2,025 research outputs found
Effects of large-scale environment on the assembly history of central galaxies
We examine whether large-scale environment affects the mass assembly history
of their central galaxies. To facilitate this, we constructed dark matter halo
merger trees from a cosmological N-body simulation and calculated the formation
and evolution of galaxies using a semi-analytic method. We confirm earlier
results that smaller halos show a notable difference in formation time with a
mild dependence on large-scale environment. However, using a semi-analytic
model, we found that on average the growth rate of the stellar mass of central
galaxies is largely insensitive to large-scale environment. Although our
results show that the star formation rate (SFR) and the stellar mass of central
galaxies in smaller halos are slightly affected by the assembly bias of halos,
those galaxies are faint, and the difference in the SFR is minute, and
therefore it is challenging to detect it in real galaxies given the current
observational accuracy. Future galaxy surveys, such as the BigBOSS experiment
and the Large Synoptic Survey Telescope, which are expected to provide
observational data for fainter objects, will provide a chance to test our model
predictions.Comment: 7 pages, 5 figure
Independent vector analysis based on overlapped cliques of variable width for frequency-domain blind signal separation
A novel method is proposed to improve the performance of independent vector analysis (IVA) for blind signal separation of acoustic mixtures. IVA is a frequency-domain approach that successfully resolves the well-known permutation problem by applying a spherical dependency model to all pairs of frequency bins. The dependency model of IVA is equivalent to a single clique in an undirected graph; a clique in graph theory is defined as a subset of vertices in which any pair of vertices is connected by an undirected edge. Therefore, IVA imposes the same amount of statistical dependency on every pair of frequency bins, which may not match the characteristics of real-world signals. The proposed method allows variable amounts of statistical dependencies according to the correlation coefficients observed in real acoustic signals and, hence, enables more accurate modeling of statistical dependencies. A number of cliques constitutes the new dependency graph so that neighboring frequency bins are assigned to the same clique, while distant bins are assigned to different cliques. The permutation ambiguity is resolved by overlapped frequency bins between neighboring cliques. For speech signals, we observed especially strong correlations across neighboring frequency bins and a decrease in these correlations with an increase in the distance between bins. The clique sizes are either fixed, or determined by the reciprocal of the mel-frequency scale to impose a wider dependency on low-frequency components. Experimental results showed improved performances over conventional IVA. The signal-to-interference ratio improved from 15.5 to 18.8 dB on average for seven different source locations. When we varied the clique sizes according to the observed correlations, the stability of the proposed method increased with a large number of cliques.open4
Merger relics of cluster galaxies
Context. Sheen and collaborators recently found that a surprisingly large
portion (38%) of massive early-type galaxies in heavy clusters show strong
merger-related disturbed features. This contradicts the general understanding
that massive clusters are hostile environments for galaxy mergers. Considering
the significance of mergers in galaxy evolution, it is important to understand
this. Aims. We aim to present a theoretical foundation that explains galaxy
mergers in massive clusters. Methods. We used the N-body simulation technique
to perform a cosmological-volume simulation and derive dark-halo merger trees.
Then, we used the semi-analytic modeling technique to populate each halo with
galaxies. We ran hydrodynamic simulations of galaxy mergers to estimate the
lifetime of merger features for the imaging condition used by Sheen and
collaborators. We applied this merger feature lifetime to our semi-analytic
models. Finally, we counted the massive early-type galaxies in heavy model
clusters that would show strong merger features. Results. While there still are
substantial uncertainties, our preliminary results are remarkably close to the
observed fraction of galaxies with merger features. Key ingredients for the
success are twofold: firstly, the subhalo motion in dark haloes has been
accurately traced, and, second, the lifetime of merger features has been
properly estimated. As a result, merger features are expected to last very long
in cluster environments. Many massive early-type galaxies in heavy clusters
therefore show merger features not because they experience mergers in the
current clusters in situ, but because they still carry their merger features
from their previous halo environments. Conclusions. Investigating the merger
relics of cluster galaxies is potentially important, because it uniquely allows
us to backtrack the halo merger history.Comment: 4 pages, 3 figures, accepted for publication in A&A Research Not
Sussing merger trees: the Merger Trees Comparison Project
Merger trees follow the growth and merger of dark-matter haloes over cosmic history. As well as giving important insights into the growth of cosmic structure in their own right, they provide an essential backbone to semi-analytic models of galaxy formation. This paper is the first in a series to arise from the Sussing Merger Trees Workshop in which 10 different tree-building algorithms were applied to the same set of halo catalogues and their results compared. Although many of these codes were similar in nature, all algorithms produced distinct results. Our main conclusions are that a useful merger-tree code should possess the following features: (i) the use of particle IDs to match haloes between snapshots; (ii) the ability to skip at least one, and preferably more, snapshots in order to recover subhaloes that are temporarily lost during merging; (iii) the ability to cope with (and ideally smooth out) large, temporary fluctuations in halo mass. Finally, to enable different groups to communicate effectively, we defined a common terminology that we used when discussing merger trees and we encourage others to adopt the same language. We also specified a minimal output format to record the results
Statistics of Two-point Correlation and Network Topology for Lyman Alpha Emitters at
We investigate the spatial distribution of Lyman alpha emitting galaxies
(LAEs) at , selected from the NOAO Deep Wide-Field Survey
(NDWFS), using two-point statistics and topological diagnostics adopted from
network science. We measure the clustering length, Mpc,
and the bias, . Fitting the clustering with halo
occupation distribution (HOD) models results in two disparate possibilities:
(1) where the fraction of central galaxies is 1% in halos of mass
; and (2) where the fraction is 20%. We refer to
these two scenarios as the `Dusty Core Scenario' for Model#1 since most of
central galaxies in massive halos are dead in Ly emission, and the
`Pristine Core Scenario' for Model#2 since the central galaxies are bright in
Ly emission. Traditional two-point statistics cannot distinguish
between these disparate models given the current data sets. To overcome this
degeneracy, we generate mock catalogs for each HOD model using a high
resolution -body simulation and adopt a network statistics approach, which
provides excellent topological diagnostics for galaxy point distributions. We
find three topological anomalies from the spatial distribution of observed
LAEs, which are not reproduced by the HOD mocks. We find that Model#2 matches
better all network statistics than Model#1, suggesting that the central
galaxies in halos at need to be
less dusty to be bright as LAEs, potentially implying some replenishing
channels of pristine gas such as the cold mode accretion.Comment: 23 pages, 18 figures, accepted by MNRA
Glaucomatous Patterns in Frequency Doubling Technology (FDT) Perimetry Data Identified by Unsupervised Machine Learning Classifiers
Purpose: The variational Bayesian independent component analysis-mixture model (VIM), an unsupervised machine-learning classifier, was used to automatically separate Matrix Frequency Doubling Technology (FDT) perimetry data into clusters of healthy and glaucomatous eyes, and to identify axes representing statistically independent patterns of defect in the glaucoma clusters.
Methods: FDT measurements were obtained from 1,190 eyes with normal FDT results and 786 eyes with abnormal FDT results from the UCSD-based Diagnostic Innovations in Glaucoma Study (DIGS) and African Descent and Glaucoma Evaluation Study (ADAGES). For all eyes, VIM input was 52 threshold test points from the 24-2 test pattern, plus age.
Results: FDT mean deviation was -1.00 dB (S.D. = 2.80 dB) and -5.57 dB (S.D. = 5.09 dB) in FDT-normal eyes and FDT-abnormal eyes, respectively (p<0.001). VIM identified meaningful clusters of FDT data and positioned a set of statistically independent axes through the mean of each cluster. The optimal VIM model separated the FDT fields into 3 clusters. Cluster N contained primarily normal fields (1109/1190, specificity 93.1%) and clusters G(1) and G(2) combined, contained primarily abnormal fields (651/786, sensitivity 82.8%). For clusters G(1) and G(2) the optimal number of axes were 2 and 5, respectively. Patterns automatically generated along axes within the glaucoma clusters were similar to those known to be indicative of glaucoma. Fields located farther from the normal mean on each glaucoma axis showed increasing field defect severity.
Conclusions: VIM successfully separated FDT fields from healthy and glaucoma eyes without a priori information about class membership, and identified familiar glaucomatous patterns of loss.open0
Melting Domain Size and Recrystallization Dynamics of Ice Revealed by Time-Resolved X-ray Scattering
The phase transition between water and ice is ubiquitous and one of the most
important phenomena in nature. Here, we performed time-resolved x-ray
scattering experiments capturing the melting and recrystallization dynamics of
ice. The ultrafast heating of ice I is induced by an IR laser pulse and probed
with an intense x-ray pulse, which provided us with direct structural
information on different length scales. From the wide-angle x-ray scattering
(WAXS) patterns, the molten fraction, as well as the corresponding temperature
at each delay, were determined. The small-angle x-ray scattering (SAXS)
patterns, together with the information extracted from the WAXS analysis,
provided the time-dependent change of the size and the number of the liquid
domains. The results show partial melting (~13 %) and superheating of ice
occurring at around 20 ns. After 100 ns, the average size of the liquid domains
grows from about 2.5 nm to 4.5 nm by the coalescence of approximately six
adjacent domains. Subsequently, we capture the recrystallization of the liquid
domains, which occurs on microsecond timescales due to the cooling by heat
dissipation and results to a decrease of the average liquid domain size
VEGFR2 but not VEGFR3 governs integrity and remodeling of thyroid angiofollicular unit in normal state and during goitrogenesis
Thyroid gland vasculature has a distinguishable characteristic of endothelial fenestrae, a critical component for proper molecular transport. However, the signaling pathway that critically governs the maintenance of thyroid vascular integrity, including endothelial fenestrae, is poorly understood. Here, we found profound and distinct expression of follicular epithelial VEGF-A and vascular VEGFR2 that were precisely regulated by circulating thyrotropin, while there were no meaningful expression of angiopoietin-Tie2 system in the thyroid gland. Our genetic depletion experiments revealed that VEGFR2, but not VEGFR3, is indispensable for maintenance of thyroid vascular integrity. Notably, blockade of VEGF-A or VEGFR2 not only abrogated vascular remodeling but also inhibited follicular hypertrophy, which led to the reduction of thyroid weights during goitrogenesis. Importantly, VEGFR2 blockade alone was sufficient to cause a reduction of endothelial fenestrae with decreases in thyrotropin-responsive genes in goitrogen-fed thyroids. Collectively, these findings establish follicular VEGF-Avascular VEGFR2 axis as a main regulator for thyrotropindependent thyroid angiofollicular remodeling and goitrogenesis.Peer reviewe
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