Compact Nuclei in Galaxies at Moderate Redshift: I. Imaging and Spectroscopy

This study explores the space density and properties of active galaxies to z=0.8. We have investigated the frequency and nature of unresolved nuclei in galaxies at moderate redshift as indicators of nuclear activity such as Active Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged galaxies with multi-component models using maximum likelihood estimate techniques to determine the best model fit. We select those galaxies requiring an unresolved, point source component in the galaxy nucleus, in addition to a disk and/or bulge component, to adequately model the galaxy light. We have searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies containing compact nuclei. In our survey of 1033 galaxies, the fraction containing an unresolved nuclear component greater than 3% of the total galaxy light is 16+/-3% corrected for incompleteness and 9+/-1% for nuclei greater than 5% of the galaxy light. Spectroscopic redshifts have been obtained for 35 of our AGN/starburst candidates and photometric redshifts are estimated to an accuracy of sigma_z=0.1 for the remaining sample. In this paper, the first of two in this series, we present the selected HST imaged galaxies having unresolved nuclei and discuss the selection procedure. We also present the ground-based spectroscopy for these galaxies as well as the photometric redshifts estimated for those galaxies without spectra.Comment: 56 pages, 22 figures, to appear in ApJ Supplement Series, April 199

The JPL MSAT mobile laboratory and the pilot field experiments

A Mobile Laboratory/Propagation Measurement Van (PMV) was developed to support the field experiments of the Mobile Satellite Experiment (MSAT-X) Project. This van was designed to provide flexibility, self-sufficiency and data acquisition to allow for both measurement of equipment performance and the mobile environment. The design philosophy and implementation of the PMV are described. The Pilot Field Experiments and an overall description of the three experiments in which the PMV was used are described

Brown dwarf populations in open clusters

We present the results of multiple simulations of open clusters, modelling the dynamics of a population of brown dwarf members. We consider the effects of a large range of primordial binary populations, including the possibilities of having brown dwarf members contained within a binary system. We also examine the effects of various cluster diameters and masses. Our examination of a population of wide binary systems containing brown dwarfs, reveals evidence for exchange reactions whereby the brown dwarf is ejected from the system and replaced by a heavier main-sequence star. We find that there exists the possibility of hiding a large fraction of the brown dwarfs contained within the primordial binary population. We conclude that it is probable that the majority of brown dwarfs are contained within primordial binary systems which then hides a large proportion of them from detection.Comment: 16 pages, 8 figures; to appear in MNRA

Natural Four-Generation Mass Textures in MSSM Brane Worlds

A fourth generation of Standard Model (SM) fermions is usually considered unlikely due to constraints from direct searches, electroweak precision measurements, and perturbative unitarity. We show that fermion mass textures consistent with all constraints may be obtained naturally in a model with four generations constructed from intersecting D6 branes on a T^6/(Z_2 x Z_2) orientifold. The Yukawa matrices of the model are rank 2, so that only the third- and fourth-generation fermions obtain masses at the trilinear level. The first two generations obtain masses via higher-order couplings and are therefore naturally lighter. In addition, we find that the third and fourth generation automatically split in mass, but do not mix at leading order. Furthermore, the SM gauge couplings automatically unify at the string scale, and all the hidden-sector gauge groups become confining in the range 10^{13}--10^{16} GeV, so that the model becomes effectively a four-generation MSSM at low energies.Comment: Accepted for publication in Physical Review

How Much Do Immigration and Trade Affect Labor Market Outcomes?

macroeconomics, trade, labor markets, immigration