We use a set of hydrodynamical and dark matter-only (DMonly) simulations to calibrate the halo mass function (HMF). We explore the impact of baryons, propose an improved parametrization for spherical ...overdensity masses, and identify differences between our DMonly HMF and previously published HMFs. We use the Magneticum simulations, which are well suited because of their accurate treatment of baryons, high resolution, and large cosmological volumes of up to (3818 Mpc)3. Baryonic effects globally decrease the masses of galaxy clusters, which, at a given mass, results in a decrease of their number density. This effect vanishes at high redshift z ∼ 2 and for high masses M
200 m ≳ 1014 M⊙. We perform cosmological analyses of three idealized approximations to the cluster surveys by the South Pole Telescope (SPT), Planck, and eROSITA. We pursue two main questions. (1) What is the impact of baryons? – for the SPT-like and the Planck-like samples, the impact of baryons on cosmological results is negligible. In the eROSITA-like case, however, neglecting the baryonic impact leads to an underestimate of Ωm by about 0.01, which is comparable to the expected uncertainty from eROSITA. (2) How does our DMonly HMF compare with previous work? – for the Planck-like sample, results obtained using our DMonly HMF are shifted by Δ(σ8) ≃ Δ(σ8(Ωm/0.27)0.3) ≃ 0.02 with respect to results obtained using the Tinker et al. fit. This suggests that using our HMF would shift results from Planck clusters towards better agreement with cosmic-microwave-background anisotropy measurements. Finally, we discuss biases that can be introduced through inadequate HMF parametrizations that introduce false cosmological sensitivity.
ABSTRACT
Luminous matter produces very energetic events, such as active galactic nuclei and supernova explosions, that significantly affect the internal regions of galaxy clusters. Although the ...current uncertainty in the effect of baryonic physics on cluster statistics is subdominant as compared to other systematics, the picture is likely to change soon as the amount of high-quality data is growing fast, urging the community to keep theoretical systematic uncertainties below the ever-growing statistical precision. In this paper, we study the effect of baryons on galaxy clusters, and their impact on the cosmological applications of clusters, using the magneticum suite of cosmological hydrodynamical simulations. We show that the impact of baryons on the halo mass function can be recast in terms on a variation of the mass of the haloes simulated with pure N-body, when baryonic effects are included. The halo mass function and halo bias are only indirectly affected. Finally, we demonstrate that neglecting baryonic effects on haloes mass function and bias would significantly alter the inference of cosmological parameters from high-sensitivity next-generations surveys of galaxy clusters.
We study the gravitational collapse of axion dark matter fluctuations in the postinflationary scenario, so-called axion miniclusters, with N-body simulations. Largely confirming theoretical ...expectations, overdensities begin to collapse in the radiation-dominated epoch and form an early distribution of miniclusters with masses up to 10−12 M⊙. After matter-radiation equality, ongoing mergers give rise to a steep power-law distribution of minicluster halo masses. The density profiles of well-resolved halos are Navarro–Frenk–White-like to good approximation. The fraction of axion dark matter in these bound structures is ∼ 0.75 at redshift z = 100 .
ABSTRACT
We constrain cosmological parameters from a joint cosmic shear analysis of peak-counts and the two-point shear correlation functions, as measured from the Dark Energy Survey (DES-Y1). We ...find the structure growth parameter $S_8\equiv \sigma _8\sqrt{\Omega _{\rm m}/0.3} = 0.766^{+0.033}_{-0.038}$ which, at 4.8 per cent precision, provides one of the tightest constraints on S8 from the DES-Y1 weak lensing data. In our simulation-based method we determine the expected DES-Y1 peak-count signal for a range of cosmologies sampled in four w cold dark matter parameters (Ωm, σ8, h, w0). We also determine the joint covariance matrix with over 1000 realizations at our fiducial cosmology. With mock DES-Y1 data we calibrate the impact of photometric redshift and shear calibration uncertainty on the peak-count, marginalizing over these uncertainties in our cosmological analysis. Using dedicated training samples we show that our measurements are unaffected by mass resolution limits in the simulation, and that our constraints are robust against uncertainty in the effect of baryon feedback. Accurate modelling for the impact of intrinsic alignments on the tomographic peak-count remains a challenge, currently limiting our exploitation of cross-correlated peak counts between high and low redshift bins. We demonstrate that once calibrated, a fully tomographic joint peak-count and correlation functions analysis has the potential to reach a 3 per cent precision on S8 for DES-Y1. Our methodology can be adopted to model any statistic that is sensitive to the non-Gaussian information encoded in the shear field. In order to accelerate the development of these beyond-two-point cosmic shear studies, our simulations are made available to the community upon request.
On the linearity of tracer bias around voids Pollina, Giorgia; Hamaus, Nico; Dolag, Klaus ...
Monthly notices of the Royal Astronomical Society,
07/2017, Volume:
469, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract
The large-scale structure of the Universe can be observed only via luminous tracers of the dark matter. However, the clustering statistics of tracers are biased and depend on various ...properties, such as their host-halo mass and assembly history. On very large scales, this tracer bias results in a constant offset in the clustering amplitude, known as linear bias. Towards smaller non-linear scales, this is no longer the case and tracer bias becomes a complicated function of scale and time. We focus on tracer bias centred on cosmic voids, i.e. depressions of the density field that spatially dominate the Universe. We consider three types of tracers: galaxies, galaxy clusters and active galactic nuclei, extracted from the hydrodynamical simulation Magneticum Pathfinder. In contrast to common clustering statistics that focus on auto-correlations of tracers, we find that void–tracer cross-correlations are successfully described by a linear bias relation. The tracer-density profile of voids can thus be related to their matter-density profile by a single number. We show that it coincides with the linear tracer bias extracted from the large-scale auto-correlation function and expectations from theory, if sufficiently large voids are considered. For smaller voids we observe a shift towards higher values. This has important consequences on cosmological parameter inference, as the problem of unknown tracer bias is alleviated up to a constant number. The smallest scales in existing data sets become accessible to simpler models, providing numerous modes of the density field that have been disregarded so far, but may help to further reduce statistical errors in constraining cosmology.
We present results from the first cosmological simulations which study the onset of primordial, metal-free (population III), cosmic star formation and the transition to the present-day, metal-rich ...star formation (population II-I), including molecular (H2, HD, etc.) evolution, tracing the injection of metals by supernovæ (SNe) into the surrounding intergalactic medium and following the change in the initial mass function (IMF) according to the metallicity of the corresponding stellar population. Our investigation addresses the role of a wide variety of parameters (critical metallicity for the transition, IMF slope and range, SN/pair-instability SN metal yields, star formation threshold, resolution, etc.) on the metal-enrichment history and the associated transition in the star formation mode. All simulations present common trends. Metal enrichment is very patchy, with rare, unpolluted regions surviving at all redshifts, inducing the simultaneous presence of metal-free and metal-rich star formation regimes. As a result of the rapid pollution within high-density regions due to the first SN/pair-instability SN, local metallicity is quickly boosted above the critical metallicity for the transition. For this reason, population III stars dominate only during the very first stages of structure formation, with an average contribution to the total star formation rate that reaches a constant value of ∼10−3 at redshift ∼11–13. If primordial supenovæ consisted only of type II ones, the contribution would be ∼10−1. Interestingly, the above conclusions are independent from many poorly constrained parameters.
State-of-the-art integral field surveys like ATLAS3D, SLUGGS, CALIFA, SAMI, and MaNGA provide large data sets of kinematical observations of early-type galaxies (ETGs), yielding constraints on the ...formation of ETGs. Using the cosmological hydrodynamical Magneticum Pathfinder simulations, we investigate the paradigm of fast- and slow-rotating ETGs in a fully cosmological context. We show that the ETGs within the Magneticum simulation are in remarkable agreement with the observations, revealing fast and slow rotators quantified by the angular momentum proxy λR and the flattening ɛ with the observed prevalence. Taking full advantage of the three-dimensional data, we demonstrate that the dichotomy between fast- and slow-rotating galaxies gets enhanced, showing an upper and lower population separated by an underpopulated region in the edge-on λ _{R_{1/2}}-ɛ plane. We show that the global anisotropy parameter inferred from the λ _{R_{1/2}}-ɛ edge-on view is a very good predictor of the true anisotropy of the system. This drives a physically based argument for the location of fast rotators in the observed plane. Following the evolution of the λ _{R_{1/2}}-ɛ plane through cosmic time, we find that, while the upper population is already in place at z = 2, the lower population gets statistically significant below z = 1 with a gradual increase. At least 50{{ per cent}} of the galaxies transition from fast to slow rotators on a short time scale, in most cases associated to a significant merger event. Furthermore, we connect the M*-j* plane, quantified by the b-value, with the λ _{R_{1/2}}-ɛ plane, revealing a strong correlation between the position of a galaxy in the λ _{R_{1/2}}-ɛ plane and the b-value. Going one step further, we classify our sample based on features in their velocity map, finding all five common kinematic groups, also including the recently observed group of prolate rotators, populating distinct regions in the λ _{R_{1/2}}-b plane.
We study the use of red-sequence-selected galaxy spectroscopy for unbiased estimation of galaxy cluster masses by using a publicly available simulated galaxy catalog. We explore the impact of ...selection using galaxy color, projected separation from the cluster center, galaxy luminosity, and spectroscopic redshift. We identify and characterize each of the following sources of bias and scatter in velocity dispersion at fixed mass: the intrinsic properties of halos in the form of halo triaxiality, sampling noise, the presence of multiple kinematic populations within the cluster, and the effect of interlopers. We show that even in red-sequence and spectroscopically selected galaxy samples, the interloper fraction is significant, and that the variations in the interloper population from cluster to cluster provide the dominant contribution to the velocity dispersion scatter at fixed mass. We present measurements of the total scatter in dispersion at fixed mass as a function of the number of redshifts. Results indicate that improvements in scatter are modest beyond samples of ~30 redshifts per cluster. Our results show that while cluster velocity dispersions extracted from a few dozen red-sequence-selected galaxies do not provide precise masses on a single cluster basis, an ensemble of cluster velocity dispersions can be combined to produce a precise calibration of a cluster survey-mass-observable relation. Currently, disagreements in the literature on simulated subhalo velocity dispersion-mass relations place a systematic floor on velocity dispersion mass calibration at the 5% level in dispersion.
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