2,277 research outputs found
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
Modeling Multi-Wavelength Stellar Astrometry. I. SIM Lite Observations of Interacting Binaries
Interacting binaries consist of a secondary star which fills or is very close
to filling its Roche lobe, resulting in accretion onto the primary star, which
is often, but not always, a compact object. In many cases, the primary star,
secondary star, and the accretion disk can all be significant sources of
luminosity. SIM Lite will only measure the photocenter of an astrometric
target, and thus determining the true astrometric orbits of such systems will
be difficult. We have modified the Eclipsing Light Curve code (Orosz &
Hauschildt 2000) to allow us to model the flux-weighted reflex motions of
interacting binaries, in a code we call REFLUX. This code gives us sufficient
flexibility to investigate nearly every configuration of interacting binary. We
find that SIM Lite will be able to determine astrometric orbits for all
sufficiently bright interacting binaries where the primary or secondary star
dominates the luminosity. For systems where there are multiple components that
comprise the spectrum in the optical bandpass accessible to SIM Lite, we find
it is possible to obtain absolute masses for both components, although
multi-wavelength photometry will be required to disentangle the multiple
components. In all cases, SIM Lite will at least yield accurate inclinations,
and provide valuable information that will allow us to begin to understand the
complex evolution of mass-transferring binaries. It is critical that SIM Lite
maintains a multi-wavelength capability to allow for the proper deconvolution
of the astrometric orbits in multi-component systems.Comment: 12 pages, 6 figures, 6 tables. Accepted for publication in the
Astrophysical Journa
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A New Calibration Of Galactic Cepheid Period-Luminosity Relations From B To K Bands, And A Comparison To LMC Relations
Context. The universality of the Cepheid period-luminosity (PL) relations has been under discussion since metallicity effects were assumed to play a role in the value of the intercept and, more recently, of the slope of these relations. Aims. The goal of the present study is to calibrate the Galactic PL relations in various photometric bands (from B to K) and to compare the results to the well-established PL relations in the LMC. Methods. We use a set of 59 calibrating stars, the distances of which are measured using five different distance indicators: Hubble Space Telescope and revised Hipparcos parallaxes, infrared surface brightness and interferometric Baade-Wesselink parallaxes, and classical Zero-Age-Main-Sequence-fitting parallaxes for Cepheids belonging to open clusters or OB stars associations. A detailed discussion of absorption corrections and projection factor to be used is given. Results. We find no significant difference in the slopes of the PL relations between LMC and our Galaxy. Conclusions. We conclude that the Cepheid PL relations have universal slopes in all photometric bands, not depending on the galaxy under study (at least for LMC and Milky Way). The possible zero-point variation with metal content is not discussed in the present work, but an upper limit of 18.50 for the LMC distance modulus can be deduced from our data.McDonald Observator
HST Fine Guidance Sensor Astrometric Parallaxes for Three Dwarf Novae: SS Aurigae, SS Cygni, and U Geminorum
We report astrometric parallaxes for three well known dwarf novae obtained
using the Fine Guidance Sensors on the Hubble Space Telescope. We found a
parallax for SS Aurigae of Pi = 5.00 +/- 0.64 mas, for SS Cygni we found Pi =
6.02 +/- 0.46 mas, and for U Geminorum we obtained Pi = 10.37 +/- 0.50 mas.
These represent the first true trigonometric parallaxes of any dwarf novae. We
briefly compare these results with previous distance estimates. This program
demonstrates that with a very modest amount of HST observing time, the Fine
Guidance Sensors can deliver parallaxes of unrivaled precision.Comment: 15 pages, 2 Table
Gene duplication and divergence produce divergent MHC genotypes without disassortative mating
Genes of the major histocompatibility complex (MHC) exhibit heterozygote advantage in immune defence, which in turn can select for MHC-disassortative mate choice. However, many species lack this expected pattern of MHC-disassortative mating. A possible explanation lies in evolutionary processes following gene duplication: if two duplicated MHC genes become functionally diverged from each other, offspring will inherit diverse multilocus genotypes even under random mating. We used locus-specific primers for high-throughput sequencing of two expressed MHC Class II B genes in Leach\u27s storm-petrels, Oceanodroma leucorhoa, and found that exon 2 alleles fall into two gene-specific monophyletic clades. We tested for disassortative vs. random mating at these two functionally diverged Class II B genes, using multiple metrics and different subsets of exon 2 sequence data. With good statistical power, we consistently found random assortment of mates at MHC. Despite random mating, birds had MHC genotypes with functionally diverged alleles, averaging 13 amino acid differences in pairwise comparisons of exon 2 alleles within individuals. To test whether this high MHC diversity in individuals is driven by evolutionary divergence of the two duplicated genes, we built a phylogenetic permutation model. The model showed that genotypic diversity was strongly impacted by sequence divergence between the most common allele of each gene, with a smaller additional impact of monophyly of the two genes. Divergence of allele sequences between genes may have reduced the benefits of actively seeking MHC-dissimilar mates, in which case the evolutionary history of duplicated genes is shaping the adaptive landscape of sexual selection
Fermions, T-duality and effective actions for D-branes in bosonic backgrounds
We find the effective action for any D-brane in a general bosonic background
of supergravity. The results are explicit in component fields up to second
order in the fermions and are obtained in a covariant manner. No interaction
terms between fermions and the field , characteristic of the bosonic
actions, are considered. These are reserved for future work. In order to obtain
the actions, we reduce directly from the M2-brane world-volume action to the
D2-brane world-volume action. Then, by means of T-duality, we obtain the other
Dp-brane actions. The resulting Dp-brane actions can be written in a single
compact and elegant expression.Comment: 22 pages, latex, version published by JHEP plus typos corrected in
eq.(44) and eq.(47
Wess-Zumino Terms in Supersymmetric Gauge Theories
The Wess-Zumino term is constructed for supersymmetric QCD with two colors
and flavors, and is shown to correctly reproduce the anomalous Ward identities.
Supersymmetric QCD is also shown not to have topologically stable skyrmion
solutions because of baryon flat directions, which allow them to unwind. The
generalization of these results to other supersymmetric theories with quantum
modified constraints is discussed.Comment: 4 pages (revtex
The Extrasolar Planet epsilon Eridani b - Orbit and Mass
Hubble Space Telescope observations of the nearby (3.22 pc), K2 V star
epsilon Eridani have been combined with ground-based astrometric and radial
velocity data to determine the mass of its known companion. We model the
astrometric and radial velocity measurements simultaneously to obtain the
parallax, proper motion, perturbation period, perturbation inclination, and
perturbation size. Because of the long period of the companion, \eps b, we
extend our astrometric coverage to a total of 14.94 years (including the three
year span of the \HST data) by including lower-precision ground-based
astrometry from the Allegheny Multichannel Astrometric Photometer. Radial
velocities now span 1980.8 -- 2006.3. We obtain a perturbation period, P = 6.85
+/- 0.03 yr, semi-major axis, alpha =1.88 +/- 0.20 mas, and inclination i =
30.1 +/- 3.8 degrees. This inclination is consistent with a previously measured
dust disk inclination, suggesting coplanarity. Assuming a primary mass M_* =
0.83 M_{\sun}, we obtain a companion mass M = 1.55 +/- 0.24 M_{Jup}. Given the
relatively young age of epsilon Eri (~800 Myr), this accurate exoplanet mass
and orbit can usefully inform future direct imaging attempts. We predict the
next periastron at 2007.3 with a total separation, rho = 0.3 arcsec at position
angle, p.a. = -27 degrees. Orbit orientation and geometry dictate that epsilon
Eri b will appear brightest in reflected light very nearly at periastron.
Radial velocities spanning over 25 years indicate an acceleration consistent
with a Jupiter-mass object with a period in excess of 50 years, possibly
responsible for one feature of the dust morphology, the inner cavity
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