1,381 research outputs found
Dynamical friction in dwarf galaxies
We present a simplified analytic approach to the problem of the spiraling of
a massive body orbiting within the dark halo of a dwarf galaxy. This dark halo
is treated as the core region of a King distribution of dark matter particles,
in consistency with the observational result of dwarf galaxies having solid
body rotation curves. Thus we derive a simple formula which provides a reliable
and general first order solution to the problem, totally analogous to the one
corresponding to the dynamical friction problem in an isothermal halo. This
analytic approach allows a clear handling and a transparent understanding of
the physics and the scaling of the problem. A comparison with the isothermal
case shows that in the core regions of a King sphere, dynamical friction
proceeds at a different rate, and is sensitive to the total core radius. Thus,
in principle, observable consequences may result. In order to illustrate the
possible effects, we apply this formula to the spiraling of globular cluster
orbits in dwarf galaxies, and show how present day globular cluster systems
could in principle be used to derive better limits on the structure of dark
halos around dwarf galaxies, when the observational situation improves. As a
second application, we study the way a massive black hole population forming a
fraction of these dark halos would gradually concentrate towards the centre,
with the consequent deformation of an originally solid body rotation curve.
This effect allows us to set limits on the fraction/mass of any massive black
hole minority component of the dark halos of dwarf galaxies. In essence, we
take advantage of the way the global matter distribution fixes the local
distribution function for the dark matter particles, which in turn determines
the dynamical friction problem.Comment: 8 pages, 1 figure, Accepted in MNRA
Deriving star formation histories: Inverting HR diagrams through a variational calculus maximum likelihood method
We introduce a new method for solving maximum likelihood problems through
variational calculus, and apply it to the case of recovering an unknown star
formation history, , from a resulting HR diagram. This approach allows
a totally non-parametric solution which has the advantage of requiring no
initial assumptions on the . As a full maximum likelihood statistical
model is used, we take advantage of all the information available in the HR
diagram, rather than concentrating on particular features such as turn off
points or luminosity functions. We test the method using a series of synthetic
HR diagrams produced from known , and find it to be quite successful
under noise conditions comparable to those present in current observations. At
this point we restrict the analysis to situations where the metallicity of the
system is known, as is the case with the resolved populations of the dwarf
spheroidal companions to the Milky Way or the solar neighbourhood Hipparcos
data. We also include tests to quantify the way uncertainties in the assumed
metallicity, binary fraction and IMF affect our inferences.Comment: 16 pages including 12 figures. Accepted for publication in MNRAS,
appendix adde
Non-parametric star formation histories for 5 dwarf spheroidal galaxies of the local group
We use recent HST colour-magnitude diagrams of the resolved stellar
populations of a sample of local dSph galaxies (Carina, LeoI, LeoII, Ursa Minor
and Draco) to infer the star formation histories of these systems, .
Applying a new variational calculus maximum likelihood method which includes a
full Bayesian analysis and allows a non-parametric estimate of the function one
is solving for, we infer the star formation histories of the systems studied.
This method has the advantage of yielding an objective answer, as one need not
assume {\it a priori} the form of the function one is trying to recover. The
results are checked independently using Saha's statistic. The total
luminosities of the systems are used to normalize the results into physical
units and derive SN type II rates. We derive the luminosity weighted mean star
formation history of this sample of galaxies.Comment: 14 pages including 7 figures. Accepted to MNRA
HST Star Counts at High Galactic Latitudes
We use star counts from 13 deep HST fields imaged with the {\it Wide Field
Camera - 2} in order to constrain the amount of dark matter in the Galaxy that
can be in the form of low-mass main sequence stars or white-dwarfs. Based on
the number of red stars found in our fields, we exclude the possibility that
more than 25\% of the massive dark halo is made up of M dwarfs or subdwarfs;
fairly massive () and yet extremely faint ( \gtsima
) stellar candidates would have to be invoked in order to make the
observed number of stars compatible with that predicted by a stellar dark halo.
White dwarfs must also be intrinsically very faint ( \gtsima ) in
order to be consistent with the observed number of faint stars in the HST
fields. We also rule out an increasing or flat stellar luminosity function
beyond . The inferred slope of the disk luminosity function is
intermediary between local, volume-limited surveys and ground-based photometric
ones. Finally, the magnitude counts are well fitted by existing models for the
structure of the Galaxy, with only small changes in the fiducial values of the
model parameters. The colour distribution, however, is not well reproduced by
the models. It is unclear at present if this reflects inadequacies of the
available models or uncertainties in the colour-magnitude diagrams for low
metallicity stars and in the photometric calibration.Comment: 18 pages plus 6 figure
HST Observations of the Field Star Population in the Large Magellanic Cloud
We present and photometry, obtained with the Hubble Space Telescope,
for stars in a field in the inner disk of the Large Magellanic
Cloud. We confirm previous results indicating that an intense star formation
event, probably corresponding to the formation of the LMC disk, occurred a few
times years ago. We find a small but real difference between our field
and one further out in the disk observed by Gallagher et al (1996): either star
formation in the inner disk commenced slightly earlier, or the stars are
slightly more metal rich. We also find evidence for a later burst, around 1 Gyr
ago, which may correspond to the formation of the LMC bar. About 5% of the
stars in our field are substantially older than either burst, and are probably
members of an old disk or halo population with age Gyr.Comment: 18 pages, 7 figures: only 3 available ellectronically - complete
copies by request from [email protected]
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Host plant recognition by the root feeding clover weevil, Sitona lepidus (Coleoptera: Curculionidae)
This study investigated the ability of neonatal larvae of the root-feeding weevil, Sitona lepidus Gyllenhal, to locate white clover Trifolium repens L. (Fabaceae) roots growing in soil and to distinguish them from the roots of other species of clover and a co-occurring grass species. Choice experiments used a combination of invasive techniques and the novel technique of high resolution X-ray microtomography to non-invasively track larval movement in the soil towards plant roots. Burrowing distances towards roots of different plant species were also examined. Newly hatched S. lepidus recognized T. repens roots and moved preferentially towards them when given a choice of roots of subterranean clover, Trifolium subterraneum L. (Fabaceae), strawberry clover Trifolium fragiferum L. (Fabaceae), or perennial ryegrass Lolium perenneL. (Poaceae). Larvae recognized T. repens roots, whether released in groups of five or singly, when released 25 mm (meso-scale recognition) or 60 mm (macro-scale recognition) away from plant roots. There was no statistically significant difference in movement rates of larvae
Defining the local nerve blocks for feline distal thoracic limb surgery: a cadaveric study
Though controversial, onychectomy remains a commonly performed distal thoracic limb surgical procedure in cats. Peripheral nerve block techniques have been proposed in cats undergoing onychectomy but evidence of efficacy is lacking. Preliminary tests of the described technique using cadavers resulted in incomplete staining of nerves. The aim of this study was to develop nerve block methods based on cadaveric dissections and test these methods with cadaveric dye injections
Characterization of G protein coupling mediated by the conserved D1343.49 of DRY motif, M2416.34, and F2516.44 residues on human CXCR1
© 2015 The Authors. CXCR1, a receptor for interleukin-8 (IL-8), plays an important role in defending against pathogen invasion during neutrophil-mediated innate immune response. Human CXCR1 is a G protein-coupled receptor (GPCR) with its characteristic seven transmembrane domains (TMs). Functional and structural analyses of several GPCRs have revealed that conserved residues on TM3 (including the highly conserved Asp-Arg-Tyr (DRY) motif) and TM6 near intracellular loops contain domains critical for G protein coupling as well as GPCR activation. The objective of this study was to elucidate the role of critical amino acid residues on TM3 near intracellular loop 2 (i2) and TM6 near intracellular loop 3 (i3), including S1323.47 (Baldwin location), D1343.49, M2416.34, and F2516.44, in G protein coupling and CXCR1 activation. The results demonstrate that mutations of D1343.49 at DRY motif of CXCR1 (D134N and D134V) completely abolished the ligand binding and functional response of the receptor. Additionally, point mutations at positions 241 and 251 between TM6 and i3 loop generated mutant receptors with modest constitutive activity via Gα15 signaling activation. Our results show that D1343.49 on the highly conserved DRY motif has a distinct role for CXCR1 compared to its homologues (CXCR2 and KSHV-GPCR) in G protein coupling and receptor activation. In addition, M2416.34 and F2516.44 along with our previously identified V2476.40 on TM6 are spatially located in a "hot spot" likely essential for CXCR1 activation. Identification of these amino acid residues may be useful for elucidating mechanism of CXCR1 activation and designing specific antagonists for the treatment of CXCR1-mediated diseases
Long-distance autumn migration across the Sahara by painted lady butterflies: exploiting resource pulses in the tropical savannah
The painted lady, Vanessa cardui, is a migratory butterfly that performs an annual multi-generational migration between Europe and North Africa. Its seasonal appearance south of the Sahara in autumn is well known and has led to the suggestion that it results from extremely long migratory flights by European butterflies to seasonally exploit the Sahel and the tropical savannah. However, this possibility has remained unproven. Here, we analyse the isotopic composition of butterflies from seven European and seven African countries to provide new support for this hypothesis. Each individual was assigned a geographical natal origin, based on its wing stable hydrogen isotope (Âż2Hw) value and a predicted Âż2Hw basemap for Europe and northern Africa. Natal assignments of autumn migrants collected south of the Sahara confirmed long-distance movements (of 4000 km or more) starting in Europe. Samples from Maghreb revealed a mixed origin of migrants, with most individuals with a European origin, but others having originated in the Sahel. Therefore, autumn movements are not only directed to northwestern Africa, but also include southward and northward flights across the Sahara. Through this remarkable behaviour, the productive but highly seasonal region south of the Sahara is incorporated into the migratory circuit of V. cardui.K.A.H. was funded by an operating grant and D.X.S. by an NSERC visiting fellowship from Environment Canada. Funding to R.V. and G.T. was provided by the Committee for Research and Exploration of National Geographic (grant no. 9528-14) and by the Spanish Ministerio de EconomĂa y Competitividad (CGL2013-48277-P). G.T. is supported by the Marie Curie Actions FP7-PEOPLE-2013-IOF (project 622716) and the grant BP-A00275 (AGAUR-Generalitat de Catalunya). Expeditions in Morocco were funded by Antoni Jonch CooperaciĂł.Peer Reviewe
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