arXiv subjects

Simon May

Publications and source records attributed to Simon May.

At least 19 recordsLinked to original sources

arXiv2026

A Finely-Binned Measurement of the Connected Even-Parity Galaxy 4-Point Correlation Function of DESI Year 1 Luminous Red Galaxies

We present the connected even-parity galaxy 4-Point Correlation Function (4PCF) of DESI Year 1 (Y1) Luminous Red Galaxies (LRGs). This is one of the first measurements of the connected 4PCF on data, and shows a clear detection at $\sim$12 to 17$\sigma$ in the full-sky analysis. We test the robustness of the signal by varying three factors: hemisphere (north vs. south), redshift range (either the full range $0.4 < z < 1.1$ or the higher-number-density interval $0.4 < z < 0.8$), and sample completeness (the full sample vs. only the most complete regions within the number density-motivated cut). Finally, we cross-correlate different patches that are spatially well-separated, and thus should have largely independent noise. This approach, at leading order, is able to remove mismatch between the covariance matrix of the data and that of the mocks; however, in certain cases at the cost of sensitivity. We find $\sim$15$\sigma$ evidence for an even-parity 4PCF in this approach. Overall, the studies of the 4PCF presented here probe sensitively any mismatch between mock catalogs and data, and also open up the prospect of fitting a model, constraining cosmological parameters and galaxy biases, and searching for Baryon Acoustic Oscillation (BAO) features.

astro-ph.CO↗

arXiv2026

Halo mass functions in mixed cold and fuzzy dark matter models

We investigate the impact of mixed cold and fuzzy dark matter (MDM) cosmologies on the halo mass function (HMF) using numerical simulations performed with the AxiREPO framework. We consider models in which an ultralight axion-like component with mass $m =10^{-24.5} \mathrm{eV}$ constitutes a fraction $f \leq 0.3$ of the total dark matter. To enable consistent halo identification in mixed-species scenarios, we develop a grid-based halo-finding pipeline that combines the particle-based cold dark matter (CDM) and wave-like fuzzy dark matter (FDM) components into a unified density field. We find that FDM traces the large-scale CDM distribution while suppressing small-scale structure through wave interference effects, leading to a reduction in the abundance of low-mass haloes and modifying the HMF in a manner dependent on redshift and FDM fraction. Increasing the FDM fraction produces a systematic downward shift in the HMF and modifies its high-mass slope. Motivated by these trends, we introduce a phenomenological model that maps CDM HMFs to their MDM counterparts using a suppression function with parameters dependent on redshift and FDM fraction. This model reproduces the simulated HMFs within approximately 0.1 to 0.2 dex across the parameter space explored ($1 \leq z \leq 4$, $f \leq 0.3$). Our results provide a computationally efficient method for predicting structure formation in MDM cosmologies without requiring dedicated simulations for each parameter choice, and establish a framework for exploring the impact of MDM on cosmological structure formation.

astro-ph.CO↗

arXiv2026

The Impact of Fuzzy Dark Matter Dynamics on the Accumulation and Fragmentation of Primordial Gas

Fuzzy Dark Matter (FDM), particularly in the $10^{-22}$ eV mass regime is frequently used to characterize wave-like interference effects. It exhibits macroscopic wave properties, which drive distinct baryonic dynamics within collapsed haloes. Using the hydrodynamical code AREPO with the AxiREPO module and primordial chemistry, we simulate the assembly of haloes with masses $3 \times 10^{8} \le M_{\mathrm{h}} \le 8 \times 10^{9} \: M_\odot$ across a range of axion masses $1 \times 10^{-22} \le m_{\mathrm{a}} \le 7 \times 10^{-22}$ eV. We investigate how small-scale dynamics of the FDM density field affect the accumulation of cold, dense gas essential for primordial star formation. We demonstrate that gas collapse is suppressed by a two-fold mechanism: a delay driven by the geometry of the FDM solitonic core and a secondary dynamical barrier caused by stochastic wave fluctuations. While the flattened solitonic potential profile itself inhibits central gas accumulation, these wave-driven dynamics provide a further layer of disruption and angular momentum support, which in certain regimes prevents gas from reaching the central, compact, high-density configurations characteristic of CDM. Consequently, sites of star formation are shifted away from a single central peak toward a population of lower-mass clusters. Our work provides a physical framework for calibrating halo mass-dependent star formation efficiencies in FDM cosmologies, where internal processes may delay Cosmic Dawn beyond the effects of the initial power spectrum cut-off. These results are essential for interpreting realistic observational constraints from future 21-cm signal observations and the faint-end luminosity functions observed by the JWST, as well as providing an upper bound on the baryonic effects in the context of Mixed Dark Matter scenarios.

astro-ph.GA↗

arXiv2025

Updated bounds on ultra-light dark matter from the tiniest galaxies

The particle mass of dark matter (DM) was previously constrained using kinematics of ultra-faint dwarf galaxies to $m > 3 \times 10^{-19}\,\mathrm{eV}$. This constraint, which excludes the "fuzzy" range of ultra-light dark matter from comprising all of the DM, relies on an estimate of the heating rate from fuzzy dark matter (FDM) wave interference using linear perturbation theory. Here, we compare the results of this perturbative calculation to full Schr\"odinger-Poisson simulations of the evolution of star particles in FDM halos. This comparison confirms theoretical expectations that FDM heating is stronger in fully nonlinear simulations due to the formation of a dense central soliton whose fluctuations enhance gravitational perturbations, and that bounds on the DM particle mass using this perturbative method are indeed conservative. We also show that these bounds are not affected by possible tidal stripping, since for dwarf satellites like Segue 1, the tidal radius is much larger than the observed size of the galaxy. We further show that the constraints on the mass cannot be evaded by invoking DM self-interactions, due to constraints on the self-interaction from large-scale structure. Lastly, we show that if the recently discovered system Ursa Major III/UNIONS I is a galaxy, the observed properties of this object strengthen the lower bound on the DM mass by over an order of magnitude, to $m > 8 \times 10^{-18}\,\mathrm{eV}$, at 95% confidence. This constraint could further be strengthened considerably by more precise measurements of the size and velocity dispersion of this and other similar galaxies, and by using full Schr\"odinger-Poisson simulations.

astro-ph.CO↗

arXiv2025

Measurement of Parity-Violating Modes of the Dark Energy Spectroscopic Instrument (DESI) Year 1 Luminous Red Galaxies' 4-Point Correlation Function

Here we report the first measurement of the parity-violating (PV) 4-Point Correlation Function (4PCF) of the Dark Energy Spectroscopic Instrument's Year 1 Luminous Red Galaxy (DESI Y1 LRG) sample, motivated by the potential detection of the PV 4PCF in the Sloan Digital Sky Survey Baryon Oscillation Spectroscopic Survey (SDSS BOSS) galaxies. In our auto-correlation ("auto") analysis, we find a statistically significant excess of the PV signal compared to mocks without any PV, at 4-10$\sigma$ depending on details of the analysis. This could arise either from genuine PV or from an underestimation of the variance in the mocks; it is unlikely to arise, at the signal level, from a systematic. We then cross-correlate ("cross") the putative PV signal between different, independent patches of sky, and there find no detection of parity violation. The two measurements are in significant tension: while the cross has somewhat larger error bars than the auto, this is not sufficient to explain the discrepancy. We thus present the current work as an intriguing addition to the PV work on BOSS and as motivation for exploring further the relationship between the auto and cross PV 4PCF analyses.

astro-ph.CO↗

arXiv2025

Early Growth of Structure in Warm Wave Dark Matter

We explore the growth of structure in wave-like dark matter models, where the field and density spectra are peaked at sub-horizon wavenumbers. Starting with the Schr\"odinger-Poisson system, we derive the scale-dependent evolution of the matter power spectrum during radiation and matter domination. We find a suppression of adiabatic perturbations during radiation domination, controlled by a free-streaming length, and scale-dependent growth of the initially white-noise isocurvature power, controlled by a Jeans scale during matter domination. The results are in qualitative, and in some regimes quantitative, agreement with the quasi-particle picture. We verify the analytic results of the power spectrum with 3+1-dimensional cosmological Schr\"odinger-Poisson simulations. We propose an analytic formula for the halo mass function, which is in rough agreement with the simulation results at early times after matter-radiation equality. Our simulations show that early halos typically host a soliton.

astro-ph.CO↗

arXiv2025

Core Collapse Beyond the Fluid Approximation: The Late Evolution of Self-Interacting Dark Matter Halos

We show that the gravothermal collapse of self-interacting dark matter (SIDM) halos can deviate from local thermodynamic equilibrium. As a consequence, the self-similar evolution predicted by the commonly adopted conducting fluid model can be altered or broken. Our results are obtained using a novel, efficient kinetic solver called KiSS-SIDM for tracing the gravothermal evolution based on the Direct Simulation Monte Carlo (DSMC) framework. In the long mean free path stage, the code is a viable alternative to the fluid model, yet requires no calibration parameters. Further, this method enables a fully kinetic treatment well into the late, short mean free path, stage of the collapse. We apply the method to a canonical case with isotropic, velocity independent scattering. We find that although a fluid treatment is appropriate deep in the short mean free path core, departures from local thermodynamic equilibrium develop in the intermediate mean free path region bounding the core, which modify the late-time evolution. KiSS-SIDM is publicly available at https://gitlab.com/Socob/KiSS-SIDM.

astro-ph.CO↗

arXiv2025

Analytic Model for Covariance Matrices of the 2-, 3-, and 4-Point Correlation Functions in the Gaussian Random Field Approximation

Analyses of the galaxy N-Point Correlation Functions (NPCFs) have a large number of degrees of freedom, meaning one cannot directly estimate an invertible covariance matrix purely from mock catalogs, as has been the standard approach for the 2PCF and power spectrum. Instead, templates are used based on assuming a Gaussian Random Field density with the true, Boltzmann-solver-computed power spectrum. The resulting covariance matrices are sparse but have notable internal structure. To understand this structure better, we seek a fully analytic, closed-form covariance matrix template, using a power law power spectrum $P(k) \propto 1/k$ and including shot noise. We obtain a simple closed-form solution for the covariance of the 2PCF, as well as closed-form solutions for the fundamental building blocks (termed ``$f$-integrals'') of the covariance matrices for the 3PCF, 4PCF, and beyond. We achieve single-digit percent level accuracy for the $f$-integrals, confirming that our power spectrum model is a suitable alternative to the true power spectrum. In the $f$-integrals, we find that the greatest contributions arise when closed triangles may be formed. When $f$-integrals are multiplied together, as needed for the covariance, the number of non-vanishing configurations reduces. We use these results to present a clearer picture of the covariance matrices' structure and sparsity, which correspond to triangular and non-triangular regions. This will be useful in guiding future NPCF analyses with spectroscopic galaxy surveys such as DESI, Euclid, Roman, and SPHEREx.

astro-ph.CO↗

arXiv2024

Improved Halo Model Calibrations for Mixed Dark Matter Models of Ultralight Axions

We study the implications of relaxing the requirement for ultralight axions to account for all dark matter in the Universe by examining mixed dark matter (MDM) cosmologies with axion fractions $f \leq 0.3$ within the fuzzy dark matter (FDM) window $10^{-25}$ eV $\lesssim m \lesssim 10^{-23}$ eV. Our simulations, using a new MDM gravity solver implemented in AxiREPO, capture wave dynamics across various scales with high accuracy down to redshifts $z\approx 1$. We identify halos with Rockstar using the CDM component and find good agreement of inferred halo mass functions (HMFs) and concentration-mass relations with theoretical models across redshifts $z=1-10$. This justifies our halo finder approach a posteriori as well as the assumptions underlying the MDM halo model AxionHMcode. Using the inferred axion halo mass-cold halo mass relation $M_{\text{a}}(M_{\text{c}})$ and calibrating a generalised smoothing parameter $\alpha$ to our MDM simulations, we present a new version of AxionHMcode. The code exhibits excellent agreement with simulations on scales $k< 20 \ h$ cMpc$^{-1}$ at redshifts $z=1-3.5$ for $f\leq 0.1$ around the fiducial axion mass $m = 10^{-24.5}$ eV $ = 3.16\times 10^{-25}$ eV, with maximum deviations remaining below 10%. For axion fractions $f\leq 0.3$, the model maintains accuracy with deviations under 20% at redshifts $z\approx 1$ and scales $k< 10 \ h$ cMpc$^{-1}$, though deviations can reach up to 30% for higher redshifts when $f=0.3$. Reducing the run-time for a single evaluation of AxionHMcode to below $1$ minute, these results highlight the potential of AxionHMcode to provide a robust framework for parameter sampling across MDM cosmologies in Bayesian constraint and forecast analyses.

astro-ph.CO↗

arXiv2024

No evidence for parity violation in BOSS

Recent studies have found evidence for parity violation in the BOSS spectroscopic galaxy survey, with statistical significance as high as $7\sigma$. These analyses assess the significance of the parity-odd four-point correlation function (4PCF) with a statistic called $\chi^2$. This statistic is biased if the parity-even eight-point correlation function (8PCF) of the data differs from the mock catalogs. We construct new statistics $\chi^2_\times$, $\chi^2_{\mathrm{null}}$ that separate the parity violation signal from the 8PCF bias term, allowing them to be jointly constrained. Applying these statistics to BOSS, we find that the parity violation signal ranges from $0$ to $2.5\sigma$ depending on analysis choices, whereas the 8PCF bias term is $\sim 6\sigma$. We conclude that there is no compelling evidence for parity violation in BOSS. Our new statistics can be used to search for parity violation in future surveys, such as DESI, without 8PCF biases.

astro-ph.CO↗

arXiv2023

Nested solitons in two-field fuzzy dark matter

Dark matter as scalar particles consisting of multiple species is well motivated in string theory where axion fields are ubiquitous. A two-field fuzzy dark matter (FDM) model features two species of ultralight axion particles with different masses, $m_1 \neq m_2$, which is extended from the standard one-field model with $m_a \sim 10^{-22}\,{\rm eV}$. Here we perform numerical simulations to explore the properties of two-field FDM haloes. We find that the central soliton has a nested structure when $m_2 \gg m_1$, which is distinguishable from the generic flat-core soliton in one-field haloes. However, the formation of this nested soliton is subject to many factors, including the density fraction and mass ratio of the two fields. Finally, we study non-linear structure formation in two-field cosmological simulations with self-consistent initial conditions and find that the small-scale structure in two-field cosmology is also distinct from the one-field model in terms of DM halo counts and soliton formation time.

astro-ph.CO↗

arXiv2023

A lensed radio jet at milli-arcsecond resolution II: Constraints on fuzzy dark matter from an extended gravitational arc

Using a single gravitational lens system observed at $\lesssim5$ milli-arcsecond resolution with very long baseline interferometry (VLBI), we place a lower bound on the mass of the fuzzy dark matter (FDM) particle, ruling out $m_\chi \leq 4.4\times10^{-21}~\mathrm{eV}$ with a 20:1 posterior odds ratio relative to a smooth lens model. We generalize our result to non-scalar and multiple-field models, such as vector FDM, with $m_{\chi,\mathrm{vec}} > 1.4 \times 10^{-21}~\mathrm{eV}$. Due to the extended source structure and high angular resolution of the observation, our analysis is directly sensitive to the presence of granule structures in the main dark matter halo of the lens, which is the most generic prediction of FDM theories. A model based on well-understood physics of ultra-light dark matter fields in a gravitational potential well makes our result robust to a wide range of assumed dark matter fractions and velocity dispersions in the lens galaxy. Our result is competitive with other lower bounds on $m_\chi$ from past analyses, which rely on intermediate modelling of structure formation and/or baryonic effects. Higher resolution observations taken at 10 to 100 GHz could improve our constraints by up to 2 orders of magnitude in the future.

astro-ph.CO↗

arXiv2022

The halo mass function and filaments in full cosmological simulations with fuzzy dark matter

Fuzzy dark matter (FDM) is a dark matter candidate consisting of ultra-light scalar particles with masses around $10^{-22} \mathrm{eV}/c^2$, a regime where cold bosonic matter behaves as a collective wave rather than individual particles. It has increasingly attracted attention due to its rich phenomenology on astrophysical scales, with implications for the small-scale tensions present within the standard cosmological model, $\Lambda$CDM. Although constraints on FDM are accumulating in many different contexts, very few have been verified by self-consistent numerical simulations. We present new large numerical simulations of cosmic structure formation with FDM, solving the full Schr\"odinger-Poisson (SP) equations using the AxiREPO code, which implements a pseudo-spectral numerical method. Combined with our previous simulations, they allow us to draw a four-way comparison of matter clustering, contrasting results (such as power spectra) for each combination of initial conditions (FDM vs. CDM) and dynamics (SP vs. $N$-body). By disentangling the impact of initial conditions and non-linear dynamics, we can gauge the validity of approximate methods used in previous works, such as ordinary $N$-body simulations with an FDM initial power spectrum. Due to the comparatively large volume achieved in our FDM simulations, we are able to measure the FDM halo mass function from full wave simulations for the first time, and compare to previous results obtained using analytic or approximate approaches. We find that, due to the cut-off of small-scale power in the FDM power spectrum, haloes are linked via continuous, smooth, and dense filaments throughout the entire simulation volume (unlike for the standard $\Lambda$CDM power spectrum), posing significant challenges for reliably identifying haloes. We also investigate the density profiles of these filaments and compare to their CDM counterparts.

astro-ph.CO↗

arXiv2022

Matrix Factorizations of the discriminant of $S_n$

Consider the symmetric group $S_n$ acting as a reflection group on the polynomial ring $k[x_1, \ldots, x_n]$, where $k$ is a field such that Char$(k)$ does not divide $n!$. We use Higher Specht polynomials to construct matrix factorizations of the discriminant of this group action: these matrix factorizations are indexed by partitions of $n$ and respect the decomposition of the coinvariant algebra into isotypical components. The maximal Cohen-Macaulay modules associated to these matrix factorizations give rise to a noncommutative resolution of the discriminant and they correspond to the nontrivial irreducible representations of $S_n$. All our constructions are implemented in Macaulay2 and we provide several examples. We also discuss extensions of these results to Young subgroups of $S_n$.

math.AC↗

arXiv2021

The Diversity of Core Halo Structure in the Fuzzy Dark Matter Model

In the fuzzy dark matter (FDM) model, gravitationally collapsed objects always consist of a solitonic core located within a virialised halo. Although various numerical simulations have confirmed that the collapsed structure can be described by a cored NFW like density profile, there is still disagreement about the relation between the core mass and the halo mass. To fully understand this relation, we have assembled a large sample of cored haloes based on both idealised soliton mergers and cosmological simulations with various box sizes. We find that there exists a sizeable dispersion in the core-halo mass relation that increases with halo mass, indicating that the FDM model allows cores and haloes to coexist in diverse configurations. We provide a new empirical equation for a core halo mass relation with uncertainties that can encompass all previously found relations in the dispersion, and emphasise that any observational constraints on the particle mass using a tight one-to-one core-halo mass relation should suffer from an additional uncertainty on the order of 50 % for halo masses $ \ge 10^9 (8 \times 10^{-23} eV/ (mc^2))^{3/2} M_\odot$. We suggest that tidal stripping may be one of the effects contributing to the scatter in the relation.

astro-ph.CO↗

arXiv2021

Non-commutative resolutions for the discriminant of the complex reflection group $G(m,p,2)$

We show that for the family of complex reflection groups $G=G(m,p,2)$ appearing in the Shephard--Todd classification, the endomorphism ring of the reduced hyperplane arrangement $A(G)$ is a non-commutative resolution for the coordinate ring of the discriminant $\Delta$ of $G$. This furthers the work of Buchweitz, Faber and Ingalls who showed that this result holds for any true reflection group. In particular, we construct a matrix factorization for $\Delta$ from $A(G)$ and decompose it using data from the irreducible representations of $G$. For $G(m,p,2)$ we give a full decomposition of this matrix factorization, including for each irreducible representation a corresponding a maximal Cohen--Macaulay module. The decomposition concludes that the endomorphism ring of the reduced hyperplane arrangement $A(G)$ will be a non-commutative resolution.

math.AC↗

arXiv2021

Structure formation in large-volume cosmological simulations of fuzzy dark matter: Impact of the non-linear dynamics

An ultra-light bosonic particle of mass around $10^{-22}\,\mathrm{eV}/c^2$ is of special interest as a dark matter candidate, as it both has particle physics motivations, and may give rise to notable differences in the structures on highly non-linear scales due to the manifestation of quantum-physical wave effects on macroscopic scales, which could address a number of contentious small-scale tensions in the standard cosmological model, $\Lambda$CDM. Using a spectral technique, we here discuss simulations of such fuzzy dark matter (FDM), including the full non-linear wave dynamics, with a comparatively large dynamic range and for larger box sizes than considered previously. While the impact of suppressed small-scale power in the initial conditions associated with FDM has been studied before, the characteristic FDM dynamics are often neglected; in our simulations, we instead show the impact of the full non-linear dynamics on physical observables. We focus on the evolution of the matter power spectrum, give first results for the FDM halo mass function directly based on full FDM simulations, and discuss the computational challenges associated with the FDM equations. FDM shows a pronounced suppression of power on small scales relative to cold dark matter (CDM), which can be understood as a damping effect due to 'quantum pressure'. In certain regimes, however, the FDM power can exceed that of CDM, which may be interpreted as a reflection of order-unity density fluctuations occurring in FDM. In the halo mass function, FDM shows a significant abundance reduction below a characteristic mass scale only. This could in principle alleviate the need to invoke very strong feedback processes in small galaxies to reconcile $\Lambda$CDM with the observed galaxy luminosity function, but detailed studies that also include baryons will be needed to ultimately judge the viability of FDM.

astro-ph.CO↗

arXiv2020

minimal-lagrangians: Generating and studying dark matter model Lagrangians with just the particle content

minimal-lagrangians is a Python program which allows one to specify the field content of an extension of the Standard Model of particle physics and, using this information, to generate the most general renormalizable Lagrangian that describes such a model. As the program was originally created for the study of minimal dark matter models with radiative neutrino masses, it can handle additional scalar or Weyl fermion fields which are $\mathrm{SU}(3)_{\mathrm{C}}$ singlets, $\mathrm{SU}(2)_{\mathrm{L}}$ singlets, doublets or triplets, and can have arbitrary $\mathrm{U}(1)_{\mathrm{Y}}$ hypercharge. It is also possible to enforce an arbitrary number of global $\mathrm{U}(1)$ symmetries (with $\mathbb{Z}_2$ as a special case) so that the new fields can additionally carry such global charges. In addition to human-readable and $\mathrm{\LaTeX}$ output, the program can generate SARAH model files containing the computed Lagrangian, as well as information about the fields after electroweak symmetry breaking (EWSB), such as vacuum expectation values (VEVs) and mixing matrices. This capability allows further detailed investigation of the model in question, with minimal-lagrangians as the first component in a tool chain for rapid phenomenological studies of "minimal" dark matter models requiring little effort and no unnecessary input from the user.

hep-ph↗