558 research outputs found
Electromagnetic follow-up of gravitational wave transient signal candidates
Pioneering efforts aiming at the development of multi-messenger gravitational
wave and electromagnetic astronomy have been made. An electromagnetic
observation follow-up program of candidate gravitational wave events has been
performed (Dec 17 2009 to Jan 8 2010 and Sep 4 to Oct 20 2010) during the
recent runs of the LIGO and Virgo gravitational wave detectors. It involved
ground-based and space electromagnetic facilities observing the sky at optical,
X-ray and radio wavelengths. The joint gravitational wave and electromagnetic
observation study requires the development of specific image analysis
procedures able to discriminate the possible electromagnetic counterpart of
gravitational wave triggers from contaminant/background events. The paper
presents an overview of the electromagnetic follow-up program and the image
analysis procedures.Comment: Proceedings of the 12th International Conference on "Topics in
Astroparticle and Underground Physics" (TAUP 2011), Munich, September 2011
(to appear in IoP Journal of Physics: Conference Series
The structures of E. coli NfsA bound to the antibiotic nitrofurantoin; to 1,4-benzoquinone and to FMN
NfsA is a dimeric flavoprotein that catalyses the reduction in nitroaromatics and quinones by NADPH. This reduction is required for the activity of nitrofuran antibiotics. The crystal structure of free Escherichia coli NfsA and several homologues have been determined previously, but there is no structure of the enzyme with ligands. We present here crystal structures of oxidised E. coli NfsA in the presence of several ligands, including the antibiotic nitrofurantoin. Nitrofurantoin binds with the furan ring, rather than the nitro group that is reduced, near the N5 of the FMN. Molecular dynamics simulations show that this orientation is only favourable in the oxidised enzyme, while potentiometry suggests that little semiquinone is formed in the free protein. This suggests that the reduction occurs by direct hydride transfer from FMNH(−) to nitrofurantoin bound in the reverse orientation to that in the crystal structure. We present a model of nitrofurantoin bound to reduced NfsA in a viable hydride transfer orientation. The substrate 1,4-benzoquinone and the product hydroquinone are positioned close to the FMN N5 in the respective crystal structures with NfsA, suitable for reaction, but are mobile within the active site. The structure with a second FMN, bound as a ligand, shows that a mobile loop in the free protein forms a phosphate-binding pocket. NfsA is specific for NADPH and a similar conformational change, forming a phosphate-binding pocket, is likely to also occur with the natural cofactor
The Outer Disks of Early-Type Galaxies. I. Surface-Brightness Profiles of Barred Galaxies
We present a study of 66 barred, early-type (S0-Sb) disk galaxies, focused on
the disk surface brightness profile outside the bar region and the nature of
Freeman Type I and II profiles, their origins, and their possible relation to
disk truncations. This paper discusses the data and their reduction, outlines
our classification system, and presents -band profiles and classifications
for all galaxies in the sample.
The profiles are derived from a variety of different sources, including the
Sloan Digital Sky Survey (Data Release 5). For about half of the galaxies, we
have profiles derived from more than one telescope; this allows us to check the
stability and repeatability of our profile extraction and classification. The
vast majority of the profiles are reliable down to levels of mu_R ~ 27 mag
arcsec^-2; in exceptional cases, we can trace profiles down to mu_R > 28. We
can typically follow disk profiles out to at least 1.5 times the traditional
optical radius R_25; for some galaxies, we find light extending to ~ 3 R_25.
We classify the profiles into three main groups: Type I (single-exponential),
Type II (down-bending), and Type III (up-bending). The frequencies of these
types are approximately 27%, 42%, and 24%, respectively, plus another 6% which
are combinations of Types II and III. We further classify Type II profiles by
where the break falls in relation to the bar length, and in terms of the
postulated mechanisms for breaks at large radii ("classical trunction" of star
formation versus the influence of the Outer Lindblad Resonance of the bar). We
also classify the Type III profiles by the probable morphology of the outer
light (disk or spheroid). Illustrations are given for all cases. (Abridged)Comment: 41 pages, 26 PDF figures. To appear in the Astronomical Journal.
Version with full-resolution figures available at
http://www.mpe.mpg.de/~erwin/research
The H alpha Galaxy Survey VI. Star-forming companions of nearby field galaxies
We present a search for star-forming satellite galaxies that are close enough
to their parent galaxies to be considered analogues of the Magellanic Clouds.
Our search technique relied on the detection of the satellites in
continuum-subtracted narrow-band H alpha imaging of the central galaxies, which
removes most of the background and foreground line-of-sight companions, thus
giving a high probability that we are detecting true satellites. The search was
performed for 119 central galaxies at distances between 20 and 40 Mpc, although
spatial incompleteness means that we have effectively searched 53 full
satellite-containing volumes. We find only 9 probable star-forming satellites,
around 9 different central galaxies, and 2 possible satellites. After
incompleteness correction, this is equivalent to 0.17/0.21 satellites per
central galaxy. The Small Magellanic Cloud is just below the median values of
both star formation rate and R-band luminosity of the 9 probable satellites.
The Large Magellanic Cloud, however, has a higher R-band luminosity than any of
the 9 and is only exceeded in star formation rate by the one satellite that
appears to be undergoing a tidally-induced starburst. Thus the Milky Way
appears to be quite unusual, both in having two star-forming satellite galaxies
and in the high luminosity of the Large Magellanic Cloud.Comment: 13 pages, accepted for publication in A&
Pre-galactic metal enrichment - The chemical signatures of the first stars
The emergence of the first sources of light at redshifts of z ~ 10-30
signaled the transition from the simple initial state of the Universe to one of
increasing complexity. We review recent progress in our understanding of the
formation of the first stars and galaxies, starting with cosmological initial
conditions, primordial gas cooling, and subsequent collapse and fragmentation.
We emphasize the important open question of how the pristine gas was enriched
with heavy chemical elements in the wake of the first supernovae. We conclude
by discussing how the chemical abundance patterns conceivably allow us to probe
the properties of the first stars and subsequent stellar generations, and allow
us to test models of early metal enrichment.Comment: 52 pages, 20 figures, clarifications, references added, accepted for
publication in the Reviews of Modern Physic
Building up the Stellar Halo of the Galaxy
We study numerical simulations of satellite galaxy disruption in a potential
resembling that of the Milky Way. Our goal is to assess whether a merger origin
for the stellar halo would leave observable fossil structure in the phase-space
distribution of nearby stars. We show how mixing of disrupted satellites can be
quantified using a coarse-grained entropy. Although after 10 Gyr few obvious
asymmetries remain in the distribution of particles in configuration space,
strong correlations are still present in velocity space. We give a simple
analytic description of these effects, based on a linearised treatment in
action-angle variables, which shows how the kinematic and density structure of
the debris stream changes with time. By applying this description we find that
a single satellite of current luminosity 10^8 L_\sun disrupted 10 Gyr ago
from an orbit circulating in the inner halo (mean apocentre kpc)
would contribute about kinematically cold streams with internal
velocity dispersions below 5 km/s to the local stellar halo. If the whole
stellar halo were built by disrupted satellites, it should consist locally of
300 - 500 such streams. Clear detection of all these structures would require a
sample of a few thousand stars with 3-D velocities accurate to better than 5
km/s. Even with velocity errors several times worse than this, the expected
clumpiness should be quite evident. We apply our formalism to a group of stars
detected near the North Galactic Pole, and derive an order of magnitude
estimate for the initial properties of the progenitor system.Comment: 28 pages, 10 figures, minor changes, matches the version to appear in
MNRAS, Vol. 307, p.495-517 (August 1999
From identification to validation to gene count
The current GENCODE gene count of ~ 30,000, including 21,727 protein-coding and 8,483 RNA genes, is significantly lower than the 100,000 genes anticipated by early estimates. Accurate annotation of protein-coding and non-coding genes and pseudogenes is essential in calculating the true gene count and gaining insight into human evolution.
As part of the GENCODE Consortium, the HAVANA team produces high quality manual gene annotation, which forms the basis for the reference gene set being used by the ENCODE project and provides a rich annotation of alternative splice variants and assignment of functional potential. However, the protein-coding potential of some splice variants is uncertain and valid splice variants can remain unannotated if they are absent from current cDNA libraries. Recent technological developments in sequencing and mass spectrometry have created a vast amount of new transcript and protein data that facilitate the identification and validation of new and existing transcripts, while harboring their own limitations and problems
Effects of Electron Correlation, Orbital Degeneracy and Jahn-Teller Coupling in Perovskite Manganites
Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling in
double-exchange models are examined for Mn perovskite oxides. We study the
undoped Mott insulator as well as metal-insulator transitions by hole doping,
and especially strong incoherence of ferromagnetic metal. We derive models
where all the spins are fully polarized in two-dimensional planes as in the
experimental indications, and investigate their ground-state properties by
quantum Monte Carlo method. At half filling where the number of
electron is one per site on average, the Coulomb interaction opens a Mott gap
and induces a staggered orbital ordering. The opening of the Mott gap is,
however, substantially slower than the mean-field results if the Jahn-Teller
coupling is absent. The synergy between the strong correlation and the
Jahn-Teller coupling largely enhances the Mott gap amplitude and reproduces
realistic amplitudes and stabilization energy of the Jahn-Teller distortion.
Upon doping, the orbital ordering stabilized by the Coulomb interaction is
destroyed immediately. Toward the metal-insulator transition, the short-ranged
orbital correlation is critically enhanced in metals, which should be related
to strong incoherence of charge dynamics observed in experiments. Our model,
moreover, exhibits a uniform ordering of orbital in a wide
region of doping in agreement with experimental indications.Comment: 4 pages LaTeX including 3 PS figures, submitted to J.Phys.Soc.Jp
HS 2134+0400 - new very metal-poor galaxy, a representative of void population?
We present the SAO 6m telescope spectroscopy of a blue compact galaxy (BCG)
HS 2134+0400 discovered in frame of the dedicated Hamburg/SAO survey for Low
Metallicity BCGs (HSS-LM). Its very low abundance of oxygen (12+log(O/H) =
7.44), as well as other heavy elements (S, N, Ne, Ar), assigns this dwarf
galaxy to the group of BCGs with the lowest metal content. There are only eight
that low metallicity among several thousand known BCGs in the nearby Universe.
The abundance ratios for the heavy elements (S/O, Ne/O, N/O, and Ar/O) are well
consistent with the typical values of other very metal-poor BCGs. The global
environment of HS 2134+0400 is atypical of the majority of BCGs. The object
falls within the Pegasus void, the large volume with the very low density of
galaxies with the normal (M_B* = -19.6) or high luminosity. Since we found in
voids a dozen more the very metal-poor galaxies, we discuss the hypothesis that
such objects can be representative of a substantial fraction of the void dwarf
galaxy population.Comment: 6 pages, including 2 tables and 2 postscript figures. Submitted to
Astronomy Letter
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