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Erwin Lau

Publications and source records attributed to Erwin Lau.

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Masses of Sunyaev-Zel'dovich Galaxy Clusters Detected by The Atacama Cosmology Telescope: Stacked Lensing Measurements with Subaru HSC Year 3 data

We present a stacked lensing analysis of 96 galaxy clusters selected by the thermal Sunyaev-Zel'dovich (SZ) effect in maps of the cosmic microwave background (CMB). We select foreground galaxy clusters with a $5\sigma$-level SZ threshold in CMB observations from the Atacama Cosmology Telescope, while we define background source galaxies for the lensing analysis with secure photometric redshift cuts in Year 3 data of the Subaru Hyper Suprime Cam survey. We detect the stacked lensing signal in the range of $0.1 < R\, [h^{-1}\mathrm{Mpc}] < 100$ in each of three cluster redshift bins, $0.092<z\le0.445$, $0.445<z\le0.695$, and $0.695<z\le1.180$, with 32 galaxy clusters in each bin. The cumulative signal-to-noise ratios of the lensing signal are $14.6$, $12.0$, and $6.6$, respectively. Using a halo-based forward model, we then constrain statistical relationships between the mass inferred from the SZ observation (i.e. SZ mass) and the total mass derived from our stacked lensing measurements. At the average SZ mass in the cluster sample ($2.1-2.4\times10^{14}\, h^{-1}M_\odot$), our likelihood analysis shows that the average total mass differs from the SZ counterpart by a factor of $1.3 \pm 0.2$, $1.6 \pm 0.2$, and $1.6 \pm 0.3$ ($68\%$) in the aforementioned redshift ranges, respectively. Our limits are consistent with previous lensing measurements, and we find that the cluster modeling choices can introduce a $1\sigma$-level difference in our parameter inferences.

astro-ph.CO

OLIMPO: a Balloon-Borne SZE Imager to Probe ICM Dynamics and the WHIM

OLIMPO is a proposed Antarctic balloon-borne Sunyaev-Zel'dovich effect (SZE) imager to study gas dynamics associated with structure formation along with the properties of the warm-hot intergalactic medium (WHIM) residing in the connective filaments. During a 25 day flight OLIMPO will image a total of 10 z~0.05 galaxy clusters and 8 bridges at 145, 250, 365, and 460 GHz at an angular resolution of 1.0'-3.3'. The maps will be significantly deeper than those planned from CMB-S4 and CCAT-P, and will have excellent fidelity to the large angular scales of our low-z targets, which are difficult to probe from the ground. In combination with X-ray data from eROSITA and XRISM we will transform our current static view of galaxy clusters into a full dynamic picture by measuring the internal intra-cluster medium (ICM) velocity structure with the kinematic SZE, X-ray spectroscopy, and the power spectrum of ICM fluctuations. Radio observations from ASKAP and MeerKAT will be used to better understand the connection between ICM turbulence and shocks with the relativistic plasma. Beyond the cluster boundary, we will combine thermal SZE maps from OLIMPO with X-ray imaging from eROSITA to measure the thermodynamics of the WHIM residing in filaments, providing a better understanding of its properties and its contribution to the total baryon budget.

astro-ph.CO

Circumgalactic Medium on the Largest Scales: Detecting X-ray Absorption Lines with Large-Area Microcalorimeters

The circumgalactic medium (CGM) plays a crucial role in galaxy evolution as it fuels star formation, retains metals ejected from the galaxies, and hosts gas flows in and out of galaxies. For Milky Way-type and more massive galaxies, the bulk of the CGM is in hot phases best accessible at X-ray wavelengths. However, our understanding of the CGM remains largely unconstrained due to its tenuous nature. A promising way to probe the CGM is via X-ray absorption studies. Traditional absorption studies utilize bright background quasars, but this method probes the CGM in a pencil beam, and, due to the rarity of bright quasars, the galaxy population available for study is limited. Large-area, high spectral resolution X-ray microcalorimeters offer a new approach to exploring the CGM in emission and absorption. Here, we demonstrate that the cumulative X-ray emission from cosmic X-ray background sources can probe the CGM in absorption. We construct column density maps of major X-ray ions from the Magneticum simulation and build realistic mock images of nine galaxies to explore the detectability of X-ray absorption lines arising from the large-scale CGM. We conclude that the OVII absorption line is detectable around individual massive galaxies at the $3\sigma-6\sigma$ confidence level. For Milky Way-type galaxies, the OVII and OVIII absorption lines are detectable at the $\sim\,6\sigma$ and $\sim\,3\sigma$ levels even beyond the virial radius when co-adding data from multiple galaxies. This approach complements emission studies, does not require additional exposures, and will allow probing of the baryon budget and the CGM at the largest scales.

astro-ph.GA

Line Emission Mapper (LEM): Probing the physics of cosmic ecosystems

The Line Emission Mapper (LEM) is an X-ray Probe for the 2030s that will answer the outstanding questions of the Universe's structure formation. It will also provide transformative new observing capabilities for every area of astrophysics, and to heliophysics and planetary physics as well. LEM's main goal is a comprehensive look at the physics of galaxy formation, including stellar and black-hole feedback and flows of baryonic matter into and out of galaxies. These processes are best studied in X-rays, and emission-line mapping is the pressing need in this area. LEM will use a large microcalorimeter array/IFU, covering a 30x30' field with 10" angular resolution, to map the soft X-ray line emission from objects that constitute galactic ecosystems. These include supernova remnants, star-forming regions, superbubbles, galactic outflows (such as the Fermi/eROSITA bubbles in the Milky Way and their analogs in other galaxies), the Circumgalactic Medium in the Milky Way and other galaxies, and the Intergalactic Medium at the outskirts and beyond the confines of galaxies and clusters. LEM's 1-2 eV spectral resolution in the 0.2-2 keV band will make it possible to disentangle the faintest emission lines in those objects from the bright Milky Way foreground, providing groundbreaking measurements of the physics of these plasmas, from temperatures, densities, chemical composition to gas dynamics. While LEM's main focus is on galaxy formation, it will provide transformative capability for all classes of astrophysical objects, from the Earth's magnetosphere, planets and comets to the interstellar medium and X-ray binaries in nearby galaxies, AGN, and cooling gas in galaxy clusters. In addition to pointed observations, LEM will perform a shallow all-sky survey that will dramatically expand the discovery space.

astro-ph.IM

Cluster outskirts and their connection to the cosmic web

We review the latest developments in our X-ray observational and theoretical understanding of the outskirts of galaxy clusters, and their connection to the cosmic web. The faint cluster outskirts are challenging regions to observe in X-rays, requiring highly sensitive telescopes with low and stable background levels. We present our latest understanding of the thermodynamic profiles of clusters in the outskirts, and the biases that gas clumping and non-thermal pressure support can introduce. Features in the outskirts due to merging activity are discussed, along with the chemical enrichment of the outskirts ICM. We describe future prospects for X-ray observations to explore further out in the cluster outskirts and probe their connections to the cosmic web.

astro-ph.CO

The Circumgalactic Medium from the CAMELS Simulations: Forecasting Constraints on Feedback Processes from Future Sunyaev-Zeldovich Observations

The cycle of baryons through the circumgalactic medium (CGM) is important to understand in the context of galaxy formation and evolution. In this study we forecast constraints on the feedback processes heating the CGM with current and future Sunyaev-Zeldovich (SZ) observations. To constrain these processes, we use a suite of cosmological simulations, the Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS), that varies four different feedback parameters of two previously existing hydrodynamical simulations, IllustrisTNG and SIMBA. We capture the dependencies of SZ radial profiles on these feedback parameters with an emulator, calculate their derivatives, and forecast future constraints on these feedback parameters from upcoming experiments. We find that for a DESI-like (Dark Energy Spectroscopic Instrument) galaxy sample observed by the Simons Observatory all four feedback parameters are able to be constrained (some within the $10\%$ level), indicating that future observations will be able to further restrict the parameter space for these sub-grid models. Given the modeled galaxy sample and forecasted errors in this work, we find that the inner SZ profiles contribute more to the constraining power than the outer profiles. Finally, we find that, despite the wide range of AGN feedback parameter variation in the CAMELS simulation suite, we cannot reproduce the tSZ signal of galaxies selected by the Baryon Oscillation Spectroscopic Survey as measured by the Atacama Cosmology Telescope.

astro-ph.CO

Multi-scale analysis of turbulence evolution in the density stratified intracluster medium

The diffuse hot medium inside clusters of galaxies typically exhibits turbulent motions whose amplitude increases with radius, as revealed by cosmological hydrodynamical simulations. However, its physical origin remains unclear. It could either be due to an excess injection of turbulence at large radii, or faster turbulence dissipation at small radii. We investigate this by studying the time evolution of turbulence in the intracluster medium (ICM) after major mergers, using the Omega500 non-radiative hydrodynamical cosmological simulations. By applying a novel wavelet analysis to study the radial dependence of the ICM turbulence spectrum, we discover that faster turbulence dissipation in the inner high density regions leads to the increasing turbulence amplitude with radius. We also find that the ICM turbulence at all radii decays in two phases after a major merger: an early fast decay phase followed by a slow secular decay phase. The buoyancy effects resulting from the ICM density stratification becomes increasingly important during turbulence decay, as revealed by a decreasing turbulence Froude number $Fr \sim \mathcal{O}(1)$. Our results indicate that the stronger density stratification and smaller eddy turn-over time are the likely causes of the faster turbulence dissipation rate in the inner regions of the cluster.

astro-ph.CO

Cold Fronts and Shocks Formed by Gas Streams in Galaxy Clusters

Cold Fronts and shocks are hallmarks of the complex intra-cluster medium (ICM) in galaxy clusters. They are thought to occur due to gas motions within the ICM and are often attributed to galaxy mergers within the cluster. Using hydro-cosmological simulations of clusters of galaxies, we show that collisions of inflowing gas streams, seen to penetrate to the very centre of about half the clusters, offer an additional mechanism for the formation of shocks and cold fronts in cluster cores. Unlike episodic merger events, a gas stream inflow persists over a period of several Gyrs and it could generate a particular pattern of multiple cold fronts and shocks.

astro-ph.GA

nIFTy galaxy cluster simulations I: dark matter & non-radiative models

We have simulated the formation of a galaxy cluster in a $Λ$CDM universe using twelve different codes modeling only gravity and non-radiative hydrodynamics (\art, \arepo, \hydra\ and 9 incarnations of GADGET). This range of codes includes particle based, moving and fixed mesh codes as well as both Eulerian and Lagrangian fluid schemes. The various GADGET implementations span traditional and advanced smoothed-particle hydrodynamics (SPH) schemes. The goal of this comparison is to assess the reliability of cosmological hydrodynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be non-radiative. We compare images of the cluster at $z=0$, global properties such as mass, and radial profiles of various dynamical and thermodynamical quantities. The underlying gravitational framework can be aligned very accurately for all the codes allowing a detailed investigation of the differences that develop due to the various gas physics implementations employed. As expected, the mesh-based codes ART and AREPO form extended entropy cores in the gas with rising central gas temperatures. Those codes employing traditional SPH schemes show falling entropy profiles all the way into the very centre with correspondingly rising density profiles and central temperature inversions. We show that methods with modern SPH schemes that allow entropy mixing span the range between these two extremes and the latest SPH variants produce gas entropy profiles that are essentially indistinguishable from those obtained with grid based methods.

astro-ph.CO

Gas Clumping in the Outskirts of Lambda-CDM Clusters

Recent Suzaku X-ray observations revealed that the observed entropy profile of the intracluster medium (ICM) deviates significantly from the prediction of hydrodynamical simulations of galaxy clusters. In this work, we show that gas clumping introduces significant biases in X-ray measurements of the ICM profiles in the outskirts of galaxy clusters. Using hydrodynamical simulations of galaxy cluster formation in a concordance Lambda-CDM model, we demonstrate that gas clumping leads to an overestimate of the observed gas density and causes flattening of the entropy profile. Our results suggest that gas clumping must be taken into account when interpreting X-ray measurements of cluster outskirts.

astro-ph.CO