1,375 research outputs found
Structural and vibrational properties of two-dimensional nanolayers on Pd(100)
Using different experimental techniques combined with density functional
based theoretical methods we have explored the formation of
interface-stabilized manganese oxide structures grown on Pd(100) at
(sub)monolayer coverage. Amongst the multitude of phases experimentally
observed we focus our attention on four structures which can be classified into
two distinct regimes, characterized by different building blocks. Two
oxygen-rich phases are described in terms of MnO(111)-like O-Mn-O trilayers,
whereas the other two have a lower oxygen content and are based on a
MnO(100)-like monolayer structure. The excellent agreement between calculated
and experimental scanning tunneling microscopy images and vibrational electron
energy loss spectra allows for a detailed atomic description of the explored
models.Comment: 14 pages, 11 figure
Quasar Parallax: a Method for Determining Direct Geometrical Distances to Quasars
We describe a novel method to determine direct geometrical distances to
quasars that can measure the cosmological constant, Lambda, with minimal
assumptions. This method is equivalent to geometric parallax, with the
`standard length' being the size of the quasar broad emission line region
(BELR) as determined from the light travel time measurements of reverberation
mapping. The effect of non-zero Lambda on angular diameter is large, 40% at
z=2, so mapping angular diameter distances vs. redshift will give Lambda with
(relative) ease. In principle these measurements could be made in the UV,
optical, near infrared or even X-ray bands. Interferometers with a resolution
of 0.01mas are needed to measure the size of the BELR in z=2 quasars, which
appear plausible given reasonable short term extrapolations of current
technology.Comment: 13 pages, with 3 figures. ApJ Letters, in press (Dec 20, 2002
Mid-Infrared line diagnostics of Active Galaxies -- A spectroscopic AGN survey with ISO-SWS
We present medium resolution (R approx. 1500) ISO-SWS 2.4--45 micron spectra
of a sample of 29 galaxies with active nuclei. This data set is rich in fine
structure emission lines tracing the narrow line regions and (circum-)nuclear
star formation regions, and it provides a coherent spectroscopic reference for
future extragalactic studies in the mid-infrared. We use the data set to
briefly discuss the physical conditions in the narrow line regions (density,
temperature, excitation, line profiles) and to test for possible differences
between AGN sub-types. Our main focus is on new tools for determining the
propertibes of dusty galaxies and on the AGN-starburst connection. We present
mid-IR line ratio diagrams which can be used to identify composite (starburst +
AGN) sources and to distinguish between emission excited by active nuclei and
emission from (circum-nuclear) star forming regions. For instance, line ratios
of high to low excitation lines like [O IV]25.9um/[Ne II]12.8um, that have been
used to probe for AGNs in dusty objects, can be examined in more detail and
with better statistics now. In addition, we present two-dimensional diagnostic
diagrams that are fully analogous to classical optical diagnostic diagrams, but
better suited for objects with high extinction. Finally, we discuss
correlations of mid-infrared line fluxes to the mid- and far-infrared
continuum. We compare these relations to similar relations in starburst
galaxies in order to examine the contribution of AGNs to the bolometric
luminosities of their host galaxies. The spectra are available in electronic
form from the authors.Comment: 24 pages, 23 figures, 5 tables. Accepted for A&
A view of the narrow-line region in the infrared: active galactic nuclei with resolved fine-structure lines in the Spitzer archive
We queried the Spitzer archive for high-resolution observations with the
Infrared Spectrograph of optically selected active galactic nuclei (AGN) for
the purpose of identifying sources with resolved fine-structure lines that
would enable studies of the narrow-line region (NLR) at mid-infrared
wavelengths. By combining 298 Spitzer spectra with 6 Infrared Space Observatory
spectra, we present kinematic information of the NLR for 81 z<=0.3 AGN. We used
the [NeV], [OIV], [NeIII], and [SIV] lines, whose fluxes correlate well with
each other, to probe gas photoionized by the AGN. We found that the widths of
the lines are, on average, increasing with the ionization potential of the
species that emit them. No correlation of the line width with the critical
density of the corresponding transition was found. The velocity dispersion of
the gas, sigma, is systematically higher than that of the stars, sigma_*, in
the AGN host galaxy, and it scales with the mass of the central black hole,
M_BH. Further correlations between the line widths and luminosities L, and
between L and M_BH, are suggestive of a three dimensional plane connecting
log(M_BH) to a linear combination of log(sigma) and log(L). Such a plane can be
understood within the context of gas motions that are driven by AGN feedback
mechanisms, or virialized gas motions with a power-law dependence of the NLR
radius on the AGN luminosity. The M_BH estimates obtained for 35 type 2 AGN
from this plane are consistent with those obtained from the M_BH-sigma_*
relation.Comment: ApJ, revised to match the print versio
Reversible Eu<sup>2+</sup> â Eu<sup>3+</sup> transitions at EuâSi interfaces
Valence switching at EuâSi interfaces is demonstrated by resonant photoemission during repeated oxidationâreduction cycles performed by roomâtemperature O2 exposure and mild heating. The Eu2+ â Eu3+ transitions are accompanied by Fermi level switching associated with changes in the stoichiometry of the surface heterostructure. The ability to cycle between two wellâdefined magnetic states at a surface may be attractive in technological applications
The X-ray Emission from the Nucleus of the Dwarf Elliptical Galaxy NGC 3226
We present the first high resolution X-ray image of the dwarf elliptical
galaxy NGC 3226. The data were obtained during an observation of the nearby
Seyfert Galaxy NGC 3227 using the Chandra X-ray Observatory. We detect a point
X-ray source spatially consistent with the optical nucleus of NGC 3226 and a
recently-detected, compact, flat-spectrum, radio source. The X-ray spectrum can
be measured up to ~10 keV and is consistent with a power law with a photon
index 1.7 <~ Gamma <~ 2.2, or thermal bremmstrahlung emission with 4 <~ kT <~
10 keV. In both cases the luminosity in the 2--10 keV band ~10^{40} h_{75}^{-1}
erg/s. We find marginal evidence that the nucleus varies within the
observation. These characteristics support evidence from other wavebands that
NGC 3226 harbors a low-luminosity, active nucleus. We also comment on two
previously-unknown, fainter X-ray sources <~ 15 arcsec from the nucleus of NGC
3226. Their proximity to the nucleus (with projected distances <~ 1.3/h_{75}
kpc) suggests both are within NGC 3226, and thus have luminosities (~few x
10^{38} -- few x 10^{39} erg/s) consistent with black-hole binary systems.Comment: Accepted for publication in ApJ. Figures in colo
The outflow in Mrk 509: A method to calibrate XMM-Newton EPIC-pn and RGS
We have analyzed three XMM-Newton observations of the Seyfert 1 galaxy Mrk
509, with the goal to detect small variations in the ionized outflow
properties. Such measurements are limited by the quality of the
cross-calibration between RGS, the best instrument to characterize the
spectrum, and EPIC-pn, the best instrument to characterize the variability. For
all three observations we are able to improve the relative calibration of RGS
and pn consistently to 4 %. In all observations we detect three different
outflow components and, thanks to our accurate cross-calibration we are able to
detect small differences in the ionization parameter and column density in the
highest ionized component of the outflow. This constrains the location of this
component of the outflow to within 0.5 pc of the central source. Our method for
modeling the relative effective area is not restricted to just this source and
can in principle be extended to other types of sources as well.Comment: 11 pages, 9 figure
The Very Highly Ionized Broad Absorption Line System of the QSO SBS1542+541
We have analyzed the broad absorption line system of the bright (V=16.5)
high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS along
with optical data from the MMT and the Steward Observatory 2.3m telescope.
These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms,
corresponding to 340 to 2480 Angstroms in the QSO rest frame. This object
therefore offers a rare opportunity to study broad absorption lines in the
rest-frame extreme UV. We find that the absorption system is dominated by very
high-ionization species, including O VI, NeVIII, and SiXII. We also identify
apparently saturated broad Lyman-series lines of order Ly-gamma and higher.
There is strong evidence for partial occultation of the QSO emission source,
particularly from the higher-order Lyman lines which indicate a covered
fraction less than 0.2. Overall, the data suggest a correlation between a
larger covered fraction and a higher state of ionization. We suggest that the
different covered fractions can be explained by either a special line of sight
through a disk-like geometry or by the existence of density fluctuations of a
factor >2 in the BAL gas. Our photoionization models of the system indicate a
large column density and high ionization state similar to that found in X-ray
``warm absorbers''.Comment: 31 pages, 13 figures, to be published in Ap
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