71 research outputs found
Gamma-rays from dark matter annihilations strongly constrain the substructure in halos
Recently, it has been shown that electrons and positrons from dark matter
(DM) annihilations provide an excellent fit to the Fermi, PAMELA, and HESS
data. Using this DM model, which requires an enhancement of the annihilation
cross section over its standard value to match the observations, we show that
it immediately implies an observable level of gamma-ray emission for the Fermi
telescope from nearby galaxy clusters such as Virgo and Fornax. We show that
this DM model implies a peculiar feature from final state radiation that is a
distinctive signature of DM. Using the EGRET upper limit on the gamma-ray
emission from Virgo, we constrain the minimum mass of substructures within DM
halos to be > 5x10^-3 M_sun -- about four orders of magnitudes larger than the
expectation for cold dark matter. This limits the cutoff scale in the linear
matter power spectrum to k < 35/kpc which can be explained by e.g., warm dark
matter. Very near future Fermi observations will strongly constrain the minimum
mass to be > 10^3 M_sun: if the true substructure cutoff is much smaller than
this, the DM interpretation of the Fermi/PAMELA/HESS data must be wrong. To
address the problem of astrophysical foregrounds, we performed high-resolution,
cosmological simulations of galaxy clusters that include realistic cosmic ray
(CR) physics. We compute the dominating gamma-ray emission signal resulting
from hadronic CR interactions and find that it follows a universal spectrum and
spatial distribution. If we neglect the anomalous enhancement factor and assume
standard values for the cross section and minimum subhalo mass, the same model
of DM predicts comparable levels of the gamma-ray emission from DM
annihilations and CR interactions. This suggests that spectral subtraction
techniques could be applied to detect the annihilation signal.Comment: 5 pages, 2 figures (published version; minor corrections to figures
and result, equation added
Labour Input in Specialist Beef Bull Production in Sweden
Labour input was investigated on 101 Swedish beef bull farms, representing 42% of all farms rearing 100-800 bulls annually in 2007. Work time studies were performed through questionnaires, supplemented by field studies on a smaller set of farms. Young bulls were of dairy or beef breed, purchased from age 7 to 365 days by specialist producers and finished to an average age of 17 (dairy breed) and 15.5 months (beef breed) and carcass weight 300 and 330 kg, respectively. Farms with different models of finishing, depending on calf age at purchase, were categorised into four groups as: 1) Pre-weaned, 7-61 days purchase age (PW), 2) weaned, 56-92 days purchase age (W1), 3) weaned, 107-168 days purchase age (W2) and 4) weaned, 180-365 days purchase age (W3). Total median labour input per bull for pre-defined work tasks was 6.4, 7.1, 4.0 and 2.7 hours, respectively, for these four different finishing models. Labour efficiency in the four models was 0.76, 0.94, 0.64 and 0.69 min/bull/day, respectively, i.e. with no difference in labour efficiency between farms rearing pre-weaned calves (PW) or calves weaned from 2-3 months of age (W1). No differences were found in total daily labour efficiency (min/bull/day) within the quarantine house and finishing sections, or in tasks common to both sections. Feeding was the most labour-intensive task, requiring 65-78% of daily labour input. Feeding time was not strongly affected by technique, but was shortest on farms operating with total mixed ration (TMR) (0.30 min/bull/day) (p=0.046). However, farms operating with TMR were also significantly larger, with 200 bulls (range 100-600) in the finishing house compared with 150 bulls (range 44-400) on farms feeding roughage and concentrates separately (labour input 0.52 min/bull/day). The effect of housing system on labour input for daily tasks was examined on the 65% of farms utilising only one type of housing system in the finishing house. Systems with slatted floor group pens (concrete or rubber flooring) had the lowest work time requirement (0.47 min/bull/day), followed by straw bedded pens with or without paved alleys (0.51 and 0.58 min/bull/day, respectively) and loose house cubicle systems (0.70 min/bull/day). A non-linear relationship was found between labour efficiency and bull unit size. Variations in work efficiency for finishing 100-200 bulls/year ranged from 0.2-3.0 min/bull/day in the finishing house, indicating possibilities for increased labour efficiency related to factors other than unit size. Labour input per bull was not significantly affected by unit size from 450 bulls/year (0.4 min/bull/day) to 960 bulls/year (0.3 min/bull/day), possibly indicating the highest level of labour efficiency achievable in Swedish beef bull production at present
Prospects of detecting gamma-ray emission from galaxy clusters: cosmic rays and dark matter annihilations
We study the possibility for detecting gamma-ray emission from galaxy
clusters. We consider 1) leptophilic models of dark matter (DM) annihilation
that include a Sommerfeld enhancement (SFE), 2) different representative
benchmark models of supersymmetric DM, and 3) cosmic ray (CR) induced pion
decay. Among all clusters/groups of a flux-limited X-ray sample, we predict
Virgo, Fornax and M49 to be the brightest DM sources and find a particularly
low CR-induced background for Fornax. For a minimum substructure mass given by
the DM free-streaming scale, cluster halos maximize the substructure boost for
which we find a factor above 1000. Since regions around the virial radius
dominate the annihilation flux of substructures, the resulting surface
brightness profiles are almost flat. This makes it very challenging to detect
this flux with imaging atmospheric Cherenkov telescopes. Assuming cold dark
matter with a substructure mass distribution down to an Earth mass and using
extended Fermi upper limits, we rule out the leptophilic models in their
present form in 28 clusters, and limit the boost from SFE in M49 and Fornax to
be < 5. This corresponds to a limit on SFE in the Milky Way of < 3, which is
too small to account for the increasing positron fraction with energy as seen
by PAMELA and challenges the DM interpretation. Alternatively, if SFE is
realized in Nature, this would imply a limiting substructure mass of M_lim >
10^4 M_sol - a problem for structure formation. Using individual cluster
observations, it will be challenging for Fermi to constrain our selection of DM
benchmark models without SFE. The Fermi upper limits are, however, closing in
on our predictions for the CR flux using an analytic model based on
cosmological hydrodynamical cluster simulations. We limit the CR-to-thermal
pressure in nearby bright galaxy clusters of the Fermi sample to < 10% and in
Norma and Coma to < 3%.Comment: 43 pages, 23 figures, 10 tables. Accepted for publication in Phys.
Rev. D: streamlined paper, added a paragraph about detectability to
introduction, few references added, and few typos correcte
Gait and Force Analysis of Provoked Pig Gait on Clean and Fouled Concrete Surfaces
Gait and force analysis have proven to be useful methods in linking claw injuries to surface material conditions. To determine the relationship between claw disorder and floor properties such as friction and surface abrasiveness, the factors controlling gait must be characterised. The effects of fouled concrete floor conditions on the gait of 10 pigs walking in a curve, using kinematics and kinetics to record gait parameters and slip frequency are described and compared with clean conditions. Pigs adapted to fouled floor conditions by reducing their walking speed and stride length, using a higher number of 3-foot support phases and by lowering diagonality. This adaption produced lower vertical forces, a twofold reduction in propulsion and outward stabilisation force and a threefold increase in braking force, without reducing the peak utilised coefficient of friction (UCOF). The UCOF values for both limbs of the curve walking pigs exceeded the recorded dynamic coefficient of friction and the corresponding UCOF values for pigs walking a straight line in fouled floor condition. As UCOF increased and available friction from the fouled floor surface decreased, this resulted in higher forward and backward slip frequency in both limbs for pigs walking in a curve. Pigs provoked to walk in a curve can adapt to fouled floor condition, but if the floor is heavily fouled this adaption is not sufficient to ensure safe walking
LOFAR discovery of a double radio halo system in Abell 1758 and radio/X-ray study of the cluster pair
Radio halos and radio relics are diffuse synchrotron sources that extend over Mpc-scales and are found in a number of merger galaxy clusters. They are believed to form as a consequence of the energy that is dissipated by turbulence and shocks in the intra-cluster medium (ICM). However, the precise physical processes that generate these steep synchrotron spectrum sources are still poorly constrained. We present a new LOFAR observation of the double galaxy cluster Abell 1758. This system is composed of A1758N, a massive cluster hosting a known giant radio halo, and A1758S, which is a less massive cluster whose diffuse radio emission is confirmed here for the first time. Our observations have revealed a radio halo and a candidate radio relic in A1758S, and a suggestion of emission along the bridge connecting the two systems which deserves confirmation. We combined the LOFAR data with archival VLA and GMRT observations to constrain the spectral properties of the diffuse emission. We also analyzed a deep archival Chandra observation and used this to provide evidence that A1758N and A1758S are in a pre-merger phase. The ICM temperature across the bridge that connects the two systems shows a jump which might indicate the presence of a transversal shock generated in the initial stage of the merger
First evidence of diffuse ultra-steep-spectrum radio emission surrounding the cool core of a cluster
Diffuse synchrotron radio emission from cosmic-ray electrons is observed at the center of a number of galaxy clusters. These sources can be classified either as giant radio halos, which occur in merging clusters, or as mini halos, which are found only in cool-core clusters. In this paper, we present the first discovery of a cool-core cluster with an associated mini halo that also shows ultra-steep-spectrum emission extending well beyond the core that resembles radio halo emission. The large-scale component is discovered thanks to LOFAR observations at 144 MHz. We also analyse GMRT observations at 610 MHz to characterise the spectrum of the radio emission. An X-ray analysis reveals that the cluster is slightly disturbed, and we suggest that the steep-spectrum radio emission outside the core could be produced by a minor merger that powers electron re-acceleration without disrupting the cool core. This discovery suggests that, under particular circumstances, both a mini and giant halo could co-exist in a single cluster, opening new perspectives for particle acceleration mechanisms in galaxy clusters
Search for extended gamma-ray emission from the Virgo galaxy cluster with Fermi-LAT
Galaxy clusters are one of the prime sites to search for dark matter (DM)
annihilation signals. Depending on the substructure of the DM halo of a galaxy
cluster and the cross sections for DM annihilation channels, these signals
might be detectable by the latest generation of -ray telescopes. Here
we use three years of Fermi Large Area Telescope (LAT) data, which are the most
suitable for searching for very extended emission in the vicinity of nearby
Virgo galaxy cluster. Our analysis reveals statistically significant extended
emission which can be well characterized by a uniformly emitting disk profile
with a radius of 3\deg that moreover is offset from the cluster center. We
demonstrate that the significance of this extended emission strongly depends on
the adopted interstellar emission model (IEM) and is most likely an artifact of
our incomplete description of the IEM in this region. We also search for and
find new point source candidates in the region. We then derive conservative
upper limits on the velocity-averaged DM pair annihilation cross section from
Virgo. We take into account the potential -ray flux enhancement due to
DM sub-halos and its complex morphology as a merging cluster. For DM
annihilating into , assuming a conservative sub-halo model
setup, we find limits that are between 1 and 1.5 orders of magnitude above the
expectation from the thermal cross section for
. In a more optimistic scenario, we
exclude
for for the same channel. Finally, we
derive upper limits on the -ray-flux produced by hadronic cosmic-ray
interactions in the inter cluster medium. We find that the volume-averaged
cosmic-ray-to-thermal pressure ratio is less than .Comment: 15 pages, 11 figures, 4 tables, accepted for publication in ApJ;
corresponding authors: T. Jogler, S. Zimmer & A. Pinzk
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