Search

We identify voids as maximal non-overlapping spheres within the haloes of the Uchuu simulation and three smaller halo simulation boxes with smaller volume and different $\sigma_{8}$ values, and galaxies with redshift in the range $0.02

We measure the two point correlation function (CF) of 1357 galaxy clusters with a mass of $\log_{10}{M_{200}}\geq 13.6$~\hm~and at a redshift of $z \leq 0.125$. This work differs from previous analyses in that it utilizes a spectroscopic cluster catalogue, $\mathtt{SDSS-GalWCat}$, to measure the CF and detect the baryon acoustic oscillation (BAO) signal. Unlike previous studies which use statistical techniques, we compute covariance errors directly by generating a set of 1086 galaxy cluster lightcones from the GLAM $N$-body simulation. Fitting the CF with a power-law model of the form $\xi(s) = (s/s_0)^{-\gamma}$, we determine the best-fit correlation length and power-law index at three mass thresholds. We find that the correlation length increases with increasing the mass threshold while the power-law index is almost constant. For $\log_{10}{M_{200}}\geq 13.6$~\hm, we find $s_0 = 14.54\pm0.87$~\h~and $\gamma=1.97\pm0.11$. We detect the BAO signal at $s = 100$~\h~with a significance of $1.60 \sigma$. Fitting the CF with a $\Lambda$CDM model, we find $D_\mathrm{V}(z = 0.089)\mathrm{r}^{fid}_d/\mathrm{r}_d = 267.62 \pm 26$ \h, consistent with Planck 2015 cosmology. We present a set of 108 high-fidelity simulated galaxy cluster lightcones from the high-resolution \U~N-body simulation, employed for methodological validation. We find $D_\mathrm{V}(z = 0.089)/r_d = 2.666 \pm 0.129$, indicating that our method does not introduce any bias in the parameter estimation for this small sample of galaxy clusters., Comment: 7 pages, 5 figures

Gravitational $N$-body simulations calculate numerous interactions between particles. The tree algorithm reduces these calculations by constructing a hierarchical oct-tree structure and approximating gravitational forces on particles. Over the last three decades, the tree algorithm has been extensively used in large-scale simulations, and its parallelization in distributed memory environments has been well studied. However, recent supercomputers are equipped with many CPU cores per node, and optimizations of the tree construction in shared memory environments are becoming crucial. We propose a novel tree construction method in contrast to the conventional top-down approach. It first creates all leaf cells without traversing the tree and then constructs the remaining cells by a bottom-up approach. We evaluated the performance of our novel method on the supercomputer Fugaku and an Intel machine. On a single thread, our method accelerates one of the most time-consuming processes of the conventional tree construction method by a factor of above 3.0 on Fugaku and 2.2 on the Intel machine. Furthermore, as the number of threads increases, our parallel tree construction time reduces considerably. Compared to the conventional sequential tree construction method, we achieve a speedup of over 45 on 48 threads of Fugaku and more than 56 on 112 threads of the Intel machine. In stark contrast to the conventional method, the tree construction with our method no longer constitutes a bottleneck in the tree algorithm, even when using many threads., Comment: 12 pages, 6 figures, accepted by MNRAS

This study investigates the clustering and bias of Luminous Red Galaxies (LRG) in the BOSS-LOWZ, -CMASS, -COMB, and eBOSS samples, using two types of simulated lightcones: (i) high-fidelity lightcones from Uchuu $N$-body simulation, employing SHAM technique to assign LRG to (sub)halos, and (ii) 16000 covariance lightcones from GLAM-Uchuu $N$-body simulations, including LRG using HOD data from Uchuu. Our results indicate that Uchuu and GLAM lightcones closely replicate BOSS/eBOSS data, reproducing correlation function and power spectrum across scales from redshifts $0.2$ to $1.0$, from $2$ to $150\,h^{-1}\mathrm{Mpc}$ in configuration space, from $0.005$ to $0.7\,h\mathrm{Mpc}^{-1}$ in Fourier space, and across different LRG stellar masses. Furthermore, comparing with existing MD-Patchy and EZmock BOSS/eBOSS lightcones based on approximate methods, our GLAM-Uchuu lightcones provide more precise clustering estimates. We identify significant deviations from observations within $20\,h^{-1}\mathrm{Mpc}$ scales in MD-Patchy and EZmock, with our covariance matrices indicating that these methods underestimate errors by between $10\%$ and $60\%$. Lastly, we explore the impact of cosmology on galaxy clustering. Our findings suggest that, given the current level of uncertainties in BOSS/eBOSS data, distinguishing models with and without massive neutrino effects on LSS is challenging. This paper highlights the Uchuu and GLAM-Uchuu simulations' robustness in verifying the accuracy of Planck cosmological parameters, providing a strong foundation for enhancing lightcone construction in future LSS surveys. We also demonstrate that generating thousands of galaxy lightcones is feasible using $N$-body simulations with adequate mass and force resolution.

Sorting is one of the most basic algorithms, and developing highly parallel sorting programs is becoming increasingly important in high-performance computing because the number of CPU cores per node in modern supercomputers tends to increase. In this study, we have implemented two multi-threaded sorting algorithms based on samplesort and compared their performance on the supercomputer Fugaku. The first algorithm divides an input sequence into multiple blocks, sorts each block, and then selects pivots by sampling from each block at regular intervals. Each block is then partitioned using the pivots, and partitions in different blocks are merged into a single sorted sequence. The second algorithm differs from the first one in only selecting pivots, where the binary search is used to select pivots such that the number of elements in each partition is equal. We compare the performance of the two algorithms with different sequential sorting and multiway merging algorithms. We demonstrate that the second algorithm with BlockQuicksort (a quicksort accelerated by reducing conditional branches) for sequential sorting and the selection tree for merging shows consistently high speed and high parallel efficiency for various input data types and data sizes., Comment: 7 pages, 6 figures, accepted by Journal of Information Processing

In their recent study, Labb\'e et al. used multi-band infrared images captured by the James Webb Space Telescope (JWST) to discover a population of red massive galaxies that formed approximately 600 million years after the Big Bang. The authors reported an extraordinarily large density of these galaxies, with stellar masses exceeding $10^{10}$ solar masses, which, if confirmed, challenges the standard cosmological model as suggested by recent studies. However, this conclusion is disputed. We contend that during the early epochs of the universe the stellar mass-to-light ratio could not have reached the values reported by Labb\'e et al. A model of galaxy formation based on standard cosmology provides support for this hypothesis, predicting the formation of massive galaxies with higher ultraviolet (UV) luminosity, which produce several hundred solar masses of stars per year and containing significant dust. These forecasts are consistent with the abundance of JWST/HST galaxies selected photometrically in the rest-frame UV wavelengths and with the properties of the recent spectroscopically-confirmed JWST/HST galaxies formed during that era. Discrepancies with Labb\'e et al. may arise from overestimation of the stellar masses, systematic uncertainties, absence of JWST/MIRI data, heavy dust extinction affecting UV luminosities, or misidentification of faint red AGN galaxies at closer redshifts. The current JWST/HST results, combined with a realistic galaxy formation model, provide strong confirmation of the standard cosmology., Comment: Submitted to Nature, matters arising

The cluster mass-richness relation (MRR) is an observationally efficient and potentially powerful cosmological tool for constraining the mean matter density of the universe and the amplitude of fluctuations using the cluster abundance technique. We derive the MRR relation using GalWCat19, a publicly available galaxy cluster catalog we created from the Sloan Digital Sky Survey-DR13 spectroscopic dataset. The MRR shows a tail at the low-richness end. Using the Illustris-TNG and mini-Uchuu cosmological numerical simulations, we demonstrate that this tail is caused by systematical uncertainties. We show that, by means of a judicious cut, identified by the use of the Hinge function, it is possible to determine a richness threshold above which the MRR is linear i.e., where cluster mass scales with richness as logM_200 = alpha + beta logN_200. We derive the MRR and show it is consistent with both sets of simulations with a slope of beta ~ 1. We use our MRR to estimate cluster masses from the GalWCat19 catalog which we then use to set constraints on omega_m and sigma_8. Utilizing the all-member MRR, we obtain constraints of omega_m = 0.31 (+0.04-0.03) and sigma_8 = 0.82 (+0.05-0.04), and utilizing the red-member MRR, we obtain omega_m = 0.31 (+0.04-0.03) and sigma_8 = 0.81 (+0.05-0.04). Our constraints on omega_m and sigma_8 are consistent and very competitive with the Planck 2018 results.

We present the public data release of the Uchuu-UM galaxy catalogues by applying the UniverseMachine algorithm to assign galaxies to the dark matter halos in the Uchuu $N$-body cosmological simulation. It includes a variety of baryonic properties for all galaxies down to $\sim 5\times10^8 M_{\odot}$ with halos in a mass range of $10^{10}

Measurements of the luminosity function of active galactic nuclei (AGN) at high redshift ($z\gtrsim 6$) are expected to suffer from field-to-field variance, including cosmic and Poisson variances. Future surveys, such as those from the Euclid telescope and James Webb Space Telescope (JWST), will also be affected by field variance. We use the Uchuu simulation, a state-of-the-art cosmological $N$-body simulation with 2.1 trillion particles in a volume of $25.7~\mathrm{Gpc}^3$, combined with a semi-analytic galaxy and AGN formation model, to generate the Uchuu-$\nu^2$GC catalog, publicly available, that allows us to investigate the field-to-field variance of the luminosity function of AGN. With this Uchuu-$\nu^2$GC model, we quantify the cosmic variance as a function of survey area, AGN luminosity, and redshift. In general, cosmic variance decreases with increasing survey area and decreasing redshift. We find that at $z\sim6-7$, the cosmic variance depends weakly on AGN luminosity. This is because the typical mass of dark matter haloes in which AGN reside does not significantly depend on luminosity. Due to the rarity of AGN, Poisson variance dominates the total field-to-field variance, especially for bright AGN. We also examine the effect of parameters related to galaxy formation physics on the field variance. We discuss uncertainties present in the estimation of the faint-end of the AGN luminosity function from recent observations, and extend this to make predictions for the expected number of AGN and their variance for upcoming observations with Euclid, JWST, and the Legacy Survey of Space and Time (LSST)., Comment: 18 pages, 16 figures, accepted for publication in MNRAS, Figs. 3, 4, 11, 13, and 14 revised, Figs. 14 and 16 in the original manuscript moved to the Appendix, All Uchuu-$\nu^2$GC data used in this paper are publicly available on our Skies & Universes website https://www.skiesanduniverses.org/Simulations/Uchuu/GalaxyCatalogues/

Elucidating dark matter density profiles in the Galactic dwarf satellites is essential to understanding not only the quintessence of dark matter, but also the evolution of the satellites themselves. In this work, we present the current constraints on dark matter densities in the Galactic ultra-faint dwarf (UFD) and diffuse galaxies. Applying our constructed non-spherical mass models to the currently available kinematic data of the 25 UFDs and 2 diffuse satellites, we find that whereas most of the galaxies have huge uncertainties on the inferred dark matter density profiles, Eridanus~II, Segue~I, and Willman~1 favor cuspy central profiles even considering effects of a prior bias. We compare our results with the simulated subhalos on the plane between the dark matter density at 150~pc and the pericenter distance. We find that the most observed satellites and the simulated subhalos are similarly distributed on this plane, except for Antlia~2, Crater~2, and Tucana~3, which are less than one tenth of the density. Despite considerable tidal effects, the subhalos detected by commonly-used subhalo finders have difficulty in explaining such a huge deviation. We also estimate the dynamical mass-to-light ratios of the satellites and confirm the ratio is linked to stellar mass and metallicity. Tucana~3 deviates largely from these relations, while it follows the mass-metallicity relation. This indicates that Tucana~3 has a cored dark matter halo, despite a significant uncertainty in its ratios., Comment: 19 pages, 13 figures, accepted for publication in ApJ

Substructures of dark matter halo, called subhalos, provide important clues to understand the nature of dark matter. We construct a useful model to describe the properties of subhalo mass functions based on the well-known analytical prescriptions, the extended Press-Schechter theory. The unevolved subhalo mass functions at arbitrary mass scales become describable without introducing free parameters. The different host halo evolution histories are directly recast to their subhalo mass functions. As applications, we quantify the effects from (i) the Poisson fluctuation, (ii) the host mass scatter, and the (iii) different tidal evolution models on observables in the current Universe with this scheme. The Poisson fluctuation dominates in the number count of the mass ratio to the host of $\sim {\cal O}(10^{-2})$, where the intrinsic scatter is smaller by a factor of a few. The host-mass scatter around its mean does not affect the subhalo mass function. Different models of the tidal evolution predict a factor of $\sim2$ difference in numbers of subhalos with $\lesssim {\cal O}(10^{-5})$, while the dependence of the Poisson fluctuation on the tidal evolution models is subtle. The scheme provides a new tool for investigating the smallest-scale structures of our Universe which are to be observed in near future experiments., Comment: to appear in Monthly Notices of the Royal Astronomical Society

We investigate the structural properties, distribution and abundance of LCDM dark matter subhaloes using the Phi-4096 and Uchuu suite of N-body cosmological simulations. Thanks to the combination of their large volume, high mass resolution and superb statistics, we are able to quantify -- for the first time consistently over more than seven decades in ratio of subhalo-to-host-halo mass -- dependencies of subhalo properties with mass, maximum circular velocity, Vmax, host halo mass and distance to host halo centre. We also dissect the evolution of these dependencies over cosmic time. We provide accurate fits for the subhalo mass and velocity functions, both exhibiting decreasing power-law slopes in the expected range of values and with no significant dependence on redshift. We also find subhalo abundance to depend weakly on host halo mass. We explore the distribution of subhaloes within their hosts and its evolution over cosmic time for subhaloes located as deep as ~0.1 per cent of the host virial radius. Subhalo structural properties are codified via a concentration parameter, cV, that does not depend on any specific, pre-defined density profile and relies only on Vmax. We derive the cV-Vmax relation in the range 7-1500 km/s and find an important dependence on distance of the subhalo to the host halo centre, as already described in Molin\'e et al. (2017). Interestingly, we also find subhaloes of the same mass to be significantly more concentrated into more massive hosts. Finally, we investigate the redshift evolution of cV, and provide accurate fits that take into account all mentioned dependencies. Our results offer an unprecedented detailed characterization of the subhalo population, consistent over a wide range of subhalo and host halo masses, as well as cosmic times. Our work enables precision work in any future research involving dark matter halo substructure., Comment: 17 pages, 15 figures, 5 tables. Comments welcome!

We study halo mass functions with high-resolution $N$-body simulations under a $\Lambda$CDM cosmology. Our simulations adopt the cosmological model that is consistent with recent measurements of the cosmic microwave backgrounds with the ${\it Planck}$ satellite. We calibrate the halo mass functions for $10^{8.5} \lower.5ex\hbox{$\; \buildrel < \over \sim \;$} M_\mathrm{vir} / (h^{-1}M_\odot) \lower.5ex\hbox{$\; \buildrel < \over \sim \;$} 10^{15.0 - 0.45 \, z}$, where $M_\mathrm{vir}$ is the virial spherical overdensity mass and redshift $z$ ranges from $0$ to $7$. The halo mass function in our simulations can be fitted by a four-parameter model over a wide range of halo masses and redshifts, while we require some redshift evolution of the fitting parameters. Our new fitting formula of the mass function has a 5\%-level precision except for the highest masses at $z\le 7$. Our model predicts that the analytic prediction in Sheth $\&$ Tormen would overestimate the halo abundance at $z=6$ with $M_\mathrm{vir} = 10^{8.5-10}\, h^{-1}M_\odot$ by $20-30\%$. Our calibrated halo mass function provides a baseline model to constrain warm dark matter (WDM) by high-$z$ galaxy number counts. We compare a cumulative luminosity function of galaxies at $z=6$ with the total halo abundance based on our model and a recently proposed WDM correction. We find that WDM with its mass lighter than $2.71\, \mathrm{keV}$ is incompatible with the observed galaxy number density at a $2\sigma$ confidence level., Comment: 19 pages, 11 figures, 2 tables. Accepted for publication in ApJ

We study the density structures of dark matter subhalos for both cold dark matter and self-interacting dark matter models using high-resolution cosmological $N$-body simulations. We quantify subhalo's central density at 150 pc from the center of each subhalo at the classical dwarf spheroidal and ultrafaint dwarf scales found in Milky-Way sized halos. By comparing them with observations, we find that the self-interacting scattering cross-section of $\sigma/m<3\ \rm{cm^{2}g^{-1}}$ is favored. Due to the combination of hosts' tide and self-interactions, the central density of subhalos with small pericenter shows a noticeable difference between the cold and the self-interacting models, indicating that Milky-Way satellites with small pericenter are ideal sites to further constrain the nature of dark matter by future large spectroscopic surveys., Comment: 8 pages, 5 figures, accepted by PRD

We introduce the Uchuu suite of large high-resolution cosmological $N$-body simulations. The largest simulation, named Uchuu, consists of 2.1 trillion ($12800^3$) dark matter particles in a box of side-length 2.0 Gpc/h, with particle mass $3.27 \times 10^{8}$ Msun/h. The highest resolution simulation, Shin-Uchuu, consists of 262 billion ($6400^3$) particles in a box of side-length 140 Mpc/h, with particle mass $8.97 \times 10^{5}$ Msun/h. Combining these simulations we can follow the evolution of dark matter halos and subhalos spanning those hosting dwarf galaxies to massive galaxy clusters across an unprecedented volume. In this first paper, we present basic statistics, dark matter power spectra, and the halo and subhalo mass functions, which demonstrate the wide dynamic range and superb statistics of the Uchuu suite. From an analysis of the evolution of the power spectra we conclude that our simulations remain accurate from the Baryon Acoustic Oscillation scale down to the very small. We also provide parameters of a mass-concentration model, which describes the evolution of halo concentration and reproduces our simulation data to within 5 per cent for halos with masses spanning nearly eight orders of magnitude at redshift 00.5, whereas no upturn is detected at z<0.5. We make publicly available various $N$-body products as part of Uchuu Data Release 1 on the Skies & Universes site. Future releases will include gravitational lensing maps and mock galaxy, X-ray cluster, and active galactic nuclei catalogues., Comment: 22 pages, 14 figures, accepted by MNRAS. We release various $N$-body products as data release 1 on http://skiesanduniverses.org/ such as subsets of simulation particles, matter power spectra, halo/subhalo catalogues, and their merger trees

The core-cusp problem is one of the controversial issues in the standard paradigm of $\Lambda$ cold dark matter ($\Lambda$CDM) theory. However, under the assumption of conventional spherical symmetry, the strong degeneracy among model parameters makes it unclear whether dwarf spheroidal (dSph) galaxies indeed have cored dark matter density profiles at the centers. In this work, we revisit this problem using non-spherical mass models, which have the advantage of being able to alleviate the degeneracy. Applying our mass models to the currently available kinematic data of the eight classical dSphs, we find that within finite uncertainties, most of these dSphs favor cusped central profiles rather than cored ones. In particular, Draco has a cusped dark matter halo with high probability even considering a prior bias. We also find the diversity of the inner slopes in their dark matter halos. To clarify the origin of this diversity, we investigate the relation between the inner dark matter density slope and stellar-to-halo mass ratio for the sample dSphs and find this relation is generally in agreement with the predictions from recent $\Lambda$CDM and hydrodynamical simulations. We also find that the simulated subhalos have anti-correlation between the dark matter density at 150 pc and pericenter distance, which is consistent with the observed one. We estimate their astrophysical factors for dark matter indirect searches and circular velocity profiles, associated with huge uncertainties. To more precisely estimate their dark matter profiles, wide-field spectroscopic surveys for the dSphs are essential., Comment: 23 pages, 13 figures, accepted for publication in ApJ

We construct a model of H$\alpha$ emitters (HAEs) based on a semi-analytic galaxy formation model, the New Numerical Galaxy Catalog ($\nu^2$GC). In this paper, we report our estimate for the field variance of the HAE distribution. By calculating the H$\alpha$ luminosity from the star-formation rate of galaxies, our model well reproduces the observed H$\alpha$ luminosity function (LF) at $z=0.4$. The large volume of the $\nu^2$GC makes it possible to examine the spatial distribution of HAEs over a region of (411.8 Mpc)$^3$ in the comoving scale. The surface number density of $z=0.4$ HAEs with $L_{\rm H\alpha} \geq 10^{40}$ erg s$^{-1}$ is 308.9 deg$^{-2}$. We have confirmed that the HAE is a useful tracer for the large-scale structure of the Universe because of their significant overdensity ($>$ 5$\sigma$) at clusters and the filamentary structures. The H$\alpha$ LFs within a survey area of $\sim$2 deg$^2$ (typical for previous observational studies) show a significant field variance up to $\sim$1 dex. Based on our model, one can estimate the variance on the H$\alpha$ LFs within given survey areas., Comment: 13 pages, 11 figures, accepted for publication in the ApJ

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., Comment: 20 pages, 13 figures, accepted in MNRAS. Error estimates of the 2PCFs and the fitting to them have been improved. Further discussions and references have been added. Main results remain unchanged

Dark matter haloes are formed through hierarchical mergers of smaller haloes in large-scale cosmic environments, and thus anisotropic subhalo accretion through cosmic filaments have some impacts on halo structures. Recent studies using cosmological simulations have shown that the orientations of haloes correlate with the direction of cosmic filaments, and these correlations significantly depend on the halo mass. Using high-resolution cosmological $N$-body simulations, we quantified the strength of filamentary subhalo accretion for galaxy- and group-sized host haloes ($M_{\rm host}=5\times10^{11-13}M_{\odot}$) by regarding the entry points of subhaloes as filaments and present statistical studies that how the shape and orientation of host haloes at redshift zero correlate with the strength of filamentary subhalo accretion. We confirm previous studies that found the host halo mass dependence of the alignment between orientations of haloes and filaments. We also show that, for the first time, the shape and orientation of haloes weakly correlate with the strength of filamentary subhalo accretion even if the host halo masses are the same. Minor-to-major axis ratios of haloes tend to decrease as their filamentary accretion gets stronger. Haloes with highly anisotropic accretion become more spherical or oblate, while haloes with isotropic accretion become more prolate or triaxial. For haloes with strong filamentary accretion, their major axes are preferentially aligned with the filaments, while their angular momentum vectors tend to be slightly more misaligned., Comment: Accepted for publication in MNRAS, 9 pages, 6 figures, 1 table

We investigate the basic properties of voids from high resolution, cosmological N-body simulations of {\Lambda}-dominated cold dark matter ({\Lambda}CDM) models, in order to compare with the analytical model of Sheth and van de Weygaert (SvdW) for void statistics. For the subsample of five dark matter simulations in the {\Lambda}CDM cosmology with box sizes ranging from 1000 Mpc/h to 8 Mpc/h, we find that the standard void-in-cloud effect is too simplified to explain several properties of identified small voids in simulations. (i) The number density of voids is found to be larger than the prediction of the analytical model up to 2 orders of magnitude below 1 Mpc/h scales. The Press-Schechter model with the linear critical threshold of void {\delta}_v = -2.71, or a naive power law, is found to provide an excellent agreement with the void size function, suggesting that the void-in-cloud effect does not suppress as much voids as predicted by the SvdW model. (ii) We then measured the density and velocity profiles of small voids, and find that they are mostly partially collapsing underdensities, instead of being completely crushed in the standard void-in-cloud scenario. (iii) Finally, we measure the void distributions in four different tidal environments, and find that the void-in-void effect alone can explain the correlation between distribution and environments, whereas the void-in-cloud effect is only weakly influencing the abundance of voids, even in filaments and clusters., Comment: 10 pages, 7 figures

We investigate the distribution of metals in the cosmological volume at $z\sim3$, in particular, provided by massive population III (Pop III) stars using a cosmological $N$-body simulation in which a model of Pop III star formation is implemented. Owing to the simulation, we can choose minihaloes where Pop III star formation occurs at $z>10$ and obtain the spatial distribution of the metals at lower-redshifts. To evaluate the amount of heavy elements provided by Pop III stars, we consider metal yield of pair-instability or core-collapse supernovae (SNe) explosions of massive stars. By comparing our results to the Illustris-1 simulation, we find that heavy elements provided by Pop III stars often dominate those from galaxies in low density regions. The median value of the volume averaged metallicity is $Z\sim 10^{-4.5 - -2} Z_{\odot}$ at the regions. Spectroscopic observations with the next generation telescopes are expected to detect the metals imprinted on quasar spectra., Comment: 10 pages, 8 figures, accepted for publication in MNRAS

The free streaming motion of dark matter particles imprints a cutoff in the matter power spectrum and set the scale of the smallest dark matter halo. Recent cosmological $N$-body simulations have shown that the central density cusp is much steeper in haloes near the free streaming scale than in more massive haloes. Here, we study the abundance and structure of subhaloes near the free streaming scale at very high redshift using a suite of unprecedentedly large cosmological $N$-body simulations, over a wide range of the host halo mass. The subhalo abundance is suppressed strongly below the free streaming scale, but the ratio between the subhalo mass function in the cutoff and no cutoff simulations is well fitted by a single correction function regardless of the host halo mass and the redshift. In subhaloes, the central slopes are considerably shallower than in field haloes, however, are still steeper than that of the NFW profile. Contrary, the concentrations are significantly larger in subhaloes than haloes and depend on the subhalo mass. We compare two methods to extrapolate the mass-concentration relation of haloes and subhaloes to z=0 and provide a new simple fitting function for subhaloes, based on a suite of large cosmological $N$-body simulations. Finally, we estimate the annihilation boost factor of a Milky-Way sized halo to be between 1.8 and 6.2., Comment: 11 pages, 9 figures, accepted by MNRAS

We investigate metal pollution onto the surface of low-mass population III stars (Pop. III survivors) via interstellar objects floating in the Galactic interstellar medium. Only recently, Tanikawa et al. analytically estimated how much metal should collide to an orbiting Pop. III survivor encouraged by the recent discovery of 'Oumuamua and suggested that ISOs are the most dominant contributor of metal enrichment of Pop. III survivors. When we consider a distribution of interstellar objects in the Galactic disc, Pop. III survivors' orbits are significant properties to estimate the accretion rate of them though Tanikawa et al. assumed one modelled orbit. To take more realistic orbits into calculating the accretion rate, we use a high-resolution cosmological $N$-body simulation that resolves dark matter minihaloes. Pop. III survivors located at solar neighbourhood have a number of chances of ISO($> 100$ m) collisions, typically $5\times10^6$ times in the last $5$ Gyr, which is one order of magnitude greater than estimated in the previous study. When we assume a power-law parameter $\alpha$ of the ISO cumulative number density with size greater than $D$ as $n \propto D^{-\alpha}$, $0.80 \, M_{\odot}$ stars should be typically polluted [Fe/H]$\sim -2$ for the case of $\alpha=2.0$. Even in the cases of $0.70$ and $0.75 \, M_{\odot}$ stars, the typical surface metallicity are around [Fe/H]$=-6 \sim -5$. From the presence of stars with their [Fe/H], we can constrain on the lower limit of the power $\alpha$, as $\alpha \gtrsim 2.0$, which is consistent with $\alpha$ of km-size asteroids and comets in the solar system. Furthermore, we provide six candidates as the ISO-polluted Pop. III stars in the case of $\alpha \sim 2.5$. Metal-poor stars so far discovered are possible to be metal-free Pop. III stars on birth., Comment: 8 pages, 3 figures, Accepted for publication in MNRAS

The presence of dark matter substructure will boost the signatures of dark matter annihilation. We review recent progress on estimates of this subhalo boost factor---a ratio of the luminosity from annihilation in the subhalos to that originating the smooth component---based on both numerical $N$-body simulations and semi-analytic modelings. Since subhalos of all the scales, ranging from the Earth mass (as expected, e.g., the supersymmetric neutralino, a prime candidate for cold dark matter) to galaxies or larger, give substantial contribution to the annihilation rate, it is essential to understand subhalo properties over a large dynamic range of more than twenty orders of magnitude in masses. Even though numerical simulations give the most accurate assessment in resolved regimes, extrapolating the subhalo properties down in sub-grid scales comes with great uncertainties---a straightforward extrapolation yields a very large amount of the subhalo boost factor of $\gtrsim$100 for galaxy-size halos. Physically motivated theoretical models based on analytic prescriptions such as the extended Press-Schechter formalism and tidal stripping modeling, which are well tested against the simulation results, predict a more modest boost of order unity for the galaxy-size halos. Giving an accurate assessment of the boost factor is essential for indirect dark matter searches and thus, having models calibrated at large ranges of host masses and redshifts, is strongly urged upon., Comment: 30 pages, 5 figures; invited review published in Galaxies, special issue "The Role of Halo Substructure in Gamma-Ray Dark Matter Searches"

Stellar streams originating in disrupted dwarf galaxies and star clusters are observed around the Milky Way and nearby galaxies. Such substructures are the important tracers that record how the host haloes have accreted progenitor galaxies. Based on the cosmological context, we investigate the relationship between structural properties of substructures such as length and thinness at $z=0$, and orbits of their progenitors. We model stellar components of a large sample of substructures around Milky Way-sized haloes by combining semi-analytic models with a high-resolution cosmological $N$-body simulation. Using the Particle Tagging method, we embed stellar components in progenitor haloes and trace phase-space distributions of the substructures down to $z=0$. We find that the length and thinness of substructures vary smoothly as the redshift when the host haloes accrete their progenitors. For substructures observed like streams at $z=0$, a large part of the progenitors is accreted by their host haloes at redshift $0.5\lesssim z\lesssim 2.5$. Substructures with progenitors out of this accretion redshift range are entirely or less disrupted by $z=0$ and cannot be observed as streams. We also find that the distributions of length and thinness of substructures vary smoothly as pericenter and apocenter of the progenitors. Substructures observed like streams tend to have the specific range of $10\ {\rm kpc} \lesssim r_{\rm peri}\lesssim100\ {\rm kpc}$ and $50\ {\rm kpc} \lesssim r_{\rm apo}\lesssim300\ {\rm kpc}$., Comment: Accepted for publication in MNRAS, 13 pages, 10 figures, 1 table

We have developed a highly-tuned software library that accelerates the calculation of quadrupole terms in the Barnes-Hut tree code by use of a SIMD instruction set on the x86 architecture, Advanced Vector eXtensions 2 (AVX2). Our code is implemented as an extension of Phantom-GRAPE software library (Tanikawa et al. 2012, 2013) that significantly accelerates the calculation of monopole terms. If the same accuracy is required, the calculation of quadrupole terms can accelerate the evaluation of forces than that of only monopole terms because we can approximate gravitational forces from closer particles by quadrupole moments than by only monopole moments. Our implementation can calculate gravitational forces about 1.1 times faster in any system than the combination of the pseudoparticle multipole method and Phantom-GRAPE. Our implementation allows simulating homogeneous systems up to 2.2 times faster than that with only monopole terms, however, speed up for clustered systems is not enough because the increase of approximated interactions is insufficient to negate the increased calculation cost by computing quadrupole terms. We have estimated that improvement in performance can be achieved by the use of a new SIMD instruction set, AVX-512. Our code is expected to be able to accelerate simulations of clustered systems up to 1.08 times faster on AVX-512 environment than that with only monopole terms., Comment: 14 pages, 6 figures, accepted by PASJ. The code is publicly available at the official website of Phantom-GRAPE https://bitbucket.org/kohji/phantom-grape

We study evolution of dark matter substructures, especially how they lose the mass and change density profile after they fall in gravitational potential of larger host halos. We develop an analytical prescription that models the subhalo mass evolution and calibrate it to results of N-body numerical simulations of various scales from very small (Earth size) to large (galaxies to clusters) halos. We then combine the results with halo accretion histories, and calculate the subhalo mass function that is physically motivated down to Earth-mass scales. Our results --- valid for arbitrary host masses and redshifts --- show reasonable agreement with those of numerical simulations at resolved scales. Our analytical model also enables self-consistent calculations of the boost factor of dark matter annhilation, which we find to increase from tens of percent at the smallest (Earth) and intermediate (dwarfs) masses to a factor of several at galaxy size, and to become as large as a factor of $\sim$10 for the largest halos (clusters) at small redshifts. Our analytical approach can accommodate substructures in the subhalos (sub-subhalos) in a consistent framework, which we find to give up to a factor of a few enhancement to the annihilation boost. Presence of the subhalos enhances the intensity of the isotropic gamma-ray background by a factor of a few, and as the result, the measurement by Fermi Large Area Telescope excludes the annihilation cross section greater than $\sim$$4\times 10^{-26}$ cm$^3$ s$^{-1}$ for dark matter masses up to $\sim$200 GeV., Comment: 14 pages, 9 figures, to appear in PRD

We present the latest results of a semi-analytic model of galaxy formation, "New Numerical Galaxy Catalogue", which is combined with large cosmological N-body simulations. This model can reproduce statistical properties of galaxies at z < 6.0. We focus on the properties of active galactic nuclei (AGNs) and supermassive black holes, especially on the accretion timescale onto black holes. We find that the number density of AGNs at z < 1.5 and at hard X-ray luminosity 10^{ 44 }< erg/s is underestimated compared with recent observational estimates when we assume the exponentially decreasing accretion rate and the accretion timescale which is proportional to the dynamical time of the host halo or the bulge, as is often assumed in semi-analytic models. We show that to solve this discrepancy, the accretion timescale of such less luminous AGNs instead should be a function of the black hole mass and the accreted gas mass. This timescale can be obtained from a phenomenological modelling of the gas angular momentum loss in the circumnuclear torus and/or the accretion disc. Such models predict a longer accretion timescale for less luminous AGNs at z < 1.0 than bright QSOs whose accretion timescale would be 10^{ 7-8 } yr. With this newly introduced accretion timescale, our model can explain the observed luminosity functions of AGNs at z < 6.0., Comment: 31 pages and 29 figures. Accepted for publication in MNRAS

The survey of Lyman $\alpha$ emitters (LAEs) with Subaru Hyper Suprime-Cam, called SILVERRUSH (Ouchi et al.), is producing massive data of LAEs at $z\gtrsim6$. Here we present LAE simulations to compare the SILVERRUSH data. In 162$^3$ comoving Mpc$^3$ boxes, where numerical radiative transfer calculations of reionization were performed, LAEs have been modeled with physically motivated analytic recipes as a function of halo mass. We have examined $2^3$ models depending on the presence or absence of dispersion of halo Ly$\alpha$ emissivity, dispersion of the halo Ly$\alpha$ optical depth, $\tau_\alpha$, and halo mass dependence of $\tau_\alpha$. The unique free parameter in our model, a pivot value of $\tau_\alpha$, is calibrated so as to reproduce the $z=5.7$ Ly$\alpha$ luminosity function (LF). We compare our model predictions with Ly$\alpha$ LFs at $z=6.6$ and $7.3$, LAE angular auto-correlation functions (ACFs) at $z=5.7$ and $6.6$, and LAE fractions in Lyman break galaxies at $5

Super-Eddington mass accretion has been suggested as an efficient mechanism to grow supermassive black holes (SMBHs). We investigate the imprint left by the radiative efficiency of the super-Eddington accretion process on the clustering of quasars using a new semi-analytic model of galaxy and quasar formation based on large-volume cosmological $N$-body simulations. Our model includes a simple model for the radiative efficiency of a quasar, which imitates the effect of photon trapping for a high mass accretion rate. We find that the model of radiative efficiency affects the relation between the quasar luminosity and the quasar host halo mass. The quasar host halo mass has only weak dependence on quasar luminosity when there is no upper limit for quasar luminosity. On the other hand, it has significant dependence on quasar luminosity when the quasar luminosity is limited by its Eddington luminosity. In the latter case, the quasar bias also depends on the quasar luminosity, and the quasar bias of bright quasars is in agreement with observations. Our results suggest that the quasar clustering studies can provide a constraint on the accretion disc model., Comment: 6 pages, 4 figures, to appear in MNRAS Letters

Motivated by a recently found interesting property of the dark halo surface density within a radius, $r_{\rm max}$, giving the maximum circular velocity, $V_{\rm max}$, we investigate it for dark halos of the Milky Way's and Andromeda's dwarf satellites based on cosmological simulations. We select and analyze the simulated subhalos associated with Milky Way-sized dark halos and find that the values of their surface densities, $\Sigma_{V_{\rm max}}$, are in good agreement with those for the observed dwarf spheroidal satellites even without employing any fitting procedures. This implies that this surface density would not be largely affected by any baryonic feedbacks and thus universal. Moreover, all subhalos on the small scales of dwarf satellites are expected to obey the relation $\Sigma_{V_{\rm max}}\propto V_{\rm max}$, irrespective of differences in their orbital evolutions, host halo properties, and observed redshifts. Therefore, we find that the universal scaling relation for dark halos on dwarf galaxy mass scales surely exists and provides us important clues to understanding fundamental properties of dark halos. We also investigate orbital and dynamical evolutions of subhalos to understand the origin of this universal dark halo relation and find that most of subhalos evolve generally along the $r_{\rm max}\propto V_{\rm max}$ sequence, even though these subhalos have undergone different histories of mass assembly and tidal stripping. This sequence, therefore, should be the key feature to understand the nature of the universality of $\Sigma_{V_{\rm max}}$., Comment: 12 pages, 5 figures and 3 tables, submitted to ApJ

We explore the effect of varying the mass of the seed black hole on the resulting black hole mass - bulge mass relation at z ~ 0, using a semi-analytic model of galaxy formation combined with large cosmological N-body simulations. We constrain our model by requiring the observed properties of galaxies at z ~ 0 are reproduced. In keeping with previous semi-analytic models, we place a seed black hole immediately after a galaxy forms. When the mass of the seed is set at 10^5 M_sun, we find that the model results become inconsistent with recent observational results of the black hole mass - bulge mass relation for dwarf galaxies. In particular, the model predicts that bulges with ~ 10^9 M_sun harbour larger black holes than observed. On the other hand, when we employ seed black holes with 10^3 M_sun, or randomly select their mass within a 10^(3-5) M_sun range, the resulting relation is consistent with observation estimates, including the observed dispersion. We find that to obtain stronger constraints on the mass of seed black holes, observations of less massive bulges at z ~ 0 are a more powerful comparison than the relations at higher redshifts., Comment: 6 pages, 2 tables, 4 figures. Accepted for publication by MNRAS Letter

Primordial dark matter (DM) haloes are the smallest gravitationally bound DM structures from which the first stars, black holes, and galaxies form and grow in the early universe. However, their structures are sensitive to the free streaming scale of DM, which in turn depends on the nature of DM particles. In this work, we test the hypothesis that the slope of the central cusps in primordial DM haloes near the free streaming scale depends on the nature of merging process. By combining and analysing data from a cosmological simulation with the cutoff in the small-scale matter power spectrum as well as a suite of controlled, high-resolution simulations of binary mergers, we find that (1) the primordial DM haloes form preferentially through major mergers in radial orbits; (2) their central DM density profile is more susceptible to a merging process compared to that of galaxy and cluster-size DM haloes; (3) consecutive major mergers drive the central density slope to approach the {\it universal} form characterized by the NFW profile, which is shown to be robust to the impacts of mergers and serves an attractor solution for the density structure of DM haloes. Our work highlights the importance of dynamical processes on the structure formation during the Dark Ages., Comment: 13 pages, 11 figures, 4 tables, accepted for publication in MNRAS

Cosmic reionization is known to be a major phase transition of the gas in the Universe. Since astronomical objects formed in the early Universe, such as the first stars, galaxies and black holes, are expected to have caused cosmic reionization, the formation history and properties of such objects are closely related to the reionization process. In spite of the importance of exploring reionization, our understandings regarding reionization is not sufficient yet. Square Kilometre Array (SKA) is a next-generation large telescope that will be operated in the next decade. Although several programs of next-generation telescopes are currently scheduled, the SKA will be the unique telescope with a potential to directly observe neutral hydrogen up to z~30, and provide us with valuable information on the Cosmic Dawn (CD) and the Epoch of Reionization (EoR). The early science with the SKA will start in a few years; it is thus the time for us to elaborate a strategy for CD/EoR Science with the SKA. The purpose of this document is to introduce Japanese scientific interests in the SKA project and to report results of our investigation., Comment: 14 pages, 12 figures, Japan SKA consortium (SKA-JP) Cosmic Dawn/Epoch of Reionization Science book

We study the number and the distribution of low mass Pop III stars in the Milky Way. In our numerical model, hierarchical formation of dark matter minihalos and Milky Way sized halos are followed by a high resolution cosmological simulation. We model the Pop III formation in H2 cooling minihalos without metal under UV radiation of the Lyman-Werner bands. Assuming a Kroupa IMF from 0.15 to 1.0 Msun for low mass Pop III stars, as a working hypothesis, we try to constrain the theoretical models in reverse by current and future observations. We find that the survivors tend to concentrate on the center of halo and subhalos. We also evaluate the observability of Pop III survivors in the Milky Way and dwarf galaxies, and constraints on the number of Pop III survivors per minihalo. The higher latitude fields require lower sample sizes because of the high number density of stars in the galactic disk, the required sample sizes are comparable in the high and middle latitude fields by photometrically selecting low metallicity stars with optimized narrow band filters, and the required number of dwarf galaxies to find one Pop III survivor is less than ten at <100 kpc for the tip of redgiant stars. Provided that available observations have not detected any survivors, the formation models of low mass Pop III stars with more than ten stars per minihalo are already excluded. Furthermore, we discuss the way to constrain the IMF of Pop III star at a high mass range of > 10 Msun., Comment: accepted by ApJ, 11 pages, 8 figures

We investigate clustering properties of quasars using a new version of our semi-analytic model of galaxy and quasar formation with state-of-the-art cosmological N-body simulations. In this study, we assume that a major merger of galaxies triggers cold gas accretion on to a supermassive black hole and quasar activity. Our model can reproduce the downsizing trend of the evolution of quasars. We find that the median mass of quasar host dark matter haloes increases with cosmic time by an order of magnitude from z=4 (a few 1e+11 Msun) to z=1 (a few 1e+12 Msun), and depends only weakly on the quasar luminosity. Deriving the quasar bias through the quasar--galaxy cross-correlation function in the model, we find that the quasar bias does not depend on the quasar luminosity, similar to observed trends. This result reflects the fact that quasars with a fixed luminosity have various Eddington ratios and thus have various host halo masses that primarily determine the quasar bias. We also show that the quasar bias increases with redshift, which is in qualitative agreement with observations. Our bias value is lower than the observed values at high redshifts, implying that we need some mechanisms that make quasars inactive in low-mass haloes and/or that make them more active in high-mass haloes., Comment: 6 pages, 3 figures, to appear in MNRAS Letters

We perform dry merger simulations to investigate the role of dry mergers in the size growth of early-type galaxies in high density environments. We replace the virialized dark matter haloes obtained by a large cosmological $N$-body simulation with $N$-body galaxy models consisting of two components, a stellar bulge and a dark matter halo, which have higher mass resolution than the cosmological simulation. We then re-simulate nine cluster forming regions, whose masses range from 1e+14 Msun to 5e+14 Msun. Masses and sizes of stellar bulges are also assumed to satisfy the stellar mass--size relation of high-z compact massive early-type galaxies. We find that dry major mergers considerably contribute to the mass and size growth of central massive galaxies. One or two dry major mergers double the average stellar mass and quadruple the average size between $z=2$ and $z=0$. These growths favorably agree with observations. Moreover, the density distributions of our simulated central massive galaxies grow from the inside-out, which is consistent with recent observations. The mass--size evolution is approximated as R propto M_{*}^{alpha}, with alpha ~ 2.24. Most of our simulated galaxies are efficiently grown by dry mergers, and their stellar mass--size relations match the ones observed in the local Universe. Our results show that the central galaxies in the cluster haloes are potential descendants of high-z (z ~ 2-3) compact massive early-type galaxies. This conclusion is consistent with previous numerical studies which investigate the formation and evolution of compact massive early-type galaxies., Comment: 15 pages, 10 figures, accepted for MNRAS; adjusted to match the published version

We present a new cosmological galaxy formation model, $\nu^2$GC, as an updated version of our previous model $\nu$GC. We adopt the so-called "semi-analytic" approach, in which the formation history of dark matter halos is computed by ${\it N}$-body simulations, while the baryon physics such as gas cooling, star formation and supernova feedback are simply modeled by phenomenological equations. Major updates of the model are as follows: (1) the merger trees of dark matter halos are constructed in state-of-the-art ${\it N}$-body simulations, (2) we introduce the formation and evolution process of supermassive black holes and the suppression of gas cooling due to active galactic nucleus (AGN) activity, (3) we include heating of the intergalactic gas by the cosmic UV background, and (4) we tune some free parameters related to the astrophysical processes using a Markov chain Monte Carlo method. Our ${\it N}$-body simulations of dark matter halos have unprecedented box size and mass resolution (the largest simulation contains 550 billion particles in a 1.12 Gpc/h box), enabling the study of much smaller and rarer objects. The model was tuned to fit the luminosity functions of local galaxies and mass function of neutral hydrogen. Local observations, such as the Tully-Fisher relation, size-magnitude relation of spiral galaxies and scaling relation between the bulge mass and black hole mass were well reproduced by the model. Moreover, the model also well reproduced the cosmic star formation history and the redshift evolution of rest-frame ${\it K}$-band luminosity functions. The numerical catalog of the simulated galaxies and AGNs is publicly available on the web., Comment: 25 pages, 21 figures, 2 tables, accepted to PASJ. The numerical catalog of galaxies and AGNs are available at http://cdsarc.u-strasbg.fr/cgi-bin/VizieR?-source=J/PASJ/68/25

We present the evolution of dark matter halos in six large cosmological N-body simulations, called the $\nu^2$GC (New Numerical Galaxy Catalog) simulations on the basis of the LCDM cosmology consistent with observational results obtained by the Planck satellite. The largest simulation consists of $8192^3$ (550 billion) dark matter particles in a box of $1.12 \, h^{-1} \rm Gpc$ (a mass resolution of $2.20 \times 10^{8} \, h^{-1} M_{\odot}$). Among simulations utilizing boxes larger than $1 \, h^{-1} \rm Gpc$, our simulation yields the highest resolution simulation that has ever been achieved. A $\nu^2$GC simulation with the smallest box consists of eight billions particles in a box of $70 \, h^{-1} \rm Mpc$ (a mass resolution of $3.44 \times 10^{6} \, h^{-1} M_{\odot}$). These simulations can follow the evolution of halos over masses of eight orders of magnitude, from small dwarf galaxies to massive clusters. Using the unprecedentedly high resolution and powerful statistics of the $\nu^2$GC simulations, we provide statistical results of the halo mass function, mass accretion rate, formation redshift, and merger statistics, and present accurate fitting functions for the Planck cosmology. By combining the $\nu^2$GC simulations with our new semi-analytic galaxy formation model, we are able to prepare mock catalogs of galaxies and active galactic nuclei, which will be made publicly available in the near future., Comment: 18 pages, 16 figures, accepted, simulation data can be downloaded on http://hpc.imit.chiba-u.jp/~nngc/

We have investigated effects of dust attenuation on quasar luminosity functions using a semi-analytic galaxy formation model combined with a large cosmological N-body simulation. We estimate the dust attenuation of quasars self-consistently with that of galaxies by considering the dust in their host bulges. We find that the luminosity of the bright quasars is strongly dimmed by the dust attenuation, about 2 mag in the B-band. Assuming the empirical bolometric corrections for active galactic nuclei (AGNs) by Marconi et al., we find that this dust attenuation is too strong to explain the B-band and X-ray quasar luminosity functions simultaneously. We consider two possible mechanisms that weaken the dust attenuation. As such a mechanism, we introduce a time delay for AGN activity, that is, gas fueling to a central black hole starts some time after the beginning of the starburst induced by a major merger. The other is the anisotropy in the dust distribution. We find that in order to make the dust attenuation of the quasars negligible, either the gas accretion into the black holes has to be delayed at least three times the dynamical timescale of their host bulges or the dust covering factor is as small as 0.1., Comment: 5 pages, 4 figures, 1 tables. Accepted for publication by MNRAS Letters

We have simulated, for the first time, the long term evolution of the Milky Way Galaxy using 51 billion particles on the Swiss Piz Daint supercomputer with our $N$-body gravitational tree-code Bonsai. Herein, we describe the scientific motivation and numerical algorithms. The Milky Way model was simulated for 6 billion years, during which the bar structure and spiral arms were fully formed. This improves upon previous simulations by using 1000 times more particles, and provides a wealth of new data that can be directly compared with observations. We also report the scalability on both the Swiss Piz Daint and the US ORNL Titan. On Piz Daint the parallel efficiency of Bonsai was above 95%. The highest performance was achieved with a 242 billion particle Milky Way model using 18600 GPUs on Titan, thereby reaching a sustained GPU and application performance of 33.49 Pflops and 24.77 Pflops respectively., Comment: 12 pages, 4 figures, Published in: 'Proceeding SC '14 Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis'. Gordon Bell Prize 2014 finalist

We investigate the weak lensing effect by line-of-sight structures with a surface mass density of <~10^8 solar mass/arcsec^2 in QSO-galaxy quadruple lens systems. Using high-resolution N-body simulations in warm dark matter (WDM) models and observed four quadruple lenses that show anomalies in the flux ratios, we obtain constraints on the mass of thermal WDM, m_WDM>= 1.3keV(95%CL) assuming that the density of the primary lens is described by a singular isothermal ellipsoid (SIE). The obtained constraint is consistent with those from Lyman-$\alpha$ forests and the number counts of high-redshift galaxies at z>4. Our results show that WDM with a free-streaming comoving wavenumber k_{fs} <= 27 h/Mpc is disfavored as the major component of cosmological density at redshifts 0.5 <~ z <~ 4 provided that the SIE models describe the gravitational potentials of the primary lenses correctly., Comment: 14 pages, 8 figures, version accepted in MNRAS. Information about k_cut is added in table 5 and figure 4

Recent observations show that the space density of luminous active galactic nuclei (AGNs) peaks at higher redshifts than that of faint AGNs. This downsizing trend in the AGN evolution seems to be contradictory to the hierarchical structure formation scenario. In this study, we present the AGN space density evolution predicted by a semi-analytic model of galaxy and AGN formation based on the hierarchical structure formation scenario. We demonstrate that our model can reproduce the downsizing trend of the AGN space density evolution. The reason for the downsizing trend in our model is a combination of the cold gas depletion as a consequence of star formation, the gas cooling suppression in massive halos and the AGN lifetime scaling with the dynamical timescale. We assume that a major merger of galaxies causes a starburst, spheroid formation, and cold gas accretion onto a supermassive black hole (SMBH). We also assume that this cold gas accretion triggers AGN activity. Since the cold gas is mainly depleted by star formation and gas cooling is suppressed in massive dark halos, the amount of cold gas accreted onto SMBHs decreases with cosmic time. Moreover, AGN lifetime increases with cosmic time. Thus, at low redshifts, major mergers do not always lead to luminous AGNs. Because the luminosity of AGNs is correlated with the mass of accreted gas onto SMBHs, the space density of luminous AGNs decreases more quickly than that of faint AGNs. We conclude that the anti-hierarchical evolution of the AGN space density is not contradictory to the hierarchical structure formation scenario., Comment: 10 pages, 5 figures. Accepted for publication in ApJ. Minor editorial changes and typos corrected

The smallest dark matter halos are formed first in the early universe. According to recent studies, the central density cusp is much steeper in these halos than in larger halos and scales as $\rho \propto r^{-(1.5-1.3)}$. We present results of very large cosmological $N$-body simulations of the hierarchical formation and evolution of halos over a wide mass range, beginning from the formation of the smallest halos. We confirmed early studies that the inner density cusps are steeper in halos at the free streaming scale. The cusp slope gradually becomes shallower as the halo mass increases. The slope of halos 50 times more massive than the smallest halo is approximately $-1.3$. No strong correlation exists between inner slope and the collapse epoch. The cusp slope of halos above the free streaming scale seems to be reduced primarily due to major merger processes. The concentration, estimated at the present universe, is predicted to be $60-70$, consistent with theoretical models and earlier simulations, and ruling out simple power law mass-concentration relations. Microhalos could still exist in the present universe with the same steep density profiles., Comment: 13 pages, 13 figures, accepted by ApJ

We present a method to couple N-body star cluster simulations to a cosmological tidal field, using the Astrophysical Multipurpose Software Environment. We apply this method to star clusters embedded in the CosmoGrid dark matter-only LambdaCDM simulation. Our star clusters are born at z = 10 (corresponding to an age of the Universe of about 500 Myr) by selecting a dark matter particle and initializing a star cluster with 32,000 stars on its location. We then follow the dynamical evolution of the star cluster within the cosmological environment. We compare the evolution of star clusters in two Milky-Way size haloes with a different accretion history. The mass loss of the star clusters is continuous irrespective of the tidal history of the host halo, but major merger events tend to increase the rate of mass loss. From the selected two dark matter haloes, the halo that experienced the larger number of mergers tends to drive a smaller mass loss rate from the embedded star clusters, even though the final masses of both haloes are similar. We identify two families of star clusters: native clusters, which become part of the main halo before its final major merger event, and the immigrant clusters, which are accreted upon or after this event; native clusters tend to evaporate more quickly than immigrant clusters. Accounting for the evolution of the dark matter halo causes immigrant star clusters to retain more mass than when the z=0 tidal field is taken as a static potential. The reason for this is the weaker tidal field experienced by immigrant star clusters before merging with the larger dark matter halo., Comment: 14 pages, 14 figures, accepted for publication in MNRAS

Observations have revealed interesting universal properties of dark matter (DM) halos especially around low-mass galaxies. Strigari et al. (2008) showed that DM halos have common enclosed masses within 300pc (Strigari relation). Kormendy & Freeman (2004) reported DM halos having almost identical central surface densities (the $\mu_{\rm 0D}$ relation). In addition, there exists a core-cusp problem, a discrepancy of the central density distribution between simulated halos and observations. We investigate whether a scenario where cuspy halos transform into cores by some dynamical processes can also explain their universal structural properties. It is shown that a cusp-to-core transformation model naturally reproduces the $\mu_{\rm 0D}$ relation and that Strigari relation follows from the $\mu_{\rm 0D}$ relation for dwarf galaxies. We also show that the central densities of cored dark halos provide valuable information about their formation redshifts., Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letters

As an entry for the 2012 Gordon-Bell performance prize, we report performance results of astrophysical N-body simulations of one trillion particles performed on the full system of K computer. This is the first gravitational trillion-body simulation in the world. We describe the scientific motivation, the numerical algorithm, the parallelization strategy, and the performance analysis. Unlike many previous Gordon-Bell prize winners that used the tree algorithm for astrophysical N-body simulations, we used the hybrid TreePM method, for similar level of accuracy in which the short-range force is calculated by the tree algorithm, and the long-range force is solved by the particle-mesh algorithm. We developed a highly-tuned gravity kernel for short-range forces, and a novel communication algorithm for long-range forces. The average performance on 24576 and 82944 nodes of K computer are 1.53 and 4.45 Pflops, which correspond to 49% and 42% of the peak speed., Comment: 10 pages, 6 figures, Proceedings of Supercomputing 2012 (http://sc12.supercomputing.org/), Gordon Bell Prize Winner. Additional information is http://www.ccs.tsukuba.ac.jp/CCS/eng/gbp2012

We present the results of the "Cosmogrid" cosmological N-body simulation suites based on the concordance LCDM model. The Cosmogrid simulation was performed in a 30Mpc box with 2048^3 particles. The mass of each particle is 1.28x10^5 Msun, which is sufficient to resolve ultra-faint dwarfs. We found that the halo mass function shows good agreement with the Sheth & Tormen fitting function down to ~10^7 Msun. We have analyzed the spherically averaged density profiles of the three most massive halos which are of galaxy group size and contain at least 170 million particles. The slopes of these density profiles become shallower than -1 at the inner most radius. We also find a clear correlation of halo concentration with mass. The mass dependence of the concentration parameter cannot be expressed by a single power law, however a simple model based on the Press-Schechter theory proposed by Navarro et al. gives reasonable agreement with this dependence. The spin parameter does not show a correlation with the halo mass. The probability distribution functions for both concentration and spin are well fitted by the log-normal distribution for halos with the masses larger than ~10^8 Msun. The subhalo abundance depends on the halo mass. Galaxy-sized halos have 50% more subhalos than ~10^{11} Msun halos have., Comment: 15 pages, 18 figures, accepted by ApJ