Julien Devriendt

Abstract:

Under ideal MHD conditions the magnetic field strength should be correlated with density in the interstellar medium (ISM). However, observations indicate that this correlation is weak. Ambipolar diffusion can decrease the flux-to-mass ratio in weakly ionized media; however, it is generally thought to be too slow to play a significant role in the ISM except in the densest molecular clouds. Turbulence is often invoked in astrophysical problems to increase transport rates above the (very slow) laminar values predicted by kinetic theory. We describe a series of numerical experiments addressing the problem of turbulent transport of magnetic fields in weakly ionized gases. We show, subject to various geometrical and physical restrictions, that turbulence in a weakly ionized medium rapidly diffuses the magnetic flux-to-mass ratio B/ρ through the buildup of appreciable ion-neutral drifts on small scales. These results are applicable to the field strength-density correlation in the ISM, as well as the merging of flux systems such as protostar and accretion disk fields or protostellar jets with ambient matter, and the vertical transport of galactic magnetic fields.

Abstract:

The colour-magnitude relation (CMR) of cluster ellipticals has been widely used to constrain their star formation histories (SFHs) and to discriminate between the monolithic and merger paradigms of elliptical galaxy formation. We investigate the elliptical CMR predicted in the merger paradigm by using a LCDM hierarchical merger model. We first highlight sections of the literature which indicate that the traditional use of fixed apertures to derive colours gives a distorted view of the CMR due to the presence of colour gradients in galaxies. Fixed aperture observations make the CMR steeper and tighter than it really is. We then show that the star formation history (SFH) of cluster ellipticals predicted by the model is quasi-monolithic, with over 95 percent of the total stellar mass formed before a redshift of 1. The quasi-monolithic SFH produces a predicted CMR that agrees well at all redshifts with its observed counterpart once the fixed aperture effect is removed. More importantly, we present arguments to show that the elliptical-only CMR can be used to constrain the SFHs of present-day cluster ellipticals only if we believe a priori in the monolithic collapse model. It is not a meaningful tool for constraining the SFH in the merger paradigm, because a progressively larger fraction of the progenitor set of present-day cluster ellipticals is contained in late-type star forming systems at higher redshift, which cannot be ignored when deriving the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between the two competing theories of elliptical galaxy evolution.