92 research outputs found
Measuring the undetectable: Proper motions and parallaxes of very faint sources
The near future of astrophysics involves many large solid-angle, multi-epoch,
multi-band imaging surveys. These surveys will, at their faint limits, have
data on large numbers of sources that are too faint to be detected at any
individual epoch. Here we show that it is possible to measure in multi-epoch
data not only the fluxes and positions, but also the parallaxes and proper
motions of sources that are too faint to be detected at any individual epoch.
The method involves fitting a model of a moving point source simultaneously to
all imaging, taking account of the noise and point-spread function in each
image. By this method it is possible to measure the proper motion of a point
source with an uncertainty close to the minimum possible uncertainty given the
information in the data, which is limited by the point-spread function, the
distribution of observation times (epochs), and the total signal-to-noise in
the combined data. We demonstrate our technique on multi-epoch Sloan Digital
Sky Survey imaging of the SDSS Southern Stripe. We show that we can distinguish
very red brown dwarfs by their proper motions from very high-redshift quasars
more than 1.6\mag fainter than with traditional technique on these SDSS data,
and with better better fidelity than by multi-band imaging alone. We
re-discover all 10 known brown dwarfs in our sample and present 9 new candidate
brown dwarfs, identified on the basis of high proper motion.Comment: AJ, in pres
Microsurgery in pediatric upper limb reconstructions: An overview
Pediatric microsurgery; Brachial plexus palsy; Upper limbMicrocirugÃa pediátrica; Parálisis del plexo braquial; Extremidad superiorMicrocirurgia pedià trica; Parà lisi del plexe braquial; Extremitat superiorThe use of microsurgery has spread during the last decades, making resolvable many complex defects considered hitherto inapproachable. Although the small vessel diameter in children was initially considered a technical limitation, the increase in microsurgical expertise over the past three decades allowed us to manage many pediatric conditions by means of free tissue transfers. Pediatric microsurgery has been shown to be feasible, gaining a prominent place in the treatment of children affected by limb malformations, tumors, nerve injuries, and post-traumatic defects. The aim of this current concepts review is to describe the more frequent pediatric upper limb conditions in which the use of microsurgical reconstructions should be considered in the range of treatment options
A simple test for the existence of two accretion modes in Active Galactic Nuclei
By analogy to the different accretion states observed in black-hole X-ray
binaries (BHXBs), it appears plausible that accretion disks in active galactic
nuclei (AGN) undergo a state transition between a radiatively efficient and
inefficient accretion flow. If the radiative efficiency changes at some
critical accretion rate, there will be a change in the distribution of black
hole masses and bolometric luminosities at the corresponding transition
luminosity. To test this prediction, I consider the joint distribution of AGN
black hole masses and bolometric luminosities for a sample taken from the
literature. The small number of objects with low Eddington-scaled accretion
rates mdot < 0.01 and black hole masses Mbh < 10^9 Msun constitutes tentative
evidence for the existence of such a transition in AGN. Selection effects, in
particular those associated with flux-limited samples, systematically exclude
objects in particular regions of the black hole mass-luminosity plane.
Therefore, they require particular attention in the analysis of distributions
of black hole mass, bolometric luminosity, and derived quantities like the
accretion rate. I suggest further observational tests of the BHXB-AGN
unification scheme which are based on the jet domination of the energy output
of BHXBs in the hard state, and on the possible equivalence of BHXB in the very
high (or "steep power-law") state showing ejections and efficiently accreting
quasars and radio galaxies with powerful radio jets.Comment: Accepted by ApJ; 14 pages, 4 figures, uses emulateap
Spectral Decomposition of Broad-Line AGNs and Host Galaxies
Using an eigenspectrum decomposition technique, we separate the host galaxy
from the broad line active galactic nucleus (AGN) in a set of 4666 spectra from
the Sloan Digital Sky Survey (SDSS), from redshifts near zero up to about 0.75.
The decomposition technique uses separate sets of galaxy and quasar
eigenspectra to efficiently and reliably separate the AGN and host
spectroscopic components. The technique accurately reproduces the host galaxy
spectrum, its contributing fraction, and its classification. We show how the
accuracy of the decomposition depends upon S/N, host galaxy fraction, and the
galaxy class. Based on the eigencoefficients, the sample of SDSS broad-line AGN
host galaxies spans a wide range of spectral types, but the distribution
differs significantly from inactive galaxies. In particular, post-starburst
activity appears to be much more common among AGN host galaxies. The
luminosities of the hosts are much higher than expected for normal early-type
galaxies, and their colors become increasingly bluer than early-type galaxies
with increasing host luminosity. Most of the AGNs with detected hosts are
emitting at between 1% and 10% of their estimated Eddington luminosities, but
the sensitivity of the technique usually does not extend to the Eddington
limit. There are mild correlations among the AGN and host galaxy
eigencoefficients, possibly indicating a link between recent star formation and
the onset of AGN activity. The catalog of spectral reconstruction parameters is
available as an electronic table.Comment: 18 pages; accepted for publication in A
Temporal fluctuations in excimer-like interactions between pi-conjugated chromophores
Inter- or intramolecular coupling processes between chromophores such as
excimer formation or H- and J-aggregation are crucial to describing the
photophysics of closely packed films of conjugated polymers. Such coupling is
highly distance dependent, and should be sensitive to both fluctuations in the
spacing between chromophores as well as the actual position on the chromophore
where the exciton localizes. Single-molecule spectroscopy reveals these
intrinsic fluctuations in well-defined bi-chromophoric model systems of
cofacial oligomers. Signatures of interchromophoric interactions in the excited
state - spectral red-shifting and broadening, and a slowing of
photoluminescence decay - correlate with each other but scatter strongly
between single molecules, implying an extraordinary distribution in coupling
strengths. Furthermore, these excimer-like spectral fingerprints vary with
time, revealing intrinsic dynamics in the coupling strength within one single
dimer molecule, which constitutes the starting point for describing a molecular
solid. Such spectral sensitivity to sub-Angstrom molecular dynamics could prove
complementary to conventional FRET-based molecular rulers
A Survey of z ~ 6 Quasars in the Sloan Digital Sky Survey Deep Stripe. II. Discovery of Six Quasars at z AB>21
We present the discovery of six new quasars at z ~ 6 selected from the Sloan Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by repeatedly scanning a stripe along the celestial equator. The six quasars are about 2 mag fainter than the luminous z ~ 6 quasars found in the SDSS main survey and 1 mag fainter than the quasars reported in Paper I. Four of them comprise a complete flux-limited sample at 21 < z_(AB) < 21.8 over an effective area of 195 deg^2. The other two quasars are fainter than z_(AB) = 22 and are not part of the complete sample. The quasar luminosity function at z ~ 6 is well described as a single power law Φ(L_(1450))α L^β_(1450) over the luminosity range –28 < M_(1450) < –25. The best-fitting slope β varies from –2.6 to –3.1, depending on the quasar samples used, with a statistical error of 0.3-0.4. About 40% of the quasars discovered in the SDSS southern survey have very narrow Lyα emission lines, which may indicate small black hole masses and high Eddington luminosity ratios, and therefore short black hole growth timescales for these faint quasars at early epochs
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