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Amandine M. C. Le Brun

Publications and source records attributed to Amandine M. C. Le Brun.

15 recordsLinked to original sources
arXiv2026

Probing modified gravity with galaxy-cluster-lensed gravitational-wave time delays: impact of systematic effects

Strongly lensed gravitational waves provide exceptionally precise measurements of relative macroimage arrival times, making their time delays a promising time-domain probe of gravity on galaxy-cluster scales. This precision places stringent demands on the control of lensing systematic effects. We present a proof-of-concept pipeline for constraining a phenomenological modification, $D$, of the galaxy cluster Weyl potential using the macroimage time-delay span of a simulated sample of binary neutron star (BNS) sources and cluster lenses described by a Navarro--Frenk--White cluster halo and a singular isothermal sphere for the brightest cluster galaxy (BCG). Closure tests recover the injected general-relativistic (GR) limit. Adopting a fiducial 5 per cent precision on the reconstructed time-delay span, we assess controlled mis-specifications of the halo mass and concentration, BCG contribution, source and lens positions, redshifts, line-of-sight environment, projected ellipticity and secondary cluster-scale structure. Within the adopted gravity model, neglecting halo ellipticity, secondary cluster-scale structure, cluster mis-centring and line-of-sight perturbations can induce significant, strongly configuration-dependent biases in the inferred parameter $D$. Biased estimates of the cluster halo mass and concentration, as well as systematic BNS source-position errors at the level considered here, produce significant coherent shifts. By contrast, uncertainties in the BCG contribution and in the lens and source redshifts are comparatively subdominant. These results highlight the need for explicit treatment of the above uncertainties before cluster-lensed gravitational wave time delays can be used as precision tests of gravity.

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arXiv2022

Internal dark matter structure of the most massive galaxy clusters since redshift 1

We investigate the evolution of the dark matter density profiles of the most massive galaxy clusters in the Universe. Using a `zoom-in' procedure on a large suite of cosmological simulations of total comoving volume of $3\,(h^{-1}\,\rm Gpc)^3$, we study the 25 most massive clusters in four redshift slices from $z\sim 1$ to the present. The minimum mass is $M_{500} > 5.5 \times 10^{14}$ M$_{\odot}$ at $z=1$. Each system has more than two million particles within $r_{500}$. Once scaled to the critical density at each redshift, the dark matter profiles within $r_{500}$ are strikingly similar from $z\sim1$ to the present day, exhibiting a low dispersion of 0.15 dex, and showing little evolution with redshift in the radial logarithmic slope and scatter. They have the running power law shape typical of the NFW-type profiles, and their inner structure, resolved to $3.8\,h^{-1}$ comoving kpc at $z=1$, shows no signs of converging to an asymptotic slope. Our results suggest that this type of profile is already in place at $z>1$ in the highest-mass haloes in the Universe, and that it remains exceptionally robust to merging activity.

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arXiv2021

Feedback from Active Galactic Nuclei in Galaxy Groups

The co-evolution between supermassive black holes and their environment is most directly traced by the hot atmospheres of dark matter halos. Cooling of the hot atmosphere supplies the central regions with fresh gas, igniting active galactic nuclei (AGN) with long duty cycles. Outflows from the central engine tightly couple with the surrounding gaseous medium and provide the dominant heating source preventing runaway cooling by carving cavities and driving shocks across the medium. The AGN feedback loop is a key feature of all modern galaxy evolution models. Here we review our knowledge of the AGN feedback process in the specific context of galaxy groups. Galaxy groups are uniquely suited to constrain the mechanisms governing the cooling-heating balance. Unlike in more massive halos, the energy supplied by the central AGN to the hot intragroup medium can exceed the gravitational binding energy of halo gas particles. We report on the state-of-the-art in observations of the feedback phenomenon and in theoretical models of the heating-cooling balance in galaxy groups. We also describe how our knowledge of the AGN feedback process impacts on galaxy evolution models and on large-scale baryon distributions. Finally, we discuss how new instrumentation will answer key open questions on the topic.

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arXiv2019

Quantifying baryon effects on the matter power spectrum and the weak lensing shear correlation

Feedback processes from baryons are expected to strongly affect weak-lensing observables of current and future cosmological surveys. In this paper we present a new parametrisation of halo profiles based on gas, stellar, and dark matter density components. This parametrisation is used to modify outputs of gravity-only $N$-body simulations (following the prescription of Schneider and Teyssier [1]) in order to mimic baryonic effects on the matter density field. The resulting baryonic correction model relies on a few well motivated physical parameters and is able to reproduce the redshift zero clustering signal of hydrodynamical simulations at two percent accuracy below $k\sim10$ h/Mpc. A detailed study of the baryon suppression effects on the matter power spectrum and the weak lensing shear correlation reveals that the signal is dominated by two parameters describing the slope of the gas profile in haloes and the maximum radius of gas ejection. We show that these parameters can be constrained with the observed gas fraction of galaxy groups and clusters from X-ray data. Based on these observations we predict a beyond percent effect on the power spectrum above $k=0.2-1.0$ h/Mpc with a maximum suppression of 15-25 percent around $k\sim 10$ h/Mpc. As a result, the weak lensing angular shear power spectrum is suppressed by 15-25 percent at scales beyond $\ell\sim 100-600$ and the shear correlations $ξ_{+}$ and $ξ_{-}$ are affected at the 10-25 percent level below 5 and 50 arc-minutes, respectively. The relatively large uncertainties of these predictions are a result of the poorly known hydrostatic mass bias of current X-ray observations as well as the generic difficulty to observe the low density gas outside of haloes.

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arXiv2018

The impact of baryons on the matter power spectrum from the Horizon-AGN cosmological hydrodynamical simulation

Accurate cosmology from upcoming weak lensing surveys relies on knowledge of the total matter power spectrum at percent level at scales $k < 10$ $h$/Mpc, for which modelling the impact of baryonic physics is crucial. We compare measurements of the total matter power spectrum from the Horizon cosmological hydrodynamical simulations: a dark matter-only run, one with full baryonic physics, and another lacking Active Galactic Nuclei (AGN) feedback. Baryons cause a suppression of power at $k\simeq 10$ $h/$Mpc of $<15\%$ at $z=0$, and an enhancement of a factor of a few at smaller scales due to the more efficient cooling and star formation. The results are sensitive to the presence of the highest mass haloes in the simulation and the distribution of dark matter is also impacted up to a few percent. The redshift evolution of the effect is non-monotonic throughout $z=0-5$ due to an interplay between AGN feedback and gas pressure, and the growth of structure. We investigate the effectiveness of an analytic `baryonic correction model' in describing our results. We require a different redshift evolution and propose an alternative fitting function with $4$ free parameters that reproduces our results within $5\%$. Compared to other simulations, we find the impact of baryonic processes on the total matter power spectrum to be smaller at $z=0$. Correspondingly, our results suggest that AGN feedback is not strong enough in the simulation. Total matter power spectra from the Horizon simulations are made publicly available at https://www.horizon-simulation.org/catalogues.html

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arXiv2018

Resolving the hydrostatic mass profiles of galaxy clusters at z~1 with XMM-Newton and Chandra

We present a detailed study of the integrated total hydrostatic mass profiles of the five most massive ($M^{\mathrm{SZ}}_{500} > 5 \times 10^{14}$ M$_{\odot}$) galaxy clusters selected at $z\sim1$ via the Sunyaev-Zel'dovich effect. These objects represent an ideal laboratory to test structure formation models where the primary driver is gravity. Optimally exploiting spatially-resolved spectroscopic information from XMM-Newton and Chandra observations, we used both parametric (forward, backward) and non-parametric methods to recover the mass profiles, finding that the results are extremely robust when density and temperature measurements are both available. Our X-ray masses at $R_{500}$ are higher than the weak lensing masses obtained from the Hubble Space Telescope (HST), with a mean ratio of $1.39^{+0.47}_{-0.35}$. This offset goes in the opposite direction to that expected in a scenario where the hydrostatic method yields a biased, underestimated, mass. We investigated halo shape parameters such as sparsity and concentration, and compared to local X-ray selected clusters, finding hints for evolution in the central regions (or for selection effects). The total baryonic content is in agreement with the cosmic value at $R_{500}$. Comparison with numerical simulations shows that the mass distribution and concentration are in line with expectations. These results illustrate the power of X-ray observations to probe the statistical properties of the gas and total mass profiles in this high-mass, high-redshift regime.

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arXiv2017

The diversity of assembly histories leading to disc galaxy formation in a LambdaCDM model

[Abridged] Typical disc galaxies forming in a LambdaCDM cosmology encounter a violent environment, where they often experience mergers with massive satellites. The fact that disc galaxies are ubiquitous in the local Universe suggests that a quiescent history is not necessary for their formation. Modern cosmological simulations can now obtain relatively realistic populations of disc galaxies, but it still remains to be clarified how discs manage to survive massive mergers. Here we use a suite of high-resolution hydrodynamical simulations set in a LambdaCDM cosmology to elucidate the fate of discs encountering massive mergers. We extract a sample of approximately 100 disc galaxies and follow the changes in their post-merger morphologies, as tracked by their disc-to-total ratios (D/T). We also examine the relations between their present-day morphology, assembly history and gas fractions. We find that approximately half of present-day disc galaxies underwent at least one merger with a satellite of total mass exceeding the host system's stellar mass, a third had mergers with satellites of mass exceeding 3 times the host's stellar mass, and approximately one-sixth had mergers with satellites of mass exceeding 10 times of the host's stellar mass. These mergers lead to a sharp, but often temporary, decrease in the D/T of the hosts, implying that discs are usually disrupted but then quickly re-grow. To do so, high cold gas fractions are required post-merger, as well as a relatively quiescent recent history (over a few Gyrs before z=0). Our results show that discs can form via diverse merger pathways and that quiescent histories are not the dominant mode of disc formation.

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arXiv2017

Internal dark matter structure of the most massive galaxy clusters

We investigate the evolution of the dark matter density profiles of the most massive galaxy clusters in the Universe. Using a `zoom-in' procedure on a large suite of cosmological simulations of total comoving volume of $3\,(h^{-1}\,\rm Gpc)^3$, we study the 25 most massive clusters in four redshift slices from $z\sim 1$ to the present. The minimum mass is $M_{500} > 5.5 \times 10^{14}$ M$_{\odot}$ at $z=1$. Each system has more than two million particles within $r_{500}$. Once scaled to the critical density at each redshift, the dark matter profiles within $r_{500}$ are strikingly similar from $z\sim1$ to the present day, exhibiting a low dispersion of 0.15 dex, and showing little evolution with redshift in the radial logarithmic slope and scatter. They have the running power law shape typical of the NFW-type profiles, and their inner structure, resolved to $3.8\,h^{-1}$ comoving kpc at $z=1$, shows no signs of converging to an asymptotic slope. Our results suggest that this type of profile is already in place at $z>1$ in the highest-mass haloes in the Universe, and that it remains exceptionally robust to merging activity.

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arXiv2016

The scatter and evolution of the global hot gas properties of simulated galaxy cluster populations

We use the cosmo-OWLS suite of cosmological hydrodynamical simulations to investigate the scatter and evolution of the global hot gas properties of large simulated populations of galaxy groups and clusters. Our aim is to compare the predictions of different physical models and to explore the extent to which commonly-adopted assumptions in observational analyses (e.g. self-similar evolution) are violated. We examine the relations between (true) halo mass and the X-ray temperature, X-ray luminosity, gas mass, Sunyaev-Zel'dovich (SZ) flux, the X-ray analogue of the SZ flux ($Y_X$) and the hydrostatic mass. For the most realistic models, which include AGN feedback, the slopes of the various mass-observable relations deviate substantially from the self-similar ones, particularly at late times and for low-mass clusters. The amplitude of the mass-temperature relation shows negative evolution with respect to the self-similar prediction (i.e. slower than the prediction) for all models, driven by an increase in non-thermal pressure support at higher redshifts. The AGN models predict strong positive evolution of the gas mass fractions at low halo masses. The SZ flux and $Y_X$ show positive evolution with respect to self-similarity at low mass but negative evolution at high mass. The scatter about the relations is well approximated by log-normal distributions, with widths that depend mildly on halo mass. The scatter decreases significantly with increasing redshift. The exception is the hydrostatic mass-halo mass relation, for which the scatter increases with redshift. Finally, we discuss the relative merits of various hot gas-based mass proxies.

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arXiv2016

The BAHAMAS project: Calibrated hydrodynamical simulations for large-scale structure cosmology

The evolution of the large-scale distribution of matter is sensitive to a variety of fundamental parameters that characterise the dark matter, dark energy, and other aspects of our cosmological framework. Since the majority of the mass density is in the form of dark matter that cannot be directly observed, to do cosmology with large-scale structure one must use observable (baryonic) quantities that trace the underlying matter distribution in a (hopefully) predictable way. However, recent numerical studies have demonstrated that the mapping between observable and total mass, as well as the total mass itself, are sensitive to unresolved feedback processes associated with galaxy formation, motivating explicit calibration of the feedback efficiencies. Here we construct a new suite of large-volume cosmological hydrodynamical simulations (called BAHAMAS, for BAryons and HAloes of MAssive Systems) where subgrid models of stellar and Active Galactic Nucleus (AGN) feedback have been calibrated to reproduce the present-day galaxy stellar mass function and the hot gas mass fractions of groups and clusters in order to ensure the effects of feedback on the overall matter distribution are broadly correct. We show that the calibrated simulations reproduce an unprecedentedly wide range of properties of massive systems, including the various observed mappings between galaxies, hot gas, total mass, and black holes, and represent a significant advance in our ability to mitigate the primary systematic uncertainty in most present large-scale structure tests.

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arXiv2016

Dissecting the thermal Sunyaev-Zeldovich-gravitational lensing cross-correlation with hydrodynamical simulations

We use the cosmo-OWLS suite of cosmological hydrodynamical simulations, which includes different galactic feedback models, to predict the cross-correlation signal between weak gravitational lensing and the thermal Sunyaev-Zeldovich (tSZ) $y$-parameter. The predictions are compared to the recent detection reported by van Waerbeke and collaborators. The simulations reproduce the weak lensing-tSZ cross-correlation, $ξ_{yκ}(θ)$, well. The uncertainty arising from different possible feedback models appears to be important on small scales only ($θ\lesssim 10$ arcmin), while the amplitude of the correlation on all scales is sensitive to cosmological parameters that control the growth rate of structure (such as $σ_8$, $Ω_m$ and $Ω_b$). This study confirms our previous claim (in Ma et al.) that a significant proportion of the signal originates from the diffuse gas component in low-mass ($M_{\rm{halo}} \lesssim 10^{14} M_{\odot}$) clusters as well as from the region beyond the virial radius. We estimate that approximately 20$\%$ of the detected signal comes from low-mass clusters, which corresponds to about 30$\%$ of the baryon density of the Universe. The simulations also suggest that more than half of the baryons in the Universe are in the form of diffuse gas outside halos ($\gtrsim 5$ times the virial radius) which is not hot or dense enough to produce a significant tSZ signal or be observed by X-ray experiments. Finally, we show that future high-resolution tSZ-lensing cross-correlation observations will serve as a powerful tool for discriminating between different galactic feedback models.

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arXiv2015

Testing Sunyaev-Zel'dovich measurements of the hot gas content of dark matter haloes using synthetic skies

The thermal Sunyaev-Zel'dovich (tSZ) effect offers a means of probing the hot gas in and around massive galaxies and galaxy groups and clusters, which is thought to constitute a large fraction of the baryon content of the Universe. The Planck collaboration recently performed a stacking analysis of a large sample of `locally brightest galaxies' (LBGs) and, surprisingly, inferred an approximately self-similar relation between the tSZ flux and halo mass. At face value, this implies that the hot gas mass fraction is independent of halo mass, a result which is in apparent conflict with resolved X-ray observations. We test the robustness of the inferred trend using synthetic tSZ maps generated from cosmological hydrodynamical simulations and using the same tools and assumptions applied in the Planck study. We show that, while the detection and the estimate of the `total' flux (within $5 r_{500}$) is reasonably robust, the inferred flux originating from within $r_{500}$ (i.e. the limiting radius to which X-ray observations typically probe) is highly sensitive to the assumed pressure distribution of the gas. Using our most realistic simulations with AGN feedback, that reproduce a wide variety of X-ray and optical properties of groups and clusters, we estimate that the derived tSZ flux within $r_{500}$ is biased high by up to to an order of magnitude for haloes with masses $M_{500} \sim 10^{13}$ M$_{\odot}$. Moreover, we show that the AGN simulations are consistent with the total tSZ flux-mass relation observed with Planck, whereas a self-similar model is ruled out.

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arXiv2014

The impact of galaxy formation on the total mass, mass profile and abundance of haloes

We use cosmological hydrodynamical simulations to investigate how the inclusion of physical processes relevant to galaxy formation (star formation, metal-line cooling, stellar winds, supernovae and feedback from Active Galactic Nuclei, AGN) change the properties of haloes, over four orders of magnitude in mass. We find that gas expulsion and the associated dark matter (DM) expansion induced by supernova-driven winds are important for haloes with masses M200 < 10^13 Msun, lowering their masses by up to 20% relative to a DM-only model. AGN feedback, which is required to prevent overcooling, has a significant impact on halo masses all the way up to cluster scales (M200 ~ 10^15 Msun). Baryonic physics changes the total mass profiles of haloes out to several times the virial radius, a modification that cannot be captured by a change in the halo concentration. The decrease in the total halo mass causes a decrease in the halo mass function of about 20%. This effect can have important consequences for abundance matching technique as well as for most semi-analytic models of galaxy formation. We provide analytic fitting formulae, derived from simulations that reproduce the observed baryon fractions, to correct halo masses and mass functions from DM-only simulations. The effect of baryonic physics (AGN feedback in particular) on cluster number counts is about as large as changing the cosmology from WMAP7 to Planck, even when a moderately high mass limit of M500 ~ 10^14 Msun is adopted. Thus, for precision cosmology the effects of baryons must be accounted for.

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arXiv2014

Towards a realistic population of simulated galaxy groups and clusters

We present a new suite of large-volume cosmological hydrodynamical simulations called cosmo-OWLS. They form an extension to the OverWhelmingly Large Simulations (OWLS) project, and have been designed to help improve our understanding of cluster astrophysics and non-linear structure formation, which are now the limiting systematic errors when using clusters as cosmological probes. Starting from identical initial conditions in either the Planck or WMAP7 cosmologies, we systematically vary the most important `sub-grid' physics, including feedback from supernovae and active galactic nuclei (AGN). We compare the properties of the simulated galaxy groups and clusters to a wide range of observational data, such as X-ray luminosity and temperature, gas mass fractions, entropy and density profiles, Sunyaev-Zel'dovich flux, I-band mass-to-light ratio, dominance of the brightest cluster galaxy, and central massive black hole (BH) masses, by producing synthetic observations and mimicking observational analysis techniques. These comparisons demonstrate that some AGN feedback models can produce a realistic population of galaxy groups and clusters, broadly reproducing both the median trend and, for the first time, the scatter in physical properties over approximately two decades in mass ($10^{13} \lesssim M_{500} \lesssim 10^{15}~\textrm{M}_{\odot}$) and 1.5 decades in radius ($0.05 \lesssim r/r_{500} \lesssim 1.5$). However, in other models, the AGN feedback is too violent (even though they reproduce the observed BH scaling relations), implying calibration of the models is required. The production of realistic populations of simulated groups and clusters, as well as models that bracket the observations, opens the door to the creation of synthetic surveys for assisting the astrophysical and cosmological interpretation of cluster surveys, as well as quantifying the impact of selection effects.

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arXiv2014

The thermal Sunyaev Zel'dovich effect power spectrum in light of Planck

(Abridged) The amplitude of the thermal Sunyaev Zel'dovich effect (tSZ) power spectrum is extremely sensitive to the abundance of galaxy clusters and therefore to fundamental cosmological parameters that control their growth, such as sigma_8 and Omega_m. Here we explore the sensitivity of the tSZ power spectrum to important non-gravitational ('sub-grid') physics by employing the cosmo-OWLS suite of large-volume cosmological hydrodynamical simulations, run in both the Planck and WMAP7 best-fit cosmologies. On intermediate and small angular scales (ell > ~1000, or theta < ~10 arcmin), accessible with the South Pole Telescope and the Atacama Cosmology Telescope, the predicted tSZ power spectrum is highly model dependent, with AGN feedback having a particularly large effect. However, at large scales, observable with the Planck telescope, the effects of sub-grid physics are minor. Comparing the simulations with observations, we find a significant amplitude offset on all measured angular scales (including large scales), if the Planck best-fit cosmology is assumed by the simulations. This is shown to be a generic result for all current tSZ models. By contrast, if the WMAP7 cosmology is adopted, there is full consistency with the Planck power spectrum measurements on large scales and agreement at the 2 sigma level with the SPT/ACT measurements at intermediate scales for our fiducial AGN model, which Le Brun et al. (2014) have shown reproduces the 'resolved' properties of the local cluster population remarkably well. These findings strongly suggest that there are significantly fewer massive galaxy clusters than expected for the Planck best-fit cosmology, which is consistent with recent measurements of the tSZ number counts. Our findings therefore pose a significant challenge to the cosmological parameter values preferred (and/or the model adopted) by the Planck primary CMB analyses.

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