Cosmology

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:

We estimate the bispectrum of the Very Small Array data from the compact and extended configuration observations released in December 2002, and compare our results to those obtained from Gaussian simulations. There is a slight excess of large bispectrum values for two individual fields, but this does not appear when the fields are combined. Given our expected level of residual point sources, we do not expect these to be the source of the discrepancy. Using the compact configuration data, we put an upper limit of 5400 on the value of f_NL, the non-linear coupling parameter, at 95 per cent confidence. We test our bispectrum estimator using non-Gaussian simulations with a known bispectrum, and recover the input values.

Abstract:

We present results of a Gemini adaptive optics (AO) imaging program to investigate the host galaxies of typical QSOs at z~2. Our aim is to study the host galaxies of typical, L*_qso QSOs at the epoch of peak QSO and star formation activity. The large database of faint QSOs provided by the 2dF QSO Redshift Survey allows us to select a sample of QSOs at z=1.75-2.5 which have nearby (<12 arcsecond separation) bright stars suitable for use as AO guide stars. We have observed a sample of 9 QSOs. The images of these sources have AO corrected full-width at half-maximum of between 0.11 and 0.25 arcseconds. We use multiple observations of point spread function (PSF) calibration star pairs in order to quantify any uncertainty in the PSF. We then factored these uncertainties into our modelling of the QSO plus host galaxy. In only one case did we convincingly detect a host (2QZ J133311.4+001949, at z=1.93). This host galaxy has K=18.5+-0.2 mag with a half-light radius, r_e=0.55+-0.1'', equivalent to ~3L*_gal assuming a simple passively evolving model. From detailed simulations of our host galaxy modelling process, we find that for four of our targets we should be sensitive to host galaxies that are equivalent to ~2L*_gal (passively evolved). Our non-detections therefore place tight constraints on the properties of L*_qso QSO host galaxies, which can be no brighter (after allowing for passive evolution) than the host galaxies of L*_qso AGN at low redshift, although the QSOs themselves are a factor of ~50 brighter. This implies that either the fueling efficiency is much greater at high redshift, or that more massive black holes are active at high redshift.