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An excursion set model of hierarchical clustering: Ellipsoidal collapse and the moving barrier

Astrophysics · Submitted 7 May 2001 · 26 references

Pith's verdict: The analytic first-crossing formula is the real contribution and it holds up; the 'wide variety of barriers' line is two shapes and an ad hoc truncation, which the authors half-admit.

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arXiv IDastro-ph/0105113 (version 1)
Pith IDY3HNQ6RI, Pith's permanent record of this paper and its review
CitationGiuseppe Tormen (Padova), Ravi K. Sheth (Fermilab). "An excursion set model of hierarchical clustering: Ellipsoidal collapse and the moving barrier." arXiv:astro-ph/0105113 (2001). https://pith.science/paper/Y3HNQ6RI
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The paper

The paper claims

The paper argues that dark matter halo abundances and progenitor counts are better described by random walks crossing a barrier that rises with the smoothing scale than by the classical flat barrier of spherical collapse, and gives a closed

The reading

The excursion set approach turns the counting of collapsed dark matter haloes into a random-walk problem: a walk starts at zero overdensity and a halo forms where the walk first crosses a barrier. For most of the model's history that barrier was taken to be flat, which is what spherical collapse implies. This paper asks what happens when the barrier rises with the variance, the shape ellipsoidal collapse implies, on the reasoning that a low-mass patch must hold itself together against stronger tides than a massive one, and it supplies a formula that works across a whole family of such rising barriers rather than for one hand-tuned shape. If the claim holds, one short expression yields the abundance of haloes, the abundance of their progenitors inside a given parent, and the way halo counts shift between underdense and overdense regions, without running a simulation for each case.

What carries the argument

The load-bearing object is the moving barrier $B(S,z)=sqrt{a},delta_{sc}(z)[1+beta(anu)^{-alpha}]$ with $nu equiv [delta_{sc}(z)/sigma(m)]^2$, together with the ansatz that the first-crossing distribution is the constant-barrier Gaussian form multiplied by a truncated Taylor series $T(S)=sum_{n=0}^{5}(-S)^n/n!,partial^n B/partial S^n$ of the barrier. The same substitution rule, $B(s)to B(s)-B(S)$ and $Sto s-S$, carries the formula over to the two-barrier problem that defines the conditional mass function, which is what makes progenitor counts and local-density mass functions accessible without a fresh numerical fit for every parent mass. Two auxiliary devices do real work: a sampling trick that turns a Gaussian variate $x$ into a mass via $anu=|x|^{3.6}/(1+|x|^{1.6})$, accurate to about a percent, and a six-dimensional random walk built from the deformation tensor, which is used to check that replacing the scatter in the shear field by its mean is a fair reduction.

Extended reading notes

Core claim

The central claim is that the first-crossing distribution of a Brownian walk absorbed on a moving barrier $B(S)$ is well approximated by $f(S)=|T(S)|e^{-B(S)^2/2S}/(Ssqrt{2pi S})$, where $T(S)$ is the sum of the first five terms of the Taylor expansion of $B(S)$ in $S$; the expression is exact for constant and linear barriers, where the series terminates, and remains accurate for the ellipsoidal barrier $B=sqrt{a},delta_{sc}(z)[1+beta(anu)^{-alpha}]$ with $beta approx 0.485$, $alpha approx 0.615$ and $a approx 0.7$. Feeding it the ellipsoidal barrier reproduces both the unconditional mass function and the conditional (progenitor) mass function measured in the GIF $N$-body simulations better than the constant-barrier spherical-collapse prediction, and the same machinery, with local density mapped to an effective redshift, gives the mass function inside dense and underdense cells. The authors are explicit that the improvement holds at large lookback times: at small lookback times, and for progenitors more than half the parent mass, the moving-barrier conditional mass function has the wrong shape, and they trace that to the excursion set neglect of correlations between smoothing scales rather than to the barrier shape itself.

Load-bearing premise

The premise everything rests on is that the collapse condition can be written as a barrier depending only on the smoothing scale, with the scatter a real patch feels from its surrounding shear field averaged away, and that the barrier's overall amplitude $a approx 0.7$ is legitimately fixed against the same simulations the resulting formula is then tested on.

Original abstract

The excursion set approach allows one to estimate the abundance and spatial distribution of virialized dark matter haloes efficiently and accurately. The predictions of this approach depend on how the nonlinear processes of collapse and virialization are modelled. We present simple analytic approximations which allow us to compare the excursion set predictions associated with spherical and ellipsoidal collapse. In particular, we present formulae for the universal unconditional mass function of bound objects and the conditional mass function which describes the mass function of the progenitors of haloes in a given mass range today. We show that the ellipsoidal collapse based moving barrier model provides a better description of what we measure in the numerical simulations than the spherical collapse based constant barrier model, although the agreement between model and simulations is better at large lookback times. Our results for the conditional mass function can be used to compute accurate approximations to the local-density mass function which quantifies the tendency for massive haloes to populate denser regions than less massive haloes. This happens because low density regions can be thought of as being collapsed haloes viewed at large lookback times, whereas high density regions are collapsed haloes viewed at small lookback times. Although we have only applied our analytic formulae to two simple barrier shapes, we show that they are, in fact, accurate for a wide variety of moving barriers. We suggest how they can be used to study the case in which the initial dark matter distribution is not completely cold.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 7,006 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.94 · human participants 0.01 · controlled comparison 0.52 · randomized assignment 0.01 · states quantitative result 0.09 · compares with prior results 0.92 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

The excursion set approach allows one to estimate the abundance and spatial distribution of virialized dark matter haloes efficiently and accurately. The predictions of this approach depend on how the nonlinear processes of collapse and virialization are modelled. We present simple analytic approximations which allow us to compare the excursion set predictions associated with spherical and ellipsoidal collapse. In particular, we present formulae for the universal unconditional mass function of bound objects and the conditional mass function which describes the mass function of the progenitors of haloes in a given mass range today. We show that the ellipsoidal collapse based moving barrier model provides a better description of what we measure in the numerical simulations than the spherical collapse based constant barrier model, although the agreement between model and simulations is better at large lookback times. Our results for the conditional mass function can be used to compute accurate approximations to the local-density mass function which quantifies the tendency for massive haloes to populate denser regions than less massive haloes. This happens because low density regions can be thought of as being collapsed haloes viewed at large lookback times, whereas high density regions are collapsed haloes viewed at small lookback times. Although we have only applied our analytic formulae to two simple barrier shapes, we show that they are, in fact, accurate for a wide variety of moving barriers. We suggest how they can be used to study the case in which the initial dark matter distribution is not completely cold.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 The excursion set approach allows one to estimate the abundance and spatial distribution of virialized dark matter haloes efficiently and accurately. background 1.00 0.10 0.04 asserted 0.98
s1 The predictions of this approach depend on how the nonlinear processes of collapse and virialization are modelled. background 0.99 0.06 0.02 asserted 1.00
s2 We present simple analytic approximations which allow us to compare the excursion set predictions associated with spherical and ellipsoidal collapse. contribution 0.88 0.95 0.06 asserted 0.97
s3 In particular, we present formulae for the universal unconditional mass function of bound objects and the conditional mass function which describes the mass function of the progenitors of haloes in a given mass range today. contribution 0.98 0.93 0.06 asserted 0.99
s4 We show that the ellipsoidal collapse based moving barrier model provides a better description of what we measure in the numerical simulations than the spherical collapse based constant barrier model, although the agreement between model and simulations is better at large lookback times. result 1.00 0.94 0.78 asserted 1.00
s5 Our results for the conditional mass function can be used to compute accurate approximations to the local-density mass function which quantifies the tendency for massive haloes to populate denser regions than less massive haloes. result 0.38 0.85 0.07 asserted 0.54
s6 This happens because low density regions can be thought of as being collapsed haloes viewed at large lookback times, whereas high density regions are collapsed haloes viewed at small lookback times. interpretation 1.00 0.29 0.04 asserted 0.92
s7 Although we have only applied our analytic formulae to two simple barrier shapes, we show that they are, in fact, accurate for a wide variety of moving barriers. result 0.82 0.94 0.10 asserted 1.00
s8 We suggest how they can be used to study the case in which the initial dark matter distribution is not completely cold. future work 1.00 0.49 0.04 hedged possibility 0.97

What kind of paper the classifier reports this is: theoretical result 0.98 · evidence from proof or derivation 0.55.

What was examined, and what was not

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  • figures and tables
  • references and citing sentences
  • relations to other papers
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  • P abstract
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  • S sections
  • T body sentences
  • C citations
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Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. First crossing distributions and the universal uncondi￾tional halo mass function. Histogram shows the distribution ob￾tained by simulating random walks which are absorbed on the ellipsoidal collapse moving barrier; solid curve shows our analytic approximation to this distribution (equation 4). Dashed curve shows the distribution which fits the halo mass function in cos￾mological simulations (equation 2), and dotted curve shows the distribution associated with spherical collapse. and linear (Sheth 1998 and Appendix A of this paper) bar￾riers. These solutions suggest the following…
Figure 2. Examples of random walks used to construct the con￾ditional mass functions associated with the ellipsoidal collapse moving barriers at z = 0 (dotted curve) and z = 2 (solid curve). Bond et al. (1991) and Lacey & Cole (1993) argued that conditional mass functions could be estimated by consider￾ing the successive crossings of boundaries associated with different redshifts. The first crossing of two constant barri￾ers of different heights has an analytic solution, so they were able to provide analytic estimates for the conditional mass function associated with the spherical collapse…
Figure 3. Excursion set conditional mass functions at z, for parent haloes identified at the present time. The two sets of curves in each panel are for parent haloes in the mass range 1 ≤ M/M∗ ≤ 2 (upper curves) and 16 ≤ M/M∗ ≤ 32 (lower curves); the upper curves have been shifted upwards by a factor of ten for clarity. Symbols with error bars show the distributions measured in the ΛCDM simulations, histograms show the result of generating the first crossing distribution by simulating an ensemble of 104 random walks, smooth solid curves show the analytic approximation discussed in the text,…
Figure 4. Conditional mass functions in the SCDM simulations (symbols with error bars). The two sets of curves in each panel show parent haloes with masses in the range 1 ≤ M/M∗ ≤ 2 (upper curves) and 16 ≤ M/M∗ ≤ 32 (lower curves); the upper curves have been shifted upwards by a factor of ten for clarity. Dotted curves show the spherical collapse prediction (Lacey & Cole 1993), dashed curves show the distribution one gets by rescaling the unconditional mass function (equation 2), and solid curves show our analytic approximation to the random walk with moving-barrier simulations (equation 7).…
Figure 5. Mass functions as a function of local density in the SCDM simulations (symbols with error bars), plotted as a func￾tion of relative mass, so as to resemble the conditional mass functions presented earlier. Dotted curves show the spherical col￾lapse prediction, dashed curves show the distribution one gets by rescaling the unconditional mass function (equation 2), and solid curves show our analytic approximation to the random walk with moving-barrier simulations (equation 7). The curves have been offset upwards by a factor of ten and a hundred, in the case of the middle and topmost…
Figure 9. Rescaled conditional mass functions in the SCDM model. Panels show the different redshift bins we studied earlier. Symbols in each panel show the different mass ranges we considered. Dotted line shows the constant barrier prediction (in these variables it is the same as the unconditional mass function), dashed line shows the result of rescaling the actual unconditional mass function, and solid curves show the result of rescaling the moving barrier predictions. the rescaled conditional mass function constructed using a friends-of-friends algorithm look very similar to the spheri￾cal…
Figure 10. Same as the previous figure, but for ΛCDM. the mass which is got by summing up the masses of all the progenitor subclumps (which may depend on redshift). The figures above are for the case in which a subclump is any clump which donates at least one particle to the final ob￾ject, and the parent mass is defined as the mass at the final time (so it is independent of redshift). Although the actual conditional mass functions depend slightly on which com￾bination of the above choices we make, the generic results shown above are independent of this choice. Before moving on, we think it…
Figure 11. Distribution of overdensities and scales at which the six-dimensional random walks crossed the ellipsoidal collapse bar￾rier. Solid curve shows the approximation used by Sheth, Mo & Tormen (2001) in their, considerably simpler, one-dimensional random walks. patch may differ from patch to patch. Appendix B provides a simple prescription for choosing a set of patches which have the correct ensemble averaged properties—in essence, this requires studying the first crossing distribution of six￾dimensional random walks. Because a six-dimensional walk is computationally ex￾pensive, rather…
Figure 13. Comparison of the various mass functions described in this paper, with the one which fits the Hubble volume simu￾lations presented in Jenkins et al (2001). Upper curves show the residuals between our analytic formulae and the Jenkins et al. fitting formula
Figure 14. The unconditional mass function associated with a model of collapse which was presented in Monaco (1997a,b). Whereas the height of the barrier studied in our paper increases with distance, the one proposed by Monaco decreases with dis￾tance. Despite the dramatic differences between the two barriers, inserting this shape into our equation (4) provides a good fit (solid curve) to the exact result (histogram). their shape is not so different from ours, whereas Monaco’s really is quite different. The fact that this barrier decreases with S means that all walks are guaranteed to cross…

Showing 10 of the paper's 14 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:astro-ph/0105113 by the authors.

References
How later papers used it

233 later papers cite it

Evidence: substantial. 225 of the citing papers share no author with it, and 112 of those carry a confident label from Pith's citation classifier.

Pith read 39 of the 354 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Uses its method or tool 25 of 89 papers with this label
How the literature uses it

109 arXiv papers cite this in 172 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 92 sentences without a confident label

Uses its method or tool 45 papers
  • Confronting JWST Massive Galaxies. — By assuming that each dark-matter halo at high $z$ contains a single central galaxy, the expected number of galaxies with stellar mass larger than some observational threshold $M_⋆^obs$ can be obtained as [ref] : [equation] where Vol $=Ω(r_max^3 - r_min^3)/3$ is the comoving survey volume, determined by the surveyed redshift range (from $r(z_min)$ to $r(z_max)$) and the survey angular coverage $Ω$, and $dn/dM_h$ is the halo mass function[We will use the Sheth-Tormen [ref] mass function as in Ref. [ref] , but with a fit calibrated to high-redshift N-body simulations from Ref. [ref] .].

    Insights from HST into Ultra-Massive Galaxies and Early-Universe Cosmology · Introduction · 0.92

  • To this purpose we use the Sheth & Tormen mass function [ref] dndM = -√(2ãπ)A ł[1+ł(ã δ_c^2D^2σ_M^2)̊^-p]̊ ρ_mM^2δ_cDσ_M × dlnσ_Mdln M expł(-ã δ_c^22D^2σ_M^2)̊ , where $ã=0.707$ [We changed the commonly adopted notation to avoid confusion with the scale factor $a$.], $p=0.3$, $A=0.2162$, $δ_c$ is the linear critical density contrast derived in Section [ref], $D=δ^L_m/δ^L_m(a=1)$ is the linear growth factor, and $σ_M$ is the variance of the linear matter power spectrum defined as [ref] σ_M^2 = 12π^2∫^∞_0 dk k^2W^2(kR) P^L(k) , where the window function is defined as W(kR) = 3sin(kR)-kRcos(kR)(kR)^3 , being $R$ the comoving radius enclosing the mass $M=4π3ρ_mR^3$.

    Growth of non-linear structures and spherical collapse in the Galileon Ghost Condensate model · Mass function · 0.98

  • Since our simulations do not resolve all the low mass halos that are expected to host HI, we have used the coarse density field in each pixel to statistically sample these halos using a halo mass function conditioned on the overdensity of the pixel following [ref] .

    Simulating the Large-Scale Structure of HI Intensity Maps · Conclusions · 1.00

Uses its data or values 2 papers
  • The bias terms are given as (for the Sheth-Tormen and Tinker mass functions respectively) [ref] [equation] where $a_2=-17/21$ and we list other constants below [equation] We re-emphasize that all these constants are fit to Newtonian, $Λ$CDM simulations.

    Modelling the matter bispectrum at small scales in modified gravity · Details of the halo model spectra · 0.80

  • The corresponding halo masses and number densities ($M_h, dn/dlnM_h$) are approximately ($7 ×10^10 , 7 ×10^-6 Mpc^-3$) at $z = 12$, ($2 ×10^11 , 4 ×10^-6 Mpc^-3$) at $z = 10$, and ($2 ×10^12 , 9 ×10^-7 Mpc^-3$) at $z = 7$ [ref] .

    Grain-size evolution and rapid dust growth in high-redshift galaxies · Model parameters and fiducial setup · 0.85

Compares with it 9 papers
  • Our main arguments for the validity of our approach are the good agreement of the resulting unconditional halo mass function (see Figure [ref]) and large scale bias (see Figures [ref] and [ref]) with the analytical results from [ref] .

    Simulating the Large-Scale Structure of HI Intensity Maps · HI intensity maps · 0.97

  • In Fig. [ref] we compare our HMFs calculated from $N$-body simulations with HMFs from the Sheth-Tormen (ST) semi-analytic formulae [ref] .[See [ref] for other semi-analytic fits to the mass function, which we found to work less well for models with massive neutrinos.] The ST fit is based on the fact that, as first pointed out by Press and Schechter [ref] , the HMF can be written as [equation] with $ν≡[δ_sc(z)/σ(M)]^2$, where $δ_sc(z)=1.686$ is the overdensity required for spherical collapse at $z$, and $ρ̅= Ω_m ρ_c$. $dn(M,z)$ is the number density of halos in the mass interval $M$ to $M + dM$.

    Neutrinos in Non-linear Structure Formation - The Effect on Halo Properties · The halo mass function · 0.99

  • Note that this expression matches the first two terms of [ref] which are equivalent to [ref] for $D_B=0$.

    Spherical collapse and halo mass function in f(R) theories · Halo mass function prediction for uncorrelated random walk and generic barrier · 1.00

Cites it as support 1 paper
Cites it as background 14 papers
  • The excursion set formalism predicts the abundance of objects crossing a critical density threshold as a consequence of hierarchical structure formation in an initially linear Gaussian field [ref] .

    VERSUS: An excursion-set-inspired void-finder for the Stage-IV era · Voids in the matter field · 0.95

  • A large amount of research has been devoted to identifying mass functions that accurately describe the simulations and data of galactic halos, see, e.g., [ref] .

    Toward More Realistic Mass Functions For Ultradense Dark Matter Halos · Introduction · 0.98

  • Over the years, several layers of complexity have been added to the initial model, with the inclusion of the peak constraint [ref] , halos preferentially form at peaks in the initial field, the excursion set constraint to solve the cloud in cloud problem [ref] , and more realistic collapse models, halos are likely to form in a ellipsoidal fashion [ref] .

    Weighing neutrinos with the halo environment · Analytical considerations on environmental bias · 0.84

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