Galactic magnetic fields and the large-scale anisotropy at MILAGRO

The air-shower observatory Milagro has detected a large-scale anisotropy of unknown origin in the flux of TeV cosmic rays. We propose that this anisotropy is caused by galactic magnetic fields, in particular, that it results from the combined effects of the regular and the turbulent (fluctuating) magnetic fields in our vicinity. Instead of a diffusion equation, we integrate Boltzmann's equation to show that the turbulence may define a preferred direction in the cosmic-ray propagation that is orthogonal to the local regular magnetic field. The approximate dipole anisotropy that we obtain explains well Milagro's data.Comment: 12 pages, version to appear in ApJ

Magnetic fields and the outer rotation curve of M31

Recent observations of the rotation curve of M31 show a rise of the outer part that can not be understood in terms of standard dark matter models or perturbations of the galactic disc by M31's satellites. Here, we propose an explanation of this dynamical feature based on the influence of the magnetic field within the thin disc. We have considered standard mass models for the luminous mass distribution, a NFW model to describe the dark halo, and we have added up the contribution to the rotation curve of a magnetic field in the disc, which is described by an axisymmetric pattern. Our conclusion is that a significant improvement of the fit in the outer part is obtained when magnetic effects are considered. The best-fit solution requires an amplitude of ~4 microG with a weak radial dependence between 10 and 38 kpc.Comment: Accepted for publication in ApJ

Vertical motions in the disk of NGC 5668 as seen with optical Fabry-Perot spectroscopy

We have observed the nearly face-on spiral galaxy NGC 5668 with the TAURUS II Fabry-Perot interferometer at the William Herschel Telescope using the $H\alpha$ line to study the kinematics of the ionized gas. From the extracted data cube we construct intensity, velocity and velocity dispersion maps. We calculate the rotation curve in the innermost 2 arcmin and we use the residual velocity field to look for regions with important vertical motions. By comparing the geometry of these regions in the residual velocity field with the geometry in the intensity and velocity dispersion maps we are able to select some regions which are very likely to be shells or chimneys in the disk. The geometry and size of these regions are very similar to the shells or chimneys detected in other galaxies by different means. Moreover, it is worth noting than this galaxy has been reported to have a population of neutral hydrogen high velocity clouds (Schulman et al. 1996) which, according to these observations, could have been originated by chimneys similar to those reported in this paper.Comment: 7 pages with 9 figures. LaTeX file using A&A v4.0 macro

Are rotation curves in NGC 6946 and the Milky Way magnetically supported?

Following the model of magnetically supported rotation of spiral galaxies, the inner disk rotation is dominated by gravity but magnetism is not negligible at radii where the rotation curve becomes flat, and indeed becomes dominant at very large radii. Values of the order of 1 $\mu$G, or even less, produce a centripetal force when the absolute value of the slope of the curve [$B_\phi$, R] (azimuthal field strength versus radius) is less than $R^{-1}$. The $R^{-1}$-profile is called the critical profile. From this hypothesis, the following is to be expected: at large radii, a ``subcritical'' profile (slope flatter than $R^{-1}$); at still larger radii a $B_\phi$-profile becoming asymptotically critical as the density becomes asymptotically vanishing. Recent observations of magnetic fields in NGC 6946 and the Milky Way are in very good agreement with these predictions. This magnetic alternative requires neither galactic dark matter (DM) nor modification of fundamental laws of physics, but it is not in conflict with these hypotheses, especially with the existence of cosmological cold dark matter (CDM).Comment: 11 pages, 2 figures, accepted for publication in Astron. Astrophy