288 research outputs found
Calculation of the optical response of C60 and Na8 using time-dependent density functional theory and local orbitals
We report on a general method for the calculation of the frequency-dependent
optical response of clusters based upon time-dependent density functional
theory (TDDFT). The implementation is done using explicit propagation in the
time domain and a self-consistent program that uses a linear combination of
atomic orbitals (LCAO). Our actual calculations employ the SIESTA program,
which is designed to be fast and accurate for large clusters. We use the
adiabatic local density approximation to account for exchange and correlation
effects. Results are presented for the imaginary part of the linear
polarizability, Im [\alpha(w)], and the dipole strength function, S(w), of C60
and Na8, compared to previous calculations and to experiment. We also show how
to calculate the integrated frequency-dependent second order non-linear
polarizability for the case of a step function electric field,
\gamma_{step}(w), and present results for C60.Comment: 11 pages with 6 postscript figures. Submitted for publicatio
Double non-equivalent chain structure on vicinal Si(557)-Au surface
We study electronic and topographic properties of the vicinal Si(557)-Au
surface using scanning tunneling microscopy and reflection of high energy
electron diffraction technique. STM data reveal double wire structures along
terraces. Moreover behavior of the voltage dependent STM tip - surface distance
is different in different chains. While the one chain shows oscillations of the
distance which are sensitive to the sign of the voltage bias, the oscillations
in the other chain remain unchanged with respect to the positive/negative
biases. This suggests that one wire has metallic character while the other one
- semiconducting. The experimental results are supplemented by theoretical
calculations within tight binding model suggesting that the observed chains are
made of different materials, one is gold and the other one is silicon chain.Comment: 9 pages, 12 figures, accepted for publication in Phys. Rev.
Chandra and optical/IR observations of CXOJ1415.2+3610, a massive, newly discovered galaxy cluster at z~1.5
(Abridged) We report the discovery of CXO J1415.2+3610, a distant (z~1.5)
galaxy cluster serendipitously detected in a deep, high-resolution Chandra
observation targeted to study the cluster WARP J1415.1+3612 at z=1.03. This is
the highest-z cluster discovered with Chandra so far. Moreover, the total
exposure time of 280 ks with ACIS-S provides the deepest X-ray observation
currently achieved on a cluster at z>1.5. We perform an X-ray spectral fit of
the extended emission of the intracluster medium (ICM) with XSPEC, and we
detect at a 99.5% confidence level the rest frame 6.7-6.9 keV Iron K_\alpha
line complex, from which we obtain z_X=1.46\pm0.025. The analysis of the
z-3.6\mu m color-magnitude diagram shows a well defined sequence of red
galaxies within 1' from the cluster X-ray emission peak with a color range [5 <
z-3.6 \mu m < 6]. The photometric redshift obtained by spectral energy
distribution (SED) fitting is z_phot=1.47\pm 0.25. After fixing the redshift to
z=1.46, we perform the final spectral analysis and measure the average gas
temperature with a 20% error, kT=5.8^{+1.2}_{-1.0} keV, and the Fe abundance
Z_Fe = 1.3_{-0.5}^{+0.8}Z_\odot. We fit the background subtracted surface
brightness with a single beta--model out to 35" and derive the deprojected
electron density profile. The ICM mass is 1.09_{-0.2}^{+0.3}\times 10^{13}
M_\odot within 300 kpc. The total mass is M_{2500}= 8.6_{-1.7}^{+2.1} \times 10
^{13} M_\odot for R_{2500}=(220\pm 55) kpc. Extrapolating the profile at larger
radii we find M_{500}= 2.1_{-0.5}^{+0.7} \times 10 ^{14} M_\odot for R_{500} =
510_{-50}^{+55}$ kpc. This analysis establishes CXOJ1415.2+3610 as one of the
best characterized distant galaxy clusters based on X-ray data alone.Comment: 12 pages, 9 figures, A\&A in press, minor modifications in the tex
Study of star-forming galaxies in SDSS up to redshift 0.4: I. Metallicity evolution
The chemical composition of the gas in galaxies versus cosmic time provides a
very important tool for understanding galaxy evolution. Although there are many
studies at high redshift, they are rather scarce at lower redshifts. However,
low redshift studies can provide important clues about the evolution of
galaxies, furnishing the required link between local and high redshift
universe. In this work we focus on the metallicity of the gas of star-forming
galaxies at low redshift, looking for signs of chemical evolution.
To analyze the metallicity contents star-forming galaxies of similar
luminosities and masses at different redshifts. With this purpose, we present a
study of the metallicity of relatively massive (log(M_star/M_sun)>10.5) star
forming galaxies from SDSS--DR5 (Sloan Digital Sky Survey--Data Release 5),
using different redshift intervals from 0.04 to 0.4.
We used data processed with the STARLIGHT spectral synthesis code, correcting
the fluxes for dust extinction, estimating metallicities using the R_23 method,
and segregating the samples with respect to the value of the
[NII]6583/[OII]3727 line ratio in order to break the R_23 degeneracy selecting
the upper branch. We analyze the luminosity and mass-metallicity relations, and
the effect of the Sloan fiber diameter looking for possible biases.
By dividing our redshift samples in intervals of similar magnitude and
comparing them, significant signs of metallicity evolution are found.
Metallicity correlates inversely with redshift: from redshift 0 to 0.4 a
decrement of ~0.1 dex in 12+log(O/H) is found.Comment: 11 pages, 9 figures, Accepted for publication in A&
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