Astron. Astrophys. 355, 128-137 (2000)

3. Results

3.1. Total power and polarized emission at 8.46 GHz

The total power map of NGC 4449 at 8.46 GHz with apparent B-vectors of polarized intensity is shown in Fig. 1. As no correction for Faraday rotation was applied, the orientation of observed B-vectors may differ from the magnetic field directions by some - on average in the disk, the maximum difference reaching in small regions of high Faraday rotation measures (see Sect. 3.3). The map shows details of the radio structure in the inner disk. The total power emission shows strong peaks at the position of bright star-forming regions. In addition to that diffuse radio emission away from the optically bright star-forming body has been detected as well. This radio envelope extends along the galaxy's minor axis up to (corresponding to  kpc) from the main plane. The extent of the radio envelope at 8.46 GHz is larger than that of the faint diffuse H emission (see Fig. 1). In the southern disk the radio emission has an extension towards a nebulous object at RA₂₀₀₀ = , Dec₂₀₀₀ = (probably a supernova remnant), forming a faint peak at its position.

Fig. 1. The total power contour map of NGC 4449 at 8.46 GHz with observed B-vectors of polarized intensity (taken as perpendicular to E-vectors) superimposed onto the H image of Bomans et al. (1997), digitally enhanced to show the low-brightness details of the ionized gas distribution. The resolution of the radio map is 12". The contour levels are (3, 5, 7, 10, 15, 20, 30, 60, 90, 140, 200)Jy/b.a., the r.m.s. noise level of the total power map. Vectors of 10" correspond to 50 µJy/b.a.

The contour map of polarized brightness with apparent B-vectors proportional to the polarization degree is shown in Fig. 2. The extended radio emission is substantially polarized (locally up to 50%), with extended ( kpc) domains of highly aligned B-vectors. The magnetic field structure in the inner disk looks unusual at first glance (Figs. 1 and 2). The projected magnetic vectors in NGC 4449 show two distinct kinds of structure. From the bright central star-forming region they are directed radially outwards, on each side forming a polarized "fan". The B-vectors are parallel to the H filaments discussed in detail by Sabbadin & Bianchini (1979) and by Bomans et al. (1997). In the galaxy's outskirts, the magnetic vectors run along a polarized ridge encircling the galaxy on the northern, north-eastern and eastern side. Between this structure and the eastern "fan" an elongated unpolarized "valley" is due to a geometrical superposition of mutually perpendicular polarization directions in the "fan" and in the polarized ridge.

Fig. 2. The contours of polarized intensity of NGC 4449 at 8.46 GHz with superimposed B-vectors (perpendicular to the observed E-vectors) of the polarization degree. The contour levels are (2, 4, 6, 9)Jy/b.a., the r.m.s. noise of the polarized intensity map. Vectors of 10" correspond to 20%

3.2. Total power and polarization at 4.86 GHz

Our maps at 4.86 GHz have a considerably worse resolution than those at 8.46 GHz, and the orientations of the B-vectors may be subject to stronger Faraday rotation (on average about but locally up to , see Fig. 5). However, due to a higher signal-to-noise ratio at 4.86 GHz the radio emission is traced much further out (Fig. 3). At this frequency we can trace the radio envelope in the sky plane out to (3.5 kpc) from the galaxy's major axis. The nonthermal emission thus extends into the halo beyond one isophotal (at the level of ) major axis radius, which is a rare (though not exceptional) phenomenon among spiral galaxies (e.g. Hummel et al. 1991).

Fig. 3. The contour map of the total power of NGC 4449 at 4.86 GHz with B-vectors of the polarized intensity superimposed onto an optical image obtained by one of us (SK) at the Hoher List Observatory. The resolution is 19". The contour levels are (3, 5, 10, 20, 40, 60, 80, 100, 150, 200, 250, 300)Jy/b.a., the r.m.s. noise in the total power map. Vectors of 10" correspond to 50 µJy/b.a.. No correction for Faraday rotation was applied (see Sect. 3.2)

The map at 4.86 GHz again shows the polarized "fans", however the eastern one is less conspicuous at this frequency than at 8.46 GHz, which suggests stronger Faraday depolarization in this region. The polarized ridge in the northwestern portion of the galaxy, already visible in Fig. 2, turns out to be part of a larger polarized ring surrounding the galaxy from the northeast through north, east and south down to the southwest, with a well-organized, coherent pattern of magnetic vectors (Fig. 4). Another weak fragment of the polarized ring is visible west of the centre. The ring coincides well with a similar feature visible in HI (Hunter, priv. comm., Fig. 4), with one of the brightest polarization peaks lying close to the densest neutral gas clump. Along the ring the polarization B-vectors are not exactly tangential to the azimuthal directions or to the HI shell. They deviate systematically from the azimuthal directions by some - . A detailed discussion of the magnetic field directions is presented in Sect. 4.

Fig. 4. Contours and B-vectors of the polarized intensity of NGC 4449 at 4.86 GHz with a resolution 19" superimposed onto a colour plot of the HI column density made from the data kindly supplied by Dr. D.A. Hunter from the Lovell Observatory. The contour levels of the polarized intensity are (3, 10, 20, 30, 35)Jy/b.a., the r.m.s. noise level in the polarized intensity map

Due to the higher sensitivity at 4.86 GHz our map shows very well the unpolarized "valley" not only at the interface of the eastern "fan" and the ridge but also a similar feature in the NW disk. In both cases they result from a geometrical superposition of magnetic field directions in the "fans" and in the polarized ring, seen almost perpendicular to each other when projected to the sky plane.

3.3. Faraday rotation

The distribution of Faraday rotation measures between 8.46 and 4.86 GHz is shown in Fig. 5. The northern and eastern parts of the polarized ring, as well as the "magnetic fan" east of the central star-forming complex show coherently positive Faraday rotation measures (RM) over areas with sizes of about , with a mean value of about +50 rad/m². The values of RM are rising locally up to +200 rad/m². The western "fan" and the southern part of the polarized ring are dominated by negative RMs, on average of about -50 rad/m² but also reaching -150 rad/m² locally. The errors in these regions vary from to rad/m² in regions of low RMs, exceeding rad/m² in regions of high rotation measures. However, though in individual points the values of RM do not generally exceed the errors by more than 2 - 2.5 r.m.s. errors, coherent areas of the same sign of RM extend over many beam sizes. The statistical significance of our determinations of RM is discussed in detail in Sect. 4.

Fig. 5. The distribution of the Faraday rotation measures (RM) in the disk of NGC 4449, computed between 8.44 and 4.86 GHz. All data were convolved to a common beam of 19". Positive and negative values of RM are marked by circles and crosses, respectively. The symbol sizes indicate the absolute value of RM as indicated in the figure legend. The contour line divides the regions with positive and negative values of RM. The underlying greyscale plot shows the RM errors

© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000
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