Kinetic and magnetic alpha effects in nonlinear dynamo theory

The backreaction of the Lorentz force on the alpha-effect is studied in the limit of small magnetic and fluid Reynolds numbers, using the first order smoothing approximation (FOSA) to solve both the induction and momentum equations. Both steady and time dependent forcings are considered. In the low Reynolds number limit, the velocity and magnetic fields can be expressed explicitly in terms of the forcing function. The nonlinear alpha-effect is then shown to be expressible in several equivalent forms in agreement with formalisms that are used in various closure schemes. On the one hand, one can express alpha completely in terms of the helical properties of the velocity field as in traditional FOSA, or, alternatively, as the sum of two terms, a so-called kinetic alpha-effect and an oppositely signed term proportional to the helical part of the small scale magnetic field. These results hold for both steady and time dependent forcing at arbitary strength of the mean field (abridged).Comment: Accepted for publication in MNRA

The formation of supermassive black holes in the first galaxies

We discuss the formation of supermassive black holes in the early universe, and how to probe their subsequent evolution with the upcoming mm/sub-mm telescope ALMA. We first focus on the chemical and radiative conditions for black hole formation, in particular considering radiation trapping and molecular dissociation effects. We then turn our attention towards the magnetic properties in the halos where the first black holes form, and show that the presence of turbulence may lead to a magnetic dynamo, which could support the black hole formation process by providing an efficient means of transporting the angular momentum. We finally focus on observable properties of high-redshift black holes with respect to ALMA, and discuss how to distinguish between chemistry driven by the starburst and chemistry driven by X-rays from the black hole.Comment: Contribution to AIP conference proceedings "First Stars and Galaxies: Challenges in the Next Decade". 4 pages, 3 figure

Galactic dynamo action in presence of stochastic alpha and shear

Using a one-dimensional $\alpha\omega$-dynamo model appropriate to galaxies, we study the possibility of dynamo action driven by a stochastic alpha effect and shear. To determine the field evolution, one needs to examine a large number of different realizations of the stochastic component of $\alpha$. The net growth or decay of the field depends not only on the dynamo parameters but also on the particular realization, the correlation time of the stochastic $\alpha$ compared to turbulent diffusion timescale and the time over which the system is evolved. For dynamos where both a coherent and fluctuating $\alpha$ are present, the stochasticity of $\alpha$ can help alleviate catastrophic dynamo quenching, even in the absence of helicity fluxes. One can obtain final field strengths up to a fraction $\sim 0.01$ of the equipartition field $B_{eq}$ for dynamo numbers $| D| \sim 40$, while fields comparable to $B_{eq}$ require much larger degree of $\alpha$ fluctuations or shear. This type of dynamo may be particularly useful for amplifying fields in the central regions of disk galaxies.Comment: Accepted for publication in MNRAS Letter