Geraint F Lewis

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Recent observations have revealed streams of gas and stars in the halo of the Milky Way that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies, and that(More)
Seven cosmological hydrodynamic simulations of disk galaxy formation are analyzed to determine the alignment of the disk within the dark matter halo and the internal structure of the halo. We find that the orientation of the outer halo, beyond ∼ 0.1 r vir , is unaffected by the presence of the disk. In contrast, the inner halo is aligned such that the halo(More)
In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10-100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide(More)
An all high-latitude sky survey for cool carbon giant stars in the Galactic halo has revealed 75 such stars, of which the majority are new detections. Of these, more than half are clustered on a Great Circle on the sky which intersects the center of Sagittarius dwarf galaxy and is parallel to its proper motion vector, while many of the remainder are(More)
We present the results of a kinematic survey of the dwarf spheroidal satellite of M31, Andromeda IX, which appears to be the lowest surface brightness and also the faintest galaxy (M V = −8.3) found to date. Using Keck/DEIMOS spectroscopic data, we have measured its velocity relative to M31, its velocity dispersion, and its metallicity. It exhibits a(More)
Two studies have recently reported the discovery of pronounced Halo substructure in the Sloan Digital Sky Survey (SDSS) commissioning data. Here we show that this Halo substructure is almost in its entirety due to the expected tidal stream torn off the Sagittarius dwarf galaxy during the course of its many close encounters with the Milky Way. This(More)
We report on the discovery of three new dwarf galaxies in the Local Group. These galaxies are found in new CFHT/MegaPrime g, i imaging of the southwestern quadrant of M31, extending our extant survey area to include the majority of the southern hemisphere of M31's halo out to 150 kpc. All these galaxies have stellar populations which appear typical of dwarf(More)
Images of the molecular CO 2-1 line emission and the radio continuum emission from the redshift 4.12 gravitationally lensed quasi-stellar object (QSO) PSS J2322+1944 reveal an Einstein ring with a diameter of 1.5". These observations are modeled as a star-forming disk surrounding the QSO nucleus with a radius of 2 kiloparsecs. The implied massive star(More)
The concept of expanding space has come under fire recently as being inadequate and even misleading in describing the motion of test particles in the universe. Previous investigations have suffered from a number of shortcomings, which we seek to correct. We study the motion of test particles in the universe in detail, solving the geodesic equations of(More)
Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way. It has previously been suspected that dwarf galaxies may not be isotropically distributed around our Galaxy, because several are correlated with streams of H I emission, and may form coplanar groups.(More)