7,744 research outputs found
Composition of Jupiter irregular satellites sheds light on their origin
Irregular satellites of Jupiter with their highly eccentric, inclined and
distant orbits suggest that their capture took place just before the giant
planet migration. We aim to improve our understanding of the surface
composition of irregular satellites of Jupiter to gain insight into a narrow
time window when our Solar System was forming. We observed three Jovian
irregular satellites, Himalia, Elara, and Carme, using a medium-resolution
0.8-5.5 micro m spectrograph on the National Aeronautics and Space
Administration (NASA) Infrared Telescope Facility (IRTF). Using a linear
spectral unmixing model we have constrained the major mineral phases on the
surface of these three bodies. Our results confirm that the surface of Himalia,
Elara, and Carme are dominated by opaque materials such as those seen in
carbonaceous chondrite meteorites. Our spectral modeling of NIR spectra of
Himalia and Elara confirm that their surface composition is the same and
magnetite is the dominant mineral. A comparison of the spectral shape of
Himalia with the two large main C-type asteroids, Themis (D 176 km) and Europa
(D 352 km), suggests surface composition similar to Europa. The NIR spectrum of
Carme exhibits blue slope up to 1.5 microm and is spectrally distinct from
those of Himalia and Elara. Our model suggests that it is compositionally
similar to amorphous carbon. Himalia and Elara are compositionally similar but
differ significantly from Carme. These results support the hypotheses that the
Jupiter irregular satellites are captured bodies that were subject to further
breakup events and clustered as families based on their similar physical and
surface compositions
CHANDRA reveals galaxy cluster with the most massive nearby cooling core, RXCJ1504.1-0248
A CHANDRA follow-up observation of an X-ray luminous galaxy cluster with a
compact appearance, RXCJ1504.1-0248 discovered in our REFLEX Cluster Survey,
reveals an object with one of the most prominent cluster cooling cores. With a
core radius of ~30 kpc smaller than the cooling radius with ~140 kpc more than
70% of the high X-ray luminosity of Lbol = 4.3 10e45 erg s-1 of this cluster is
radiated inside the cooling radius. A simple modeling of the X-ray morphology
of the cluster leads to a formal mass deposition rate within the classical
cooling flow model of 1500 - 1900 Msun yr-1 (for h=0.7), and 2300 - 3000 Msun
yr-1 (for h=0.5). The center of the cluster is marked by a giant elliptical
galaxy which is also a known radio source. Thus it is very likely that we
observe one of the interaction systems where the central cluster AGN is heating
the cooling core region in a self-regulated way to prevent a massive cooling of
the gas, similar to several such cases studied in detail in more nearby
clusters. The interest raised by this system is then due to the high power
recycled in RXCJ1504-0248 over cooling time scales which is about one order of
magnitude higher than what occurs in the studied, nearby cooling core clusters.
The cluster is also found to be very massive, with a global X-ray temperature
of about 10.5 keV and a total mass of about 1.7 10e15 Msun inside 3 Mpc.Comment: accepted for publication in Astrophys. Journal, 10 figure
Mass-Temperature Relation of Galaxy Clusters: A Theoretical Study
Combining conservation of energy throughout nearly-spherical collapse of
galaxy clusters with the virial theorem, we derive the mass-temperature
relation for X-ray clusters of galaxies . The normalization factor
and the scatter of the relation are determined from first principles with
the additional assumption of initial Gaussian random field. We are also able to
reproduce the recently observed break in the M-T relation at T \sim 3 \keV,
based on the scatter in the underlying density field for a low density
CDM cosmology. Finally, by combining observational data of high
redshift clusters with our theoretical formalism, we find a semi-empirical
temperature-mass relation which is expected to hold at redshifts up to unity
with less than 20% error.Comment: 43 pages, 13 figures, One figure is added and minor changes are made.
Accepted for Publication in Ap
On the possibility to supercool molecular hydrogen down to superfluid transition
Recent calculations by Vorobev and Malyshenko (JETP Letters, 71, 39, 2000)
show that molecular hydrogen may stay liquid and superfluid in strong electric
fields of the order of . I demonstrate that strong local
electric fields of similar magnitude exist beneath a two-dimensional layer of
electrons localized in the image potential above the surface of solid hydrogen.
Even stronger local fields exist around charged particles (ions or electrons)
if surface or bulk of a solid hydrogen crystal is statically charged.
Measurements of the frequency shift of the photoresonance transition
in the spectrum of two-dimensional layer of electrons above positively or
negatively charged solid hydrogen surface performed in the temperature range 7
- 13.8 K support the prediction of electric field induced surface melting. The
range of surface charge density necessary to stabilize the liquid phase of
molecular hydrogen at the temperature of superfluid transition is estimated.Comment: 5 pages, 2 figure
Problems for MOND in Clusters and the Ly-alpha Forest
The observed dynamics of gas and stars on galactic and larger scales cannot
be accounted for by self-gravity, indicating that there are large quantities of
unseen matter, or that gravity is non-Newtonian in these regimes. Milgrom's
MOdified Newtonian Dynamics (MOND) postulates that Newton's laws are modified
at very low acceleration, and can account for the rotation curves of galaxies
and some other astrophysical observations, without dark matter. Here we apply
MOND to two independent physical systems: Ly-alpha absorbers and galaxy
clusters. While physically distinct, both are simple hydrodynamical systems
with characteristic accelerations in the MOND regime. We find that Ly-alpha
absorbers are somewhat smaller than in Newtonian gravity with dark matter, but
the result depends crucially on the (unknown) background acceleration field in
which they are embedded. In clusters MOND appears to explain the observed
(baryonic) mass-temperature relation. However, given observed gas density and
enclosed mass profiles and the assumption of hydrostatic equilibrium, MOND
predicts radial temperature profiles which disagree badly with observations. We
show this explicitly for the Virgo, Abell 2199 and Coma clusters, but the
results are general, and seem very difficult to avoid. If this discrepancy is
to be resolved by positing additional (presumably baryonic) dark matter, then
this dark matter must have ~1-3 times the cluster gas mass within 1 Mpc. This
result strongly disfavors MOND as an alternative to dark matter (Abridged).Comment: Revised version. Important caveat in Ly-alpha calculation discussed;
conclusions weakened. Coma cluster and calculation of dark matter mass
required by MOND added; cluster conclusions strengthened. 11 EmulateApJ pages
with 3 embedded figures. Accepted by Ap
Stripped Spiral Galaxies as Promising Targets for the Determination of the Cepheid distance to the Virgo Cluster
The measurement of precise galaxy distances by Cepheid observations out to
the distance of the Virgo cluster is important for the determination of the
Hubble constant (). The Virgo cluster is thereby often used as an
important stepping stone. The first HST measurement of the distance of a Virgo
galaxy (M100) using Cepheid variables provided a value for
km/s/Mpc (Freedman et al. 1994). This measurement was preceeded by a ground
based study of the Virgo spiral NGC4571 (Pierce et al. 1994) formally providing
km/s/Mpc. These determinations rely on the accuracy with which
the position of this observed spiral galaxy can be located with respect to the
Virgo cluster center. This uncertainty introduces a major error in the
determination of , together with the uncertainty in the adopted Virgo
infall velocity of the Local Group. Here we propose the use of spiral galaxies
which show clear signs of being stripped off their interstellar medium by the
intracluster gas of the Virgo cluster as targets for the Cepheid distance
measurements. We show that the stripping process and the knowledge of the
intracluster gas distribution from ROSAT X-ray observations allow us to locate
these galaxies with an at least three times higher precision with respect to
M87 than in the case of other spirals like M100. The X-ray observations further
imply that M87 is well centered within the intracluster gas halo of the Virgo
cluster and that M86 is associated with a group of galaxies and a larger dark
matter halo. The combination of these informations could enable us to locate
the two stripped spiral galaxies quite precisely within the Virgo cluster and
could greatly improve the determination of the Virgo cluster distance.Comment: 21 pages, Latex(aaspp.sty), including 6 figures, accepted for
publication in ApJL (shortened abstract:
Chandra Observations of Gas Stripping in the Elliptical Galaxy NGC 4552 in the Virgo Cluster
We use a 54.4 ks Chandra observation to study ram-pressure stripping in
NGC4552 (M89), an elliptical galaxy in the Virgo Cluster. Chandra images in the
0.5-2 keV band show a sharp leading edge in the surface brightness 3.1 kpc
north of the galaxy center, a cool (kT =0.51^{+0.09}_{-0.06} keV) tail with
mean density n_e ~5.4 +/- 1.7 x 10^{-3} cm^{-3} extending ~10 kpc to the south
of the galaxy, and two 3-4 kpc horns of emission extending southward away from
the leading edge. These are all features characteristic of supersonic
ram-pressure stripping of galaxy gas, due to NGC4552's motion through the
surrounding Virgo ICM. Fitting the surface brightness profile and spectra
across the leading edge, we find the galaxy gas inside the edge is cooler (kT =
0.43^{+0.03}_{-0.02} keV) and denser (n_e ~ 0.010 cm^{-3}) than the surrounding
Virgo ICM (kT = 2.2^{+0.7}_{-0.4} keV and n_e = 3.0 +/- 0.3 x 10^{-4} cm^{-3}).
The resulting pressure ratio between the free-streaming ICM and cluster gas at
the stagnation point is ~7.6^{+3.4}_{-2.0} for galaxy gas metallicities of
0.5^{+0.5}_{-0.3} Zsolar, which suggests that NGC4552 is moving supersonically
through the cluster with a velocity v ~ 1680^{+390}_{-220} km/s (Mach
2.2^{+0.5}_{-0.3}) at an angle xi ~ 35 +/- 7 degrees towards us with respect to
the plane of the sky.Comment: 31 pages, 12 figures, ApJ, in press; paper split into 2 parts, Paper
I(sec 1-3) here, added figs and discussion to conform to published version;
Paper II (sec. 4) in astro-ph/060440
Structure of adsorbed Fe on Ni{111}
Using photoelectron diffraction in the scanned energy mode we have established that Fe atoms adsorb in the fcc hollow sites of the Ni{111} surface even at low temperatures. Total-energy calculations had suggested that the hcp hollow sites were more stable
Elastic fluctuations as observed in a confocal slice
Recent confocal experiments on colloidal solids motivate a fuller study of
the projection of three-dimensional fluctuations onto a two-dimensional
confocal slice. We show that the effective theory of a projected crystal
displays several exceptional features, such as non-standard exponents in the
dispersion relations. We provide analytic expressions for the effective
two-dimensional elastic properties which allow one to work back from sliced
experimental observations to three-dimensional elastic constants.Comment: 5 pages, 2 figure
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