arXiv subjects

Wojciech A. Hellwing

Publications and source records attributed to Wojciech A. Hellwing.

At least 37 records · Page 2Linked to original sources

arXiv2023

Deep learning based photometric redshifts for the Kilo-Degree Survey Bright Galaxy Sample

In cosmological analyses, precise redshift determination remains pivotal for understanding cosmic evolution. However, with only a fraction of galaxies having spectroscopic redshifts (spec-$z$s), the challenge lies in estimating redshifts for a larger number. To address this, photometry-based redshift (photo-$z$) estimation, employing machine learning algorithms, is a viable solution. Identifying the limitations of previous methods, this study focuses on implementing deep learning (DL) techniques within the Kilo-Degree Survey (KiDS) Bright Galaxy Sample for more accurate photo-$z$ estimations. Comparing our new DL-based model against prior `shallow' neural networks, we showcase improvements in redshift accuracy. Our model gives mean photo-$z$ bias $\langle Δz\rangle= 10^{-3}$ and scatter $\mathrm{SMAD}(Δz)=0.016$, where $Δz = (z_\mathrm{phot}-z_\mathrm{spec})/(1+z_\mathrm{spec})$. This research highlights the promising role of DL in revolutionizing photo-$z$ estimation.

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arXiv2023

Ghostly galaxies: accretion-dominated stellar systems in low-mass dark matter halos

Wide-area deep imaging surveys have discovered large numbers of extremely low surface brightness dwarf galaxies, which challenge galaxy formation theory and, potentially, offer new constraints on the nature of dark matter. Here we discuss one as-yet unexplored formation mechanism that may account for a fraction of low surface brightness dwarfs. We call this the `ghost galaxy' scenario. In this scenario, inefficient radiative cooling prevents star formation in the `main branch' of the merger tree of a low mass dark matter halo, such that almost all its stellar mass is acquired through mergers with less massive (but nevertheless star-forming) progenitors. Present-day systems formed in this way would be `ghostly' isolated stellar halos with no central galaxy. We use merger trees based on the Extended Press-Schechter formalism and the COCO cosmological N-body simulation to demonstrate that mass assembly histories of this kind can occur for low-mass halos in Lambda-CDM, but they are rare. They are most probable in isolated halos of present-day mass ~4x10^9 M_sun, occurring for ~5 per cent of all halos of that mass under standard assumptions about the timing and effect of cosmic reionization. The stellar masses of star-forming progenitors in these systems are highly uncertain; abundance-matching arguments imply a bimodal present-day mass function having a brighter population (median M_star ~3x10^6 M_sun) consistent with the tail of the observed luminosity function of ultra-diffuse galaxies. This suggests observable analogues of these systems may await discovery. We find that a stronger ionizing background (globally or locally) produces brighter and more extended ghost galaxies.

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arXiv2023

A Local Universe model for constrained simulations

The aim of cosmological simulations is to reproduce the properties of the observed Universe, serving as tools to test structure and galaxy formation models. Constrained simulations of our local cosmological region up to a few hundred Mpc/h , the local Universe, are designed to reproduce the actual cosmic web of structures as observed. A question that often arises is how to judge the quality of constrained simulations against the observations of the Local Universe. Here we introduce the Local Universe model (LUM), a new methodology, whereby many constrained simulations can be judged and the ''best'' initial conditions can be identified. By characterising the Local Universe as a set of rich clusters, the model identifies haloes that serve as simulated counterparts to the observed clusters. Their merit is determined against a null hypothesis, the probability that such a counterpart could be identified in a random, unconstrained simulation. This model is applied to 100 constrained simulations using the Cosmicflows-3 data. Cluster counterparts are found for all constrained simulations, their distribution of separation from the true observed cluster position and their mass distribution are investigated. Lastly, the ''best'' constrained simulation is selected using the LUM and discussed in more detail.

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arXiv2023

Constrained simulations of the local Universe with Modified Gravity

We present a methodology for constructing modified gravity (MG) constrained simulations of the local Universe using positions and peculiar velocities from the CosmicFlows data set. Our analysis focuses on the following MG models: the normal branch of the Dvali-Gabadadze-Porrati (nDGP) model and Hu-Sawicki $f(R)$ model. We develop a model independent methodology for constructing constrained simulations with any given power spectra and numerically calculated linear growth functions. Initial conditions (ICs) for a set of constrained simulations are constructed for the standard cosmological model $Λ$CDM and the MG models. Differences between the model's reconstructed Wiener filtered density and the resultant simulation density are presented showing the importance for the generation of MG constrained ICs to study the subtle effects of MG in the local Universe. These are the first MG constrained simulations ever produced. The current work paves the way to improved approximate methods for models with scale-dependent growth functions, such as $f(R)$, and for high-resolution hydrodynamical MG zoom-in simulations of the local Universe.

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arXiv2023

The spatial distribution of Milky Way satellites, gaps in streams and the nature of dark matter

The spatial distribution of Milky Way (MW) subhaloes provides an important set of observables for testing cosmological models. These include the radial distribution of luminous satellites, planar configurations, and the abundance of dark subhaloes whose existence or absence is key to distinguishing amongst dark matter models. We use the COCO $N$-body simulations of cold dark matter (CDM) and 3.3keV thermal relic warm dark matter (WDM) to predict the satellite spatial distribution in the limit that the impact of baryonic physics is minimal. We demonstrate that the radial distributions of CDM and 3.3keV-WDM luminous satellites are identical if the minimum pre-infall halo mass to form a galaxy is $>10^{8.5}\mathrm{M}_{\odot}$. The distribution of dark subhaloes is significantly more concentrated in WDM due to the absence of low mass, recently accreted substructures that typically inhabit the outer parts of a MW halo in CDM. We show that subhaloes of mass $[10^{7},10^{8}]\mathrm{M}_{\odot}$ and within 30kpc of the centre are the stripped remnants of larger haloes in both models. Therefore their abundance in WDM is $3\times$ higher than one would anticipate from the overall WDM subhalo population. We estimate that differences between CDM and WDM concentration-mass relations can be probed for subhalo-stream impact parameters $<2$kpc. Finally, we find that the impact of WDM on planes of satellites is likely negligible. Comprehensive comparisons with observations will require further work with high resolution, self-consistent hydrodynamical simulations.

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arXiv2022

Universality of the halo mass function in modified gravity cosmologies

We study the halo mass function (HMF) in modified gravity (MG) models using a set of large $N$-body simulations -- the ELEPHANT suite. We consider two popular beyond-general relativity scenarios: the Hu-Sawicki chameleon $f(R)$ model and the normal branch of the Dvali-Gabadadze-Porrati braneworld (nDGP). We show that in MG, analytic formulation based on the Press-Schechter framework offers a grossly inaccurate description of the HMF. We find, however, that once the HMF is expressed in terms of the dimensionless multiplicity function, it approximately assumes a redshift-independent universal character for all the models. Exploiting this property, we propose universal fits for the MG HMF in terms of their fractional departures from the $\mathrm{Λ\text{CDM}}$ case. We find two enclosed formulas, one for $f(R)$ and another for nDGP, that provide a reliable description of the HMF over the mass range covered by the simulations. These are accurate to a few percent with respect to the $N$-body data. We test the extrapolation potential of our fits against separate simulations with a different cosmological background and mass resolution, and find very good accuracy, within $\sim 10\%$. A particularly interesting finding from our analysis is a Gaussian-like shape of the HMF deviation that seems to appear universally across the whole $f(R)$ family, peaking at a mass variance scale characteristic for each $f(R)$ variant. We attribute this behavior to the specific physics of the environmentally-dependent chameleon screening models.

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arXiv2022

COWS: A filament finder for Hessian cosmic web identifiers

The large scale galaxy and matter distribution is often described by means of the cosmic web made up of voids, sheets, filaments and knots. Many different recipes exist for identifying this cosmic web. Here we focus on a sub-class of cosmic web identifiers, based on the analysis of the Hessian matrix, and proposed a method, called COsmic Web Skeleton (COWS), of separating a set of filaments cells into an ensemble of individual discreet filaments. Specifically, a thinning algorithm is applied to velocity shear tensor based cosmic web (V-web) to identify the spine of the filaments. This results in a set of filaments with well defined end-point and length. It is confirmed that these sit at local density ridges and align with the appropriate direction defined by the underlying velocity field. The radial density profile of these curved cylindrical filaments, as well as the distribution of their lengths is also examined. The robustness of all results is checked against resolution and the V-web threshold. The code for the COWS method has been made publicly available.

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arXiv2021

Testing the theory of gravity with DESI: estimators, predictions and simulation requirements

Shortly after its discovery, General Relativity (GR) was applied to predict the behavior of our Universe on the largest scales, and later became the foundation of modern cosmology. Its validity has been verified on a range of scales and environments from the Solar system to merging black holes. However, experimental confirmations of GR on cosmological scales have so far lacked the accuracy one would hope for -- its applications on those scales being largely based on extrapolation and its validity sometimes questioned in the shadow of the unexpected cosmic acceleration. Future astronomical instruments surveying the distribution and evolution of galaxies over substantial portions of the observable Universe, such as the Dark Energy Spectroscopic Instrument (DESI), will be able to measure the fingerprints of gravity and their statistical power will allow strong constraints on alternatives to GR. In this paper, based on a set of $N$-body simulations and mock galaxy catalogs, we study the predictions of a number of traditional and novel estimators beyond linear redshift distortions in two well-studied modified gravity models, chameleon $f(R)$ gravity and a braneworld model, and the potential of testing these deviations from GR using DESI. These estimators employ a wide array of statistical properties of the galaxy and the underlying dark matter field, including two-point and higher-order statistics, environmental dependence, redshift space distortions and weak lensing. We find that they hold promising power for testing GR to unprecedented precision. The major future challenge is to make realistic, simulation-based mock galaxy catalogs for both GR and alternative models to fully exploit the statistic power of the DESI survey and to better understand the impact of key systematic effects. Using these, we identify future simulation and analysis needs for gravity tests using DESI.

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arXiv2021

Probing gravity with redshift-space distortions: effects of tracer bias and sample selection

We investigate clustering properties of dark matter halos and galaxies to search for optimal statistics and scales where possible departures from general relativity (GR) could be found. We use large N-body cosmological simulations to perform measurements based on the two-point correlation function (2PCF) in GR and in selected modified gravity (MG) structure formation scenarios. As a test-bed, we employ two popular beyond-GR models: $f(R)$ gravity and the normal branch of the Dvali-Gabadadze-Porrati (nDGP) braneworld. We study a range of simulated halo and galaxy populations and reveal a noticeable MG signal in the monopole and quadrupole moments of the redshift-space 2PCF, and in the so-called clustering wedges. However, once expressed in terms of the linear distortion parameter, $β$, the statistical significance of these signals largely diminishes due to a strong degeneracy between MG-enhanced clustering and modified tracer bias. To circumvent this, we consider statistics less dependent on the bias: relative clustering ratios. We generalize the monopole ratio proposed in earlier work to multipole moments and clustering wedges, and introduce a new estimator of the $β$ parameter. The clustering ratios we extract foster noticeable differences between MG and GR models, reaching a maximum deviation of 10\% at 2$σ$ significance for specific variants of $f(R)$ and nDGP. We show that such departures could be measured for $β$ if non-linear effects at intermediate scales are correctly modeled. Our study indicates that the clustering ratios give great promise to search for signatures of MG in the large-scale structure. We also find that the selection of an optimal tracer sample depends on a particular statistics and gravity model to be considered.

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arXiv2021

Caught in the cosmic web: Environmental effect on halo concentrations, shape, and spin

Using a set of high-resolution simulations we study the statistical correlation of dark matter halo properties with the large-scale environment. We consider halo populations split into four Cosmic Web (CW) elements: voids, walls, filaments, and nodes. For the first time we present a study of CW effects for halos covering six decades in mass: $10^{8}-10^{14}{h^{-1}{\rm M}_{\odot}}$. We find that the fraction of halos living in various web components is a strong function of mass, with the majority of $M>10^{12}{h^{-1}{\rm M}_{\odot}}$ halos living in filaments and nodes. Low mass halos are more equitably distributed in filaments, walls, and voids. For halo density profiles and formation times we find a universal mass threshold of $M_{th}\sim6\times10^{10}{h^{-1}{\rm M}_{\odot}}$ below which these properties vary with environment. Here, filament halos have the steepest concentration-mass relation, walls are close to the overall mean, and void halos have the flattest relation. This amounts to $c_{200}$ for filament and void halos that are respectively $14\%$ higher and $7\%$ lower than the mean at $M=2\times10^8{h^{-1}{\rm M}_{\odot}}$, with low-mass node halos being most likely splashed-back. We find double power-law fits that very well describe $c(M)$ for the four environments in the whole probed mass range. A complementary picture is found for the average formation times, with the mass-formation time relations following trends shown for the concentrations: the nodes halos being the oldest and void halo the youngest. The CW environmental effect is much weaker when studying the halo spin and shapes. The trends with halo mass is reversed: the small halos with $M<10^{10}{h^{-1}{\rm M}_{\odot}}$ seem to be unaffected by the CW environment. Some weak trends are visible for more massive void and walls halos, which, on average, are characterized by lower spin and higher triaxiality parameters.

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arXiv2020

The skewness of z=0.5 redshift-space galaxy distribution in Modified Gravity

We study the reduced skewness, $S_{3,g}\equiv\barξ_{3,g}/\barξ_{2,g}^2$ of galaxy distribution at $z=0.5$ in two families of modfied gravity models: the Hu-Sawicki $f(R)$-gravity and normal-branch of Dvali-Gabadadze-Porrati (nDGP) models. We use a set of mock galaxy catalogues specifally designed to match CMASS spectroscopic galaxy sample. For the first time we investigate the third reduced moment of such galaxy distributions both in the redshift space. Our analysis confirms that the signal previously indicated only for dark matter halo catalogues persists also in realistic mock galaxy samples. This result offers a possibility to extract a potential modified gravity signal in $S_3$ from spectroscopic galaxy data without a need for a very precise and self-consistent RSD models constructed for each and every modified gravity scenario separately. We show that the relative deviations from $Λ$CDM~ $S_{3,g}$ of various modified gravity models can vary from $7$ down to $\sim 2-3\%$ effects. Albeit, the effect looks small, we show that for considered models it can foster a $2-3σ$ falsification. Finally we argue that galaxy sample of a significantly higher number density should provide even stronger constraints by limiting shot-noise effects affecting the $S_{3,g}$ estimates at small comoving separations.

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arXiv2019

Simulating the dark matter decay signal from the Perseus galaxy cluster

The nearby Perseus galaxy cluster is a key target for indirect detection searches for decaying dark matter. We use the C-EAGLE simulations of galaxy clusters to predict the flux, width and shape of a dark matter decay line, paying particular attention to the unexplained 3.55keV line detected in the spectra of some galaxies and clusters, and the upcoming XRISM X-ray observatory mission. We show that the line width in C-EAGLE clusters similar to Perseus is typically [600-800]$\mathrm{kms^{-1}}$, and therefore narrower than the amplitude of the velocity dispersion of galaxies in the cluster. Haloes that are significantly disturbed can, however, exhibit galaxy velocity dispersions higher than $1000\mathrm{kms^{-1}}$, and in this case will show a large difference between the line profiles of on- and off-center observations. We show that the line profile is likely to be slightly asymmetric, but still well approximated by a Gaussian at the 10% level, and that the halo asymmetry can lead to fluxes that vary by a factor of two. In summary, we predict that, if the previously reported 3.55keV line detections do originate from dark matter decay, the XRISM mission will detect a line with a roughly Gaussian profile at a rest frame energy of 3.55keV, with a width $>600\mathrm{kms^{-1}}$ and flux approximately in the range $[4-9]\times10^{-8}\mathrm{counts/sec/cm^{2}}$.

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arXiv2018

Uneven flows: On cosmic bulk flows, local observers, and gravity

Using N-body simulations we study the impact of various systematic effects on the bulk flow (BF) and the Cosmic Mach Number (CMN). We consider two types of systematics: those related to survey properties and those induced by observer's location in the Universe. In the former category we model sparse sampling, velocity errors, and survey incompleteness. In the latter, we consider Local Group (LG) analogue observers, placed in a specific location within the Cosmic Web, satisfying various observational criteria. We differentiate such LG observers from Copernican ones, who are at random locations. We report strong systematic effects on the measured BF and CMN induced by sparse sampling, velocity errors and radial incompleteness. For BF most of these effects exceed 10\% for scales $R\leq100h^{-1}$Mpc. For CMN some of these systematics can be catastrophically large ($>50\%$) also on bigger scales. Moreover, we find that the position of the observer in the Cosmic Web significantly affects the locally measured BF (CMN), with effects as large as $\sim20\%$ ($30\%)$ at $R\leq50h^{-1}$Mpc for a LG-like observer as compared to a random one. This effect is comparable to the sample variance. To highlight the importance of these systematics, we additionally study a model of modified gravity (MG) with $\sim15\%$ enhanced growth rate. We found that the systematic effects can mimic the modified gravity signal. The worst-case scenario is realized for a case of a LG-like observer, when the effects induced by local structures are degenerate with the enhanced growth rate fostered by MG. Our results indicate that dedicated constrained simulations and realistic mock galaxy catalogs will be absolutely necessary to fully benefit from the statistical power of the forthcoming peculiar velocity data from surveys such as TAIPAN, WALLABY, Cosmic Flows-4 and SKA.

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arXiv2017

Properties of Local Group galaxies in hydrodynamical simulations of sterile neutrino dark matter cosmologies

We study galaxy formation in sterile neutrino dark matter models that differ signifi- cantly from both cold and from `warm thermal relic' models. We use the EAGLE code to carry out hydrodynamic simulations of the evolution of pairs of galaxies chosen to resemble the Local Group, as part of the APOSTLE simulations project. We compare cold dark matter (CDM) with two sterile neutrino models with 7 keV mass: one, the warmest among all models of this mass (LA120) and the other, a relatively cold case (LA10). We show that the lower concentration of sterile neutrino subhalos compared to their CDM counterparts makes the inferred inner dark matter content of galaxies like Fornax (or Magellanic Clouds) less of an outlier in the sterile neutrino cosmologies. In terms of the galaxy number counts the LA10 simulations are indistinguishable from CDM when one takes into account halo-to-halo (or `simulation-to-simulation') scatter. In order for the LA120 model to match the number of Local Group dwarf galaxies, a higher fraction of low mass haloes is required to form galaxies than is predicted by the EAGLE simulations. As the census of the Local Group galaxies nears completion, this population may provide a strong discriminant between cold and warm dark matter models.

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arXiv2017

Theoretical Accuracy in Cosmological Growth Estimation

We elucidate the importance of the consistent treatment of gravity-model specific non-linearities when estimating the growth of cosmological structures from redshift space distortions (RSD). Within the context of standard perturbation theory (SPT), we compare the predictions of two theoretical templates with redshift space data from COLA (COmoving Lagrangian Acceleration) simulations in the normal branch of DGP gravity (nDGP) and General Relativity (GR). Using COLA for these comparisons is validated using a suite of full N-body simulations for the same theories. The two theoretical templates correspond to the standard general relativistic perturbation equations and those same equations modelled within nDGP. Gravitational clustering non-linear effects are accounted for by modelling the power spectrum up to one loop order and redshift space clustering anisotropy is modelled using the Taruya, Nishimichi and Saito (TNS) RSD model. Using this approach, we attempt to recover the simulation's fiducial logarithmic growth parameter $f$. By assigning the simulation data with errors representing an idealised survey with a volume of $10\mbox{Gpc}^3/h^3$, we find the GR template is unable to recover fiducial $f$ to within 1$σ$ at $z=1$ when we match the data up to $k_{\rm max}=0.195h$/Mpc. On the other hand, the DGP template recovers the fiducial value within $1σ$. Further, we conduct the same analysis for sets of mock data generated for generalised models of modified gravity using SPT, where again we analyse the GR template's ability to recover the fiducial value. We find that for models with enhanced gravitational non-linearity, the theoretical bias of the GR template becomes significant for stage IV surveys. Thus, we show that for the future large data volume galaxy surveys, the self-consistent modelling of non-GR gravity scenarios will be crucial in constraining theory parameters.

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arXiv2017

Tracing the cosmic web

The cosmic web is one of the most striking features of the distribution of galaxies and dark matter on the largest scales in the Universe. It is composed of dense regions packed full of galaxies, long filamentary bridges, flattened sheets and vast low density voids. The study of the cosmic web has focused primarily on the identification of such features, and on understanding the environmental effects on galaxy formation and halo assembly. As such, a variety of different methods have been devised to classify the cosmic web -- depending on the data at hand, be it numerical simulations, large sky surveys or other. In this paper we bring twelve of these methods together and apply them to the same data set in order to understand how they compare. In general these cosmic web classifiers have been designed with different cosmological goals in mind, and to study different questions. Therefore one would not {\it a priori} expect agreement between different techniques however, many of these methods do converge on the identification of specific features. In this paper we study the agreements and disparities of the different methods. For example, each method finds that knots inhabit higher density regions than filaments, etc. and that voids have the lowest densities. For a given web environment, we find substantial overlap in the density range assigned by each web classification scheme. We also compare classifications on a halo-by-halo basis; for example, we find that 9 of 12 methods classify around a third of group-mass haloes (i.e. $M_{\rm halo}\sim10^{13.5}h^{-1}M_{\odot}$) as being in filaments. Lastly, so that any future cosmic web classification scheme can be compared to the 12 methods used here, we have made all the data used in this paper public.

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arXiv2017

The galaxy population in cold and warm dark matter cosmologies

We use a pair of high resolution N-body simulations implementing two dark matter models, namely the standard cold dark matter (CDM) cosmogony and a warm dark matter (WDM) alternative where the dark matter particle is a 1.5keV thermal relic. We combine these simulations with the GALFORM semi-analytical galaxy formation model in order to explore differences between the resulting galaxy populations. We use GALFORM model variants for CDM and WDM that result in the same z=0 galaxy stellar mass function by construction. We find that most of the studied galaxy properties have the same values in these two models, indicating that both dark matter scenarios match current observational data equally well. Even in under-dense regions, where discrepancies in structure formation between CDM and WDM are expected to be most pronounced, the galaxy properties are only slightly different. The only significant difference in the local universe we find is in the galaxy populations of "Local Volumes", regions of radius 1 to 8Mpc around simulated Milky Way analogues. In such regions our WDM model provides a better match to observed local galaxy number counts and is five times more likely than the CDM model to predict sub-regions within them that are as empty as the observed Local Void. Thus, a highly complete census of the Local Volume and future surveys of void regions could provide constraints on the nature of dark matter.

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arXiv2017

Speeding up $N$-body simulations of modified gravity: Chameleon screening models

We describe and demonstrate the potential of a new and very efficient method for simulating certain classes of modified gravity theories, such as the widely studied $f(R)$ gravity models. High resolution simulations for such models are currently very slow due to the highly nonlinear partial differential equation that needs to be solved exactly to predict the modified gravitational force. This nonlinearity is partly inherent, but is also exacerbated by the specific numerical algorithm used, which employs a variable redefinition to prevent numerical instabilities. The standard Newton-Gauss-Seidel iterative method used to tackle this problem has a poor convergence rate. Our new method not only avoids this, but also allows the discretised equation to be written in a form that is analytically solvable. We show that this new method greatly improves the performance and efficiency of $f(R)$ simulations. For example, a test simulation with $512^3$ particles in a box of size $512 \, \mathrm{Mpc}/h$ is now 5 times faster than before, while a Millennium-resolution simulation for $f(R)$ gravity is estimated to be more than 20 times faster than with the old method. Our new implementation will be particularly useful for running very high resolution, large-sized simulations which, to date, are only possible for the standard model, and also makes it feasible to run large numbers of lower resolution simulations for covariance analyses. We hope that the method will bring us to a new era for precision cosmological tests of gravity.

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