Colloquiums and Seminars(2021)

Three cosmic tracer fields are measured from imaging surveys: galaxy density, weak gravitation lensing shear, and cluster density. The joint analysis of the auto and cross correlations of the first two fields, often referred to as the 3x2pt analysis, has become a popular and competitive cosmological test of the standard cosmological model. The abundances and spatial distributions of galaxy clusters, which are associated with the highest peaks in the matter density field, provide another powerful probe of cosmic structure formation and evolution; thus, the combination of cluster abundances and 3x2pt analysis is expected to yield precise cosmological constraints. In the first part of the talk, I will present a novel multi-probe cluster cosmology analysis, which focuses exclusively on large scales. This new cluster cosmology analysis yields competitive cosmological constraints while being robust against several systematics. I will describe the extensive validation of the measurements, modeling, and inferences using N-body simulations populated with galaxies. In the second part of the talk, I will present cosmological constraints from the first joint analysis of cluster abundances and auto/cross correlations of all three cosmic tracer fields measured from the first year of the Dark Energy Survey (DES-Y1). The talk will be concluded by a discussion on the implication of the result, potential improvements, and expected constraining powers in the up-coming DES-Y3 analysis and future wide imaging surveys.

Binary Supermassive Black Holes (BSBHs) are important objects for studying galaxy evolution, gravitational wave, and cosmology. However, searching close BSBHs at high redshift (z>1) is difficult due to the angular resolution of the telescopes. For the first half of the talk, I will present our new discovery of candidate periodic quasars as possible BSBHs at milli-pc scale. The search is based on 20-yr light curves by combining DES and SDSS. I will also discuss the implications of those candidates. For the second half of the talk, I will show a new astrometric technique to find the unresolved dual/lensed quasars at (sub)kpc scale using Gaia. The follow-up HST observations returned handful dual/lensed quasar candidates proving the feasibility of the technique. Those candidates if confirmed could form a pilot sample for studying close dual AGNs at high redshift.

(Sub)millimeter dust thermal emission is a traditional and successful method to study the magnetic fields on scales larger than disks. Thanks to the recent development of interferometric polarimetry, especially ALMA, we have been able to study resolved polarization image on disk scales. The results, however, have been surprising. Instead of dichroic thermal emission from grains aligned with magnetic fields, disks show mostly scattering-induced polarization pattern, especially at shorter wavelengths. In this talk, I will first discuss the feasibility of the alignment of dust grains with magnetic fields in PPDs, with the so-called superparamagnetic inclusions taken into account. I will show that large millimeter-sized dust grains can hardly be aligned with magnetic fields, due to the dense environment and frequent gas bombardment. With similar grain alignment analysis, we perform synthetic polarimetric observation using a disk formation simulation. Our results show a transition from magnetically aligned polarization to scattering-induced polarization going from the envelope scale to the disk scale. This is in agreement with a recent polarization survey in the Perseus molecular cloud. At last, I will present some preliminary results on the explanation of a non-detection of polarization from a debris disk system and discuss its implication on grain alignment theory.

Along with the significant development of astronomical data in 4V aspects (Volume, Velocity, Variety, and Value/Veracity), machine learning techniques, as an analysis tool, are applied in a broad range of astronomical studies. In my PhD, I studied galaxy morphology with supervised and unsupervised machine learning techniques for two types of tasks - classification and exploration. In this talk, I will give an overview about how I think machine learning techniques can be used in astronomical studies, in particular, galaxy morphological classification, with four projects I accomplished during my PhD using a broad range of data (DES, SDSS, simulated data for Euclid). Additionally, I would like to bring up a discussion about a possibility of a new astronomical approach may be carried out through machine’s perspective.

Time-domain observations now offer a promising new way to study accretion and jet physics in X-ray binaries. Through detecting and characterizing rapid flux variability in these systems across a wide range of wavelength/energy bands (probing emission from different regions of the accretion flow and jet), we can measure properties that are difficult, if not impossible, to measure by traditional spectral and imaging methods (e.g., size scales, geometry, jet speeds, the sequence of events leading to jet launching). While variability studies in the X-ray bands are a staple in the X-ray binary community, there are many challenges that accompany such studies at longer wavelengths. However, with recent advances to observing techniques/instrumentation, the availability of new computational tools, and today's improved coordination capabilities, we are no longer limited by these challenges. In this talk, I will discuss new results from multi-wavelength fast timing observations of Cygnus X-1 and MAXI J1820+070, highlighting how we can directly connect variability properties to internal jet physics. Additionally, I will discuss future prospects for obtaining more of these invaluable data sets, and the key role that next-generation instruments will play in driving new discoveries through this science.

Astrophysical observations at (sub-)mm wavelengths (λ from ~300 μm to ~3mm) allow us to study the cold and dense material in the Universe, hence probing the formation of stars and planets, and the interstellar and circumgalactic medium within galaxies across cosmic time. The current generation of 10-meter-class single dish telescopes has delivered some of the first surveys at (sub-)mm wavelengths, allowing us to go far beyond the previously optical-biased view of the Universe. Follow-up observations with interferometers then revealed in exquisite detail the morphology and kinematics of such (sub-)mm sources, enabling tests and revisions of theoretical models for the formation and evolution of planets, stars, and galaxies. However, it is now clear that without a transformative change in the capabilities of single-dish facilities in the 2030s, interferometers (like the ALMA observatory) will soon become source-starved. The current generation of 10-m class single dish telescopes, with their limited fields of view, spatial resolutions, and sensitivities, can only reveal the ‘tip of the iceberg’ of the (sub-)mm source population, both for Galactic and extragalactic studies. These limitations cannot be fully mitigated by interferometers, which are all intrinsically affected by a low mapping speed and by the loss of diffuse extended signals. The Atacama Large Aperture Submillimeter telescope (AtLAST; http://atlast-telescope.org/;https://ui.adsabs.harvard.edu/abs/2020arXiv201107974K/abstract) project is a concept for a 50-meter diameter single dish observatory to be built near the ALMA site. With its extremely large field of view (the goal is ~ 2 degrees), spatial resolution (up to ~1.5” at 350 μm), and sensitivity to both point sources and large-scale structures, AtLAST will be transformational for all fields of Astronomy in the 2030s. Here we will describe the recently approved EU Horizon2020 project to deliver a comprehensive design study for such a next-generation single-single dish facility. Beyond the EU, AtLAST would welcome an international consortium, and is beginning to garner broad support, with support from the Japanese 50-meter Large Submm Telescope community as well as many US Astro2020 decadal and Canadian Long Range Plan 2020 science case submissions.

The Hubble constant is one of the most important parameters in cosmology. Its value directly sets the age, the size, and the critical density of the Universe. Despite the success of the flat LCDM model, the derived Hubble constant from Planck data under the assumption of a flat LCDM model has 4.4-sigma tension with the direct measurements. If this tension is not due to the systematics, it may indicate the new physics beyond the standard cosmological model. H_0 from time-delay lensing is a powerful independent tool for addressing the H_0 tension since it is independent of both Planck and the distance ladder. One way to do this is to increase the number of high-quality lens systems since this allows us to look for correlations and other effects due to systematics, and to do hierarchical approaches to assess known systematic effects. Keck AO data is not only the key component to increase the precision of H0 measurement but also provides systematic checks with the H_0 results based on HST imaging. In this talk, I will present the current H_0 measurement, the systematic checks, and the future prospective of TDCOSMO collaboration.

Galaxies are systems that convert gas into stars. In order to understand galaxy evolution it is necessary to obtain an accurate account of the efficiency of star formation of galaxies as a function of time and environment. This in turn requires robust mass measurements. Gravitational lensing is one of the few available tools for measuring masses at cosmological distances. Weak lensing probes the dark matter distribution, which sets the amount and the temperature of the initial gas reservoir, while strong lensing is sensitive to the stellar component of galaxies. The combination of weak and strong lensing can therefore help greatly in our understanding of the baryon cycle. I will give an overview of the most recent discoveries enabled by gravitational lensing observations in the field of galaxy evolution and discuss the possibilities that the LSST will open up in this important area of research.

I will give a broad overview of the science questions that were motivating my research on astrophysical compact objects throughout the last decade. These involve stability of accretion disks and analytic and numerical treatment of radiation in general relativity. I will highlight my recent work on the Event Horizon Telescope project, from designing the data processing pipeline, through investigating consistency with the archival observations, all the way to the theoretical interpretations and testing Kerr paradigm. I will also outline several exciting projects that are nearing completion, as well as some future plans.

We present S82-20, an unusual object identified at redshift z~ 3 from excess g-band imaging in SDSS-Stripe 82. The rest frame ultraviolet spectrum of this object shows emission lines from highly ionized species, including HeII λ1640 and CIV λλ 1548, 1550 doublets and OVI λλ 1032, 1038 doublets. The high Lyα luminosity (3.5x10^44 erg/s), the high emission line EWs ( >200\AA for Lyα), the FWHM of the emission lines ( <800 km/s), and the detection of the high ionization OVI line strongly support the interpretation that S82-20 is a Type II QSO. However, some evidence suggests that the Type II QSO interpretation may not be sufficient to explain all the available observables, as the CIV/HeII ratio is not fully reproduced by photonionization models of AGN only, and requires either some contribution from metal rich star-formation or high velocity shocks. Fully understanding the ionization power of S82-20 is crucial. If indeed it is a Type~II QSO, it would be likely missed in searches based on the detection at WISE wavelengths. This in turn would imply that our current estimates of the overall accretion rate of the QSO population may be underestimated.

The formation of stars, galaxies, and the large-scale structure in the Universe drives complex energy density flows over a wide range of scales from atomic nuclei to the Hubble length. The net effect could be summarized by a census of the density parameters, Ω, for different entries in the cosmic inventory over time. I will present my ongoing effort to probe the history of several key cosmic constituents, including stars, dust, thermal, and gravitational energy associated with the large-scale structure. To do so, we deproject the cosmic UV, IR, and the Sunyaev-Zeldovich effect backgrounds over redshift using a new tomographic intensity mapping approach. While the results are already pushing our understanding of the Universe, they represent only the beginning of a new chapter of cosmology and astrophysics using the entire radiation field without necessarily resolving individual galaxies. Looking forward, I will describe the infrastructure that we are building in facilitating a close synergy between galaxy surveys and intensity mapping. I will close by highlighting some exciting science opportunities in the next decade.

Current and upcoming facilities like SKA, HERA, LOFAR and JWST will unleash a flood of high redshift (z>5) observational data that will usher the study of the Epoch of Reionization (EoR) into a new era, making it the next big frontier in cosmological structure formation. It is therefore important for theoretical models to achieve sufficient accuracy and physical fidelity to meaningfully interpret these observational results. This requires a realistic description of galaxy formation in an evolving radiation field. In this talk, I will introduce the THESAN simulation project that couples state-of-the-art galaxy formation models (like IllustrisTNG & SMUGGLE) with radiative transfer, dust, and non-equilibrium thermochemistry to comprehensively study high redshift galaxy formation and reionization. The suite consists of both large volume reionization simulations (~100 Mpcs) focused on studying the large-scale properties of the IGM and zoom-in simulations of individual galaxies at unprecedented (~ 10 pc) resolution, designed to investigate the sources responsible for the reionization process. I will present results from this suite and discuss the advantages of using this multi-pronged approach to studying high redshift structure formation.

Planet formation starts in the earliest stages of star formation. Recent observations of exoplanets revealed a diverse planetary system architecture. The key to understanding the observed diversity seems to lie in the physics that govern star and planet formation in the first million years. I will present observational and numerical studies that dwell into the earliest stages of planetary system formation. It has been shown that some of the physics (gravity, rotation, magnetic field, turbulence, radiation) of star formation can be explored by comparing the composition of the environment around young stars to solar system objects. The composition of the dust and gas provides a view into the processing of material as the young protoplanetary disk forms and, also, the formation of the early planet-building materials. Thus, the physics of star formation can only be unveiled by combining astrophysical studies with the chemistry and geochemical processes of planetary bodies. In this talk, I will also discuss future challenges to overcome in star and planet formation.

(canceled)

We investigate the mass-velocity dispersion relation (MVDR) in 29 galaxy clusters in the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS). We measure the spatially resolved, line-of-sight velocity dispersion profiles of these clusters, which we find to be mostly flat at large radii, reminiscent of the rotation curves of galaxies. We discover a tight empirical relation between the baryonic mass Mbar and the flat velocity dispersion of the member galaxies. The residuals of the MVDR are uncorrelated with other cluster properties. These characteristics are reminiscent of the MVDR for individual galaxies, albeit about ten times larger characteristic acceleration scale. The cluster baryon fraction falls short of the cosmic value, exposing a problem: the discrepancy increases systematically for clusters of lower mass and lower baryonic acceleration.

Intensity mapping (IM) has emerged as a promising tool to study the high redshift universe and the faint, diffuse extragalactic populations. Without resolving individual galaxies, IM measures the integrated light from all sources and uses that to statistically probe the emission properties of the sources as well as the underlying large-scale structure they trace. In this talk, I will cover several topics on intensity mapping technique. First, I will introduce line intensity mapping (LIM), which probes the 3D large-scale structure of the universe, and a [C II] LIM experiment, TIME, that I am currently working on. Next I will talk about two recent projects on studying extragalactic background light (EBL), the aggregate emission from all extragalactic sources throughout cosmic time. I will present our latest stacking analysis results on probing the extended stellar halo with CIBER, a sounding rocket experiment that measures the near-IR EBL. Finally, I will conclude with future outlook for IM using SPHEREx, an upcoming all-sky near-infrared spectro-imaging survey.

The discovery of Gravitational Wave (GW) marked an opening of a new era of multi-messenger GW astronomy.Exploring the new physics under the extreme gravity condition and eventually challenge the fundamental physics such as General Relativity and Cosmology in high precision is essential. Our group is working on LIGO. We expect to solve the degeneracies of parameters with world wide observation, and in particular, the polarization of GWs. Joint LIGO-Virgo-KAGRA observations from 2022 will put us on the frontiers of GW astronomy and cosmology. In this talk, we report the progress of the KAGRA and LIGO calibration and future plans for accurate measurement of GWs with their impact on the fundamental physics. We also talk about the future R&D of gravitational wave detector system.

Dust plays an important role in shaping the interstellar radiation field and chemistry in the interstellar medium (ISM). In observations, one key factor that reveals dust evolution is the dust-to-metals ratio (D/M), which describes the fraction of heavy elements contained in dust grains. In this talk, I will introduce our observations of resolved (~kpc scales) D/M in the nearby galaxies. D/M values of 0.40-0.58 are found in our previous studies. I will also talk about the main technical challenges in resolved D/M observations, namely the environmental dependence of CO-to-H2 conversion factor, the calibration of dust emissivity, and the measurements of oxygen abundance.

The Kepler Space Telescope has brought major advances in the study of exoplanets by producing photometric light curves of stars with high precision. One important byproduct has to do with the discovery of super flares in 2012 that were detected in some stars with energies many orders of magnitude larger than the greatest solar flares ever observed. In this talk we will give an overview covering some of the major steps in our understanding of the physical nature of the flare effects and the rotation-age-activity relation of low-mass stars. The super flares and the related stellar outflows and radiation could have important implications on the atmospheric evolution of exoplanets and their habit- ability and thus are certainly scientific topics of interest for future observations. (The presentation will be given online due to the covid-19 situation. Please find the video link on this page: https://web.phys.ntu.edu.tw/colloquium/index.html/ )

Fast Radio Bursts (FRBs) are bright (~1 Jy) radio flashes with millisecond-duration, which randomly appear across the whole sky from cosmological origin. About six hundred FRBs have been published since the first discovery in 2007, of which two dozen sources have been observed with complex repetitions. Through interferometry efforts, about twenty FRBs have been localized to their host galaxies, and the follow-up observations have played an important role in revealing the local environment of FRBs. However, the nature of FRBs is still puzzling. In this talk, I will review the progress of the FRB field in the past decade, discuss a few highlighted repeaters, and introduce a next generation FRB telescope -- Broadbeam Radio Survey Telescope (BRST). I will discuss the potential scientific impact from the BRST project. The BRST with 200 elements is approved, which will open up new windows for repeating FRBs. We also welcome everyone to participate in the BRST, decipher the mystery of FRBs, and use FRBs to probe the universe together in the upcoming FRB era.

Protoplanetary disk formation is a natural outcome of prestellar core collapse. During the embedded class 0/1 phase, the close interaction with the infalling envelope should not be neglected when considering the dynamics and evolution of the disk. While evolved disks show larger sizes and a wide diversity of morphologies, recent observations suggested that young disks are mostly small (<50 au). I will present our recent work (https://arxiv.org/abs/2109.05823) on the self-regulated disk formation, where the magnetic braking is moderated by the nonideal magnetohydrodynamic effects. The resulting disk radius is slowly varying for a wide range of physical parameters, which explains for the observed small disk size.

Spiral structures are frequently observed in circumstellar disks. These spirals are sometimes associated with the self-gravitating nature of the disk, when it is still massive relative to its central star. In that case, they could be used to infer constraints on the physical properties inside the disk. I will look back at the results of numerical simulations of self-gravitating turbulence in disks, with a focus on its morphological properties and on the role of the spirals in the accretion process.

Galaxy clusters are the largest structures held by gravitational force, and their formation involves an amount of energy that is only comparable to the Big Bang. For this reason, merging galaxy clusters are often called the largest known particle colliders. These make them important laboratories for the investigation of their three main components: galaxies, hot gas and dark matter. In this presentation, I will show how the study of interacting galaxy clusters can offer some insights into the nature of dark matter and discuss the efforts we have made to provide a reliable description of post-merger systems.

Accretion discs around binary star systems are ubiquitous in the galaxy and planet formation is thought to occur within these discs. Circumbinary discs are commonly observed to be misaligned with respect to the binary orbital plane. A misaligned circumbinary disc eventually evolves to a stable orientation, either coplanar or polar to the binary orbital plane. The process of disc alignment has important implications for planet formation. By understanding the structure and evolution of these discs and also debris discs, I shed light on the observed characteristics of exoplanets. My focus is to study the gas dynamics around binary and higher-order star systems with an emphasis on explaining observations and developing theoretical models to better constrain planet formation mechanisms. My results unravel robust planet formation scenarios, which have far reaching implications for the present and upcoming observations from space telescope TESS. Furthermore, the next-generation telescopes, such as James Webb Space Telescope and Thirty Meter Telescope will fuel the discovery of planets within binary and higher-order star systems.

Non-axisymmetric potentials induce the formation of some structural components of galaxies such as spiral arms and stellar bars. These kinds of perturbations over axisymmetric potentials affect the dynamics of particles on the disk by changing their orbits, being able to induce deviations in the circular rotation. In this talk I will address on the incidence of bar-like flows in galaxies observed with integral field spectroscopic data (MUSE and MaNGA). I will show that gas and stellar velocity fields of barred galaxies are affected by the bar-potential, and they are better described with elongated orbits instead of pure circular rotation models.

One of the fundamental constraints on studying galaxy evolution is that we are not able to monitor individual galaxies to track the evolution. The stars in galaxies provide fossil records on the build-up processes of galaxies. I will demonstrate that with current observing facilities, we are able to track the evolution of individual galaxies in the distant Universe from their stellar populations. The high precision allows a quantitative comparison to cosmological hydro-dynamical simulations and validates current models of galaxy formation. The archeological method can date the ages of galaxies, pinpoint where they are in their evolutionary process, and identify galaxies in the key phase of galaxy evolution for follow-up studies. With the coming survey projects and observing facilities, the archeological method will be able to track the evolution of galaxies in the even earlier Universe, and with better statistics and precision.

It has long been known that filaments have been connected to the star formation process but Herschel observations have sparked a recent interest in them. However, the role they play has not been clear, nor has the effect of their geometry on the star formation process. Here I will present two recent works, one numerical and one observational, looking at helping to understand the place filaments occupy in star formation. First, I will present a numerical study looking at the fragmentation of an accreting, non-equilibrium filament. I show that filaments are prone to fragment into smaller sub-filaments due to internal accretion-driven turbulence. These sub-filaments play a key role in the formation of cores, affecting both their number and mass. Moreover, unlike in equilibrium models of filament fragmentation, no characteristic fragmentation length-scale is observed. Second, I will present a Herschel study of the outer Galaxy giant molecular filament (GMF) G214.5-1.8. I find that G214.5 has a mass of ~16,000 solar masses, similar to clouds such as Serpens and Mon R2, yet is mostly quiescent while those are highly star forming. I find that G214.5 has fragmented into numerous clumps and that there appears to be no characteristic fragmentation length-scale present. I show that the GMF is unusual in its high aspect ratio, narrowness, and paucity of dense gas, as well as having a highly asymmetric radial profile consistent with compression. Using the LAB-HI survey data, I find that G214.5 lies on the edge of a HI superbubble and this may be the cause of G214.5's properties, highlighting external effects and how they shape filaments.