arXiv subjects

Takahiro Nishimichi

Publications and source records attributed to Takahiro Nishimichi.

At least 73 records · Page 4Linked to original sources

arXiv2021

Power Spectrum of Intrinsic Alignments of Galaxies in IllustrisTNG

We present the 3-{\it dimensional} intrinsic alignment power spectra between the projected 2d galaxy shape/spin and the 3d tidal field across $0.1<k/h{\rm Mpc}^{-1}<60$ using cosmological hydrodynamical simulation, Illustris-TNG300, at redshifts ranging from $0.3$ to $2$. The shape-tidal field alignment increases with galaxy mass and the linear alignment coefficient $A_{\rm IA}$, defined with respect to the primordial tidal field, is found to have weak redshift dependence. We also show a promising detection of the shape/spin-tidal field alignments for stellar mass limited samples and a weak or almost null signal for star-forming galaxies for the TNG300 volume, $\sim 0.01~(h^{-1}{\rm Gpc})^3$. We further study the morphology and environmental dependence of the intrinsic alignment power spectra. The shape of massive disk- and spheroid-galaxies tend to align with the tidal field. The spin of low mass disks (and spheroids at low redshifts) tend to be parallel with the tidal field, while the spin of massive spheroids and disks tend to be perpendicular to tidal field. The shape and spin of massive centrals align with the tidal field at both small and large scales. Satellites show a radial alignment within the one-halo term region, and low mass satellites have an intriguing alignment signal in the two-halo term region. We also forecast a feasibility to measure the intrinsic alignment power spectrum for spectroscopic and imaging surveys such as Subaru HSC/PFS and DESI. Our results thus suggest that galaxy intrinsic alignment can be used as a promising tool for constraining the galaxy formation models.

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arXiv2021

Covariance of the matter power spectrum including the survey window function effect: N-body simulations vs. fifth-order perturbation theory on grid

We present a Next-to-next-to-leading (fifth or NNLO) order calculation for the covariance matrix of the matter power spectrum, taking into account the effect of survey window functions. Using the grid-based calculation scheme for the standard perturbation theory, GridSPT, we quickly generate multiple realizations of the nonlinear density fields to fifth order in perturbation theory, then estimate the power spectrum and the covariance matrix from the sample. To the end, we have obtained the non-Gaussian covariance originated from the one-loop trispectrum without explicitly computing the trispectrum. By comparing the GridSPT calculations with the N-body results, we show that NNLO GridSPT result reproduces the N-body results on quasi-linear scales where SPT accurately models nonlinear matter power spectrum. Incorporating the survey window function effect to GridSPT is rather straightforward, and the resulting NNLO covariance matrix also matches well with the N-body results.

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arXiv2020

Cosmological inference from emulator based halo model I: Validation tests with HSC and SDSS mock catalogs

We present validation tests of emulator-based halo model method for cosmological parameter inference, assuming hypothetical measurements of the projected correlation function of galaxies, $w_{\rm p}(R)$, and the galaxy-galaxy weak lensing, $Δ\!Σ(R)$, from the spectroscopic SDSS galaxies and the Hyper Suprime-Cam Year1 (HSC-Y1) galaxies. To do this, we use \textsc{Dark Emulator} developed in Nishimichi et al. based on an ensemble of $N$-body simulations, which is an emulation package enabling a fast, accurate computation of halo clustering quantities for flat-geometry $w$CDM cosmologies. Adopting the halo occupation distribution, the emulator allows us to obtain model predictions of $Δ\!Σ$ and $w_{\rm p}$ for the SDSS-like galaxies at a few CPU seconds for an input set of parameters. We present performance and validation of the method by carrying out Markov Chain Monte Carlo analyses of the mock signals measured from a variety of mock catalogs that mimic the SDSS and HSC-Y1 galaxies. We show that the halo model method can recover the underlying true cosmological parameters to within the 68\% credible interval, except for the mocks including the assembly bias effect (although we consider the unrealistically-large amplitude of assembly bias effect). Even for the assembly bias mock, we demonstrate that the cosmological parameters can be recovered {\it if} the analysis is restricted to scales $R\gtrsim 10~h^{-1}{\rm Mpc}$. We also show that, by using a single population of source galaxies to infer the relative strengths of $Δ\!Σ$ for multiple lens samples at different redshifts, the joint probes method allows for self-calibration of photometric redshift errors and multiplicative shear bias. Thus we conclude that the emulator-based halo model method can be safely applied to the HSC-Y1 dataset, achieving a precision of $σ(S_8)\simeq 0.04$.

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arXiv2020

Blinded challenge for precision cosmology with large-scale structure: results from effective field theory for the redshift-space galaxy power spectrum

An accurate theoretical template for the galaxy power spectrum is a key for the success of ongoing and future spectroscopic surveys. We examine to what extent the Effective Field Theory of Large Scale Structure is able to provide such a template and correctly estimate cosmological parameters. To that end, we initiate a blinded challenge to infer cosmological parameters from the redshift-space power spectrum of high-resolution mock catalogs mimicking the BOSS galaxy sample but covering a hundred times larger cumulative volume. This gigantic simulation volume allows us to separate systematic bias due to theoretical modeling from the statistical error due to sample variance. The challenge task was to measure three unknown input parameters used in the simulation: the Hubble constant, the matter density fraction, and the clustering amplitude. We present analyses done by two independent teams, who have fitted the mock simulation data generated by yet another independent group. This allows us to avoid any confirmation bias by analyzers and pin down possible tuning of the specific EFT implementations. Both independent teams have recovered the true values of the input parameters within sub-percent statistical errors corresponding to the total simulation volume.

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arXiv2020

Power spectrum of halo intrinsic alignments in simulations

We use a suite of $N$-body simulations to study intrinsic alignments (IA) of halo shapes with the surrounding large-scale structure in the $Λ$CDM model. For this purpose, we develop a novel method to measure multipole moments of the three-dimensional power spectrum of the $E$-mode field of halo shapes with the matter/halo distribution, $P_{δE}^{(\ell)}(k)$ (or $P^{(\ell)}_{{\rm h}E}$), and those of the auto-power spectrum of the $E$ mode, $P^{(\ell)}_{EE}(k)$, based on the $E$/$B$-mode decomposition. The IA power spectra have non-vanishing amplitudes over the linear to nonlinear scales, and the large-scale amplitudes at $k\lesssim 0.1~h~{\rm Mpc}^{-1}$ are related to the matter power spectrum via a constant coefficient ($A_{\rm IA}$), similar to the linear bias parameter of galaxy or halo density field. We find that the cross- and auto-power spectra $P_{δE}$ and $P_{EE}$ at nonlinear scales, $k\gtrsim 0.1~h~{\rm Mpc}^{-1}$, show different $k$-dependences relative to the matter power spectrum, suggesting a violation of the nonlinear alignment model commonly used to model contaminations of cosmic shear signals. The IA power spectra exhibit baryon acoustic oscillations, and vary with halo samples of different masses, redshifts and cosmological parameters ($Ω_{\rm m}, S_8$). The cumulative signal-to-noise ratio for the IA power spectra is about 60% of that for the halo density power spectrum, where the super-sample covariance is found to give a significant contribution to the total covariance. Thus our results demonstrate that the IA power spectra of galaxy shapes, measured from imaging and spectroscopic surveys for an overlapping area of the sky, can be used to probe the underlying matter power spectrum, the primordial curvature perturbations, and cosmological parameters, in addition to the standard galaxy density power spectrum.

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arXiv2020

Impacts of pre-initial conditions on anisotropic separate universe simulations: a boosted tidal response in the epoch of reionization

To generate initial conditions for cosmological $N$-body simulations, one needs to prepare a uniform distribution of simulation particles, so-called the pre-initial condition (pre-IC). The standard method to construct the pre-IC is to place the particles on the lattice grids evenly spaced in the three-dimensional spatial coordinates. However, even after the initial displacement of each particle according to cosmological perturbations, the particle distribution remains to display an artificial anisotropy. Such an artifact causes systematic effects in simulations at later time until the evolved particle distribution sufficiently erases the initial anisotropy. In this paper, we study the impacts of the pre-IC on the anisotropic separate universe simulation, where the effect of large-scale tidal field on structure formation is taken into account using the anisotropic expansion in a local background (simulation volume). To quantify the impacts, we compare the simulations employing the standard grid pre-IC and the glass one, where the latter is supposed to suppress the initial anisotropy. We show that the artificial features in the grid pre-IC simulations are seen until $z\sim 9$, while the glass pre-IC simulations appear to be stable and accurate over the range of scales we study. From these results we find that a coupling of the large-scale tidal field with matter clustering is enhanced compared to the leading-order prediction of perturbation theory in the quasi non-linear regime in the redshift range $5\lesssim z\lesssim 15$, indicating the importance of tidal field on structure formation at such high redshifts, e.g. during the epoch of reionization.

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arXiv2020

Accurate emulator for the redshift-space power spectrum of dark matter halos and its application to galaxy power spectrum

An accurate theoretical template of the redshift-space galaxy power spectrum, if applicable out to nonlinear scales, enables us to extract more stringent and robust constraints on cosmological parameters from the measured galaxy clustering. In this work, we develop a simulation-based template, so-called emulator, for the redshift-space power spectrum of dark matter halos. Using the redshift-space halo power spectra measured from the Dark Quest $N$-body simulation suite that covers 101 flat-geometry $w$CDM cosmologies around the Planck $Λ$CDM model, we feed these data into a feed-forward neural network to build the fast and accurate emulation of the power spectrum from the linear to nonlinear scales up to $k \simeq 0.6 \,h \,{\rm Mpc}^{-1}$. Our emulator achieves about 1% and 5% fractional accuracies in predicting the monopole and quadrupole moments of the power spectrum, respectively, for halos of $\sim 10^{13}h^{-1}M_\odot$ that correspond to host halos of the SDSS LOWZ- and CMASS-like galaxies, where the achieved accuracies are sufficient compared to the statistical errors of SDSS volume. The validation and performance of the emulator are given by the comparison of the emulator predictions with the power spectra directly measured from the simulations for validation sets that are not used in the training. We demonstrate that the emulator outputs can be used to make model predictions for the redshift-space power spectrum of galaxies by employing user-fed models for the halo-galaxy connection, such as the halo occupation distribution. The emulator allows us to easily incorporate the Finger-of-God effect due to the virial motions of galaxies and the Alcock-Paczyński distortions. Our code can compute the redshift-space galaxy power spectrum in a CPU subseconds and is ready to perform the emulator-based cosmological analysis for the exiting and upcoming galaxy redshift surveys.

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arXiv2020

Power spectrum response of large-scale structure in 1D and in 3D: tests of prescriptions for post-collapse dynamics

The power spectrum response function of the large-scale structure of the Universe describes how the evolved power spectrum is modified by a small change in initial power through non-linear mode coupling of gravitational evolution. It was previously found that the response function for the coupling from small to large scales is strongly suppressed in amplitude, especially at late times, compared to predictions from perturbation theory (PT) based on the single-stream approximation. One obvious explanation for this is that PT fails to describe the dynamics beyond shell-crossing. We test this idea by comparing measurements in $N$-body simulations to prescriptions based on PT but augmented with adaptive smoothing to account for the formation of non-linear structures of various sizes in the multi-stream regime. We first start with one-dimensional (1D) cosmology, where the Zel'dovich approximation provides the exact solution in the single stream regime. Similarly to the 3D case, the response function of the large-scale modes exhibits a strong suppression in amplitude at small scales which cannot be explained by the Zel'dovich solution alone. However, by performing adaptive smoothing of initial conditions to identify haloes of different sizes and solving approximately post-collapse dynamics in the 3-streams regime, agreement between theory and simulations drastically improves. We extend our analyses to the 3D case using PINOCCHIO algorithm, in which similar adaptive smoothing is implemented on the Lagrangian PT fields to identify haloes and is combined with a spherical halo prescription to account for post-collapse dynamics. Again, a suppression is found in the coupling between small- and large-scale modes and the agreement with simulations is improved.

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arXiv2020

Validating a minimal galaxy bias method for cosmological parameter inference using HSC-SDSS mock catalogs

We assess the performance of a perturbation theory inspired method for inferring cosmological parameters from the joint measurements of galaxy-galaxy weak lensing ($ΔΣ$) and the projected galaxy clustering ($w_{\rm p}$). To do this, we use a wide variety of mock galaxy catalogs constructed based on a large set of $N$-body simulations that mimic the Subaru HSC-Y1 and SDSS galaxies, and apply the method to the mock signals to address whether to recover the underlying true cosmological parameters in the mocks. We find that, as long as the appropriate scale cuts, $12$ and $8~h^{-1}{\rm Mpc}$ for $ΔΣ$ and $w_{\rm p}$ respectively, are adopted, a "minimal-bias" model using the linear bias parameter $b_1$ alone and the nonlinear matter power spectrum can recover the true cosmological parameters (here focused on $Ω_{\rm m}$ and $σ_8$) to within the 68% credible interval, for all the mocks we study including one in which an assembly bias effect is implemented. This is as expected if physical processes inherent in galaxy formation/evolution are confined to local, small scales below the scale cut, and thus implies that real-space observables have an advantage in filtering out the impact of small-scale nonlinear effects in parameter estimation, compared to their Fourier-space counterparts. In addition, we find that a theoretical template including the higher-order bias contributions such as nonlinear bias parameter $(b_2)$ does not improve the cosmological constraints, but rather leads to a larger parameter bias compared to the baseline $b_1$-method.

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arXiv2020

Towards a non-Gaussian model of redshift space distortions

To understand the nature of the accelerated expansion of the Universe, we need to combine constraints on the expansion rate and growth of structure. The growth rate is usually extracted from three dimensional galaxy maps by exploiting the effects of peculiar motions on galaxy clustering. However, theoretical models of the probability distribution function (PDF) of galaxy pairwise peculiar velocities are not accurate enough on small scales to reduce the error on theoretical predictions to the level required to match the precision expected for measurements from future surveys. Here, we improve the modelling of the pairwise velocity distribution by using the Skew-T PDF, which has nonzero skewness and kurtosis. Our model accurately reproduces the redshift-space multipoles (monopole, quadrupole and hexadecapole) predicted by N-body simulations, above scales of about $10\,h^{-1}{\rm Mpc}$. We illustrate how a Taylor expansion of the streaming model can reveal the contributions of the different moments to the clustering multipoles, which are independent of the shape of the velocity PDF. The Taylor expansion explains why the Gaussian streaming model works well in predicting the first two redshift-space multipoles, although the velocity PDF is non-Gaussian even on large scales. Indeed, any PDF with the correct first two moments would produce precise results for the monopole down to scales of about $10\,h^{-1}{\rm Mpc}$, and for the quadrupole down to about $30\,h^{-1}{\rm Mpc}$. An accurate model for the hexadecapole needs to include higher-order moments.

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arXiv2020

Semi-analytic modelling of AGNs: auto-correlation function and halo occupation

The spatial clustering of active galactic nuclei (AGNs) is considered to be one of the important diagnostics for the understanding of the underlying processes behind their activities complementary to measurements of the luminosity function (LF). We analyse the AGN clustering from a recent semi-analytic model performed on a large cosmological $N$-body simulation covering a cubic gigaparsec comoving volume. We have introduced a new time-scale of gas accretion on to the supermassive black holes to account for the loss of the angular momentum on small scales, which is required to match the faint end of the observed X-ray LF. The large simulation box allows us accurate determination of the auto-correlation function of the AGNs. The model prediction indicates that this time-scale plays a significant role in allowing massive haloes to host relatively faint population of AGNs, leading to a higher bias factor for those AGNs. The model predictions are in agreement with observations of X-ray selected AGNs in the luminosity range $10^{41.5}~\mathrm{erg} \ \mathrm{s}^{-1} \leq L_{2-10\mathrm{keV}} \leq 10^{44.5}~\mathrm{erg} \ \mathrm{s}^{-1}$, with the typical host halo mass of $10^{12.5-13.5} h^{-1}\,{\rm M}_{\odot}$ at $z \lesssim 1$. This result shows that the observational clustering measurements impose an independent constraint on the accretion time-scale complementary to the LF measurements. Moreover, we find that not only the effective halo mass corresponding to the overall bias factor, but the extended shape of the predicted AGN correlation function shows remarkable agreement with those from observations. Further observational efforts towards the low luminosity end at $z \sim 1$ would give us stronger constraints on the triggering mechanisms of AGN activities through their clustering.

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arXiv2020

The impact of projection effects on cluster observables: stacked lensing and projected clustering

An optical cluster finder inevitably suffers from projection effects, where it misidentifies a superposition of galaxies in multiple halos along the line-of-sight as a single cluster. Using mock cluster catalogs built from cosmological N-body simulations, we quantify the impact of these projection effects with a particular focus on the observables of interest for cluster cosmology, namely the cluster lensing and the cluster clustering signals. We find that "observed" clusters, i.e. clusters identified by our cluster finder algorithm, exhibit lensing and clustering signals that deviate from expectations based on a statistically isotropic halo model -- while both signals agree with halo model expectations on small scales, they show unexpected boosts on large scales, by up to a factor of 1.2 or 1.4 respectively. We identify the origin of these boosts as the inherent selection bias of optical cluster finders for clusters embedded within filaments aligned with the line-of-sight, and show that a minority ($\sim 30\%$) of such clusters within the entire sample is responsible for this observed boost. We discuss the implications of our results on previous studies of optical cluster, as well as prospects for identifying and mitigating projection effects in future cluster cosmology analyses.

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arXiv2020

Anisotropic separate universe simulations

The long-wavelength coherent overdensity and tidal force, which are not direct observables for a finite-volume survey, affect time evolution of cosmic structure formation and therefore clustering observables through the mode coupling. In this paper we develop an "anisotropic" separate universe (SU) simulation technique to simulate large-scale structure formation taking into account the effect of large-scale tidal force into the anisotropic expansion of local background. We modify the TreePM N-body simulation code to implement the anisotropic SU simulations, and then study the "response" function of matter power spectrum that describes how the matter power spectrum responds to the large-scale tidal effect as a function of wavenumber and redshift for a given global cosmology. We test and validate the SU simulation results from the comparison with the perturbation theory predictions and the results from high-resolution PM simulations. We find that the response function displays characteristic scale dependences over the range of scales down to nonlinear scales, up to k ~ 6 h/Mpc.

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arXiv2020

Shapes and alignments of dark matter haloes and their brightest cluster galaxies in 39 strong lensing clusters

We study shapes and alignments of 45 dark matter (DM) haloes and their brightest cluster galaxies (BCGs) using a sample of 39 massive clusters from Hubble Frontier Field (HFF), Cluster Lensing And Supernova survey with Hubble (CLASH), and Reionization Lensing Cluster Survey (RELICS). We measure shapes of the DM haloes by strong gravitational lensing, whereas BCG shapes are derived from their light profiles in Hubble Space Telescope images. Our measurements from a large sample of massive clusters presented here provide new constraints on dark matter and cluster astrophysics. We find that DM haloes are on average highly elongated with the mean ellipticity of $0.482\pm 0.028$, and position angles of major axes of DM haloes and their BCGs tend to be aligned well with the mean value of alignment angles of $22.2\pm 3.9$ deg. We find that DM haloes in our sample are on average more elongated than their BCGs with the mean difference of their ellipticities of $0.11\pm 0.03$. In contrast, the Horizon-AGN cosmological hydrodynamical simulation predicts on average similar ellipticities between DM haloes and their central galaxies. While such a difference between the observations and the simulation may well be explained by the difference of their halo mass scales, other possibilities include the bias inherent to strong lensing measurements, limited knowledge of baryon physics, or a limitation of cold dark matter.

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arXiv2020

Model independent measurement of the growth rate from the consistency relations of the LSS

The Consistency Relations for the Large Scale Structure provide a link between the amplitude of Baryonic Acoustic Oscillations in the squeezed bispectrum (BS) and in the power spectrum (PS). This relation depends on the large scale bias of the considered tracer, $b_α$, and on the growth rate of structures, $f$. Remarkably, originating from basic symmetry principles, this relation is exact and independent on the underlying cosmological model. By analysing data from large volume simulations, both for dark matter and for haloes, we illustrate how BS and PS measurements can be used to extract $b_α$ and $f$ without the need of any theoretical approximation scheme for the computation of the BS and the PS. We show that, combining measurements of the squeezed BS with the quadrupole to monopole ratios for the PS at large scales can successfully break the $b_α-f$ degeneracy. We forecast that this method, applied to a Euclid-like survey, will be able to measure bias, and then the growth rate, at better than $10\%$ level, with no extra assumption.

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arXiv2020

Fitting the nonlinear matter bispectrum by the Halofit approach

We provide a new fitting formula of the matter bispectrum in the nonlinear regime calibrated by high-resolution cosmological $N$-body simulations of $41$ cold dark matter ($w$CDM, $w=$ constant) models around the Planck 2015 best-fit parameters. As the parameterization in our fitting function is similar to that in Halofit, our fitting is named BiHalofit. The simulation volume is sufficiently large ($> 10 \, {\rm Gpc}^3$) to cover almost all measurable triangle bispectrum configurations in the universe. The function is also calibrated using one-loop perturbation theory at large scales ($k<0.3 \, h \, {\rm Mpc}^{-1}$). Our formula reproduced the matter bispectrum to within $10 \, (15) \, \%$ accuracy in the Planck 2015 model at wavenumber $k< 3 \, (10) \, h \, {\rm Mpc}^{-1}$ and redshifts $z=0-3$. The other $40$ $w$CDM models obtained poorer fits, with accuracy approximating $20 \, \%$ at $k<3 \, h \, {\rm Mpc}^{-1}$ and $z=0-1.5$ (the deviation includes the $10 \, \%$-level sample variance of the simulations). We also provide a fitting formula that corrects the baryonic effects such as radiative cooling and active galactic nucleus feedback, using the latest hydrodynamical simulation IllustrisTNG. We demonstrate that our new formula more accurately predicts the weak-lensing bispectrum than the existing fitting formulas. This formula will assist current and future weak-lensing surveys and cosmic microwave background lensing experiments. Numerical codes of the formula are available, written in Python, C and Fortran.

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arXiv2020

Measuring Bias via the Consistency Relations of the Large Scale Structure

Consistency Relations (CR) for the Large Scale Structure are exact equalities between correlation functions of different order. These relations descend from the equivalence principle and hold for primordial perturbations generated by single-field models of inflation. They are not affected by nonlinearities and hold also for biased tracers and in redshift space. We show that Baryonic Acoustic Oscillations (BAO) in the bispectrum (BS) in the squeezed limit are suppressed with respect to those in the power spectrum (PS) by a coefficient that depends on the BS configuration and on the bias parameter (and, in redshift space, also on the growth rate). We test these relations using large volume N-body simulations and show that they provide a novel way to measure large scale halo bias and, potentially, the growth rate. Since bias is obtained by comparing two directly observable quantities, the method is free from theoretical uncertainties both on the computational scheme and on the underlying cosmological model.

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arXiv2020

Lagrangian-space Gaussian ansatz for the matter redshift-space power spectrum and correlation function

We study the predictions for the matter redshift-space power spectrum and correlation function of a Lagrangian-space Gaussian ansatz introduced in a previous work. This model is a natural extension of the Zeldovich approximation, where the displacement and velocity power spectra are determined by the equations of motion, instead of being set equal to the linear power spectrum. It does not contain any free parameter. As for the real-space statistics, we find that this Lagrangian-space approach is much more efficient for the correlation functions than for the power spectra. The damping of the BAO oscillations is well recovered but there is a large smooth drift from the simulations in the power spectra. The multipoles of the correlation functions are well recovered on BAO scales, with an accuracy of $2\%$ for $ξ^s_0$ down to $10 h^{-1}$ Mpc, and of $3\%$ for $ξ^s_2$ down to $26 h^{-1}$ Mpc, at $z \geq 0.35$.

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