25,810 research outputs found
An Optimal Strategy for Accurate Bulge-to-disk Decomposition of Disk Galaxies
The development of two-dimensional (2D) bulge-to-disk decomposition
techniques has shown their advantages over traditional one-dimensional (1D)
techniques, especially for galaxies with non-axisymmetric features. However,
the full potential of 2D techniques has yet to be fully exploited. Secondary
morphological features in nearby disk galaxies, such as bars, lenses, rings,
disk breaks, and spiral arms, are seldom accounted for in 2D image
decompositions, even though some image-fitting codes, such as GALFIT, are
capable of handling them. We present detailed, 2D multi-model and
multi-component decomposition of high-quality -band images of a
representative sample of nearby disk galaxies selected from the Carnegie-Irvine
Galaxy Survey, using the latest version of GALFIT. The sample consists of five
barred and five unbarred galaxies, spanning Hubble types from S0 to Sc.
Traditional 1D decomposition is also presented for comparison. In detailed case
studies of the 10 galaxies, we successfully model the secondary morphological
features. Through a comparison of best-fit parameters obtained from different
input surface brightness models, we identify morphological features that
significantly impact bulge measurements. We show that nuclear and inner
lenses/rings and disk breaks must be properly taken into account to obtain
accurate bulge parameters, whereas outer lenses/rings and spiral arms have a
negligible effect. We provide an optimal strategy to measure bulge parameters
of typical disk galaxies, as well as prescriptions to estimate realistic
uncertainties of them, which will benefit subsequent decomposition of a larger
galaxy sample.Comment: 30 pages, 14 figures, published in ApJ; minor typos correcte
Chandra Survey of Nearby Galaxies: A Significant Population of Candidate Central Black Holes in Late-type Galaxies
Based on the Chandra data archive as of March 2016, we have identified 314
candidate active galactic nuclei in 719 galaxies located closer than 50 Mpc,
among them late-type (Hubble types Sc and later) galaxies that previously had
been classified from optical observations as containing star-forming (H II)
nuclei. These late-type galaxies comprise a valuable subsample to search for
low-mass (<~ 10^6 solar masses) central black holes. For the sample as a whole,
the overall dependence of the fraction of active nuclei on galaxy type and
nuclear spectral classification is consistent with previous results based on
optical surveys. We detect 51 X-ray cores among the 163 H II nuclei and
estimate that, very conservatively, ~74% of them with luminosities above 10^38
erg/s are not contaminated by X-ray binaries; the fraction increases to ~92%
for X-ray cores with a luminosity of 10^39 erg/s or higher. This allows us to
estimate a black hole occupation fraction of >~ 21% in these late-type, many
bulgeless, galaxies.Comment: ApJ to appea
Chandra Survey of Nearby Galaxies: The Catalog
We searched in the public archive of the Chandra X-ray Observatory as of
March 2016 and assembled a sample of 719 galaxies within 50 Mpc with ACIS
observations available. By cross-correlation with the optical or near-infrared
nuclei of these galaxies, 314 of them are identified to have an X-ray active
galactic nucleus (AGN). The majority of them are low-luminosity AGNs and are
unlikely X-ray binaries based upon their spatial distribution and luminosity
functions. The AGN fraction is around 60% for elliptical galaxies and
early-type spirals, but drops to roughly 20% for Sc and later types, consistent
with previous findings in the optical. However, the X-ray survey is more
powerful in finding weak AGNs, especially from regions with active star
formation that may mask the optical AGN signature. For example, 31% of the H II
nuclei are found to harbor an X-ray AGN. For most objects, a single power-law
model subject to interstellar absorption is adequate to fit the spectrum, and
the typical photon index is found to be around 1.8. For galaxies with a
non-detection, their stacked Chandra image shows an X-ray excess with a
luminosity of a few times 10^37 erg/s on average around the nuclear region,
possibly composed of faint X-ray binaries. This paper reports on the technique
and results of the survey; in-depth analysis and discussion of the results will
be reported in forthcoming papers.Comment: Accepted for publication in the Astrophysical Journa
Predicting species' tolerance to salinity and alkalinity using distribution data and geochemical modelling: a case study using Australian grasses
BACKGROUND AND AIMS: Salt tolerance has evolved many times independently in different plant groups. One possible explanation for this pattern is that it builds upon a general suite of stress-tolerance traits. If this is the case, then we might expect a correlation between salt tolerance and other tolerances to different environmental stresses. This association has been hypothesized for salt and alkalinity tolerance. However, a major limitation in investigating large-scale patterns of these tolerances is that lists of known tolerant species are incomplete. This study explores whether species' salt and alkalinity tolerance can be predicted using geochemical modelling for Australian grasses. The correlation between taxa found in conditions of high predicted salinity and alkalinity is then assessed. METHODS: Extensive occurrence data for Australian grasses is used together with geochemical modelling to predict values of pH and electrical conductivity to which species are exposed in their natural distributions. Using parametric and phylogeny-corrected tests, the geochemical predictions are evaluated using a list of known halophytes as a control, and it is determined whether taxa that occur in conditions of high predicted salinity are also found in conditions of high predicted alkalinity. KEY RESULTS: It is shown that genera containing known halophytes have higher predicted salinity conditions than those not containing known halophytes. Additionally, taxa occurring in high predicted salinity tend to also occur in high predicted alkalinity. CONCLUSIONS: Geochemical modelling using species' occurrence data is a potentially useful approach to predict species' relative natural tolerance to challenging environmental conditions. The findings also demonstrate a correlation between salinity tolerance and alkalinity tolerance. Further investigations can consider the phylogenetic distribution of specific traits involved in these ecophysiological strategies, ideally by incorporating more complete, finer-scale geochemical information, as well as laboratory experiments.This work was supported by the Australian Research Council
Steady-State Two Atom Entanglement in a Pumped Cavity
In this paper we explore the possibility of a steady-state entanglement of
two two-level atoms inside a pumped cavity by taking into account cavity
leakage and the spontaneous emission of photons by the atoms. We describe the
system in the dressed state picture in which the coherence is built into the
dressed states while transitions between the dressed states are incoherent. Our
model assumes the vacuum Rabi splitting of the dressed states to be much larger
than any of the decay parameters of the system which allows atom-field
coherence to build up before any decay process takes over. We show that, under
our model, a pumping field cannot entangle two closed two-level atoms inside
the cavity in the steady-state, but a steady-state entanglement can be achieved
with two open two-level atoms.Comment: 19 pages, 5 figure
Next-to-leading order QCD corrections to a heavy resonance production and decay into top quark pair at the LHC
We present a complete next-to-leading order (NLO) QCD calculation to a heavy
resonance production and decay into a top quark pair at the LHC, where the
resonance could be either a Randall-Sundrum (RS) Kaluza-Klein (KK) graviton
or an extra gauge boson . The complete NLO QCD corrections can enhance the
total cross sections by about and for the and
the , respectively, depending on the resonance mass. We also explore in
detail the NLO corrections to the polar angle distributions of the top quark,
and our results show that the shapes of the NLO distributions can be different
from the leading order (LO) ones for the KK graviton. Moreover, we study the
NLO corrections to the spin correlations of the top quark pair production via
the above process, and find that the corrections are small.Comment: Published version in PR
Pole expansion of self-energy and interaction effect on topological insulators
We study effect of interactions on time-reversal-invariant topological
insulators. Their topological indices are expressed by interacting Green's
functions. Under the local self-energy approximation, we connect topological
index and surface states of an interacting system to an auxiliary
noninteracting system, whose Hamiltonian is related to the pole-expansions of
the local self-energy. This finding greatly simplifies the calculation of
interacting topological indices and gives an noninteracting pictorial
description of interaction driven topological phase transitions. Our results
also bridge studies of the correlated topological insulating materials with the
practical dynamical-mean-field-theory calculations.Comment: 4.2 pages, 3 figures, reference added, typos correcte
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