358 research outputs found
The Star Clusters in the Irregular Galaxy NGC 4449
We examine the star clusters in the irregular galaxy NGC 4449. We use a
near-infrared spectrum and broad-band images taken with the HST to place a
limit of 8--15 Myrs on the age of the bright central ojbect in NGC 4449. Its
luminosity and size suggest that it is comparable to young super star clusters.
However, there is a peculiar nucleated-bar structure at the center of this star
cluster, and we suggest that this structure is debris from the interaction that
has produced the counter-rotating gas systems and extended gas streamers in the
galaxy.
From the images we identify 60 other candidate compact star clusters in NGC
4449. Fourteen of these could be background elliptical galaxies or old globular
star clusters. Of the star clusters, three, in addition to the central object,
are potentially super star clusters, and many others are comparable to the
populous clusters found in the LMC. The star clusters span a large range in
ages with no obvious peak in cluster formation that might be attributed to the
interaction that the galaxy has experienced.Comment: To be published in PASP, Feb. 2001; also attainable from
ftp.lowell.edu, cd pub/dah/n4449pape
Statistical mechanics of budget-constrained auctions
Finding the optimal assignment in budget-constrained auctions is a
combinatorial optimization problem with many important applications, a notable
example being the sale of advertisement space by search engines (in this
context the problem is often referred to as the off-line AdWords problem).
Based on the cavity method of statistical mechanics, we introduce a message
passing algorithm that is capable of solving efficiently random instances of
the problem extracted from a natural distribution, and we derive from its
properties the phase diagram of the problem. As the control parameter (average
value of the budgets) is varied, we find two phase transitions delimiting a
region in which long-range correlations arise.Comment: Minor revisio
Evaluating Exposure of Northern Fur Seals, Callorhinus Ursinus, to Microplastic Pollution Through Fecal Analysis
Environmental microplastics are widely documented in marine life and bioaccumulation may present risks to marine predators. Investigations of microplastics in marine mammals are increasing, though none have examined animals routinely consumed by humans. Here, we investigate microplastic exposure in the northern fur seal (Callorhinus ursinus), a species consumed by humans, using fecal material. We examined 44 feces (scat) at sites encompassing the seals\u27 eastern Pacific range. Multiple contamination control measures were implemented, including field and laboratory controls. Fragments were the most common microplastic recovered, in 55% (24/44) of scat and no controls (range 1 to 86 fragments/scat, mean 16.6, sd 19.1). Microplastic fibers were recovered from 41% of scats (18/44), though some controls contained fibers confounding fiber results. Fecal analysis documented northern fur seal exposure to microplastics throughout their eastern Pacific range. © 201
Contrasting patterns of genetic diversity at three different genetic markers in a marine mammal metapopulation
Many studies use genetic markers to explore population structure and variability within species. However, only a minority use more than one type of marker and, despite increasing evidence of a link between heterozygosity and individual fitness, few ask whether diversity correlates with population trajectory. To address these issues, we analyzed data from the Steller’s sea lion, Eumetiopias jubatus, where three stocks are distributed over a vast geographical range and where both genetic samples and detailed demographic data have been collected from many diverse breeding colonies. To previously published mitochondrial DNA(mtDNA) and microsatellite data sets,we have added new data for amplified fragment length polymorphism (AFLP) markers, comprising 238 loci scored in 285 sea lions sampled from 23 natal rookeries. Genotypic diversity was low relative to most vertebrates, with only 37 loci (15.5%) being polymorphic. Moreover, contrasting geographical patterns of genetic diversity were found at the three markers, with Nei’s gene diversity tending to be higher for AFLPs and microsatellites in rookeries of the western and Asian stocks, while the highest mtDNA values were found in the eastern stock. Overall, and despite strongly contrasting demographic histories, after applying phylogenetic correction we found little correlation between genetic diversity and either colony size or demography. In contrast, we were able to show a highly significant positive relationship between AFLP diversity and current population size across a range of pinniped species, even though equivalent analyses did not reveal significant trends for either microsatellites or mtDNA
Electronic structure and magnetic properties of RMnX (R= Mg, Ca, Sr, Ba, Y; X= Si, Ge) studied by KKR method
Electronic structure calculations, using the charge and spin self-consistent
Korringa- Kohn-Rostoker (KKR) method, have been performed for several Mn
compounds ( = Mg, Ca, Sr, Ba, Y; = Si, Ge) of the CeFeSi-type structure.
The origin of their magnetic properties has been investigated emphasizing the
role of the Mn sublattice. The significant influence of the Mn-Mn and Mn-
interatomic distances on the Mn magnetic moment value is delineated from our
computations, supporting many neutron diffraction data. We show that the marked
change of with the Mn-Mn and Mn- distances resulted from a
redistribution between spin-up and spin-down -Mn DOS rather than from
different fillings of the Mn 3-shell. Bearing in mind that the neutron
diffraction data reported for the Mn compounds are rather scattered, the
KKR computations of are in fair agreement with the experimental
values. Comparing density of states near obtained in different magnetic
orderings, one can notice that the entitled Mn systems seem to 'adapt'
their magnetic structures to minimize the DOS in the vicinity of the Fermi
level. Noteworthy, the SrMnGe antiferromagnet exhibits a pseudo-gap behaviour
at , suggesting anomalous electron transport properties. In addition,
the F-AF transition occurring in the disordered LaYMnSi alloy for
the range is well supported by the DOS features of
LaYMnSi. In contrast to the investigated Mn compounds,
YFeSi was found to be non-magnetic, which is in excellent agreement with the
experimental data.Comment: 10 pages + 14 figures, to appear in Eur. Phys. Jour.
Young Massive Clusters in Non-Interacting Galaxies
Young star clusters with masses well in excess of 100.000 Msun have been
observed not only in merger galaxies and large-scale starbursts, but also in
fairly normal, undisturbed spiral and irregular galaxies. Here we present
virial mass estimates for a sample of 7 such clusters and show that the derived
mass-to-light ratios are consistent with "normal" Kroupa-type stellar mass
distributions.Comment: 4 pages, 1 figure, to appear in "Starbursts from 30 Doradus to Lyman
Break Galaxies'', eds. R. de Grijs, R. M. Gonzalez Delgad
Influence of age on ocular biomechanical properties in a canine glaucoma model with ADAMTS10 mutation
<div><p>Soft tissue often displays marked age-associated stiffening. This study aims to investigate how age affects scleral biomechanical properties in a canine glaucoma model with <i>ADAMTS10</i> mutation, whose extracellular matrix is concomitantly influenced by the mutation and an increased mechanical load from an early age. Biomechanical data was acquired from <i>ADAMTS10-</i>mutant dogs (n = 10, 21 to 131 months) and normal dogs (n = 5, 69 to 113 months). Infusion testing was first performed in the whole globes to measure ocular rigidity. After infusion experiments, the corneas were immediately trephined to prepare scleral shells that were mounted on a pressurization chamber to measure strains in the posterior sclera using an inflation testing protocol. Dynamic viscoelastic mechanical testing was then performed on dissected posterior scleral strips and the data were combined with those reported earlier by our group from the same animal model (Palko et al, IOVS 2013). The association between age and scleral biomechanical properties was evaluated using multivariate linear regression. The relationships between scleral properties and the mean and last measured intraocular pressure (IOP) were also evaluated. Our results showed that age was positively associated with complex modulus (p<0.001) and negatively associated with loss tangent (p<0.001) in both the affected and the normal groups, suggesting an increased stiffness and decreased mechanical damping with age. The regression slopes were not different between the groups, although the complex modulus was significantly lower in the affected group (p = 0.041). The posterior circumferential tangential strain was negatively correlated with complex modulus (R = -0.744, p = 0.006) showing consistent mechanical evaluation between the testing methods. Normalized ocular rigidity was negatively correlated with the last IOP in the affected group (p = 0.003). Despite a mutation that affects the extracellular matrix and a chronic IOP elevation in the affected dogs, age-associated scleral stiffening and loss of mechanical damping were still prominent and had a similar rate of change as in the normal dogs.</p></div
Hubble Space Telescope study of resolved red giant stars in the outer halos of nearby dwarf starburst galaxies
[abridged] Aims. We observed the outer parts of NGC 1569 and NGC 4449, two of
the closest and strongest dwarf starburst galaxies in the local universe, to
characterize their stellar density and populations, and obtain new insights
into the structure, formation, and evolution of starburst galaxies and galaxy
halos. Methods. We obtained HST/WFPC2 images between 5 and 8 scale radii from
the center, along the intermediate and minor axes. We performed point-source
photometry to determine color magnitude diagrams of I vs. V-I. We compared the
results at different radii, including also our prior HST/ACS results for more
centrally located fields. Results. We detect stars in the RGB and TP-AGB
(carbon star) phases in all outer fields, but not younger stars such as those
present at smaller radii. The RGB star density profile is well fit by either a
de Vaucouleurs profile or a power-law profile, but has more stars at large
radii than a single exponential. To within the uncertainties, there are no
radial gradients in the RGB color or carbon-to-RGB-star ratio at large radii.
Conclusions. The galaxies have faint outer stellar envelopes that are not
tidally truncated within the range of radii addressed by our study. The density
profiles suggest that these are not outward extensions of the inner disks, but
are instead distinct stellar halos. This agrees with other work on galaxies of
similar morphology. The presence of such halos is consistent with predictions
of hierarchical galaxy formation scenarios. The halos consist of
intermediate-age/old stars, confirming the results of other studies that have
shown the starburst phenomenon to be very centrally concentrated. There is no
evidence for stellar-population age/metallicity gradients within the halos
themselves.Comment: 13 pages, 13 figures and 2 tables. Accepted for publication in A&
Trends in Metal Oxide Stability for Nanorods, Nanotubes, and Surfaces
The formation energies of nanostructures play an important role in
determining their properties, including the catalytic activity. For the case of
15 different rutile and 8 different perovskite metal oxides, we find that the
density functional theory (DFT) calculated formation energies of (2,2)
nanorods, (3,3) nanotubes, and the (110) and (100) surfaces may be described
semi-quantitatively by the fraction of metal--oxygen bonds broken and the
bonding band centers in the bulk metal oxide
Endovascular interventions in the perioperative management of the Fontan procedure in a developing congenital heart surgery program
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