645 research outputs found
Parameter identification for soil simulation based on the discrete element method and application to small scale shallow penetration tests
The Discrete Element Method (DEM) is well-established and widely used in soil-tool interaction related applications. As for all simulation tools, a proper calibration of the model parameters is crucial. In this contribution, we present the parametrization procedure of the DEM software GRAnular Physics Engine (GRAPE), developed and implemented at Fraunhofer ITWM, and attempt to use two parametrized soil samples for the simulation of small scale shallow penetration tests. The results are compared to laboratory measurements
Triaxial compression and direct shear tests in the parametrization of soil modeled via the Discrete Element Method
Investigation in the parametrization of soil modeled with DEM based on measurements in triaxial compression and direct shear tests
Massive Galaxies in COSMOS: Evolution of Black hole versus bulge mass but not versus total stellar mass over the last 9 Gyrs?
We constrain the ratio of black hole (BH) mass to total stellar mass of
type-1 AGN in the COSMOS survey at 1<z<2. For 10 AGN at mean redshift z~1.4
with both HST/ACS and HST/NICMOS imaging data we are able to compute total
stellar mass M_(*,total), based on restframe UV-to-optical host galaxy colors
which constrain mass-to-light ratios. All objects have virial BH mass-estimates
available from the COSMOS Magellan/IMACS and zCOSMOS surveys. We find zero
difference between the M_BH--M_(*,total)-relation at z~1.4 and the
M_BH--M_(*,bulge)-relation in the local Universe.
Our interpretation is: (a) If our objects were purely bulge-dominated, the
M_BH--M_(*,bulge)-relation has not evolved since z~1.4. However, (b) since we
have evidence for substantial disk components, the bulges of massive galaxies
(logM_(*,total)=11.1+-0.25 or logM_BH~8.3+-0.2) must have grown over the last 9
Gyrs predominantly by redistribution of disk- into bulge-mass. Since all
necessary stellar mass exists in the galaxy at z=1.4, no star-formation or
addition of external stellar material is required, only a redistribution e.g.
induced by minor and major merging or through disk instabilities. Merging, in
addition to redistributing mass in the galaxy, will add both BH and
stellar/bulge mass, but does not change the overall final M_BH/M_(*,bulge)
ratio.
Since the overall cosmic stellar and BH mass buildup trace each other tightly
over time, our scenario of bulge-formation in massive galaxies is independent
of any strong BH-feedback and means that the mechanism coupling BH and bulge
mass until the present is very indirect.Comment: Published in ApJL; 7 pages, 2 figures; updated to accepted version
(methods changed, results unchanged
Inter-comparison of Radio-Loudness Criteria for Type 1 AGNs in the XMM-COSMOS Survey
Limited studies have been performed on the radio-loud fraction in X-ray
selected type 1 AGN samples. The consistency between various radio-loudness
definitions also needs to be checked. We measure the radio-loudness of the 407
type 1 AGNs in the XMM-COSMOS quasar sample using nine criteria from the
literature (six defined in the rest-frame and three defined in the observed
frame): , ,
, ,
, ,
(observed frame),
(observed frame), and (observed frame). Using any single criterion
defined in the rest-frame, we find a low radio-loud fraction of
in the XMM-COSMOS type 1 AGN sample, except for . Requiring that any
two criteria agree reduces the radio-loud fraction to for about
3/4 of the cases. The low radio-loud fraction cannot be simply explained by the
contribution of the host galaxy luminosity and reddening. The
gives the smallest radio-loud fraction. Two
of the three radio-loud fractions from the criteria defined in the observed
frame without k-correction ( and ) are much larger than
the radio-loud fractions from other criteria.Comment: 12 pages, 7 figures, MNRAS submitte
The bulk of the black hole growth since z ~ 1 occurs in a secular universe: no major merger-AGN connection
What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGNs)
activity? To answer this long-standing question, we analyze 140 XMM-Newton-selected AGN host galaxies and
a matched control sample of 1264 inactive galaxies over z ~ 0.3–1.0 and M_∗ < 10^(11.7) M_⊙ with high-resolution
Hubble Space Telescope/Advanced Camera for Surveys imaging from the COSMOS field. The visual analysis of
their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies
in the AGN and control samples, i.e., quantifying the signature of recent mergers which might potentially be
responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies
do not show strong distortions and (2) there is no significant difference in the distortion fractions between active
and inactive galaxies. Our findings provide the best direct evidence that, since z ~ 1, the bulk of black hole (BH)
accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e.,
internal secular processes and minor interactions, are the leading triggers for the episodes of major BH growth.We
also exclude an alternative interpretation of our results: a substantial time lag between merging and the observability
of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement
of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to (1) recent major
mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGNs, (2) the lack of
a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low
levels of soft X-ray obscuration for AGNs hosted by interacting galaxies, in contrast to model predictions
Spectropolarimetric Evidence for Radiatively Inefficient Accretion in an Optically Dull Active Galaxy
We present Subaru/FOCAS spectropolarimetry of two active galaxies in the
Cosmic Evolution Survey. These objects were selected to be optically dull, with
the bright X-ray emission of an AGN but missing optical emission lines in our
previous spectroscopy. Our new observations show that one target has very weak
emission lines consistent with an optically dull AGN, while the other object
has strong emission lines typical of a host-diluted Type 2 Seyfert galaxy. In
neither source do we observe polarized emission lines, with 3-sigma upper
limits of P_BLR < 2%. This means that the missing broad emission lines (and
weaker narrow emission lines) are not due to simple anisotropic obscuration,
e.g., by the canonical AGN torus. The weak-lined optically dull AGN exhibits a
blue polarized continuum with P = 0.78 +/- 0.07% at 4400 A < lambda_rest < 7200
A (P = 1.37 +/- 0.16% at 4400 A < lambda_rest < 5050 A). The wavelength
dependence of this polarized flux is similar to that of an unobscured AGN
continuum and represents the intrinsic AGN emission, either as synchrotron
emission or the outer part of an accretion disk reflected by a clumpy dust
scatterer. Because this intrinsic AGN emission lacks emission lines, this
source is likely to have a radiatively inefficient accretion flow.Comment: Accepted to ApJ. 6 pages, 2 figure
Atomistic spin textures on-demand in the van der Waals layered magnet CrSBr
Controlling magnetism in low dimensional materials is essential for designing
devices that have feature sizes comparable to several critical length scales
that exploit functional spin textures, allowing the realization of low-power
spintronic and magneto-electric hardware. [1] Unlike conventional
covalently-bonded bulk materials, van der Waals (vdW)-bonded layered magnets
[2-4] offer exceptional degrees of freedom for engineering spin textures. [5]
However, their structural instability has hindered microscopic studies and
manipulations. Here, we demonstrate nanoscale structural control in the layered
magnet CrSBr creating novel spin textures down to the atomic scale. We show
that it is possible to drive a local structural phase transformation using an
electron beam that locally exchanges the bondings in different directions,
effectively creating regions that have vertical vdW layers embedded within the
horizontally vdW bonded exfoliated flakes. We calculate that the newly formed
2D structure is ferromagnetically ordered in-plane with an energy gap in the
visible spectrum, and weak antiferromagnetism between the planes. Our study
lays the groundwork for designing and studying novel spin textures and related
quantum magnetic phases down to single-atom sensitivity, potentially to create
on-demand spin Hamiltonians probing fundamental concepts in physics, [6-10] and
for realizing high-performance spintronic, magneto-electric and topological
devices with nanometer feature sizes. [11,12]Comment: Main manuscript: 11 pages, 4 figures ; Extended data: 22 pages, 19
figure
Accretion Rate and the Physical Nature of Unobscured Active Galaxies
We show how accretion rate governs the physical properties of a sample of
unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs).
We avoid the systematic errors plaguing previous studies of AGN accretion rate
by using accurate accretion luminosities (L_int) from well-sampled
multiwavelength SEDs from the Cosmic Evolution Survey (COSMOS), and accurate
black hole masses derived from virial scaling relations (for broad-line AGNs)
or host-AGN relations (for narrow-line and lineless AGNs). In general, broad
emission lines are present only at the highest accretion rates (L_int/L_Edd >
0.01), and these rapidly accreting AGNs are observed as broad-line AGNs or
possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower
specific accretion rates (L_int/L_Edd < 0.01) are unobscured and yet lack a
broad line region. The disappearance of the broad emission lines is caused by
an expanding radiatively inefficient accretion flow (RIAF) at the inner radius
of the accretion disk. The presence of the RIAF also drives L_int/L_Edd < 10^-2
narrow-line and lineless AGNs to 10 times higher ratios of radio to optical/UV
emission than L_int/L_Edd > 0.01 broad-line AGNs, since the unbound nature of
the RIAF means it is easier to form a radio outflow. The IR torus signature
also tends to become weaker or disappear from L_int/L_Edd < 0.01 AGNs, although
there may be additional mid-IR synchrotron emission associated with the RIAF.
Together these results suggest that specific accretion rate is an important
physical "axis" of AGN unification, described by a simple model.Comment: Accepted for publication in the Astrophysical Journal. 15 pages, 9
figure
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