1,714 research outputs found
Newcomers Meet the Intracluster Medium in the Coma Cluster
A main topic at this meeting is how galaxies are affected when they enter for
the first time the cluster environment from the outskirts. Most of the times we
are forced to infer the environmental effects indirectly, relying on systematic
variations of galaxy properties with environment, but there aren't many
examples of direct observations able to unveil ongoing transformations taking
place, and the corresponding mechanism producing it. We present a case in which
it is possible to identify the cluster environment, and in particular the
intracluster medium and the recent infall history of galaxies onto the cluster,
as the cause for a recent, abrupt change in the evolutionary history of
galaxies.Comment: 5 pages, 1 postscript figure -- to appear in "Outskirts of Galaxy
Clusters: intense life in the suburbs", IAU Colloquium N. 195, 2004, ed. A
Diaferi
Relationship between macroscopic physical properties and local distortions of low doping La{1-x}Ca{x}MnO3: an EXAFS study
A temperature-dependent EXAFS investigation of La{1-x}Ca{x}MnO3 is presented
for the concentration range that spans the ferromagnetic-insulator (FMI) to
ferromagnetic-metal (FMM) transition region, x = 0.16-0.22. The samples are
insulating for x = 0.16-0.2 and show a metal/insulator transition for x = 0.22.
All samples are ferromagnetic although the saturation magnetization for the 16%
Ca sample is only ~ 70% of the expected value at 0.4T. We find that the FMI
samples have similar correlations between changes in the local Mn-O distortions
and the magnetization as observed previously for the colossal magnetoresistance
(CMR) samples (0.2 < x < 0.5) - except that the FMI samples never become fully
magnetized. The data show that there are at least two distinct types of
distortions. The initial distortions removed as the insulating sample becomes
magnetized are small and provides direct evidence that roughly 50% of the Mn
sites have a small distortion/site and are magnetized first. The large
remaining Mn-O distortions at low T are attributed to a small fraction of
Jahn-Teller-distorted Mn sites that are either antiferromagnetically ordered or
unmagnetized. Thus the insulating samples are very similar to the behavior of
the CMR samples up to the point at which the M/I transition occurs for the CMR
materials. The lack of metallic conductivity for x <= 0.2, when 50% or more of
the sample is magnetic, implies that there must be preferred magnetized Mn
sites and that such sites do not percolate at these concentrations.Comment: 27 pages, 8 figures, to be submitted to Phys. Rev.
A comparison of the galaxy populations in the Coma and distant clusters: the evolution of k+a galaxies and the role of the intracluster medium
The spectroscopic properties of galaxies in the Coma cluster are compared
with those of galaxies in rich clusters at , to investigate the
evolution of the star formation history in clusters. Luminous galaxies with
and post-starburst/post-starforming (k+a) spectra which
constitute a significant fraction of galaxies in distant cluster samples are
absent in Coma, where spectacular cases of k+a spectra are found instead at
and represent a significant proportion of the cluster dwarf galaxy
population. A simple inspection of their positions on the sky indicates that
this type of galaxy does not show a preferential location within the cluster,
but the bluest and strongest-lined group of k+a's lies in projection towards
the central 1.4 Mpc of Coma and have radial velocities significantly higher
than the cluster mean. We find a striking correlation between the positions of
these young and strong post-starburst galaxies and substructure in the hot
intracluster medium (ICM) identified from {\it XMM-Newton} data, with these
galaxies lying close to the edges of two infalling substructures. This result
strongly suggests that the interaction with the dense ICM could be responsible
for the quenching of the star formation (thus creating the k+a spectrum), and
possibly, for any previous starburst. The evolution with redshift of the
luminosity distribution of k+a galaxies can be explained by a ``downsizing
effect'', with the maximum luminosity/mass of actively star-forming galaxies
infalling onto clusters decreasing at lower redshift. We discuss the possible
physical origin of this downsizing effect and the implications of our results
for current scenarios of environmental effects on the star formation in
galaxies.Comment: 21 pages, 7 figures, to appear in ApJ, version after referee's
change
Local Structure of La1-xSrxCoO3 determined from EXAFS and neutron PDF studies
The combined local structure techniques, extended x-ray absorption fine
structure (EXAFS) and neutron pair distribution function analysis, have been
used for temperatures 4 <= T <= 330 K to rule out a large Jahn-Teller (JT)
distortion of the Co-O bond in La1-xSrxCoO3 for a significant fraction of Co
sites (x <= 0.35), indicating few, if any, JT-active, singly occupied e_g Co
sites exist.Comment: 5 page
Correlated local distortions of the TlO layers in TlBaCuO: An x-ray absorption study
We have used the XAFS (x-ray-absorption fine structure) technique to
investigate the local structure about the Cu, Ba, and Tl atoms in orthorhombic
Tl-2201 with a superconducting transition temperature T=60 K. Our results
clearly show that the O(1), O(2), Cu, and Ba atoms are at their ideal sites as
given by the diffraction measurements, while the Tl and O(3) atoms are more
disordered than suggested by the average crystal structure. The Tl-Tl distance
at 3.5 \AA{ } between the TlO layers does not change, but the Tl-Tl distance at
3.9 \AA{ } within the TlO layer is not observed and the Tl-Ba and Ba-Tl peaks
are very broad. The shorter Tl-O(3) distance in the TlO layer is about 2.33
\AA, significantly shorter than the distance calculated with both the Tl and
O(3) atoms at their ideal sites ( 0 or ). A model based
on these results shows that the Tl atom is displaced along the
directions from its ideal site by about 0.11 \AA; the displacements of
neighboring Tl atoms are correlated. The O(3) atom is shifted from the $4e$
site by about 0.53 \AA{ } roughly along the directions. A comparison of
the Tl L-edge XAFS spectra from three samples, with T=60 K, 76 K,
and 89 K, shows that the O environment around the Tl atom is sensitive to T
while the Tl local displacement is insensitive to T and the structural
symmetry. These conclusions are compared with other experimental results and
the implications for charge transfer and superconductivity are discussed. This
paper has been submitted to Phys. Rev. B.Comment: 20 pages plus 14 ps figures, REVTEX 3.
- …