931 research outputs found
The Capture of Centaurs as Trojans
Large scale simulations of Centaurs have yielded vast amounts of data, the
analysis of which allows interesting but uncommon scenarios to be studied. One
such rare phenomenon is the temporary capture of Centaurs as Trojans of the
giant planets. Such captures are generally short (10 kyr to 100 kyr), but occur
with sufficient frequency (about 40 objects larger than 1 km in diameter every
Myr) that they may well contribute to the present-day populations. Uranus and
Neptune seem to have great difficulty capturing Centaurs into the 1:1
resonance, while Jupiter captures some, and Saturn the most (80 %). We
conjecture that such temporary capture from the Centaur population may be the
dominant delivery route into the Saturnian Trojans. Photometric studies of the
Jovian Trojans may reveal outliers with Centaur-like as opposed to asteroidal
characteristics, and these would be prime candidates for captured Centaurs.Comment: 5 pages, 2 figures, submitted to MNRAS (Letters
A Dark Matter Hurricane: Measuring the S1 Stream with Dark Matter Detectors
The recently discovered S1 stream passes through the Solar neighbourhood on a
low inclination, counter-rotating orbit. The progenitor of S1 is a dwarf galaxy
with a total mass comparable to the present-day Fornax dwarf spheroidal, so the
stream is expected to have a significant DM component. We compute the effects
of the S1 stream on WIMP and axion detectors as a function of the density of
its unmeasured dark component. In WIMP detectors the S1 stream supplies more
high energy nuclear recoils so will marginally improve DM detection prospects.
We find that even if S1 comprises less than 10% of the local density, multi-ton
xenon WIMP detectors can distinguish the S1 stream from the bulk halo in the
relatively narrow mass range between 5 and 25 GeV. In directional WIMP
detectors such as CYGNUS, S1 increases DM detection prospects more
substantially since it enhances the anisotropy of the WIMP signal. Finally, we
show that axion haloscopes possess by far the greatest potential sensitivity to
the S1 stream. Once the axion mass has been discovered, the distinctive
velocity distribution of S1 can easily be extracted from the axion power
spectrum.Comment: 21 pages, 11 figure
The mass of dwarf spheroidal galaxies and the missing satellite problem
We present the results from a suite of N-body simulations of the tidal
stripping of two-component dwarf galaxies comprising some stars and dark
matter. We show that recent kinematic data from the local group dwarf
spheroidal (dSph) galaxies suggests that dSph galaxies must be sufficiently
massive (M) that tidal stripping is of little
importance for the stars. We discuss the implications of these massive dSph
galaxies for cosmology and galaxy formation.Comment: 4 pages, 1 figure, to appear in the proceedings of the IAUC198
"Near-Field Cosmology with Dwarf Elliptical Galaxies", H. Jerjen & B.
Binggeli (eds.). Comments welcom
First Clear Signature of an Extended Dark Matter Halo in the Draco Dwarf Spheroidal
We present the first clear evidence for an extended dark matter halo in the
Draco dwarf spheroidal galaxy based on a sample of new radial velocities for
159 giant stars out to large projected radii. Using a two parameter family of
halo models spanning a range of density profiles and velocity anisotropies, we
are able to rule out (at about the 2.5 sigma confidence level) haloes in which
mass follows light. The data strongly favor models in which the dark matter is
significantly more extended than the visible dwarf. However, haloes with
harmonic cores larger than the light distribution are also excluded. When
combined with existing measurements of the proper motion of Draco, our data
strongly suggest that Draco has not been tidally truncated within ~1 kpc. We
also show that the rising velocity dispersion at large radii represents a
serious problem for modified gravity (MOND).Comment: to be published in ApJL; 5 pages, 4 figure
Are there sextuplet and octuplet image systems?
We study gravitational lensing by the family of scale-free galaxies with flat
rotation curves. The models are defined by a shape function, which prescribes
the radius of the isophote as a function of position angle from the major axis.
The critical curves are analytic, while the caustic network is reducible to a
simple quadrature. The cusps are always located at the turning points of the
shape function. We show that the models with exactly elliptic isophotes never
admit butterfly or swallowtail cusps and so have at most 4 (or 5) images.
Higher order imaging is brought about by deviations of the isophotes from pure
ellipses -- such as pointedness caused by embedded disks or boxiness caused by
recent merging. The criteria for the onset of sextuple and octuple imaging can
be calculated analytically in terms of the ellipticity and the fourth-order
Fourier coefficients (a_4 and b_4) used by observers to parametrise the
isophote shapes. The 6 or 8 images are arranged roughly in a circle, which
appears as an incomplete Einstein ring if inadequately resolved. Using data on
the shapes of elliptical galaxies and merger remnants, we estimate that 1% of
all multiply imaged quasars may be sextuplet systems or higher. Forthcoming
satellites like the Global Astrometric Interferometer for Astrophysics (GAIA)
will provide datasets of roughly 4000 multiply imaged systems, and so about 40
will show sextuple imaging or higher.Comment: 10 pages, 9 figures, MNRAS (in press
Market Power in UK Food Retailing: Theory and Evidence from Seven Product Groups
Establishing the presence of market power in food chains has become an increasingly pertinent line of enquiry given the trend towards increasing concentration that has been observed in many parts of the world. This paper presents a theoretical model of price transmission in vertically related markets under imperfect competition. The model delivers a quasi-reduced form representation that is empirically tractable using readily available market data to test for the presence of market power. In particular, we show that the hypothesis of perfect competition can be rejected if shocks to the demand and supply function are significant and correctly signed in price transmission equations. Using a cointegrated vector autoregression, we find empirical results that are consistent with downstream market power in six out of seven food products investigated, supporting both the findings of the UK competition authority's recent investigation in to supermarkets and renewed calls for further scrutiny of supermarket behaviour by the UK's Office of Trading.imperfect competition, Cointegrated VARs, UK food industry, Marketing, D4, L81,
Temperature effects on the vertical movements of the Severn Suspension Bridgeâs suspension cables measured by GNSS
The use of GNSS for the deflection monitoring of large bridges has been an ongoing field of research for 20 years. The Severn Suspension Bridge, in the UK, has a main span length of 988 metres. Datasets were gathered in both March 2010 and July 2015 by placing GNSS antennas on the tops of the support towers, as well as on the suspension cables. The data were gathered over four days and three days respectively during these surveys. In addition to the GNSS data, weigh in motion data of the traffic loading, the temperature of the bridgeâs steel work, and the air temperature and wind speed and direction at a number of locations were collected. In 2010, the temperature during the survey varied between 0.335ÂșC to 13.750ÂșC for the air temperature, and between 0.886ÂșC to 12.390ÂșC for the steel temperature. During the survey in 2015, the temperature for the air varied between 10.800ÂșC to 22.160ÂșC, and the steel temperature varied between 13.820ÂșC to 20.410ÂșC.
This paper analyses the vertical movements at the mid span of the bridgeâs suspension cable using the data from 2010 and also 2015. The vertical movements are due to a number of reasons. Firstly, the traffic flow will cause rapid changes in the height of the cable, of the order ofdecimetres due to changes in traffic loading over a time period of seconds or minutes. Secondly, the wind will also cause movements in the cable, but mainly in the horizontal direction. Vertical movements due to the vibrating nature of the cable will also be present, at a rate of 0.1Hz or so. Finally, the cable will expand and contract due to the change in temperature. This will take place over a period of tens of minutes. The relation between the antenna location in 2010 and 2015 are calculated against changes in temperature, and correlation between the movements are shown. The overall movements due to the change in temperature during the survey in 2010 can be shown to be of the order of decimetres, and similarly in 2015.
The change in a bridgeâs height, due to a change in temperature is an important parameter to be known. A bridge in the UK could experience changes in temperature from almost 30ÂșC to -10ÂșC in a period of a year during its lifetime. In other parts of the world, this differential could be even more. This could result in a very significant vertical movement of the bridge, which in turn could affect the clearance space under the bridge for passing ships. This type of movement tied with changing tides could result in large ships colliding with such bridges if the clearance is not fully understood
Kinematically Cold Populations at Large Radii in the Draco and Ursa Minor Dwarf Spheroidals
We present projected velocity dispersion profiles for the Draco and Ursa
Minor (UMi) dwarf spheroidal galaxies based on 207 and 162 discrete stellar
velocities, respectively. Both profiles show a sharp decline in the velocity
dispersion outside ~30 arcmin (Draco) and ~40 arcmin (UMi). New, deep
photometry of Draco reveals a break in the light profile at ~25 arcmin. These
data imply the existence of a kinematically cold population in the outer parts
of both galaxies. Possible explanations of both the photometric and kinematic
data in terms of both equilibrium and non-equilibrium models are discussed in
detail. We conclude that these data challenge the picture of dSphs as simple,
isolated stellar systems.Comment: 5 pages, accepted for publication in ApJ Letter
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