7,589 research outputs found
Improved Method for Detecting Local Discontinuities in CMB data by Finite Differencing
An unexpected distribution of temperatures in the CMB could be a sign of new
physics. In particular, the existence of cosmic defects could be indicated by
temperature discontinuities via the Kaiser-Stebbins effect. In this paper, we
show how performing finite differences on a CMB map, with the noise regularized
in harmonic space, may expose such discontinuities, and we report the results
of this process on the 7-year Wilkinson Microwave Anisotropy Probe data.Comment: 5 pages, 6 figures; Text has been edited, in line with the PRD
articl
The core flux of the brightest 10 micron galaxies in the southern sky
Aims. Near diffraction-limited images have been taken at 8.9, 11.9, and 12.9
micron for the brightest extragalactic sources in the southern sky, in order to
optimally plan N-band observations with MIDI (MID-infrared Interferometric
instrument) at the VLTI. Methods. We have assembled a sample of 21 objects
consisting of all the AGNs observable from Paranal observatory, Chile, plus
three non-AGN objects, with an estimated N-band flux greater than 400mJy. We
used the TIMMI2 Mid Infrared instrument mounted on the ESO's 3.6m telescope to
obtain near diffraction-limited images in order to establish the unresolved
core flux within < 0.5 arscsec. Results. Positions and core total fluxes were
obtained for all sources in our sample and compared with similar investigations
in the literature. We find that 15 AGN and the nuclear starburst in NGC 253
exhibit an unresolved core flux < 300mJy at 11.9 micron, making them promising
targets for MIDI at the VLTI. For extended sources, near diffraction-limited
images are presented and discussed.Comment: Accepted to A&
WQ 2059-247: An unusual high redshift X-ray cluster
X-ray, optical, and radio observations of a high redshift, Bautz-Morgan type I cluster of galaxies are reported. The cD galaxy contains a powerful, flat spectrum radio source coincident with the possibly stellar nucleus. The cluster is an extremely luminous X-ray source; however, unlike nearby luminous X-ray clusters the X-ray spectrum appears to be rather soft. Two possible interpretations of the soruces are suggested: either the intracluster gas is much cooler in high redshift clusters because they are less relaxed, or the X-ray and radio emissions from WQ 2059-247 are the result of a non thermal QSO/BL Lac type object in the nucleus of the cD
On the Internal Absorption of Galaxy Clusters
A study of the cores of galaxy clusters with the Einstein SSS indicated the
presence of absorbing material corresponding to 1E+12 Msun of cold cluster gas,
possibly resulting from cooling flows. Since this amount of cold gas is not
confirmed by observations at other wavelengths, we examined whether this excess
absorption is present in the ROSAT PSPC observations of 20 bright galaxy
clusters. For 3/4 of the clusters, successful spectral fits were obtained with
absorption due only to the Galaxy, and therefore no extra absorption is needed
within the clusters, in disagreement with the results from the Einstein SSS
data for some of the same clusters. For 1/4 of the clusters, none of our
spectral fits was acceptable, suggesting a more complicated cluster medium than
the two-temperature and cooling flow models considered here. However, even for
these clusters, substantial excess absorption is not indicated.Comment: accepted by the Astrophysical Journa
A Thin HI Circumnuclear Disk in NGC4261
We report on high sensitivity, spectral line VLBI observations of the HI
absorption feature in the radio galaxy NGC4261. Although absorption is only
detectable on the most sensitive baseline, it can be unambiguously associated
with the counterjet and is interpreted to originate in a thin atomic
circumnuclear disk. This structure is probably a continuation of the dusty
accretion disk inferred from HST imaging, which could be feeding the massive
black hole. HI column densities in front of the counterjet of the order of
10^{21}(T_sp/100 K) cm^{-2} are derived, consistent with X-ray data and VLBI
scale free-free absorption. The data presented here are the result of the first
scientific project processed on the new EVN MkIV data processor.Comment: 4 pages, 3 postscript figures, Astronomy and Astrophysics Letters, in
pres
Casimir interaction between a plate and a cylinder
We find the exact Casimir force between a plate and a cylinder, a geometry
intermediate between parallel plates, where the force is known exactly, and the
plate--sphere, where it is known at large separations. The force has an
unexpectedly weak decay \sim L/(H^3 \ln(H/R)) at large plate--cylinder
separations H (L and R are the cylinder length and radius), due to transverse
magnetic modes. Path integral quantization with a partial wave expansion
additionally gives a qualitative difference for the density of states of
electric and magnetic modes, and corrections at finite temperatures.Comment: 4 pages, 3 figure
Constraints on the large-x d/u ratio from electron-nucleus scattering at x>1
Recently the ratio of neutron to proton structure functions F_2n/F_2p was
extracted from a phenomenological correlation between the strength of the
nuclear EMC effect and inclusive electron-nucleus cross section ratios at x>1.
Within conventional models of nuclear smearing, this "in-medium correction"
(IMC) extraction constrains the size of nuclear effects in the deuteron
structure functions, from which the neutron structure function F_2n is usually
extracted. The IMC data determine the resulting proton d/u quark distribution
ratio, extrapolated to x=1, to be 0.23 +- 0.09 with a 90% confidence level.
This is well below the SU(6) symmetry limit of 1/2 and significantly above the
scalar diquark dominance limit of 0.Comment: 4 pages, 3 figure
Shock heating in the nearby radio galaxy NGC 3801
Original article can be found at: http://www.journals.uchicago.edu/ApJ/--Copyright American Astronomical SocietyPeer reviewe
The Casimir Energy for a Hyperboloid Facing a Plate in the Optical Approximation
We study the Casimir energy of a massless scalar field that obeys Dirichlet
boundary conditions on a hyperboloid facing a plate. We use the optical
approximation including the first six reflections and compare the results with
the predictions of the proximity force approximation and the semi-classical
method. We also consider finite size effects by contrasting the infinite with a
finite plate. We find sizable and qualitative differences between the new
optical method and the more traditional approaches.Comment: v2: 14 pages, 11 eps figures; typo in eq. (21) removed, clarification
added, fig. 10 improved; version published in Phys. Rev.
HST/STIS Spectroscopy of the Lyman-Alpha Emission Line in the Central Dominant Galaxies in A426, A1795, and A2597: Constraints on Clouds in the Intracluster Medium
We report on HST/STIS spectra of the Lyman-alpha emission in the central
dominant galaxies in three rich clusters of galaxies. We find evidence for a
population of clouds in the intracluster medium.We detect 10 Ly-alpha
absorption systems towards the nucleus of NGC1275 with columns of N(HI)
1E12-1E14 cm-2. The detected absorption features are most consistent with
associated nuclear absorption systems. There is very little nuclear absorption
at the systemic velocity in NGC1275. This implies that the large columns
detected in the 21 cm line towards the parsec scale radio source avoid the line
of sight to the nucleus. This gas may be located in a circumnuclear disk or
torus. We detect at least one and possibly two absorption features towards the
extended Ly-alpha in A426. We do not detect absorption towards the extended
Ly-alpha emission in A1795, and A2597 with upper limits N(HI) 1E13 cm-2 for
optically thin absorbers. Our data constrain the covering factor of any high
column density gas in the ICM to be less than 25%. Our results suggest that the
lack of observed intermediate temperature gas is not explained by obscuration.
In addition, the low columns of gas on the 100 kpc scales in the ICM suggests
that (1) the rate at which cold gas accumulates in the ICM on these scales is
very low, and (2) the dense nebulae in the central 10 kpc must have cooled or
been deposited in situ.Comment: 6 figure
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