Possible High-Redshift, Low-Luminosity AGN Activity in the Hubble Deep Field

In the Hubble Deep Field (HDF), twelve candidate sources of high-redshift (z > 3.5) AGN activity have been identified. The color selection criteria were established by passing spectra of selected quasars and Seyfert galaxies (appropriately redshifted and modified for "Lyman forest" absorption), as well as stars, observed normal and starburst galaxies, and galaxy models for various redshifts through the filters used for the HDF observations. The actual identification of AGN candidates also involved convolving a Laplacian-of-Gaussian filter with the HDF images, thereby removing relatively flat galactic backgrounds and leaving only the point-like components in the centers. Along with positions and colors, estimated redshifts and absolute magnitudes are reported, with the candidates falling toward the faint end of the AGN luminosity function. One candidate has been previously observed spectroscopically, with a measured redshift of 4.02. The number of sources reported here is consistent with a simple extrapolation of the observed quasar luminosity function to magnitude 30 in B_Johnson. Implications for ionization of the intergalactic medium and for gravitational lensing are discussed.Comment: 10 pages LaTex plus 2 separate files (Table 1 which is a two-page landscape LaTex file; and Figure 6 which is a large (0.7 MB) non-encapsulated postscript file). Accepted for publication in the Astronomical Journa

A guide for performing system safety analysis

A general guide is presented for performing system safety analyses of hardware, software, operations and human elements of an aerospace program. The guide describes a progression of activities that can be effectively applied to identify hazards to personnel and equipment during all periods of system development. The general process of performing safety analyses is described; setting forth in a logical order the information and data requirements, the analytical steps, and the results. These analyses are the technical basis of a system safety program. Although the guidance established by this document cannot replace human experience and judgement, it does provide a methodical approach to the identification of hazards and evaluation of risks to the system

Automated static magnetic cleanliness screening for the TRACERS small-satellite mission

The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) Small Explorers mission requires high-fidelity magnetic field measurements for its magnetic reconnection science objectives and for its technology demonstration payload Magnetometers for Innovation and Capability (MAGIC). TRACERS needs to minimize the local magnetic noise through a magnetic cleanliness program such that the stray fields from the spacecraft and its instruments do not distort the local geophysical magnetic field of interest. Here we present an automated magnetic screening apparatus and procedure to enable technicians to routinely and efficiently measure the magnetic dipole moments of potential flight parts to determine whether they are suitable for spaceflight. This procedure is simple, replicable, and accurate down to a dipole moment of 1.59 × 10−3 N m T−1. It will be used to screen parts for the MAGIC instrument and other subsystems of the TRACERS satellite mission to help ensure magnetically clean measurements on orbit.</p

Bromethalin toxicosis--evaluation of aminophylline treatment and an epidemiolgic assessment

The compound bromethalin is described and characterized in terms of mode of action when used as a rodenticide. It is hypothesized that an antimetabolite which inhibits the conversion of bromethalin to its desmethylbromethalin metabolite could represent a potential treatment for bromethalin toxicosis. An ideal antimetabolite of bromethalin is described, and based on these considerations, aminophylline (theophylline ethylenediamine) was chosen as a potential antimetabolite. In lab trials, aminophylline was administered to rats given a lethal dose of bromethalin, but it was not effective in prolonging rat survival. However, it did result in mean survival time being increase to nearly double. Further studies aminophylline or similar drugs may results in identifying a suitable treatment for bromethalin-induced toxic syndrome

The Effect of Mobile Element IS10 on Experimental Regulatory Evolution in Escherichia coli

Mobile genetic elements are widespread in bacteria, where they cause several kinds of mutations. Although their effects are on the whole negative, rare beneficial mutations caused by insertion sequence elements are frequently selected in some experimental evolution systems. For example, in earlier work, we found that strains of Escherichia coli that lack the sigma factor RpoS adapt to a high-osmolarity environment by the insertion of element IS10 into the promoter of the otsBA operon, rewiring expression from RpoS dependent to RpoS independent. We wished to determine how the presence of IS10 in the genome of this strain shaped the evolutionary outcome. IS10 could influence the outcome by causing mutations that confer adaptive phenotypes that cannot be achieved by strains without the element. Alternatively, IS10 could influence evolution by increasing the rate of appearance of certain classes of beneficial mutations even if they are no better than those that could be achieved by a strain without the element. We found that populations evolved from an IS10-free strain did not upregulate otsBA. An otsBA-lacZY fusion facilitated the recovery of a number of mutations that upregulate otsB without involving IS10 and found that two caused greater fitness increases than IS10 insertion, implying that evolution could have upregulated otsBA in the IS10-free strain. Finally, we demonstrate that there is epistasis between the IS10 insertion into the otsBA promoter and the other adaptive mutations, implying that introduction of IS10 into the otsBA promoter may alter the trajectory of adaptive evolution. We conclude that IS10 exerts its effect not by creating adaptive phenotypes that could not otherwise occur but by increasing the rate of appearance of certain adaptive mutations