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We seek the conditions for a {\it steady} mean field galactic dynamo. The parameter set is reduced to those appearing in the $\alpha^2$ and $\alpha/\omega$ dynamo, namely velocity amplitudes, and the ratio of sub-scale helicity to diffusivity. The parameters can be allowed to vary on conical spirals. We analyze the mean field dynamo equations in terms of scale invariant logarithmic spiral modes and special exact solutions. Compatible scale invariant gravitational spiral arms are introduced and illustrated in an appendix, but the detailed dynamical interaction with the magnetic field is left for another work. As a result of planar magnetic spirals `lifting' into the halo, multiple sign changes in average rotation measures forming a regular pattern on each side of the galactic minor axis, are predicted. Such changes have recently been detected in the CHANG-ES survey., Comment: 44 double-spaced pages, 10 figures plus 1 appendix figure, accepted to MNRAS

We present an analytic model of a magnetized galactic halo surrounding a Mestel gravitating disc. The magnetic field is taken to be in energy equipartition with the pressure dominant rotating halo gas ({\it not} with the cosmic rays), and the whole system is in a steady state. A more flexible `anisotropic equipartition' model is also explored. A definite pressure law is required to maintain the equilibrium, but the halo density is constant. The velocity/magnetic system is scale-free. The objective is to find the rotational velocity lag in such a halo. The magnetic field is not force-free so that angular momentum may be transported from the halo to the intergalactic medium. We find that the `X'-shaped structure observed for halo magnetic fields can be obtained together with a simple analytic formula for the rate of decline of the velocity with height $z$. The formula also predicts the change in lag with radius, $r$., Comment: 25 pages, 5 figures. Accepted (March 9, 2016) to Monthly Notices of the Royal Astronomical Society

We present the first maps of NGC 3044 and NGC 4157 at $\lambda\,450 \mu$m and $\lambda\,850 \mu$m from the JCMT as well as the first maps at 617 MHz from the GMRT. High latitude emission has been detected in both the radio continuum and sub-mm for NGC 3044 and in the radio continuum for NGC 4157, including several new features. The dust spectrum at long wavelengths required fitting with a two-temperature model for both galaxies, implying the presence of cold dust. Dust masses are $M_d\,=\,1.6\,\times\,10^8 M_\odot$ and $M_d\,=\,2.1\,\times\,10^7 M_\odot$ for NGC 3044 and NGC 4157, respectively, and are dominated by the cold component. There is a clear correlation between the 617 MHz and $\lambda\,850 \mu$m emission in the two galaxies. In the case of NGC 3044, this implies a relation between the non-thermal synchrotron emission and cold dust. The 617 MHz component represents an integration of massive star formation over the past $10^{7-8}$ yr and the $\lambda \,850 \mu$m emission represents heating from the diffuse interstellar radiation field (ISRF). The 617 MHz -- $\lambda\,850 \mu$m correlation improves when a smoothing kernel is applied to the $\lambda\,850 \mu$m data to account for differences between the CR electron diffusion scale and the mean free path of an ISRF photon to dust. The best-fit relation is $L_{617_{\rm MHz}}\,\propto\,{L_{850 \mu{\rm m}}}^{2.1\,\pm\,0.2}$ for NGC 3044. If variations in the cold dust emissivity are dominated by variations in dust density, and the synchrotron emission depends on magnetic field strength (a function of gas density) as well as CR electron generation (a function of massive star formation rate and therefore density via the Schmidt law) then the expected correlation for NGC 3044 is $L_{617_{\rm MHz}}\,\propto\,{L_{850 \mu{\rm m}}}^{2.2}$, in agreement with the observed correlation., Comment: 19 pages; Figure 1 is included as an associated jpg file. Accepted to mnras

In this paper, we study the distribution functions that arise naturally during self-similar radial infall of collisionless matter. Such matter may be thought of either as stars or as dark matter particles. If a rigorous steady state is assumed, then the system is infinite and is described by a universal distribution function given the self-similar index. The steady logarithmic potential case is exceptional and yields the familiar Gaussian for an infinite system with an inverse-square density profile. We show subsequently that for time-dependent radial self-similar infall, the logarithmic case is accurately described by the Fridmann and Polyachenko distribution function. The system in this case is finite but growing. We are able to embed a central mass in the universal steady distribution only by iteration, except in the case of massless particles. The iteration yields logarithmic corrections to the massless particle case and requires a `renormalization' of the central mass. A central spherical mass may be accurately embedded in the Fridmann and Polyachenko growing distribution however. Some speculation is given concerning the importance of radial collisionless infall in actual galaxy formation.

We compare the conclusions reached using the coarse-graining technique employed by Henriksen (2004) for a one degree of freedom (per particle) collisionless system, to those presented in a paper by Binney (2004) based on an exact one degree of freedom model. We find agreement in detail but in addition we show that the isolated 1D system is self-similar and therefore unrelaxed. Fine graining of this system recovers much less prominent wave-like structure than in a spherically symmetric isotropic 3D system. The rate of central flattening is also reduced in the 1D system. We take this to be evidence that relaxation of collisionless systems proceeds ultimately by way of short wavelength Landau damping. N-body systems, both real and simulated, can be trapped in an incompletely relaxed state because of a break in the cascade of energy to small scales. This may be due to the rapid dissipation of the small scale oscillations in an isolated system, to the existence of conserved quantities such as angular momentum, or to the failure in simulations to resolve sub-Jeans length scales. Such a partially relaxed state appears to be the NFW state, and is to be expected especially in young systems. The NFW core is shown to be isolated. In non-isolated systems continuing coarse-grained relaxation should be towards a density core in solid body rotation.

Navarro, Frenk, and White have suggested that the density profiles of simulated dark matter halos have a ``universal'' shape so that a given halo can be characterized by a single free parameter which fixes its mass. In this paper, we revisit the spherical infall model in the hope of recognizing in detail the existence and origin of any such universality. A system of particles is followed from linear perturbation, through first shell crossing, then through an accretion or infall phase, and finally to virialization. During the accretion phase, the system relaxes through a combination of phase mixing, phase space instability, and moderate violent relation. It is driven quickly, by the flow of mass through its surface, toward self-similar evolution. The self-similar solution plays its usual role of intermediate attractor and can be recognized from a virial-type theorem in scaled variables and from our numerical simulations. The transition to final equilibrium state once infall has ceased is relatively gentle, an observation which leads to an approximate form for the distribution function of the final system. The infall phase fixes the density profile in intermediate regions of the halo to be close to r^{-2}. We make contact with the standard hierarchical clustering scenario and explain how modifications of the self-similar infall model might lead to density profiles in agreement with those found in numerical simulations., 26 pages, Latex, plus 11 figures

A new model for gamma-ray pulsars presented by Higgins & Henriksen is applied to the cases of the seven known gamma-ray pulsars. Those pulsars that are not presently observed in gamma-rays, but are candidates for observation by the next generation of gamma-ray telescopes, are discussed. The case of millisecond pulsars is discussed, and it is shown that these objects should radiate at detectable levels, in opposition to the predictions of other gamma-ray pulsar models.

A new model for the high-energy emission from pulsars is developed by considering charged particle motion in the fields of a spinning, highly magnetised and conducting sphere in vacuum. A generally applicable approximation to the particle motion in strong fields is developed and applied to the numerical modelling, and the radiation emitted by curvature emission is summed to generate light curves. The model predicts many of the observed features of pulsar light curves. This paper outlines the basic properties of the model; a subsequent paper will discuss the statistical properties of a population of model pulsars and apply the model to the known gamma-ray pulsars.

Presentation de simulations numeriques hydrodynamiques 3D de collisions entre des nuages interstellaires en rotation. Les resultats montrent que la rotation etend de maniere significative les durees de vie des structures resultantes comparativement aux cas sans rotation. On note egalement l'importance des interactions spin-orbite dans le cas de collisions decentrees. Un critere, base sur le temps de chute libre et la periode initiale de rotation, est fourni afin de determiner l'issue d'une collision, les parametres se rapportant a la configuration initiale etant donnes