166 research outputs found
Temperature dependence of surface stress across an order-disorder transition: p(1x2)O/W(110)
Strain relaxations of a p(1x2) ordered oxygen layer on W(110) are measured as
a function of temperature across the disordering transition using low-energy
electron diffraction. The measured strains approach values of 0.027 in the
[1-10] and -0.053 in the [001] direction. On the basis of the measured strain
relaxations, we give quantitative information on temperature-dependent surface
stress using the results of ab initio calculations. From the surface formation
energy for different strains, determined by first-principles calculations, we
estimate that surface stress changes from -1.1 for the ordered phase to -0.2N/m
for the disordered one along [1-10], and from 5.1 to 3.4 N/m along [001].
Moreover, our observation that the strains scale inversely with domain size
confirms that the strain relaxation takes place at the domain boundaries.Comment: 8 pages, 5 figure
Quantum-size effects on chemisorption properties: CO on Cu ultrathin films
We address, by means of ab-initio calculations, the origin of the correlation
that has been observed experimentally between the chemisorption energy of CO on
nanoscale Cu(001) supported films and quantum-size effects. The calculated
chemisorption energy shows systematic oscillations, as a function of film
thickness, with a periodicity corresponding to that of quantum-well states at
the surface-Brillouin-zone center crossing the Fermi energy. We explain this
trend based on the oscillations, with film thickness, of the decay length on
the vacuum side of the quantum-well states at the Fermi energy. Contrary to
previous suggestions, we find that the actual oscillations with film thickness
of the density of states per atom of the film at the Fermi energy cannot
account for the observed trend in the chemisorption energy.Comment: 18 pages, 7 figures. Accepted for publication in Phys. Rev.
Strain relaxation in small adsorbate islands: O on W(110)
The stress-induced lattice changes in a p(1x2) ordered oxygen layer on W(110)
are measured by low-energy electron diffraction. We have observed that small
oxygen islands show a mismatch with the underlying lattice. Our results
indicate that along [1-10] the average mismatch scales inversely with the
island size as 1/L for all oxygen coverages up to 0.5 ML, while along [001] it
is significant only for the smallest oxygen islands and scales as a higher
power of the inverse island size. The behaviour along [1-10] is described by a
one-dimensional finite-size Frenkel-Kontorova model. Using this model, together
with calculated force constants, we make a quantitative estimate for the change
of surface-stress upon oxygen adsorption. The result is consistent with our
ab-initio calculations, which give a relative compressive stress of -4.72 N/m
along [1-10] and a minute relative tensile stress of 0.15 N/m along [001]. The
scaling along [001] is qualitatively explained as an effect induced by the
lattice relaxation in the [1-10] direction.Comment: 22 pages, 5 figure
The structure of elliptical galaxies in the Virgo cluster. Results from the INT Wide Field Survey
We report on a complete CCD imaging survey of 226 elliptical galaxies in the
North-East quadrant of the Virgo cluster, representative of the properties of
giant and dwarf elliptical galaxies in this cluster. We fit their radial light
profiles with the Sersic r^(1/n) model of light distribution. We confirm the
result of Graham & Guzman (2003) that the apparent dichotomy between E and dE
galaxies in the luminosity-_e plane no longer appears when other structural
parameters are considered and can be entirely attributed to the onset of "core"
galaxies at B_T sim -20.5 mag. When "core" galaxies are not considered, E and
dE form a unique family with n linearly increasing with the luminosity. For 90
galaxies we analyze the B-I color indices, both in the nuclear and in the outer
regions. Both indices are bluer toward fainter luminosities. We find also that
the outer color gradients do not show any significant correlation with the
luminosity. The scatter in all color indicators increases significantly toward
lower luminosities, e.g. galaxies fainter than B_T sim -15 have a B-I spread
0.5 mag.Comment: 21 pages, 14 figures, 2 tables. Accepted for publication on A&A. High
resolution figures at
http://goldmine.mib.infn.it/papers/WFS_04/WFS_04-frame.htm
Radial Alignment in Simulated Clusters
Observational evidence for the radial alignment of satellites with their dark
matter host has been accumulating steadily in the past few years. The effect is
seen over a wide range of scales, from massive clusters of galaxies down to
galaxy-sized systems, yet the underlying physical mechanism has still not been
established. To this end, we have carried out a detailed analysis of the shapes
and orientations of dark matter substructures in high-resolution N-body
cosmological simulations. We find a strong tendency for radial alignment of the
substructure with its host halo: the distribution of halo major axes is very
anisotropic, with the majority pointing towards the center of mass of the host.
The alignment peaks once the sub-halo has passed the virial radius of the host
for the first time, but is not subsequently diluted, even after the halos have
gone through as many as four pericentric passages. This evidence points to the
existence of a very rapid dynamical mechanism acting on these systems and we
argue that tidal torquing throughout their orbits is the most likely candidate.Comment: v2: 13 pages, 10 figures, ApJ in press. Revisions include a new
section (4.2) comparing our results with observations, and a few added
reference
Keck/LRIS Spectroscopic Confirmation of Coma Cluster Dwarf Galaxy Membership Assignments
Keck/LRIS multi-object spectroscopy has been carried out on 140 of some of
the lowest and highest surface brightness faint (19 < R < 22) dwarf galaxy
candidates in the core region of the Coma Cluster. These spectra are used to
measure redshifts and establish membership for these faint dwarf populations.
The primary goal of the low surface brightness sample is to test our ability to
use morphological and surface brightness criteria to distinguish between Coma
Cluster members and background galaxies using high resolution HST/ACS images.
Candidates were rated as expected members, uncertain, or expected background.
From 93 spectra, 51 dwarf galaxy members and 20 background galaxies are
identified. Our morphological membership estimation success rate is ~100% for
objects expected to be members and better than ~90% for galaxies expected to be
in the background. We confirm that low surface brightness is a very good
indicator of cluster membership. High surface brightness galaxies are almost
always background with confusion arising only from the cases of the rare
compact elliptical galaxies. The more problematic cases occur at intermediate
surface brightness. Many of these galaxies are given uncertain membership
ratings, and these were found to be members about half of the time. Including
color information will improve membership determination but will fail for some
of the same objects that are already mis-identified when using only surface
brightness and morphology criteria. Compact elliptical galaxies with B-V colors
~0.2 magnitudes redward of the red sequence in particular require spectroscopic
follow-up. In a sample of 47 high surface brightness, UCD candidates, 19
objects have redshifts which place them in the Coma Cluster. Redshift
measurements are presented and the use of indirect means for establishing
cluster membership is discussed.Comment: Accepted for publication in ApJ, 25 pages, 15 figure
The Dwarf Irregular Galaxy UGC 7636 Exposed: Stripping At Work In The Virgo Cluster
We present the results of optical spectroscopy of a newly discovered H II
region residing in the H I gas cloud located between the dwarf irregular galaxy
UGC 7636 and the giant elliptical galaxy NGC 4472 in the Virgo Cluster. By
comparing UGC 7636 with dwarf irregular galaxies in the field, we show that the
H I cloud must have originated from UGC 7636 because (1) the oxygen abundance
of the cloud agrees with that expected for a galaxy with the blue luminosity of
UGC 7636, and (2) M_{H I}/L_B for UGC 7636 becomes consistent with the measured
oxygen abundance of the cloud if the H I mass of the cloud is added back into
UGC 7636. It is likely that tides from NGC 4472 first loosened the H I gas,
after which ram-pressure stripping removed the gas from UGC 7636.Comment: 12 pages, 2 eps figures (AASTeX 5.0); accepted for publication in ApJ
Letter
Blueshifted galaxies in the Virgo Cluster
We examine a sample of 65 galaxies in the Virgo cluster with negative radial
velocities relative to the Local Group. Some features of this sample are
pointed out. All of these objects are positioned compactly within a virial zone
of radius 6{\deg} in the cluster, but their centroid is displaced relative to
the dynamic center of the cluster, M87, by 1.1{\deg} to the northwest. The
dwarf galaxies in this sample are clumped on a scale of ~10' (50 kpc). The
observed asymmetry in the distribution of the blueshifted galaxies may be
caused by infall of a group of galaxies around M86 onto the main body of the
cluster. We offer another attempt to explain this phenomenon, assuming a mutual
tangential velocity of ~300 km/s between the Local Group and the Virgo cluster
owing to their being repelled from the local cosmological void.Comment: 10 pages, 4 figures, 1 table. Published in Astrophysics, Vol. 53, No.
1, pp. 32-41, 201
Stress engineering at the nanometer scale: Two-component adlayer stripes
Spontaneously formed equilibrium nanopatterns with long-range order are
widely observed in a variety of systems, but their pronounced temperature
dependence remains an impediment to maintain such patterns away from the
temperature of formation. Here, we report on a highly ordered stress-induced
stripe pattern in a two-component, Pd-O, adsorbate monolayer on W(110),
produced at high temperature and identically preserved at lower temperatures.
The pattern shows a tunable period (down to 16 nm) and orientation, as
predicted by a continuum model theory along with the surface stress and its
anisotropy found in our DFT calculations. The control over thermal fluctuations
in the stripe formation process is based on the breaking/restoring of
ergodicity in a high-density lattice gas with long-range interactions upon
turning off/on particle exchange with a heat bath.Comment: 6 pages, 4 figure
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