[Show abstract][Hide abstract] ABSTRACT: The excursion set approach provides a framework for predicting how the abundance of dark matter halos depends on the initial conditions. A key ingredient of this formalism is the specification of a critical overdensity threshold (barrier) which protohalos must exceed if they are to form virialized halos at a later time. However, to make its predictions, the excursion set approach explicitly averages over all positions in the initial field, rather than the special ones around which halos form, so it is not clear that the barrier has physical motivation or meaning. In this Letter we show that once the statistical assumptions which underlie the excursion set approach are considered a drifting diffusing barrier model does provide a good self-consistent description both of halo abundance as well as of the initial overdensities of the protohalo patches.
[Show abstract][Hide abstract] ABSTRACT: In this article we compare the halo mass function predicted by the excursion
set theory with a drifting diffusive barrier against the results of N-body
simulations for several cosmological models. This includes the standard LCDM
case for a large range of halo masses, models with different types of
primordial non-Gaussianity, and a dark energy model. We show that in all those
cosmological scenarios, the abundance of dark matter halos can be described by
a drifting diffusive barrier, where the two parameters describing the barrier
have physical content. In the case of the Gaussian LCDM, the statistics is
precise enough to actually predict those parameters from the initial
conditions. Furthermore, we found that the stochasticity in the barrier is
non-negligible making the simple deterministic spherical collapse model a bad
approximation even at very high halo masses. We also show that using the
standard excursion set approach with a barrier inspired by peak patches leads
to inconsistent predictions of the halo mass function.
Journal of Cosmology and Astroparticle Physics 12/2013; 2014(10). DOI:10.1088/1475-7516/2014/10/077 · 5.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We compare analytical predictions of void volume functions to those measured from N-body simulations, detecting voids with the zobov void finder. We push to very small, non-linear voids, below few Mpc radius, by considering the unsampled dark matter density
field. We also study the case where voids are identified using haloes. We develop analytical formula for the void abundance
of both the excursion set approach and the peaks formalism. These formulas are valid for random walks smoothed with a top-hat
filter in real space, with a large class of realistic barrier models. We test the extent to which the spherical evolution
approximation, which forms the basis of the analytical predictions, models the highly aspherical voids that occur in the cosmic
web, and are found by a watershed-based algorithm such as zobov. We show that the volume function returned by zobov is quite sensitive to the choice of treatment of subvoids, a fact that has not been appreciated previously. For reasonable
choices of subvoid exclusion, we find that the Lagrangian density δv of the zobov voids – which is predicted to be a constant δv ≈ −2.7 in the spherical evolution model – is different from the predicted value, showing substantial scatter and scale dependence.
This result applies to voids identified at z = 0 with effective radius between 1 and 10 h−1 Mpc. Our analytical approximations are flexible enough to give a good description of the resulting volume function; however,
this happens for choices of parameter values that are different from those suggested by the spherical evolution assumption.
We conclude that analytical models for voids must move away from the spherical approximation in order to be applied successfully
to observations, and we discuss some possible ways forward.
Monthly Notices of the Royal Astronomical Society 09/2013; 451(4). DOI:10.1093/mnras/stv1228 · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We compute the critical density of collapse for spherically symmetric
overdensities in a class of f(R) modified gravity models. For the first time we
evolve the Einstein, scalar field and non-linear fluid equations, making the
minimal simplifying assumptions that the metric potentials and scalar field
remain quasi-static throughout the collapse. Initially evolving a top hat
profile, we find that the density threshold for collapse depends significantly
on the initial conditions imposed, specifically the choice of size and shape.
By imposing `natural' initial conditions, we obtain a fitting function for the
spherical collapse delta_c as a function of collapse redshift, mass of the
overdensity and f_{R0}, the background scalar field value at z=0. By extending
delta_c into drifting and diffusing barrier within the context of excursion set
theory, we obtain a realistic mass function that might be used to confront this
class of scalar-tensor models with observations of dark matter halos. The
proposed analytic formula for the halo mass function was tested against Monte
Carlo random walks for a wide class of moving barriers and can therefore be
applied to other modified gravity theories.
[Show abstract][Hide abstract] ABSTRACT: Distributed as an Instant Email Notice Supernovae
Credential Certification: Masao Sako (masao@sas.upenn.edu)
Subjects: Optical, Supernovae
Referred to by ATel #: 4725, 4741, 4800, 4826
First SN Discoveries from the Dark Energy Survey The Dark Energy Survey (DES) report the discovery of the first set of supernovae (SN) from the project. Images were observed as part of the DES Science Verification phase using the newly-installed 570-Megapixel Dark Energy Camera on the CTIO Blanco 4-m telescope by observers J. Annis, E. Buckley-Geer, and H. Lin. SN observations are planned throughout the observing campaign on a regular cadence of 4-6 days in each of the ten 3-deg2 fields in the DES griz filters. The SN candidates are named according to the season and field in which they were discovered. We adopt the convention -- DES{season}{field}{index} -- where {season} is the year pertaining to the beginning of each observing season, {field} denotes one of the ten SN search fields (E1,E2,S1,S2,X1,X2,X3,C1,C2,C3) in Elais-S1 (E), Stripe 82 (S), XMM-LSS (X) and CDF-S (C), and {index} is one or more lower-case letters starting from a-z, then aa-az, and so on. The DES SN Survey strategy is described in Bernstein et al. (2012, ApJ, 753, 152).
Spectroscopic classifications were performed by the OzDES collaboration from spectra (350-900 nm) obtained at the Anglo-Australian Telescope with AAOmega-2dF observed by C. Lidman, R. Sharp, and S. A. Uddin. Classifications were performed using Superfit (Howell et al 2002, BAAS, 34, 1256) or SNID (Blondin & Tonry, 2007, ApJ, 666, 1024). Redshifts measured from narrow galaxy lines are quoted to 3 significant figures. Those measured from broad SN features are quoted to 2 significant figures. SN phases are based on both the optical spectra and multi-band light curves at the time of the spectroscopic measurements.
Name | RA(J2000) | Dec(J2000) | Discovery date (UT) | Discovery r mag| Spectrum date (UT) | redshift | type | phase
DES12C1a | 03:38:54.5 | -27:32:28.2 | 2012 Dec 07 | 22.0 | 2012 Dec 13 | 0.303 | Ia | near max
DES12C1b | 03:35:05.8 | -26:45:53.9 | 2012 Dec 07 | 20.9 | 2012 Dec 13 | 0.243 | Ia | near max
DES12C2a | 03:41:13.1 | -28:59:37.9 | 2012 Dec 04 | 21.5 | 2012 Dec 14 | 0.21 | Ia | near max
[Show abstract][Hide abstract] ABSTRACT: The high-mass end of the halo mass function is a sensitive probe of
primordial non-Gaussianity (NG). In a recent study [9] we have computed the NG
halo mass function in the context of the Excursion Set theory and shown that
the primordial NG imprint is coupled to that induced by the non-linear collapse
of dark matter halos. We also found an excellent agreement with N-body
simulation results. Here, we perform a more accurate computation which accounts
for the interval validity of the bispectrum expansion to next-to-leading order
and extend the calculation to the case of a non-vanishing primordial
trispectrum.
Physical Review D 10/2012; 86(8):083011. DOI:10.1103/PhysRevD.86.083011 · 4.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The International School for Advanced Studies (SISSA) was founded in 1978 and was the first institution in Italy to promote post-graduate courses leading to a Doctor Philosophiae (or PhD) degree. A centre of excellence among Italian and international universities, the school has around 65 teachers, 100 post docs and 245 PhD students, and is located in Trieste, in a campus of more than 10 hectares with wonderful views over the Gulf of Trieste.
SISSA hosts a very high-ranking, large and multidisciplinary scientific research output. The scientific papers produced by its researchers are published in high impact factor, well-known international journals, and in many cases in the world's most prestigious scientific journals such as Nature and Science. Over 900 students have so far started their careers in the field of mathematics, physics and neuroscience research at SISSA.
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Journal of Cosmology and Astroparticle Physics 02/2012; 002(02). DOI:10.1088/1475-7516/2012/02/002 · 6.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We use the Excursion Set formalism to compute the properties of the halo mass distribution for a stochastic barrier model which encapsulates the main features of the ellipsoidal collapse of dark matter halos. Non-markovian corrections due to the sharp filtering of the linear density field in real space are computed with the path-integral technique introduced by Maggiore & Riotto (2010). Here, we provide a detailed derivation of the results presented in Corasaniti & Achitouv (2011) and extend the mass function analysis to higher redshift. We also derive an analytical expression for the linear halo bias. We find the analytically derived mass function to be in remarkable agreement with N-body simulation data from Tinker et al. (2008) with differences smaller than ~5% over the range of mass probed by the simulations. The excursion set solution from Monte Carlo generated random walks shows the same level of agreement, thus confirming the validity of the path-integral approach for the barrier model considered here. Similarly the analysis of the linear halo bias shows deviations no greater than 20%. Overall these results indicate that the Excursion Set formalism in combination with a realistic modeling of the conditions of halo collapse can provide an accurate description of the halo mass distribution.
Physical Review D 07/2011; 84(2):023009. DOI:10.1103/PhysRevD.84.023009 · 4.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We compute the dark matter halo mass function using the excursion set formalism for a diffusive barrier with linearly drifting average which captures the main features of the ellipsoidal collapse model. We evaluate the non-Markovian corrections due to the sharp filtering of the linear density field in real space with a path-integral method. We find an unprecedented agreement with N-body simulation data with deviations ≲5% over the range of masses probed by the simulations. This indicates that the excursion set in combination with a realistic modeling of the collapse threshold can provide a robust estimation of the halo mass function.