Marc Davis

The origin of the Magellanic Stream

SHORTENED ABSTRACT: We present numerical investigations designed to critically test models of the origin of the Magellanic Stream. The most developed model is the tidal model which fails to reproduce several of its characteristic properties. We suggest an alternative model for the origin of the Stream which can explain all of its observed features and dynamics, as well as provide a strong constraint on the distribution of gas within the halo of the Milky Way. We propose that the Stream consists of material which was ram-pressure stripped from the Magellanic System during its last passage through an extended ionized disk of the Galaxy. This collision took place some 500 million years ago at a galacto-centric distance of about 65 kpc, and swept

THE DEPENDENCE OF MAGNETIC RECONNECTION ON PLASMA β AND MAGNETIC SHEAR: EVIDENCE FROM SOLAR WIND OBSERVATIONS

\sim 20

A Search for Features in Early-Type Galaxies

\% of the least bound HI into the Stream. The gas with the lowest column density lost the most orbital angular momentum, and is presently at the tip of the Stream, having fallen to a distance of

Anisotropy of the Galaxies Detected by IRAS

\sim 20

The DEEP2 Redshift Survey

kpc from the Milky Way attaining a negative velocity of 200 \kms. To prevent the stripped material from leading the Magellanic Clouds and attaining too large an infall velocity, we postulate the existence of an extended dilute halo of diffuse ionized gas surrounding the Milky Way. If the halo gas is at the virial temperature of the potential well of the Milky Way, its thermal emission would contribute

An Improved Distance Indicator for Elliptical Galaxies

\sim

Galaxy Surface Brightness and Clustering: A Test for Biased Galaxy Formation Scenarios?

40\% of the observed diffuse background radiation in the 0.5-1.0 keV (M) band, consistent with recent ROSAT measurements as well as pulsar dispersion measures. Ram pressure stripping

The Formation of Galaxies

We address the conditions for the onset of magnetic reconnection based on a survey of 197 reconnection events in solar wind current sheets observed by the Wind spacecraft. We report the first observational evidence for the dependence of the occurrence of reconnection on a combination of the magnetic field shear angle, θ, across the current sheet and the difference in the plasma β values on the two sides of the current sheet, Δβ. For low Δβ, reconnection occurred for both low and high magnetic shears, whereas only large magnetic shear events were observed for large Δβ: Events with shears as low as 11° were observed for Δβ 1.5 only events with θ > 100° were detected. Our observations are in quantitative agreement with a theoretical prediction that reconnection is suppressed in high β plasmas at low magnetic shears due to super-Alfvenic drift of the X-line caused by plasma pressure gradients across the current sheet. The magnetic shear-Δβ dependence could account for the high occurrence rate of reconnection observed in current sheets embedded within interplanetary coronal mass ejections, compared to those in the ambient solar wind. It would also suggest that reconnection could occur at a substantially higher rate in solar wind current sheets closer to the Sun than at 1 AU and thus may play an important role in the generation and heating of the solar wind.

Supercluster Infall Models

We are conducting a search for dust lanes, incipient stellar disks and bars, and other deviations from elliptical symmetry in a sample ~ 250 early-type galaxies. Division by a model galaxy image is a very powerful enhancement technique for this type of work.

Is the Universe Made of Massive Neutrinos

We generate a catalogue of 6730 galaxies uniformly selected over most of the sky (> 9.5sr) using the IRAS point source catalogue. Our catalogue reveals a small (4–7%), but robust dipole anisotropy in the galaxy distribution that points towards l = 235°, b = 45°, within 30° of the microwave dipole anisotropy. Angular correlation analysis suggests that subsets of the catalogue have characteristic distances D* ≃50 – 100h-1 Mpc. The good agreement in direction of these dipoles suggests that the IRAS galaxies roughly trace the large scale mass distribution, that the microwave dipole velocity is mostly induced by the local supercluster (cz < 3000 km s -1), and that the indicated cosmological density is high (Ω ≈ 0.5). A full sky redshift survey is in progress to obtain a more precise estimate.