749 research outputs found
The dwarf galaxy DDO 47: testing cusps hiding in triaxial halos
We present HI data of the dwarf galaxy DDO 47, aimed at testing the
hypothesis that dark halo triaxiality might induce non-circular motions
resulting in rotation curves best fitted by cored halos, even if the dark
matter halo is intrinsically cuspy. We performed a harmonic decomposition of
the velocity field in order to search for alleged non-circular motions needed
to ``hide'' a cusp: in DDO 47 non-circular motions are globally at a level of
2-3 km s, far from being sufficient to reconcile the observed rotation
curve with the CDM predictions. We conclude that the dark matter halo
around DDO 47 is truly cored and that a cusp cannot be hidden by non-circular
motions. More details are shown in Gentile et al. (2006, ApJL in press,
astro-ph/0506538).Comment: 2 pages, to appear in Proc. XXIst IAP Colloquium "Mass Profiles and
Shapes of Cosmological Structures", Paris 4-9 July 2005 (EAS Publications
Series, G. Mamon, F. Combes, C. Deffayet, B. Fort eds.
Evidence for from photoproduction and consequence for chiral-symmetry restoration at high mass
We report a partial-wave analysis of new data on the double-polarization
variable for the reactions and
and of further data published earlier. The analysis within the Bonn-Gatchina
(BnGa) formalism reveals evidence for a poorly known baryon resonance, the
one-star . This is the lowest-mass resonance with
spin-parity . Its mass is significantly higher than the mass of its
parity partner which is the lowest-mass
resonance with spin-parity . It has been suggested that chiral
symmetry might be restored in the high-mass region of hadron excitations, and
that these two resonances should be degenerate in mass. Our findings are in
conflict with this prediction.Comment: 5 pages, 3 figures; Physics Letters B in pres
decays from photoproduction of -mesons off protons
A study of the partial-wave content of the
reaction in the fourth resonance region is presented, which has been prompted
by new measurements of polarization observables for that process. Using the
Bonn-Gatchina partial-wave formalism, the incorporation of new data indicates
that the , , , and are
the most significant contributors to the photoproduction process. New results
for the branching ratios of the decays of these more prominent resonances to
final states are provided; such branches have not been indicated
in the most recent edition of the Review of Particle Properties. Based on the
analysis performed here, predictions for the helicity asymmetry for the
reaction are presented.Comment: 7 pages, 5 figures, 3 table
Exchange Currents in Photoproduction of Baryon Resonances
We calculate photoexcitation amplitudes for several nucleon and delta
resonances. We use a chiral quark model including two-body exchange currents.
The two-body currents give important contributions. For the delta (1232) and
the D13 (1520) we observe that the individual exchange current contributions
considerably cancel each other while in the case of the Roper resonance and the
S11 (1535) we get a reinforcement of the two-body amplitudes. In comparison
with present experimental data, we obtain both for the S11 (1535) and for the
Roper resonance an improvement with respect to the impulse approximation.Comment: 9 pages, 1 figur
Gas Stripping of Dwarf Galaxies in Clusters of Galaxies
Many clusters of galaxies contain an appreciable amount of hot gas, the
intracluster medium. As a consequence, gas will be stripped from galaxies that
move through the inter cluster medium, if the ram pressure exceeds the internal
gravitational force. We study the interaction between the intracluster medium
and an extended gas component of dwarf galaxies confined by a surrounding cold
dark matter halo analytically and numerically, using axisymmetric
two-dimensional hydrodynamical simulations at high resolution. The results show
that the gas within the dark matter halo is totally stripped in a typical
galactic cluster. The process of ram pressure stripping therefore must have
played an important role during the chemo-dynamical evolution of dwarf galaxies
in galactic clusters. Our results predict that most of the chemical evolution
of dwarf galaxies must have occurred at high redshift, before the virialized
cluster had formed.Comment: 9 pages, 6 figures; accepted for publication in Ap
Strong evidence for nucleon resonances near 1900 MeV
Data on the reaction γp → KþΛ from the CLAS experiments are used to derive the leading multipoles,
E0þ, M1−, E1þ, and M1þ, from the production threshold to 2180 MeV in 24 slices of the invariant mass.
The four multipoles are determined without any constraints. The multipoles are fitted using a multichannel
L þ P model that allows us to search for singularities and to extract the positions of poles on the complex
energy plane in an almost model-independent method. The multipoles are also used as additional
constraints in an energy-dependent analysis of a large body of pion and photoinduced reactions within the
Bonn-Gatchina partial wave analysis. The study confirms the existence of poles due to nucleon resonances
with spin parity JP ¼ 1=2−, 1=2þ, and 3=2þ in the region at about 1.9 Ge
Photo-production of Nucleon Resonances and Nucleon Spin Structure Function in the Resonance Region
The photo-production of nucleon resonances is calculated based on a chiral
constituent quark model including both relativistic corrections H{rel} and
two-body exchange currents, and it is shown that these effects play an
important role. We also calculate the first moment of the nucleon spin
structure function g1 (x,Q^2) in the resonance region, and obtain a
sign-changing point around Q^2 ~ 0.27 {GeV}^2 for the proton.Comment: 23 pages, 5 figure
The dwarf galaxy DDO 47 as a dark matter laboratory: testing cusps hiding in triaxial halos
We present an analysis of high resolution HI data of the dwarf galaxy DDO 47,
aimed at testing the hypothesis that dark halo triaxiality might induce
non-circular motions resulting in rotation curves best fitted by cored halos,
even if the dark matter halo is intrinsically cuspy. This hypothesis could be
invoked in order to reconcile the predictions of the standard LambdaCDM theory
with the rotation curves of disk galaxies. DDO 47 is an ideal case to test this
hypothesis because it has a very regular velocity field, its rotation curve is
best fitted by a cored halo and a NFW halo is inconsistent with the data. We
analysed the velocity field through the higher-order harmonic terms in order to
search for kinematical signatures of alleged non-circular motions needed to
"hide" a cusp: the result is that globally non-circular motions are at a level
of 2-3 km/s, and they are more likely to be associated to the presence of some
spiral structure than to a global elongated potential (e.g. a triaxial halo).
These non-circular motions are far from being sufficient to account for the
discrepancy with the LambdaCDM predictions. We therefore conclude that the dark
matter halo around the dwarf galaxy DDO 47 is truly cored and that a cusp
cannot be hidden by non-circular motions.Comment: 4 pages, accepted for publication in ApJ Letters, substantial changes
from original versio
Imaging Performance of the XMM-Newton X-ray telescopes
The in-orbit imaging performance of the three X-ray telescopes on board of
the X-ray astronomy observatory XMM-Newton is presented and compared with the
performance measured on ground at the MPE PANTER test facility. The comparison
shows an excellent agreement between the on ground and in-orbit performance.Comment: 9 pages, 10 Postscript figures, for SPIE 4012, paper 8
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