10,717 research outputs found
On small homotopies of loops
Two natural questions are answered in the negative:
(1) If a space has the property that small nulhomotopic loops bound small
nulhomotopies, then are loops which are limits of nulhomotopic loops themselves
nulhomotopic?
(2) Can adding arcs to a space cause an essential curve to become
nulhomotopic?
The answer to the first question clarifies the relationship between the
notions of a space being homotopically Hausdorff and -shape injective.Comment: 12 pages, 5 figure
The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625
We present new multi-frequency radio continuum imaging of the dwarf starburst
galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm
reveal global continuum emission dominated by free-free emission, with only
mild synchrotron components. Each of the major HII regions is detected; the
individual spectral indices are thermal for the youngest regions (showing
strongest H Alpha emission) and nonthermal for the oldest. We do not detect any
sources that appear to be associated with deeply embedded, dense, young
clusters, though we have discovered one low-luminosity, obscured source that
has no luminous optical counterpart and which resides in the region of highest
optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent
star formation, these radio properties suggest that the youngest star formation
complexes have not yet evolved to the point where their thermal spectra are
significantly contaminated by synchrotron emission. The nonthermal components
are associated with regions of older star formation that have smaller ionized
gas components. These results imply a range of ages of the HII regions and
radio components that agrees with our previous resolved stellar population
analysis, where an extended burst of star formation has pervaded the disk of
NGC 625 over the last ~ 50 Myr. We compare the nature of radio continuum
emission in selected nearby dwarf starburst and Wolf-Rayet galaxies,
demonstrating that thermal radio continuum emission appears to be more common
in these systems than in typical HII galaxies with less recent star formation
and more evolved stellar clusters.Comment: ApJ, in press; 27 pages, 5 figures. Full-resolution version may be
obtained at http://www.astro.umn.edu/~cannon/n625.vla.p
Invariant Peano curves of expanding Thurston maps
We consider Thurston maps, i.e., branched covering maps
that are postcritically finite. In addition, we assume that is expanding in
a suitable sense. It is shown that each sufficiently high iterate of
is semi-conjugate to , where is equal to the
degree of . More precisely, for such an we construct a Peano curve
(onto), such that
(for all ).Comment: 63 pages, 12 figure
Radio Sources in the 2dF Galaxy Redshift Survey. I. Radio Source Populations
We present the first results from a study of the radio continuum properties
of galaxies in the 2dF Galaxy Redshift Survey, based on thirty 2dF fields
covering a total area of about 100 square degrees. About 1.5% of galaxies with
b(J) < 19.4 mag are detected as radio continuum sources in the NRAO VLA Sky
Survey (NVSS). Of these, roughly 40% are star-forming galaxies and 60% are
active galaxies (mostly low-power radio galaxies and a few Seyferts). The
combination of 2dFGRS and NVSS will eventually yield a homogeneous set of
around 4000 radio-galaxy spectra, which will be a powerful tool for studying
the distriibution and evolution of both AGN and starburst galaxies out to
redshift z=0.3.Comment: 14 pages, 7 figures, accepted for publication in PAS
Evolution of replication efficiency following infection with a molecularly cloned feline immunodeficiency virus of low virulence
The development of an effective vaccine against human immunodeficiency virus is considered to be the most practicable means of controlling the advancing global AIDS epidemic. Studies with the domestic cat have demonstrated that vaccinal immunity to infection can be induced against feline immunodeficiency virus (FIV); however, protection is largely restricted to laboratory strains of FIV and does not extend to primary strains of the virus. We compared the pathogenicity of two prototypic vaccine challenge strains of FIV derived from molecular clones; the laboratory strain PET<sub>F14</sub> and the primary strain GL8<sub>414</sub>. PET<sub>F14</sub> established a low viral load and had no effect on CD4<sup>+</sup>- or CD8<sup>+</sup>- lymphocyte subsets. In contrast, GL8<sub>414</sub> established a high viral load and induced a significant reduction in the ratio of CD4<sup>+</sup> to CD8<sup>+</sup> lymphocytes by 15 weeks postinfection, suggesting that PET<sub>F14</sub> may be a low-virulence-challenge virus. However, during long-term monitoring of the PET<sub>F14</sub>-infected cats, we observed the emergence of variant viruses in two of three cats. Concomitant with the appearance of the variant viruses, designated 627<sub>W135</sub> and 628<sub>W135</sub>, we observed an expansion of CD8<sup>+</sup>-lymphocyte subpopulations expressing reduced CD8 ß-chain, a phenotype consistent with activation. The variant viruses both carried mutations that reduced the net charge of the V3 loop (K409Q and K409E), giving rise to a reduced ability of the Env proteins to both induce fusion and to establish productive infection in CXCR4-expressing cells. Further, following subsequent challenge of naïve cats with the mutant viruses, the viruses established higher viral loads and induced more marked alterations in CD8<sup>+</sup>-lymphocyte subpopulations than did the parent F14 strain of virus, suggesting that the E409K mutation in the PET<sub>F14</sub> strain contributes to the attenuation of the virus
Probing The Multiphase Interstellar Medium Of The Dwarf Starburst Galaxy NGC 625 With FUSE Spectroscopy
We present new FUSE spectroscopy of the dwarf starburst galaxy NGC 625. These
observations probe multiple phases of the interstellar medium, including the
coronal, ionized, neutral and molecular gas. This nearby (D = 3.9 +/- 0.2 Mpc)
system shows a clear detection of outflowing coronal gas as traced by OVI 1032
Angstrom absorption. The centroid of the OVI profile is blueshifted with
respect to the galaxy systemic velocity by ~ 30 km/sec, suggesting a
low-velocity outflow. The implied OVI velocity extent is found to be 100 +/- 20
km/sec, which is fully consistent with the detected HI outflow velocity found
in radio synthesis observations. We detect multiple lines of diffuse H2
absorption from the ISM of NGC 625; this is one of only a few extragalactic
systems with FUSE detections of H2 lines in the Lyman and Werner bands. We find
a potential abundance offset between the neutral and nebular gas that exceeds
the errors on the derived column densities. Since such an offset has been found
in multiple dwarf galaxies, we discuss the implications of a lower-metallicity
halo surrounding the central star forming regions of dwarf galaxies. The
apparent offset may be due to saturation of the observed OI line, and higher
S/N observations are required to resolve this issue.Comment: ApJ, in press; full-resolution version may be obtained at
http://www.astro.umn.edu/~cannon/n625.fuse.p
THE LOW-SPEED STATIC LONGITUDINAL AND LATERAL-CHARACTERISTICS OF A DELTA-WING MODEL WITH FIXED AND FREE-FLOATING CANARD SURFACES
Low-speed static longitudinal and lateral characteristics of delta-wing model with fixed and free-floating canard surface
The relationship between macroalgae taxa and human disturbance on central Pacific coral reefs
Climate change and human disturbance threatens coral reefs across the Pacific, yet there is little consensus on what characterizes a “healthy” reef. Benthic cover, particularly low coral cover and high macroalgae cover, are often used as an indicator of reef degradation, despite uncertainty about the typical algal community compositions associated with either near-pristine or damaged reefs. In this study, we examine differences in coral and algal community compositions and their response to human disturbance and past heat stress, by analysing 25 sites along a gradient of human disturbance in Majuro and Arno Atolls of the Republic of the Marshall Islands. Our results show that total macroalgae cover indicators of reef degradation may mask the influence of local human disturbance, with different taxa responding to disturbance differently. Identifying macroalgae to a lower taxonomic level (e.g. the genus level) is critical for a more accurate measure of Pacific coral reef health
Abundances on the Main Sequence of Omega Centauri
Abundance ratios of carbon, nitrogen and strontium relative to iron,
calculated using spectrum synthesis techniques, are given for a sample of main
sequence and turnoff stars that belong to the globular cluster omega Centauri.
The variations of carbon, nitrogen and/or strontium show several different
abundance patterns as a function of [Fe/H]. The source of the
enhancements/depletions in carbon, nitrogen and/or strontium may be enrichment
from asymptotic giant branch stars of low (1--3 solar masses) and intermediate
(3--8 solar masses) mass. Massive rotating stars which produce excess nitrogen
without carbon and oxygen overabundances may also play a role. These abundances
enable different contributors to be considered and incorporated into the
evolutionary picture of omega Cen.Comment: 43 Pages, 13 Figures. Accepted for publication in Ap
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