Magnetic fields and ionized gas in the local group irregular galaxies IC 10 and NGC 6822

Abstract

Abstract. We performed a high-sensitivity search for galaxy-scale magnetic fields by radio polarimetry at 10.45 GHz and 4.85 GHz with the Effelsberg 100 m radio telescope, accompanied by Hα imaging, for the two Local Group irregular galaxies IC 10 and NGC 6822. Their star-forming bodies are small and rotate slowly. IC 10 is known to have a very high star-forming activity, resembling blue compact dwarfs, while NGC 6822 has a low overall star-formation level. Despite very different current star formation rates, our Hα imaging revealed a large web of diffuse Hα filaments and shells in both IC 10 and NGC 6822. Some of them extend far away from the galaxy's main body. The total power emission of both objects shows bright peaks either at the positions of optically strong star-forming clumps (IC 10) or individual H  regions or supernova remnants (NGC 6822).However, in both cases we detect a smoothly distributed, extended component. In IC 10 we found clear evidence for the presence of a diffuse, mostly random magnetic field of 14 µG strength, probably generated by a fluctuation dynamo. One of the Hα-emitting filaments appears to be associated with enhanced magnetic fields. We also rediscuss the reddening of IC 10 and its implications for its distance. In the case of NGC 6822 we found only very weak evidence for nonthermal emission, except perhaps for some regions associated with local gas compression. We detect in both galaxies small spots of polarized emission, indicative of regular fields ( 3 µG), at least partly associated with local compressional phenomena.Key words. polarization -galaxies: irregular -galaxies: magnetic fields, galaxies: individual: IC10, NGC 6822 -radio continuum: galaxies IntroductionIrregular galaxies are low-mass objects exhibiting a variety of rotational properties with a subclass of them rotating very slowly (rotational speeds V rot ≤ 30 km s −1 ) and often chaotically (e.g. Lo et al. 1993). They constitute important laboratories for large-scale interactions of stars with the interstellar medium: the low gravitational potential and relatively small size increase the probability that superbubbles, forming close to star-forming regions, may break out of the galaxy (e.g. Mac Low & Ferrara 1998). Many irregular galaxies exhibit giant arcs or filaments of ionized gas (e.g. Sabbadin & Bianchini 1979;Hunter et al. 1993;Bomans et al. 1997). The role of magnetic fields in the origin and confinement of these ionized structures (e.g. Hunter & Gallagher 1990) is still a matter of debate.In spiral galaxies magnetic fields, which are sufficiently strong to trigger star formation via magnetic instabilities (Blitz & Shu 1980) or to influence the superbubble expansion (Ferriere et al. 1991), are probably generated by the mean-field dynamo (see . This requires strong Coriolis Send offprint requests to: K. Chyży, e-mail: chris@oa.uj.edu.pl forces (hence a rapid rotation) to give the turbulent motions a preferred sense of twisting. A sufficient size of the ionized gas envelope is also required. Large irregulars...

The non-thermal superbubble in IC 10: the generation of cosmic ray electrons caught in the act

“…Our VLA flux density at 1.5 GHz and those at 2.6 and 10.5 GHz, measured with the 100-m Effelsberg telescope (Chyży et al 2003(Chyży et al , 2011, of 343, 277 and 156 mJy, can be fitted with a constant spectral index of −0.41. We can interpolate them to estimate the missing zero-spacing flux in each spectral window.…”

Magnetic fields in Local Group dwarf irregulars

“…For identification of background sources, we compared our radio maps with those of the Condon (1987), NVSS 1 , and FIRST 2 surveys. We then applied the "subtraction" method (Chyży et al 2003) and removed all confusing sources from the maps. For clear detections of dwarfs their total fluxes were obtained by integrating the signal in polygonal areas encompassing all visible radio emission.…”

“…1-5, which also show background sources. Polarized emission was detected in NGC 6822, IC 10 (Chyży et al 2003), andIC 1613 (Figs. 4, 5) all at 4.85 GHz.…”

“…NGC 6822 (Chyży et al 2003), IC 10 (Chyży et al 2003), and the Large Magellanic Cloud (LMC: Gaensler et al 2005). In the Small Magellanic Cloud (SMC), a weak total field of about 3 μG was found, partly on large-scales (Mao et al 2008).…”

The fate of magnetic fields in colliding galaxies

“…Typical field strengths are of order of several µG, ranging from a few µG in dwarfs (e.g. Chyży et al, 2003) up to 30 µG in the star-forming regions of grand-design spiral galaxies (Fletcher et al, 2004). Magnetic fields have also been observed at high red-shifts like e.g.…”