arXiv subjects

Daisuke Nagai

Publications and source records attributed to Daisuke Nagai.

At least 109 records · Page 6Linked to original sources

arXiv2020

Correlations between Triaxial Shapes and Formation History of Dark Matter Haloes

The shape of dark matter haloes plays a critical role in constraining cosmology with upcoming large-scale structure surveys. In this paper, we study the correlations between the triaxial shapes and formation histories in dark matter haloes in the MultiDark Planck 2 N-body cosmological simulation. We find that halo ellipticity is strongly correlated with halo properties that serve as proxies of halo formation history, such as halo concentration and the peak-centroid offset. In particular, the correlation between halo ellipticity and halo concentration is nearly independent of the halo density peak height. We present a simple model for the correlation between halo ellipticity and concentration using conditional abundance matching, and provide fitting formulae for the multi-dimensional distributions of triaxial halo shape as a function of halo peak height. We apply our halo shape model to gauge the effects of halo ellipticity and orientation bias on the excess surface mass density profiles in cluster-size halos. Our model should be useful for exploring the impact of triaxial halo shape on cosmological constraints in upcoming weak lensing surveys of galaxy clusters.

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arXiv2020

Measuring Cosmological Distances Using Cluster Edges as a Standard Ruler

The line-of-sight velocity dispersion profile of galaxy clusters exhibits a "kink" corresponding to the spatial extent of orbiting galaxies. Because the spatial extent of a cluster is correlated with the amplitude of the velocity dispersion profile, we can utilise this feature as a gravity-calibrated standard ruler. Specifically, the amplitude of the velocity dispersion data allows us to infer the physical cluster size. Consequently, observations of the angular scale of the "kink" in the profile can be translated into a distance measurement to the cluster. Assuming the relation between cluster radius and cluster velocity dispersion can be calibrated from simulations, we forecast that with existing data from the Sloan Digital Sky Survey (SDSS) we will be able to measure the Hubble constant with $3\%$ precision. Implementing our method with data from the Dark Energy Spectroscopic Instrument (DESI) will result in a $1.3\%$ measurement of the Hubble constant. Adding cosmological supernova data improves the uncertainty of the DESI measurement to $0.7\%$.

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arXiv2020

Ly$α$ Blobs from Cold Streams Undergoing Kelvin-Helmholtz Instabilities

We present an analytic toy model for the radiation produced by the interaction between the cold streams thought to feed massive halos at high redshift and their hot CGM. We begin by deriving cosmologically motivated parameters for the streams as they enter the halo virial radius, $R_{\rm v}$, as a function of halo mass and redshift. For $10^{12}M_{\odot}$ halos at $z=2$, we find the Hydrogen number density in streams to be $n_{\rm H,s}\sim (0.1-5)\times 10^{-2}{\rm cm}^{-3}$, a factor of $δ\sim (30-300)$ times denser than the hot CGM density, while the stream radii are in the range $R_{\rm s}\sim (0.03-0.50)R_{\rm v}$. As the streams accelerate towards the halo centre, they become denser and narrower. The stream-CGM interaction induces Kelvin-Helmholtz Instability (KHI), which leads to entrainment of CGM mass by the stream and therefore to stream deceleration by momentum conservation. Assuming that the entrainment rates derived by Mandelker et al. 2019 in the absence of gravity can be applied locally at each halocentric radius, we derive equations of motion for the stream in the halo. Using these, we derive the net acceleration, mass growth, and energy dissipation induced by the stream-CGM interaction, as a function of halo mass and redshift, for different CGM density profiles. For the range of model parameters considered, we find that the interaction can induce dissipation luminosities $L_{\rm diss}>10^{42}~{\rm erg~s^{-1}}$ within $\le 0.6 R_{\rm v}$ of halos with $M_{\rm v}>10^{12}M_{\odot}$ at $z=2$, with the emission scaling with halo mass and redshift approximately as $\propto M_{\rm v}\,(1+z)^2$. The magnitude and spatial extent of the emission produced in massive halos at high redshift is consistent with observed Ly$α$ blobs, though better treatment of the UV background and self-shielding is needed to solidify this conclusion.

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arXiv2020

The Formation of Ultra-Diffuse Galaxies in the RomulusC Galaxy Cluster Simulation

We study the origins of 122 ultra-diffuse galaxies (UDGs) in the {\sc RomulusC} zoom-in cosmological simulation of a galaxy cluster (M$_{200} = 1.15\times10^{14}$ M$_{\odot}$), one of the only such simulations capable of resolving the evolution and structure of dwarf galaxies (M$_{\star} < 10^9$ M$_{\odot}$). We find broad agreement with observed cluster UDGs and predict that they are not separate from the overall cluster dwarf population. UDGs in cluster environments form primarily from dwarf galaxies that experienced early cluster in-fall and subsequent quenching due to ram pressure. The ensuing dimming of these dwarf galaxies due to passive stellar evolution results in a population of very low surface brightness galaxies that are otherwise typical dwarfs. UDGs and non-UDGs alike are affected by tidal interactions with the cluster potential. Tidal stripping of dark matter, as well as mass loss from stellar evolution, results in the adiabatic expansion of stars, particularly in the lowest mass dwarfs. High mass dwarf galaxies show signatures of tidal heating while low mass dwarfs that survive until $z=0$ typically have not experienced such impulsive interactions. There is little difference between UDGs and non-UDGs in terms of their dark matter halos, stellar morphology, colors, and location within the cluster. In most respects cluster UDG and non-UDGs alike are similar to isolated dwarf galaxies, except for the fact that they are typically quenched.

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arXiv2020

PoPE: A population-based approach to model spatial structure of astronomical systems

We present a novel population-based Bayesian inference approach to model the average and population variance of spatial distribution of a set of observables from ensemble analysis of low signal-to-noise ratio measurements. The method consists of (1) inferring the average profile using Gaussian Processes and (2) computing the covariance of the profile observables given a set of independent variables. Our model is computationally efficient and capable of inferring average profiles of a large population size from noisy measurements, without stacking and binning data nor parameterizing the shape of the mean profile. We demonstrate the performance of our method using dark matter, gas and stellar profiles extracted from hydrodynamical cosmological simulations of galaxy formation. Population Profile Estimator (PoPE) is publicly available in a GitHub repository. Our new method should be useful for measuring the spatial distribution and internal structure of a variety of astrophysical systems using large astronomical surveys.

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arXiv2020

Scatter in Sunyaev--Zel'dovich effect scaling relations explained by inter-cluster variance in mass accretion histories

X-ray and microwave cluster scaling relations are immensely valuable for cosmological analysis. However, their power is limited by astrophysical systematics that bias mass estimates and introduce additional scatter. Turbulence injected into the intracluster medium via mass assembly contributes substantially to cluster non-thermal pressure support, a significant source of such uncertainties. We use an analytical model to compute the assembly-driven non-thermal pressure profiles of haloes based on Monte Carlo-generated accretion histories. We introduce a fitting function for the average non-thermal pressure fraction profile, which exhibits minimal dependence on redshift at fixed peak height. Using the model, we predict deviations from self-similarity and the intrinsic scatter in the Sunyaev--Zel'dovich effect observable-mass scaling relation ($Y_\mathrm{SZ}-M$) due solely to inter-cluster variation in mass accretion histories. We study the dependence of $Y_\mathrm{SZ}-M$ on aperture radius, cosmology, redshift, and mass limit. The model predicts $5-9\%$ scatter in $Y_\mathrm{SZ}-M$ at $z=0$, increasing as the aperture used to compute $Y_\mathrm{SZ}$ increases from $R_\mathrm{500c}$ to $5R_\mathrm{500c}$. The predicted scatter lies slightly below that of studies based on non-radiative hydro-simulations, illustrating that assembly history variance is likely responsible for a substantial fraction of scatter in $Y_\mathrm{SZ}-M$. This should be regarded as a lower bound, which will likely increase with the use of an updated gas density model that incorporates a more realistic response to halo assembly. As redshift increases, $Y_\mathrm{SZ}-M$ deviates more from self-similarity and scatter increases. We show that the $Y_\mathrm{SZ}-M$ residuals correlate strongly with the recent halo mass accretion rate, potentially providing an opportunity to infer the latter.

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arXiv2020

Dynamical heating of the X-ray emitting intracluster medium: the roles of merger shocks and turbulence dissipation

The diffuse plasma inside clusters of galaxies has X-ray emitting temperatures of a few keV. The physical mechanisms that heat this intracluster medium (ICM) to such temperatures include the accretion shock at the periphery of a galaxy cluster, the shocks driven by merger events, as well as a somewhat overlooked mechanism -- the dissipation of intracluster turbulent motions. We study the relative role of these heating mechanisms using galaxy clusters in Lagrangian tracer particle re-simulations of the Omega500 cosmological simulation. We adopt a novel analysis method of decomposing the temperature increase at each time step into the contribution from dissipative heating and that from adiabatic heating. In the high-resolution spatial-temporal map of these heating rates, merger tracks are clearly visible, demonstrating the dominant role of merger events in heating the ICM. The dissipative heating contributed by each merger event is extended in time and also occurs in the rarefaction regions, suggesting the importance of heating by the dissipation of merger-induced turbulence. Quantitative analysis shows that turbulence heating, rather than direct heating at merger shocks, dominates the temperature increase of the ICM, especially at inner radii $r < r_{\rm 500c}$. In addition, we find that many merger shocks can propagate with almost constant velocity to very large radii $r \gg r_{\rm 500c}$, some even reach and join with the accretion shock and becoming the outer boundary of the ICM. Altogether, these results suggest that the ICM is heated more in an `inside-out' fashion rather than `outside-in' as depicted in the classical smooth accretion picture.

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arXiv2020

Clusters Have Edges: The Projected Phase SpaceStructure of SDSS redMaPPer Clusters

We study the distribution of line-of-sight velocities of galaxies in the vicinity of SDSS redMaPPer galaxy clusters. Based on their velocities, galaxies can be split into two categories: galaxies that are dynamically associated with the cluster, and random line-of-sight projections. Both the fraction of galaxies associated with the galaxy clusters, and the velocity dispersion of the same, exhibit a sharp feature as a function of radius. The feature occurs at a radial scale $R_{\rm edge} \approx 2.2R_{\rmλ}$, where $R_{\rmλ}$ is the cluster radius assigned by redMaPPer. We refer to $R_{\rm edge}$ as the "edge radius." These results are naturally explained by a model that further splits the galaxies dynamically associated with a galaxy cluster into a component of galaxies orbiting the halo and an infalling galaxy component. The edge radius $R_{\rm edge}$ constitutes a true "cluster edge", in the sense that no orbiting structures exist past this radius. A companion paper (Aung et al. 2020) tests whether the "halo edge" hypothesis holds when investigating the full three-dimensional phase space distribution of dark matter substructures in numerical simulations, and demonstrates that this radius coincides with a suitably defined splashback radius.

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arXiv2020

Instability of Supersonic Cold Streams Feeding Galaxies IV: Survival of Radiatively Cooling Streams

We study the effects of Kelvin Helmholtz Instability (KHI) on the cold streams that feed massive halos at high redshift, generalizing our earlier results to include the effects of radiative cooling and heating from a UV background, using analytic models and high resolution idealized simulations. We currently do not consider self-shielding, thermal conduction or gravity. A key parameter in determining the fate of the streams is the ratio of the cooling time in the turbulent mixing layer which forms between the stream and the background following the onset of the instability, $t_{\rm cool,mix}$, to the time in which the mixing layer expands to the width of the stream in the non-radiative case, $t_{\rm shear}$. This can be converted into a critical stream radius, $R_{\rm s,crit}$, such that $R_{\rm s}/R_{\rm s,crit}=t_{\rm shear}/t_{\rm cool,mix}$. If $R_{\rm s} R_{\rm s,crit}$, which we find to almost always be the case for astrophysical cold streams, the stream is not disrupted by KHI. Rather, background mass cools and condenses onto the stream, and can increase the mass of cold gas by a factor of $\sim 3$ within 10 stream sound crossing times. The mass entrainment induces thermal energy losses from the background and kinetic energy losses from the stream, which we model analytically. Roughly half of the dissipated energy is radiated away from gas with $T<5\times 10^4{\rm K}$, suggesting much of it will be emitted in Ly$α$.

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arXiv2020

CMB-HD: Astro2020 RFI Response

CMB-HD is a proposed ultra-deep (0.5 uk-arcmin), high-resolution (15 arcseconds) millimeter-wave survey over half the sky that would answer many outstanding questions in both fundamental physics of the Universe and astrophysics. This survey would be delivered in 7.5 years of observing 20,000 square degrees, using two new 30-meter-class off-axis cross-Dragone telescopes to be located at Cerro Toco in the Atacama Desert. Each telescope would field 800,000 detectors (200,000 pixels), for a total of 1.6 million detectors.

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arXiv2020

Astrophysics with the Spatially and Spectrally Resolved Sunyaev-Zeldovich Effects: A Millimetre/Submillimetre Probe of the Warm and Hot Universe

In recent years, observations of the Sunyaev-Zeldovich (SZ) effect have had significant cosmological implications and have begun to serve as a powerful and independent probe of the warm and hot gas that pervades the Universe. As a few pioneering studies have already shown, SZ observations both complement X-ray observations -- the traditional tool for studying the intra-cluster medium -- and bring unique capabilities for probing astrophysical processes at high redshifts and out to the low-density regions in the outskirts of galaxy clusters. Advances in SZ observations have largely been driven by developments in centimetre-, millimetre-, and submillimetre-wave instrumentation on ground-based facilities, with notable exceptions including results from the Planck satellite. Here we review the utility of the thermal, kinematic, relativistic, non-thermal, and polarised SZ effects for studies of galaxy clusters and other large scale structures, incorporating the many advances over the past two decades that have impacted SZ theory, simulations, and observations. We also discuss observational results, techniques, and challenges, and aim to give an overview and perspective on emerging opportunities, with the goal of highlighting some of the exciting new directions in this field.

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arXiv2020

Cross-correlation of the thermal Sunyaev-Zel'dovich effect and weak gravitational lensing: Planck and Subaru Hyper Suprime-Cam first-year data

Cross-correlation analysis of the thermal Sunyaev-Zel'dovich (tSZ) effect and weak gravitational lensing (WL) provides a powerful probe of cosmology and astrophysics of the intra-cluster medium. We present the measurement of the cross-correlation of tSZ and WL from Planck and Subaru Hyper-Suprime Cam. The combination enables us to study cluster astrophysics at high redshift. We use the tSZ-WL cross-correlation and the tSZ auto-power spectrum measurements to place a tight constraint on the hydrostatic mass bias, which is a measure of the degree of non-thermal pressure support in galaxy clusters. With the prior on cosmological parameters derived from the analysis of the cosmic microwave background anisotropies by Planck and taking into account foreground contributions both in the tSZ auto-power spectrum and the tSZ-WL cross-correlation, the hydrostatic mass bias is estimated to be $26.9^{+8.9}_{-4.4} \%$ ($68\%$ C.L.), which is consistent with recent measurements by mass calibration techniques.

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arXiv2019

Probing Cosmology and Cluster Astrophysics with Multi-Wavelength Surveys I. Correlation Statistics

Upcoming multi-wavelength astronomical surveys will soon discover all massive galaxy clusters and provide unprecedented constraints on cosmology and cluster astrophysics. In this paper, we investigate the constraining power of the multi-band cluster surveys, through a joint analysis of three observables associated with clusters of galaxies, including thermal Sunyaev-Zel'dovich (tSZ) effect in cosmic microwave background (CMB), X-ray emission of ionized gas, and gravitational weak lensing effect of background galaxies by the cluster's gravitational potential. We develop a theoretical framework to predict and interpret two-point correlation statistics among the three observables using a semi-analytic model of intracluster medium (ICM) and halo-based approach. In this work, we show that the auto- and cross-angular power spectra in tSZ, X-ray and lensing statistics from upcoming missions (eROSITA, CMB-S4, and LSST) can help break the degeneracy between cosmology and ICM physics. These correlation statistics are less sensitive to selection biases, and are able to probe ICM physics in distant, faint and small clusters that are otherwise difficult to be detected individually. We show that the correlation statistics are able to provide cosmological constraints comparable to the conventional cluster abundance measurements, while constraining cluster astrophysics at the same time. Our results indicate that the correlation statistics can significantly enhance the scientific returns of upcoming multi-wavelength cluster surveys.

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arXiv2019

Kelvin-Helmholtz Instability in Self-Gravitating Streams

Self-gravitating gaseous filaments exist on many astrophysical scales, from sub-pc filaments in the interstellar medium to Mpc scale streams feeding galaxies from the cosmic web. These filaments are often subject to Kelvin-Helmotz Instability (KHI) due to shearing against a confining background medium. We study the nonlinear evolution of KHI in pressure-confined self-gravitating gas streams initially in hydrostatic equilibrium, using analytic models and hydrodynamic simulations, not including radiative cooling. We derive a critical line-mass as a function of the stream Mach number and density contrast with respect to the background, $μ_{cr}(M_b,δ_c)\le 1$, where $μ=1$ is normalized to the maximal line mass for which initial hydrostatic equilibrium is possible. For $μ<μ_{cr}$, KHI dominates the stream evolution. A turbulent shear layer expands into the background and leads to stream deceleration at a similar rate to the non-gravitating case. However, with gravity, penetration of the shear layer into the stream is halted at roughly half the initial stream radius by stabilizing buoyancy forces, significantly delaying total stream disruption. Streams with $μ_{cr}<μ\le 1$ fragment and form round, long-lived clumps by gravitational instability (GI), with typical separations roughly 8 times the stream radius, similar to the case without KHI. When KHI is still somewhat effective, these clumps are below the spherical Jeans mass and are partially confined by external pressure, but they approach the Jeans mass as $μ\rightarrow 1$ and GI dominates. We discuss potential applications of our results to filaments in the ISM and dense streams feeding galaxies at high redshift.

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arXiv2019

Using X-Ray Morphological Parameters to Strengthen Galaxy Cluster Mass Estimates via Machine Learning

We present a machine learning approach for estimating galaxy cluster masses, trained using both Chandra and eROSITA mock X-ray observations of 2,041 clusters from the Magneticum simulations. We train a random forest regressor, an ensemble learning method based on decision tree regression, to predict cluster masses using an input feature set. The feature set uses core-excised X-ray luminosity and a variety of morphological parameters, including surface brightness concentration, smoothness, asymmetry, power ratios, and ellipticity. The regressor is cross-validated and calibrated on a training sample of 1,615 clusters (80% of sample), and then results are reported as applied to a test sample of 426 clusters (20% of sample). This procedure is performed for two different mock observation series in an effort to bracket the potential enhancement in mass predictions that can be made possible by including dynamical state information. The first series is computed from idealized Chandra-like mock cluster observations, with high spatial resolution, long exposure time (1 Ms), and the absence of background. The second series is computed from realistic-condition eROSITA mocks with lower spatial resolution, short exposures (2 ks), instrument effects, and background photons modeled. We report a 20% reduction in the mass estimation scatter when either series is used in our random forest model compared to a standard regression model that only employs core-excised luminosity. The morphological parameters that hold the highest feature importance are smoothness, asymmetry, and surface brightness concentration. Hence, these parameters, which encode the dynamical state of the cluster, can be used to make more accurate predictions of cluster masses in upcoming surveys, offering a crucial step forward for cosmological analyses.

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arXiv2019

Microwave Spectro-Polarimetry of Matter and Radiation across Space and Time

This paper discusses the science case for a sensitive spectro-polarimetric survey of the microwave sky. Such a survey would provide a tomographic and dynamic census of the three-dimensional distribution of hot gas, velocity flows, early metals, dust, and mass distribution in the entire Hubble volume, exploit CMB temperature and polarisation anisotropies down to fundamental limits, and track energy injection and absorption into the radiation background across cosmic times by measuring spectral distortions of the CMB blackbody emission. In addition to its exceptional capability for cosmology and fundamental physics, such a survey would provide an unprecedented view of microwave emissions at sub-arcminute to few-arcminute angular resolution in hundreds of frequency channels, a data set that would be of immense legacy value for many branches of astrophysics. We propose that this survey be carried-out with a large space mission featuring a broad-band polarised imager and a moderate resolution spectro-imager at the focus of a 3.5m aperture telescope actively cooled to about 8K, complemented with absolutely-calibrated Fourier Transform Spectrometer modules observing at degree-scale angular resolution in the 10-2000 GHz frequency range. We propose two observing modes: a survey mode to map the entire sky as well as a few selected wide fields, and an observatory mode for deeper observations of regions of specific interest.

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arXiv2019

A Space Mission to Map the Entire Observable Universe using the CMB as a Backlight

This Science White Paper, prepared in response to the ESA Voyage 2050 call for long-term mission planning, aims to describe the various science possibilities that can be realized with an L-class space observatory that is dedicated to the study of the interactions of cosmic microwave background (CMB) photons with the cosmic web. Our aim is specifically to use the CMB as a backlight -- and survey the gas, total mass, and stellar content of the entire observable Universe by means of analyzing the spatial and spectral distortions imprinted on it. These distortions result from two major processes that impact on CMB photons: scattering by electrons (Sunyaev-Zeldovich effect in diverse forms, Rayleigh scattering, resonant scattering) and deflection by gravitational potential (lensing effect). Even though the list of topics collected in this White Paper is not exhaustive, it helps to illustrate the exceptional diversity of major scientific questions that can be addressed by a space mission that will reach an angular resolution of 1.5 arcmin (goal 1 arcmin), have an average sensitivity better than 1 uK-arcmin, and span the microwave frequency range from roughly 50 GHz to 1 THz. The current paper also highlights the synergy of our BACKLIGHT mission concept with several upcoming and proposed ground-based CMB experiments.

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arXiv2019

Stirred, not Clumped: Evolution of Temperature Profiles in the Outskirts of Galaxy Clusters

Recent statistical X-ray measurements of the intracluster medium (ICM) indicate that gas temperature profiles in the outskirts of galaxy clusters deviate from self-similar evolution. Using a mass-limited sample of galaxy clusters from cosmological hydrodynamical simulations, we show that the departure from self-similarity can be explained by non-thermal gas motions driven by mergers and accretion. Contrary to previous claims, gaseous substructures only play a minor role in the temperature evolution in cluster outskirts. A careful choice of halo overdensity definition in self-similar scaling mitigates these departures. Our work highlights the importance of non-thermal gas motions in ICM evolution and the use of galaxy clusters as cosmological probes.

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