6,532 research outputs found
Ground-State Properties of a Heisenberg Spin Glass Model with a Hybrid Genetic Algorithm
We developed a genetic algorithm (GA) in the Heisenberg model that combines a
triadic crossover and a parameter-free genetic algorithm. Using the algorithm,
we examined the ground-state stiffness of the Heisenberg model in three
dimensions up to a moderate size range. Results showed the stiffness constant
of in the periodic-antiperiodic boundary condition method and that
of in the open-boundary-twist method. We considered the
origin of the difference in between the two methods and suggested that
both results show the same thing: the ground state of the open system is stable
against a weak perturbation.Comment: 11 pages, 5 figure
The Influence of Bars on Nuclear Activity
We test ideas on fueling of galactic nuclei by bar-driven inflow by comparing
the detection rate and intensity of nuclear H II regions and AGNs among barred
and unbarred galaxies in a sample of over 300 spirals selected from our recent
optical spectroscopic survey of nearby galaxies. Among late-type spirals
(Sc-Sm), but not early-type (S0/a-Sbc), we observe in the barred group a very
marginal increase in the detection rate of H II nuclei and a corresponding
decrease in the incidence of AGNs. The minor differences in the detection
rates, however, are statistically insignificant, most likely stemming from
selection effects and not from a genuine influence from the bar. The presence
of a bar seems to have no noticeable impact on the likelihood of a galaxy to
host either nuclear star formation or an AGN. The nuclei of early-type barred
spirals do exhibit measurably higher star-formation rates than their unbarred
counterparts, as indicated by either the luminosity or the equivalent width of
H-alpha emission. By contrast, late-type spirals do not show such an effect.
Bars have a negligible effect on the strength of the AGNs in our sample,
regardless of the Hubble type of the host galaxy. This result confirms similar
conclusions reached by other studies based on much smaller samples.Comment: To appear in the Astrophysical Journal. LaTex, 31 pages including 6
postscript figures and 3 tables. AAStex macros include
Star formation and figure rotation in the early-type galaxy NGC2974
We present Galaxy Evolution Explorer (GALEX) far (FUV) and near (NUV)
ultraviolet imaging of the nearby early-type galaxy NGC2974, along with
complementary ground-based optical imaging. In the ultraviolet, the galaxy
reveals a central spheroid-like component and a newly discovered complete outer
ring of radius 6.2kpc, with suggestions of another partial ring at an even
larger radius. Blue FUV-NUV and UV-optical colours are observed in the centre
of the galaxy and from the outer ring outward, suggesting young stellar
populations (< 1Gyr) and recent star formation in both locations. This is
supported by a simple stellar population model which assumes two bursts of star
formation, allowing us to constrain the age, mass fraction and surface mass
density of the young component pixel by pixel. Overall, the mass fraction of
the young component appears to be just under 1per cent (lower limit,
uncorrected for dust extinction). The additional presence of a nuclear and an
inner ring (radii 1.4 and 2.9kpc, respectively), as traced by [OIII] emission,
suggests ring formation through resonances. All three rings are consistent with
a single pattern speed of km/s/kpc, typical of S0 galaxies and only
marginally slower than expected for a fast bar if traced by a small observed
surface brightness plateau. This thus suggests that star formation and
morphological evolution in NGC2974 at the present epoch are primarily driven by
a rotating asymmetry (probably a large-scale bar), despite the standard
classification of NGC2974 as an E4 elliptical.Comment: 13 pages, 10 figures, Changed content, Accepted for publication in
MNRA
Application of the Cluster Variation Method to Spin Ice Systems on the Pyrochlore Lattice
The cactus approximation in the cluster variation method is applied to the
spin ice system with nearest neighbor ferromagnetic coupling. The temperature
dependences of the entropy and the specific heat show qualitatively good
agreement with those observed by Monte Carlo simulations and experiments, and
the Pauling value is reproduced for the residual entropy. The analytic
expression of the q-dependent magnetic susceptibility is obtained, from which
the absence of magnetic phase transition is confirmed. The neutron scattering
pattern is also evaluated and found to be consistent with that obtained from
Monte Carlo simulations.Comment: 8 pages, 7 figure
A Unified Scaling Law in Spiral Galaxies
We investigate the origin of a unified scaling relation in spiral galaxies.
Observed spiral galaxies are spread on a plane in the
three-dimensionallogarithmic space of luminosity L, radius R and rotation
velocity V. The plane is expressed as in I-passband,
where is a constant. On the plane, observed galaxies are distributed
in an elongated region which looks like the shape of a surfboard. The
well-known scaling relations, L-V (Tully-Fisher relation), V-R (also the
Tully-Fisher relation) and R-L (Freeman's law), can be understood as oblique
projections of the surfboard-like plane into 2-D spaces. This unified
interpretation of the known scaling relations should be a clue to understand
the physical origin of all the relations consistently. Furthermore, this
interpretation can also explain why previous studies could not find any
correlation between TF residuals and radius.
In order to clarify the origin of this plane, we simulate formation and
evolution of spiral galaxies with the N-body/SPH method, including cooling,
star formation and stellar feedback. Initial conditions are set to isolated 14
spheres with two free parameters, such as mass and angular momentum. The CDM
(h=0.5, ) cosmology is considered as a test case. The simulations
provide the following two conclusions: (a) The slope of the plane is well
reproduced but the zero-point is not. This zero-point discrepancy could be
solved in a low density ($\Omega_00.5) cosmology.
(b) The surfboard-shaped plane can be explained by the control of galactic mass
and angular momentum.Comment: Accepted for publication in ApJ Letters. 6 pages including 2 figure
A Search for Kinematic Evidence of Radial Gas Flows in Spiral Galaxies
CO and HI velocity fields of seven nearby spiral galaxies, derived from
radio-interferometric observations, are decomposed into Fourier components
whose radial variation is used to search for evidence of radial gas flows.
Additional information provided by optical or near-infrared isophotes is also
considered, including the relationship between the morphological and kinematic
position angles. To assist in interpreting the data, we present detailed
modeling that demonstrates the effects of bar streaming, inflow, and a warp on
the observed Fourier components. We find in all of the galaxies evidence for
either elliptical streaming or a warped disk over some range in radius, with
deviations from pure circular rotation at the level of ~20-60 km/s. Evidence
for kinematic warps is observed in several cases well inside R_{25}. No
unambiguous evidence for radial inflows is seen in any of the seven galaxies,
and we are able to place an upper limit of ~5-10 km/s (3-5% of the circular
speed) on the magnitude of any radial inflow in the inner regions of NGC 4414,
5033 and 5055. We conclude that the inherent non-axisymmetry of spiral galaxies
is the greatest limitation to the direct detection of radial inflows.Comment: 22 emulateapj pages with bitmapped colour figures, to appear in ApJ
(April 2004). For full resolution figures go to
http://www.atnf.csiro.au/people/twong/preprints
Field-Induced Two-Step Phase Transitions in the Singlet Ground State Triangular Antiferromagnet CsFeBr
The ground state of the stacked triangular antiferromagnet CsFeBr is a
spin singlet due to the large single ion anisotropy . The
field-induced magnetic ordering in this compound was investigated by the
magnetic susceptibility, the magnetization process and specific heat
measurements for an external field parallel to the -axis. Unexpectedly, two
phase transitions were observed in the magnetic field higher than 3 T. The
phase diagram for temperature versus magnetic field was obtained. The mechanism
leading to the successive phase transitions is discussed.Comment: 8 pages, 9 figures, 10 eps files, jpsj styl
Dual black holes in merger remnants. I: linking accretion to dynamics
We study the orbital evolution and accretion history of massive black hole
(MBH) pairs in rotationally supported circumnuclear discs up to the point where
MBHs form binary systems. Our simulations have high resolution in mass and
space which, for the first time, makes it feasible to follow the orbital decay
of a MBH either counter- or co-rotating with respect to the circumnuclear disc.
We show that a moving MBH on an initially counter-rotating orbit experiences an
"orbital angular momentum flip" due to the gas-dynamical friction, i.e., it
starts to corotate with the disc before a MBH binary forms. We stress that this
effect can only be captured in very high resolution simulations. Given the
extremely large number of gas particles used, the dynamical range is
sufficiently large to resolve the Bondi-Hoyle-Lyttleton radii of individual
MBHs. As a consequence, we are able to link the accretion processes to the
orbital evolution of the MBH pairs. We predict that the accretion rate is
significantly suppressed and extremely variable when the MBH is moving on a
retrograde orbit. It is only after the orbital angular momentum flip has taken
place that the secondary rapidly "lights up" at which point both MBHs can
accrete near the Eddington rate for a few Myr. The separation of the double
nucleus is expected to be around ~10 pc at this stage. We show that the
accretion rate can be highly variable also when the MBH is co-rotating with the
disc (albeit to a lesser extent) provided that its orbit is eccentric. Our
results have significant consequences for the expected number of observable
double AGNs at separations of <100 pc.Comment: 8 pages, 4 figures, revised version accepted to MNRA
Unbiased large spectroscopic surveys of galaxies selected by SPICA using dust bands
The mid-infrared (IR) range contains many spectral features associated with
large molecules and dust grains such as polycyclic aromatic hydrocarbons (PAHs)
and silicates. These are usually very strong compared to fine-structure gas
lines, and thus valuable in studying the spectral properties of faint distant
galaxies. In this paper, we evaluate the capability of low-resolution mid-IR
spectroscopic surveys of galaxies that could be performed by SPICA. The surveys
are designed to address the question how star formation and black hole
accretion activities evolved over cosmic time through spectral diagnostics of
the physical conditions of the interstellar/circumnuclear media in galaxies. On
the basis of results obtained with Herschel far-IR photometric surveys of
distant galaxies and Spitzer and AKARI near- to mid-IR spectroscopic
observations of nearby galaxies, we estimate the numbers of the galaxies at
redshift z > 0.5, which are expected to be detected in the PAH features or dust
continuum by a wide (10 deg^2) or deep (1 deg^2) blind survey, both for a given
observation time of 600 hours. As by-products of the wide blind survey, we also
expect to detect debris disks, through the mid-IR excess above the photospheric
emission of nearby main-sequence stars, and we estimate their number. We
demonstrate that the SPICA mid-IR surveys will efficiently provide us with
unprecedentedly large spectral samples, which can be studied further in the
far-IR with SPICA.Comment: Paper accepted for publication on PASA on 18th October 2017, as part
of the SPICA Special Issu
Groundwater depletion embedded in international food trade
Recent hydrological modelling1 and Earth observations2,3 have located and quantified alarming rates of groundwater depletion worldwide. This depletion is primarily due to water withdrawals for irrigation1,2,4, but its connection with the main driver of irrigation, global food consumption, has not yet been explored. Here we show that approximately eleven per cent of non-renewable groundwater use for irrigation is embedded in international food trade, of which two-thirds are exported by Pakistan, the USA and India alone. Our quantification of groundwater depletion embedded in the worldâs food trade is based on a combination of global, cropspecific estimates of non-renewable groundwater abstraction and international food trade data. A vast majority of the worldâs population lives in countries sourcing nearly all their staple crop imports from partners who deplete groundwater to produce these crops, highlighting risks for global food and water security. Some countries, such as the USA, Mexico, Iran and China, are particularly exposed to these risks because they both produce and import food irrigated from rapidly depleting aquifers. Our results could help to improve the sustainability of global food production and groundwater resource management by identifying priority regions and agricultural products at risk as well as the end consumers of these products
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