Magnetic Fields Ignorable Magnetic Fields Non Ignorable Magnetic Fields Important Magnetic Fields Dominant

Magnetic fields are considered to be dominant when εB ≥ εK , being εB = B /8π the magnetic energy density and εK = 1/2ρθ 2 the rotation energy density, for a conventional moderate B= 1 μG. They are considered to be negligible when εB < εK for B ∼ 10μG. With no assumption and no theoretical calculation, we show that magnetic fields cannot be ignored in the outer parts of a galaxy like the Milky Way and in the whole disk of a dwarf galaxy. 1. Description Magnetic fields are of the order of 5-10 μG in the interior of a spiral galaxy (see, for instance, Beck 1991) and of the order of 1-3 μG in the intergalactic medium (see, for instance, Kronberg 1994). We therefore assume 1 μG in the outer region. This interpolated value is therefore a conservative one. 0 10 20 30 40 -14 -13 -12 -11 -10 -9 R (galactocentric radius) Kpc. Milky Way DDO 154 MAGNETIC FIELDS IGNORABLE MAGNETIC FIELDS NON IGNORABLE MAGNETIC FIELDS IMPORTANT MAGNETIC FIELDS DOMINANT For the Milky Way, the density data have been taken from Diplas & Savage (1991), (see also Burton 1992). For the rotation curve it was assumed a constant value of 200 km/s (for R≥ R⊙), based on the works by Homma & Sofue (1997), 1 2 Battaner, Florido & Guijarro Merrifield (1992), Olling & Merrifield (2000). For the typical irregular galaxy DDO154, the surface density and the rotation data were taken from Gelato & Sommer-Larsen (1999). A typical thickness of 1 Kpc was assumed. A 1 μG in the outer disk does not produce detectable synchrotron radiation, because a) under equipartition I ∝ B7/2, i.e. the intensity, I, decreases much more faster than B does. b) Some authors assume that the number density of relativistic electrons is proportional to the density (because their sources are supernova explosions, which are produced where density is larger). In this case, I decreases still faster, in an exponential way, or even faster as a result of the typical stellar truncation. Probably, B decreases much slower because the strength in the intergalactic medium is ∼ 1μG. c) The synchrotron spectrum suddenly steepens for large radii (Lisenfeld et al. 1996) what is easily interpreted as a truncation of relativistic electron sources.