental problem which was misleading for a long time: When linearly polarized emission is detected on the inner edge of a spiral arm, the (E+90 o ) vectors just indicate anisotropy of the magnetic field distribution in the emission region. It is well possible that the magnetic field is still random, but compressed in one dimension by the shock from the density wave (see Laing 1980). The resulting anisotropic magnetic field still produces linear polarization and ordered vectors, but is incoherent, ie. changes its direction frequently on the turbulence scale. The RMs from such a field would be random and show no large-scale structure. If there is RM coherency on a large scale on the inner edge of the spiral arms, this means the field must have been coherent already before compression, and hence there must be another physical mechanism (dynamo or primordial origin) to generate such an ordered field. The role of spiral arms would then be restricted to align the large-scale coherent field with the arms. If no coherency exists, this implies that compression by density waves would be fully responsible for the observed linear polarization, and then no other mechanisms such as dynamo will be needed to explain the field in galactic disks. From previous observations of other galaxies, no information for such a coherence of a shocked magnetic field was available. M 51 clearly shows compressed fields on the inner edges of the spiral arms (Neininger & Horellou 1996), but there is no RM information with sufficient resolution yet. The dominating negative values of the RM data in the northeastern arm (Figs. 5) of NGC 2997, for the first time, indicate that the compressed field might be coherent. If a dynamo works, the presence of density spiral arms should be taken into account (eg. Chiba & Tosa 1990). Considering the α effect from a spiral shock wave in the interstellar gas (Mestel & Subramanian 1991; Subramanian & Mestel 1993), a bi-symmetric magnetic spiral will be closely correlated with the density wave and extends for several kpc around the co-rotation radius. This result was confirmed later by Bykov et al. (1997). However, we could not find evidence for one dominating dynamo mode in NGC 2997, neither axis-symmetric nor bi-symmetric (Sect. 3.2). Rohde & Elstner (1998) and Rohde et al. (1999) allowed the next higher mode (m =2)toevolve in their dynamo model including spiral arms and found that

12 416 J.L. Han et al.: Magnetic fields in the spiral galaxy NGC 2997 this mode is more important than the bi-symmetric (m =1) one and that it generates magnetic arms between the gaseous arms. From our data, we cannot disentangle a possible superposition of several dynamo modes; this would require future RM determinations with much higher accuracy. The RM maps of the central part (Fig. 8) and the inner disk (Fig. 5b) show that the observed spiral magnetic field in the central rigidly rotating region is apparently disconnected from the large-scale field in the northeastern arm (Sect. 3.2). This indicates that the field structures of these two regions are maintained by different physical processes (eg. an α α dynamo and compression of a coherent field) or by two different coexisting dynamos (eg. α α and α Ω) Magnetic spiral arms in the outer disk The magnetic spiral arms are difficult to understand on any known theoretical background. Fan & Lou (1996) introduced the response of the azimuthal magnetic field to the density wave in a thin gas disk. Through numerical (Fan & Lou 1997) and analytical (Lou & Fan 1998) calculations for both fast and slow magnetohydrodynamic (MHD) density waves, they suggested that the slow MHD waves present in the rigidly rotating disk may explain the magnetic arms between the optical arms in NGC 6946 and the inner part of IC 342, while they emphasized that fast MHD waves exist in M51, the outer region of IC 342 as well as in NGC2997 (Lou, Han & Fan 1999). Dynamo models can also generate interarm fields if turbulence is smaller there (Rohde & Elstner 1998; Shukurov 1998). Two magnetic arms have been detected in the south(east)ern and in the north(west)ern outer disk of NGC 2997 (Figs. 2b and 3c), though they are not as prominent as those in NGC 6946 (Beck & Hoernes 1996). Both of the two magnetic arms of NGC 2997 start near the bifurcation of the optical arms. Patsis et al. (1997) simulated the response of gaseous disks to the spiral perturbations and found that the 4/1 resonance could generate bifurcation of the arms and many interarm features in a granddesign spiral such as NGC Comparing the magnetic spiral arms detected in NGC 2997 to these simulated interarm features, the arms might be the correspondence of the interarm features i1 and i2 in Fig. 21 of Patsis et al. (1997). That is to say, the observed magnetic spiral arms could be related to the interarm gaseous features produced by the spiral perturbations, with obviously frozen-in field inside. We noticed that the magnetic spiral arms in IC 342 (Krause 1993) are also related to bifurcations. We made further identifications for those in NGC6946 and suggest that its northern magnetic arm (Fig. 2 of Beck & Hoernes 1996) could be i1, and the southern magnetic arm i2 in Fig. 21 of Patsis et al. (1997). Therefore, we propose that the magnetic arms in the outer disk of spiral galaxies are probably such interarm features with frozen-in fields, which were parts of the normal spiral arms but displaced by the perturbations. Patsis et al. (1997) did not include any magnetic field in their simulations which was beyond their original purpose. We expect that the magnetic field frozen into these interarm gaseous features could result in the observed polarized emission. In the optical arm regions, gas is warm and dense; star-formation and supernova explosion tangle the field, and hence the magnetic fields in the optical arms are no longer well-ordered. In interarm gas features, magnetic fields could be well aligned. Future searches for cold gas and hot X-ray emitting gas associated with the magnetic arms should test this hypothesis. 5. Conclusions Our polarization observations of NGC 2997 revealed a regular spiral field in the central part of this galaxy, which might be the result of an α α dynamo in the centre. In the inner disk the polarized emission was detected mainly at the inner edge of the spiral arms where it is shaped by the optical spiral structure. Our RM maps indicate that the field in the northern spiral arm is coherent over a large scale and points inwards, unexpectedly opposite to the central magnetic spiral. This is the first case of a field reversal between the central region and the inner disk detected in galaxies. We detected two magnetic spiral arms in the outer disk of NGC We propose that such arms may be generated by perturbations of the spiral pattern and could be related to bifurcations and interarm gas features. Magnetic fields in the three radial regions might at present be maintained or modified by different processes. It is not clear whether such fields were created by the same mechanisms in the past. Acknowledgements. We thank Drs. Y.-Q. Lou, E.M. Berkhuijsen and K. Subramanian for their helpful discussions, and Drs. M. Krause and Y. Sofue and the referee for their comments. Dr. T. Ye is acknowledged for his help with the ATCA data reduction. This work has been done under the exchange program between the Chinese Academy of Sciences (CAS) and Max-Planck-Gesellschaft. JLH acknowledges support from the Education Ministry of China, the Education Bureau of CAS, the Su-Shu Huang Research Foundation of CAS, and the National Natural Science Foundation of China. ME is grateful to the ATNF for providing support and facilities and he acknowledges financial support by the Deutsche Forschungsgemeinschaft (grant no. Eh 154/1-1). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. 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