Ixandra Achitouv

Ludwig-Maximilians-University of Munich, München, Bavaria, Germany

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Publications (9)36.75 Total impact

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    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.
    Physical Review Letters 12/2013; 111(23):231303. · 7.73 Impact Factor
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    I. Achitouv, C. Wagner, J. Weller, Y. Rasera
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    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). · 5.88 Impact Factor
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    Ixandra Achitouv, Mark Neyrinck, Aseem Paranjape
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    ABSTRACT: We compute analytical predictions for the volume function of voids based on the excursion set approach and the peaks formalism for random walks smoothed with a top-hat filter in real space and a large class of realistic barrier models. We test our prediction by comparing with voids identified in the dark matter density field in N-body simulations using the ZOBOV void finder. This tests 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 sub-voids, a fact that has not been appreciated previously. For reasonable choices of sub-void exclusion, we find that the Lagrangian density deltav of the ZOBOV voids -- which is predicted to be a constant deltav=-2.7 in the spherical evolution model -- is quite different from the predicted value, showing substantial scatter and scale dependence. 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.
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    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.
    Physical review D: Particles and fields 06/2013; 88(8).
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    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
    The Astronomer's Telegram. 12/2012;
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    Ixandra E. Achitouv, Pier Stefano Corasaniti
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    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. · 4.69 Impact Factor
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    Ixandra E Achitouv, Pier Stefano Corasaniti
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    ABSTRACT: The mass distribution of dark matter halos is a sensitive probe of primordial non-Gaussianity (NG). We derive an analytical formula of the halo mass function by perturbatively computing excursion set path-integrals for a non-Gaussian density field with non-vanishing skewness, fNL. We assume a stochastic barrier model which captures the main features of the ellipsoidal collapse of halos. Contrary to previous results based on extensions of the Press-Schechter formalism to NG initial conditions, we find that the non-spherical collapse of halos directly alter the signature of primordial NG. This points toward a potential degeneracy between the effect of primordial non-Gaussianity and that of non-linear halo collapse. The inferred mass function is found to be in remarkable agreement with N-body simulations of NG local type. Deviations are well within numerical uncertainties for all values of fNLloc in the range of validity of the perturbative calculation (|fnlloc|200). Moreover, the comparison with simulation results suggests that for |fNL|30 the non-linear collapse of halos, as described by our barrier model, strongly deviates from that of Gaussian initial conditions. This is not surprising since the effect of non-linear gravitational processes may be altered by initially large NG. Hence, in the lack of prior theoretical knowledge, halo collapse model parameters should be included in statistical halo mass function data analysis which aim to constrain the signature of primordial NG.
    Journal of Cosmology and Astroparticle Physics 02/2012; 002(02). · 6.04 Impact Factor
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    Pier Stefano Corasaniti, Ixandra Achitouv
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    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; 84:023009. · 4.69 Impact Factor
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    P S Corasaniti, I Achitouv
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    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.
    Physical Review Letters 06/2011; 106(24):241302. · 7.73 Impact Factor

Publication Stats

56 Citations
36.75 Total Impact Points


  • 2013
    • Ludwig-Maximilians-University of Munich
      München, Bavaria, Germany
  • 2011–2012
    • Paris Diderot University
      • Laboratoire Univers et Théories (LUTH) UMR 8102
      Lutetia Parisorum, Île-de-France, France
    • French National Centre for Scientific Research
      • Laboratoire de l'univers et de ses théories (LUTH)
      Paris, Ile-de-France, France