1,615 research outputs found
The halo mass function through the cosmic ages
In this paper we investigate how the halo mass function evolves with
redshift, based on a suite of very large (with N_p = 3072^3 - 6000^3 particles)
cosmological N-body simulations. Our halo catalogue data spans a redshift range
of z = 0-30, allowing us to probe the mass function from the dark ages to the
present. We utilise both the Friends-of-Friends (FOF) and Spherical Overdensity
(SO) halofinding methods to directly compare the mass function derived using
these commonly used halo definitions. The mass function from SO haloes exhibits
a clear evolution with redshift, especially during the recent era of dark
energy dominance (z < 1). We provide a redshift-parameterised fit for the SO
mass function valid for the entire redshift range to within ~20% as well as a
scheme to calculate the mass function for haloes with arbitrary overdensities.
The FOF mass function displays a weaker evolution with redshift. We provide a
`universal' fit for the FOF mass function, fitted to data across the entire
redshift range simultaneously, and observe redshift evolution in our data
versus this fit. The relative evolution of the mass functions derived via the
two methods is compared and we find that the mass functions most closely match
at z=0. The disparity at z=0 between the FOF and SO mass functions resides in
their high mass tails where the collapsed fraction of mass in SO haloes is ~80%
of that in FOF haloes. This difference grows with redshift so that, by z>20,
the SO algorithm finds a ~50-80% lower collapsed fraction in high mass haloes
than does the FOF algorithm, due in part to the significant over-linking
effects known to affect the FOF method.Comment: v4, 16 pages, 16 colour figures. Changed to match MNRAS print
version. NOTE: v1 of this paper has a typo in the fitting function. Please
ensure you use the latest versio
Where Are the Baryons? II: Feedback Effects
Numerical simulations of the intergalactic medium have shown that at the
present epoch a significant fraction (40-50%) of the baryonic component should
be found in the (T~10^6K) Warm-Hot Intergalactic Medium (WHIM) - with several
recent observational lines of evidence indicating the validity of the
prediction. We here recompute the evolution of the WHIM with the following
major improvements: (1) galactic superwind feedback processes from galaxy/star
formation are explicitly included; (2) major metal species (O V to O IX) are
computed explicitly in a non-equilibrium way; (3) mass and spatial dynamic
ranges are larger by a factor of 8 and 2, respectively, than in our previous
simulations. Here are the major findings: (1) galactic superwinds have dramatic
effects, increasing the WHIM mass fraction by about 20%, primarily through
heating up warm gas near galaxies with density 10^{1.5}-10^4 times the mean
density. (2) the fraction of baryons in WHIM is increased modestly from the
earlier work but is ~40-50%. (3) the gas density of the WHIM is broadly peaked
at a density 10-20 times the mean density, ranging from underdense regions to
regions that are overdense by 10^3-10^4. (4) the median metallicity of the WHIM
is 0.18 Zsun for oxygen with 50% and 90% intervals being (0.040,0.38) and
(0.0017,0.83).Comment: 44 pages, 17 figures, high res version at
http://www.astro.princeton.edu/~cen/baryonII.ps.g
Reionization of the Local Group of galaxies
We present the first detailed structure formation and radiative transfer simulations of the reionization history of our cosmic neighbourhood. To this end, we follow the formation of the Local Group of galaxies and nearby clusters by means of constrained simulations, which use the available observational constraints to construct a representation of those structures which reproduces their actual positions and properties at the present time. We find that the reionization history of the Local Group is strongly dependent on the assumed photon production efficiencies of the ionizing sources, which are still poorly constrained. If sources are relatively efficient, i.e. the process is ‘photon-rich', the Local Group is primarily ionized externally by the nearby clusters. Alternatively, if the sources are inefficient, i.e. reionization is ‘photon-poor' the Local Group evolves largely isolated and reionizes itself. The mode of reionization, external versus internal, has important implications for the evolution of our neighbourhood, in terms of e.g. its satellite galaxy populations and primordial stellar populations. This therefore provides an important avenue for understanding the young universe by detailed studies of our nearby structure
Collocation analysis for UMLS knowledge-based word sense disambiguation
BACKGROUND: The effectiveness of knowledge-based word sense disambiguation (WSD) approaches depends in part on the information available in the reference knowledge resource. Off the shelf, these resources are not optimized for WSD and might lack terms to model the context properly. In addition, they might include noisy terms which contribute to false positives in the disambiguation results. METHODS: We analyzed some collocation types which could improve the performance of knowledge-based disambiguation methods. Collocations are obtained by extracting candidate collocations from MEDLINE and then assigning them to one of the senses of an ambiguous word. We performed this assignment either using semantic group profiles or a knowledge-based disambiguation method. In addition to collocations, we used second-order features from a previously implemented approach.Specifically, we measured the effect of these collocations in two knowledge-based WSD methods. The first method, AEC, uses the knowledge from the UMLS to collect examples from MEDLINE which are used to train a Naïve Bayes approach. The second method, MRD, builds a profile for each candidate sense based on the UMLS and compares the profile to the context of the ambiguous word.We have used two WSD test sets which contain disambiguation cases which are mapped to UMLS concepts. The first one, the NLM WSD set, was developed manually by several domain experts and contains words with high frequency occurrence in MEDLINE. The second one, the MSH WSD set, was developed automatically using the MeSH indexing in MEDLINE. It contains a larger set of words and covers a larger number of UMLS semantic types. RESULTS: The results indicate an improvement after the use of collocations, although the approaches have different performance depending on the data set. In the NLM WSD set, the improvement is larger for the MRD disambiguation method using second-order features. Assignment of collocations to a candidate sense based on UMLS semantic group profiles is more effective in the AEC method.In the MSH WSD set, the increment in performance is modest for all the methods. Collocations combined with the MRD disambiguation method have the best performance. The MRD disambiguation method and second-order features provide an insignificant change in performance. The AEC disambiguation method gives a modest improvement in performance. Assignment of collocations to a candidate sense based on knowledge-based methods has better performance. CONCLUSIONS: Collocations improve the performance of knowledge-based disambiguation methods, although results vary depending on the test set and method used. Generally, the AEC method is sensitive to query drift. Using AEC, just a few selected terms provide a large improvement in disambiguation performance. The MRD method handles noisy terms better but requires a larger set of terms to improve performance
The Global Earthquake Model Physical Vulnerability Database
There are almost 50 years of research on fragility and vulnerability assessment, both key
elements in seismic risk or loss estimation. This paper presents the online database of
physical vulnerability models that has been created as part of the Global Earthquake Model
(GEM) initiative. The database comprises fragility and vulnerability curves, damage-to-loss
models, and capacity curves for various types of structures. The attributes that have been
selected to characterize each function, the constraints of setting up a usable database, the
challenges in collecting these models, and the current trends in the development of
vulnerability models are discussed in this study. The current collection of models leverages
upon the outputs of several initiatives, such as GEM’s Global Vulnerability Consortium and
the European Syner-G project. This database is publicly available through the web-based
GEM OpenQuake-platform http://doi.org/10.13117/GEM.DATASET.VULN.WEB-V1.
Reionization of the Milky Way, M31, and their satellites – I. Reionization history and star formation
Observations of the Milky Way (MW), M31, and their vicinity, known as the Local Group (LG), can provide clues about the sources of reionization. We present a suite of radiative transfer simulations based on initial conditions provided by the Constrained Local UniversE Simulations (CLUES) project that are designed to recreate the Local Universe, including a realistic MW–M31 pair and a nearby Virgo. Our box size (91Mpc) is large enough to incorporate the relevant sources of ionizing photons for the LG. We employ a range of source models, mimicking the potential effects of radiative feedback for dark matter haloes between ∼108 and 109M�. Although the LG mostly reionizes in an inside-out fashion, the final 40 per cent of its ionization shows some outside influence. For the LG satellites, we find no evidence that their redshift of reionization is related to the present-day mass of the satellite or the distance from the central galaxy. We find that fewer than 20 per cent of present-day satellites for MW and M31 have undergone any star formation prior to the end of global reionization. Approximately 5 per cent of these satellites could be classified as fossils,meaning the majority of star formation occurred at these early times. The more massive satellites have more cumulative star formation prior to the end of global reionization, but the scatter is significant, especially at the low-mass end. Present-day mass and distance from the central galaxy are poor predictors for the presence of ancient stellar populations in satellite galaxies
The Jubilee ISW Project - II. Observed and simulated imprints of voids and superclusters on the cosmic microwave background
We examine the integrated Sachs–Wolfe (ISW) imprint of voids and superclusters on the cosmic microwave background. We first study results from the Jubilee N-body simulation. From Jubilee, we obtain the full-sky ISW signal from structures out to redshift z = 1.4 and a mock luminous red galaxy catalogue. We confirm that the expected signal in the concordance cold dark matter (CDM) model is very small and likely to always be much smaller than the anisotropies arising at the last scattering surface. Any current detections of such an imprint must, therefore, predominantly arise from something other than an ISW effect in a CDM universe. Using the simulation as a guide, we then look for the signal using a catalogue of voids and superclusters from the Sloan Digital Sky Survey. We find a result that is consistent with the CDM model, i.e. a signal consistent with zero
Reionization of the Local Group of Galaxies
We present the first detailed structure formation and radiative transfer
simulations of the reionization history of our cosmic neighbourhood. To this
end, we follow the formation of the Local Group of galaxies and nearby clusters
by means of constrained simulations, which use the available observational
constraints to construct a representation of those structures which reproduces
their actual positions and properties at the present time. We find that the
reionization history of the Local Group is strongly dependent on the assumed
photon production efficiencies of the ionizing sources, which are still poorly
constrained. If sources are relatively efficient, i.e. the process is
'photon-rich', the Local Group is primarily ionized externally by the nearby
clusters. Alternatively, if the sources are inefficient, i.e. reionization is
'photon-poor' the Local Group evolves largely isolated and reionizes itself.
The mode of reionization, external vs. internal, has important implications for
the evolution of our neighbourhood, in terms of e.g. its satellite galaxy
populations and primordial stellar populations. This therefore provides an
important avenue for understanding the young universe by detailed studies of
our nearby structures.Comment: 10 pages, 8 figures (all in colour), submitted to MNRA
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