Colloquiums and Seminars(2024)

Recent observations have revealed that circumbinary disks misaligned to the binary orbit could be common in the universe. Dissipation in the disc causes it to move either towards coplanar alignment or polar alignment. In the polar configuration, the disc is perpendicular to the binary orbit, with the disc angular momentum vector aligned to the binary eccentricity vector. Since planets form inside disks, circumbinary planets may also form misaligned to the binary orbit. We explore the dynamical evolution and stability of misaligned circumbinary planets. We find that around eccentric binaries, the most stable orbits are those that are close to a polar alignment. Moreover, we find that two circumbinary planets in the same system may result in complicated orbital dynamics and resonances. These interactions could efficiently lead to the formation of free-floating planets because hosting multiple planets around a binary is more challenging than in a single star system.

The Greenland Telescope [i] started scientific observations in 2018. Currently, the Telescope is located at Pituffik Space Base (PSB) in the northwestern corner of Greenland and has participated in the observing campaigns of the Event Horizon Telescope (EHT) and the Global Millimeter-wave VLBI Array(GMVA). The first scientific results featuring the GLT revealed a panoramic picture of the black hole and its jet at a 3 mm wavelength [ii]. The next GLT result will come from the 2018 EHT observations, displaying a sharper black hole image at 1.3 mm. As is expected, many new and exciting research outcomes will be published from the observations after 2018.

The next stage of the GLT project aims to capture the most distinctive signature of general relativity–the photon rings. In their current planning, the EHT and its next-generation follow-up cannot resolve the features of the photon rings because of the lack of the required angular resolution. To achieve such a scientific breakthrough, we will relocate the GLT to Summit Station and conduct observations at 600 GHz and higher. I will present the status of the GLT and its plan for achieving our next goals.

[i] Ming-Tang Chen et al 2023 PASP 135 095001
[ii] Lu, R.-S., Asada, K., Krichbaum, T. P., et al. 2023, Nature, 616, 686

Interstellar dust has long been modeled with separate silicate and graphite/amorphous carbon components. In this talk, I will argue that initially distinct populations of stardust get rapidly homogenized in the ISM into a composite material ("astrodust"). I will show that the astrodust+PAH model is compatible with current observational constraints on dust extinction and emission in the diffuse ISM, and that it provides a more natural explanation for the observed polarized emission than do two-component models. I will discuss implications for the lifecycle of dust in galaxies.

The effect of gravitational lensing of the cosmic microwave background (CMB) provides a unique opportunity to obtain a picture of the gravitational potential of the large-scale structure of the Universe at very high redshifts. Tomographic cross-correlation of the gravitational potential with other tracers of the large-scale structure at known redshifts allows tracing the evolution of the structure and testing cosmological models. However, the analysis of upcoming data will require a very good understanding of any systematic errors that may bias cross-correlation measurements. In this talk we will present studies of systematic errors arising from redshift bin mismatch of galaxies with photometric redshift uncertainties. We show their impact on the cross-correlation measurement and cosmological parameter estimates for future data sets. We also present an efficient method for removing the errors.

Over the last decade, ALMA has revolutionized our understanding of the interstellar medium (ISM) conditions of distant galaxies. For one, ALMA has now detected (sub-)millimeter continuum emission from dozens of galaxies at z > 6.5, establishing the importance of dust-obscured star formation already within the first 800 Myr after the Big Bang. Moreover, through various bright emission line diagnostics such as [CII]158 and [OIII]88, ALMA can be used to directly study the physical conditions and kinematics of the ISM within the earliest galaxies.

In this talk, I will review what we have learned about the dust and interstellar medium properties of high-redshift galaxies, focusing specifically on the Cycle 7 ALMA Large Program REBELS and its follow-up studies. In addition, I will present recently taken JWST/NIRSpec IFU observations of REBELS sources and discuss how we can use the combined power of ALMA and JWST to learn about early dust build-up in galaxies.

Fast radio bursts (FRBs) are among the newest tools in observational astrophysicists’ repertoire to study ionized gas. Their unique, millisecond-duration radio signal is subject to propagation effects in the intervening plasma. One such effect is the plasma dispersion of FRB pulses. FRB dispersion measures (DMs) quantify the net free electron column density through the sightline. FRB DMs can be precisely measured (~0.1%) and thus are sensitive to the most diffuse plasma in the intergalactic medium (IGM) that traditional probes have found challenging to illuminate. This ability to provide novel constraints on plasma has motivated studies of the circumgalactic medium (CGM) of galaxies intersecting FRB sightlines and the cosmic web filaments of the IGM. In my talk, I will highlight some of the work already done leveraging FRBs and introduce the FLIMFLAM survey. The survey is an ongoing endeavor to map foreground matter density along ~20 FRB sightlines. To this end, the survey measures spectroscopic redshifts of the foreground galaxies. Its ultimate aim is to produce statistical constraints on key parameters describing matter distribution in the universe, including the fractions of ionized baryons residing in the diffuse IGM and the virialized gas of halos. We expect our first data release by the end of 2023. Meanwhile, some of our recent work focuses on interesting sightlines that exhibit unusually large DMs. Through foreground mapping, we confirm large host galaxy DMs in some sightlines while others show numerous foreground structures. I will also discuss one sightline where foreground mapping revealed cluster gas that enhanced the DM. I will end with prospects for FRB-based analyses in the near future that I'm excited for.

Planet formation within protoplanetary disks presents an array of unsolved questions, two of which I aim to address in this talk.

First, I'll explore how dust is concentrated to densities sufficiently high for the on-set of planetesimal formation. By drawing a parallel to the formation of underwater sand ripples, I introduce a novel linear, axisymmetric instability capable of transforming turbulent dust regions in a disk's mid-plane into dense, azimuthally stretched filaments. This process hinges on a rapid decrease in diffusivity with heightened dust concentration, a premise supported by our numerical simulations where dust-gas interactions self-generate turbulent diffusion.

Second, I will discuss planet-hosting (wide) binary systems, where recent studies suggest a prevalence of coplanar arrangement of planetary and binary orbits. I propose that this alignment, along with observed obliquity distributions in exoplanetary systems, can be attributed to viscous dissipation in the disk during binary-driven precession. Moreover, our analysis predicts that the alignment tendency should weaken as the binary stellar mass ratio increases, and indeed, I will present new evidence for this trend in TESS data of exoplanet-hosting binaries.

Observation of deep fields provides a unique scope to explore multiple aspects of extragalactic astronomy by effectively detecting the faintest objects in the distant universe. We utilize the unique angular resolution, sensitivity, and field of view of the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat to perform deep imaging of the GOODS-north field in the FUV and NUV bands reaching a 3$\sigma$ depth of ~ 27.3 AB mag. Our UV flux measurements of the identified sources complement existing rich multiband data in the GOODS-N field and enable us to probe properties of galaxies between redshift ~ 0 and 1. We study the internal dust extinction of galaxies by constraining their UV continuum slope (bet). Combining with HST F275W, F336W, and KPNO U bands, the UVIT data helped us to estimate beta of 465 galaxies between redshift 0.40 and 0.75. Our beta measurements add new data points to the least-explored redshift regime, further reinforcing the gradual reddening of the galaxy UV continuum with cosmic time. Using a sub-sample of 83 galaxies, we further constrained the beta - IRX law at redshift ~0.5. I Will also discuss how observation of AstroSat UV deep field is unique to constrain the faint end slope of UV Luminosity Function, testing the nature of SFR scaling relation, and finally to search for Lyman Continuum leaking galaxies beyond redshift ~ 1, which are important to understand galaxy evolution and cosmic reionization process.

In the next decade, large cosmological surveys will map billions of galaxies and measure cosmic structure to unprecedented precision via multiple cosmological probes. Using results from the Dark Energy Survey as an example, this talk will outline the challenges and opportunities of cosmological analyses in the presence of rich astrophysics and observational systematics. In particular, I will describe different cosmological probes measured from the Dark Energy Survey and summarize recent progress on joint analyses of galaxy clustering, weak lensing, and galaxy cluster abundances. I will then highlight the challenges for future, larger experiments such as Rubin Observatory's LSST and describe our progress in tackling these challenges through sophisticated simulations and an AI-enhanced analysis pipeline. I will conclude the talk by identifying a unique opportunity for overlapping imaging surveys and Cosmic Microwave Background surveys that will unlock the constraining power of small-scale weak gravitational lensing measurements.

Variability of mass accretion provides important information about disk evolution and the geometry of the accretion flows. While there are many studies focusing on young stars, accretion monitoring in the substellar regime has been relatively uncommon. At optical wavelengths, the H-alpha emission is a bright accretion tracer, proving an opportunity to observe young brown dwarfs with small telescopes. In this talk, I will introduce our ongoing efforts of monitoring brown dwarfs with the one-meter telescope at Lulin Observatory and present the preliminary results.

Abstract: Ghostly neutrino particles continue to bring surprises to fundamental physics, from their existence to the phenomenon of neutrino oscillation, which implies their nonzero masses. Their exact masses, among the most curious unknowns beyond the Standard Model of particle physics, can soon be probed by the joint analysis of ongoing and upcoming cosmological surveys including Rubin LSST, Euclid, Roman, DESI, PFS, Simons Observatory, CMB-S4, and LiteBRID. In this talk, I will discuss ongoing works studying the effects of massive neutrinos and will draw a roadmap towards discovering the neutrino mass over the next decade. Bio: Jia Liu (https://liuxx479.github.io/) is a computational and observational cosmologist. Liu is an associate professor at Kavli IPMU in the University of Tokyo and the director of the recently established Center for Data-Driven Discovery (CD3) at Kavli IPMU. Liu received her PhD from Columbia University in 2016, was an NSF postdoctoral fellow at Princeton (2016-2019) and a BCCP postdoctoral fellow at UC Berkeley (2019-2021).

Star formation is a process covering orders of magnitude in spatial and density scales, manifesting as a hierarchy of interacting structures. This necessitates that any complete study of this process must be multi-scale by nature. In this talk I will present my work showcasing the key role that filaments play across spatial scales and how they shape star formation, from >10 pc molecular clouds down to 100 AU protostellar disks. Using a combination of numerical hydrodynamical simulations with molecular line and dust continuum observations, I will focus on addressing a number of open questions surrounding filaments in this multi-scale framework: How do filaments fragment into cores/clumps? How does feedback interact with filaments? How are filaments shaped by their environment? I will conclude by introducing the new ASIAA-SMA key project STREAMS, which will homogeneously calculate the turbulent, magnetic and gravitational energies across multiple scales for 23 massive, star-forming clumps. With its combination of multiple datasets and large sample size, STREAMS will answer the many key, open questions about multi-scale energetics and its link to fragmentation, and high-mass star and cluster formation.

Over the next decade, large galaxy surveys will map billions of galaxies and probe cosmic structure with high statistical precision. Their ultimate goal is to develop a comprehensive model that describes the Universe from end to end. In this talk, I will introduce two of the most powerful cosmological probes: galaxy clusters and galaxy clustering. These probes offer insights into the growth of cosmic structures spanning back up to 11 billion years. By harnessing ongoing and forthcoming galaxy surveys such as the Dark Energy Spectroscopic Instrument (DESI), the Rubin Observatory's Legacy Survey of Space and Time (LSST), Subaru Prime Focus Spectrograph (PFS), and the Roman Space Telescope, we can measure the growth of structure with unprecedented precision. I will discuss the opportunities and challenges inherent in these galaxy surveys.

Protoplanetary disks serve as the sites where exoplanets are formed. Observations of these sites that give birth to planets reveal a wide range of temperatures, densities, and distribution of various molecules. The Atacama Large Millimeter/submillimeter Array (ALMA) has ushered in a new era in the study of protoplanetary disks, enabling us to explore the physics and chemistry of the outer regions of these disks. With the arrival of the James Webb Space Telescope (JWST), we can now delve much deeper into the inner regions of the disks and also investigate icy volatiles in colder regions. The combined contributions of these two instruments can provide us with greater clarity on the formation of exoplanets and the chemical inventory they inherit from protoplanetary disks. Modeling protoplanetary disks can bridge the gap between our theoretical understanding of exoplanet formation and its connection to atmospheres. Here, we introduce PEGASIS (Protoplanetary Disk and Envelope model for Emissions from GAS and Ice Absorption Simulations), a physicochemical model capable of simulating dust, cold gases, hot gases, and ices in both disks and envelopes.

I will discuss the exciting perspectives related to the near future observations of supermassive black holes at mm/sub-mm wavelengths. Event Horizon Telescope (EHT) is delivering images of subsequent AGN sources, while continuously expanding its capabilities, adding new facilities such as the Greenland Telescope, and testing observations at 345 GHz frequency. These observations correspond to the most central part of AGN sources we were ever able to resolve, opening tantalizing possibilities to study the region of jet formation, collimation, and acceleration and advancing our understanding of the physics of these extreme systems. Some fundamental questions to answer are related to the role of magnetic fields and general relativistic effects around a spinning black hole, location of the jet acceleration zone and its relation to the high energy emission. There is a lot to learn from observations of individual sources such as M87, Centaurus A, 3C 84, or 3C 279, but it is also extremely interesting to study a population of sources imaged at mm wavelength for the first time. In particular, our Galactic Center black hole Sagittarius A* is an uniquely interesting mm/sub-mm source, in which horizon-scale dynamics of a low accretion rate system can be studied. This is very exciting in the context of the source flaring, which we are trying to understand in a theoretical framework of the quasiperiodic flux eruptions expected for magnetically dominated accretion systems. Near future time domain analysis of a collection of mm/sub-mm light curves of Sagittarius A*, along with new coordinated multiwavelength observations, and perhaps even resolved (both in space and in time) observations of the EHT will necessarily shed light on the physical mechanism behind flares and high energy emission, as well as on the importance of magnetic fields in the systems similar to Sagittarius A*.

The Outer Planet Atmospheres Legacy (OPAL) program with Hubble was started in 2014 with the goal of studying time-domain phenomena in Jupiter, Uranus, and Neptune, with Saturn added in 2018 once the Cassini spacecraft was de-orbited. Once a year, the OPAL program images each of our four outer planets, producing pairs of global maps in multiple filters, which are made available at the MAST archive. The OPAL team (Simon, Wong, and Orton) have used the data to discover new dark spots on Neptune, discover a UV-dark oval in Jupiter's southern polar haze cap, measure changes in Jupiter's Great Red Spot over time, detect fine-scale waves, chronicle shifts in haze and cloud layers on all four planets, measure jet streams, and study the structure and evolution of convective storms. The data have also provided a valuable resource enhancing the science return from the Juno and New Horizons spacecraft missions, also supplementing observations from a growing list of observatories including JWST, Kepler, Spitzer, VLA, Keck, ALMA, IRTF, VLT, and Gemini.

Strong gravitational lenses are massive cosmic objects, like galaxies or galaxy clusters, which can map an extended background source, like a galaxy, into several highly distorted and magnified images. Analysing the properties of those images yields important information about the distribution of the deflecting mass and the background source. Common approaches to reconstruct the source or the deflecting mass distribution model the global properties of the source and the lens. They obtain a consistent description of the entire configuration by refining the model until it matches the observation to a predefined precision.

Here, I introduce a new approach to infer local properties of the gravitational lens and to reconstruct the source only using the properties of the multiple images without assuming a lens or a source model. The approach can be applied to galaxy or galaxy-cluster lenses in the same way and yields the maximum information all lens models agree upon. Showcasing two example lenses, I highlight 1) how to obtain a smoothness scale for dark matter with it from only three multiple images in a newly discovered cluster and 2) how to identify and resolve limits of lens models that may lead to highly unrealistic dark matter properties.

Since data is still sparse at the moment but of increasing level of detail, a model selection to find an appropriate mass density profile is also needed to reduce the increasing amount of computing time when fitting a detailed lens model to the data. Most models are currently based on heuristical fitting functions inferred from simulations. Hence, they lack an explanation for their suitability based on fundamental physical principles. To overcome this issue, I will show how the most common mass density profiles, in particular the famous Navarro-Frenk-White profile found in many simulations, can be derived from symmetry arguments and conservation laws.

References: - Lens method overview: https://arxiv.org/abs/1906.05285 - Examples: https://arxiv.org/abs/2207.01630 and https://arxiv.org/abs/2306.11779 - Derivation of power-law models: https://arxiv.org/abs/2002.00960

Understanding planet formation is a major challenge in modern astrophysics. Planets form in protoplanetary disks orbiting around young stars. These disks are gas and dust residuals inherited from the parent clouds which form stars. It is only recently, with the advent of large mm/submm arrays such as ALMA (Atacama Large Millimeter Array, Chile) and its precursors, in particular the IRAM (Institute of Radio Astronomy Millimetric) array (France), that this field has slowly emerged in the early nineties. In this talk, I will present how our understanding on planet formation has evolved in the last 30 years. For this purpose, after an introduction describing the context, I will focus on the observations and analyses of two emblematic objects: the young low-mass triple system GG Tauri and the young single HAe (2.4 Msun) star AB Auriga. Starting from unresolved images of their protoplanetary disks 30 years ago, I will show how we are now beginning to unveil their nascent planetary systems.

I will present recent observational studies of emission-line galaxies using ALMA and JWST. First, I will discuss the physical and chemical properties of a highly magnified (up to μ~160) and intrinsically faint (sub-L*) galaxy at z = 6.072, known as Cosmic Grapes, located behind the massive cluster RXC J0600-2007. This unique source has been uncovered as a bright 1-mm-wave line emitter by the ALMA Lensing Cluster Survey (ALCS) and subsequent investment of approximately 160 hours of observing time with ALMA, JWST, and MUSE/VLT reveals its highly clumpy nature. I will also demonstrate how the joint analysis of emission lines by ALMA and JWST provides constraints on fundamental parameters, such as electron density, in a metal-poor, low-mass galaxy at z = 8.496, situated behind the lensing cluster SMACS 0723.3-7327. A [OIII]5007 emission line selected galaxy at z = 8.343 behind the lensing cluster MACS J0416-2403, a part of JWST cycle 2 program MAGNIF, will also be reported. A serendipitously uncovered millimeter-wave emission line galaxy reveals the presence of a submillimeter-galaxy-like, dust-enshrouded extreme starburst event hosted by an isolated yet gas-rich grand-design barred spiral galaxy at z = 2.467. Discovery of a CO-bright (i.e., gas-rich) quiescent galaxy at z = 1.146 will also be discussed. Lastly, based on these studies, I will discuss how next-generation submillimeter-wave telescopes, like the LST/AtLAST, equipped with a large-format imaging spectrograph, will revolutionize our understanding of galaxy evolution in the early universe.

I will discuss transition protoplanetary disks observed by ALMA which have a large inner dust cavity. They are believed to be in the process of planet formation although other central clearing mechanisms are possible. Several of these disks show asymmetric emission that has been associated with vortices where the dust can be trapped and grow to form planetesimals. I will discuss the analysis of the dust properties in the asymmetric transition disks HD142527 and work needed to further elucidate the properties of this dust trap.

New general relativistic MHD models of black hole accretion flows in luminous systems (such as quasars and X-ray binaries) that include full radiation transport will be described. These models are designed to study the steady-state structure of the accretion disk near the horizon, and the effect of radiation on the launching of relativistic jets from spinning black holes. Moreover, they enable not only the interpretation of the spectra and variability of these sources, but also predictions about the rate of growth of black holes in the early universe, and measurement of the energy and momentum feedback into the surrounding medium, a process likely to be important in galaxy formation. These calculations use a new version of the Athena++ adaptive mesh refinement code based on the Kokkos library that runs on both CPUs and GPUs. A brief description of this new code, as well as other applications and extensions underway, will also be given.

Terrestrial planets have traditionally been thought to form by collisions between protoplanets taking place mostly after the dissipation of the protoplanetary disc, on time-scales of 30-100 million years. I present here a new model where terrestrial planets grow instead by accreting small pebbles in the protoplanetary disc within 3-5 million years. I discuss how the immense pebble accretion heat leads to extensive melting of the growing planets and to the emergence of deep magma oceans. Volatiles such as water, carbon and nitrogen are accreted with the pebbles and partitioned between atmosphere, magma ocean and core. The end of the accretion phase leads to rapid crystallisation of the magma ocean and outgassing of the first atmosphere. I will finally discuss how the atmospheric composition of young planets is key to understanding the origin of life.

ULTIMATE-Subaru is a next large instrumentation program at Subaru, to develop a wide-field (20-arcmin in diameter) ground-layer adaptive optics (GLAO) system and a wide-field NIR imager (WFI) on Subaru, to strengthen the capability of Subaru Telescope in NIR. ULTIMATE will deliver an improved image quality of FWHM~0.2-arcsec (at K-band) in moderate conditions of Maunakea over the full 20-arcmin FoV. While HSC/PFS are the leading instruments for dark nights, ULTIMATE will be a primary facility instrument for bright nights of Subaru in late 2020s and beyond. In this series of seminar talks, we will introduce science cases of ULTIMATE for (1) galaxy and structure formation, (2) time domain astronomy, (3) Galactic Center observations - for all of which the new capabilities of deep, sharp, wide-field IR observations are critical. Taking this opportunity, we will also introduce the "SUPER-IRNET" program, to promote everyone in the community in Taiwan to join this activity. ULTIMATE-Subaru is a next large instrumentation program at Subaru, to develop a wide-field (20-arcmin in diameter) ground-layer adaptive optics (GLAO) system and a wide-field NIR imager (WFI) on Subaru, to strengthen the capability of Subaru Telescope in NIR. ULTIMATE will deliver an improved image quality of FWHM~0.2-arcsec (at K-band) in moderate conditions of Maunakea over the full 20-arcmin FoV. While HSC/PFS are the leading instruments for dark nights, ULTIMATE will be a primary facility instrument for bright nights of Subaru in late 2020s and beyond. In this series of seminar talks, we will introduce science cases of ULTIMATE for (1) galaxy and structure formation, (2) time domain astronomy, (3) Galactic Center observations - for all of which the new capabilities of deep, sharp, wide-field IR observations are critical. Taking this opportunity, we will also introduce the "SUPER-IRNET" program, to promote everyone in the community in Taiwan to join this activity.

Upcoming large-scale structure (LSS) and cosmic microwave background (CMB) experiments offer a unique opportunity to turn the Universe into a particle physics laboratory and determine the nature of dark matter, dark energy, and the masses of the neutrinos. I will present innovative methods to jointly analyze these datasets and unleash their full constraining power. My group's research explores two powerful ways of using the CMB as a backlight for the LSS: revealing the invisible dark matter (gravitational lensing) and baryons (Sunyaev-Zel'dovich and patchy screening effects) via their shadows on the CMB. These methods will yield percent-precision maps of the dark and baryonic matter on cosmic scales, from combinations of CMB experiments like the Atacama Cosmology Telescope, Simons Observatory and CMB-S4 with LSS experiments like the Dark Energy Spectroscopic Instrument and the Rubin Observatory. These will not only shed light on dark matter, dark energy and the neutrinos, but they will also constrain models of inflation and transform our understanding or galaxy formation.

The current generation of redshift surveys will provide three-dimensional maps of the Universe with tens of millions of galaxies spanning much of the observable universe. These maps explore physics beyond the standard model, including the physics of dark energy and early universe inflation. I will present new measurements of cosmic expansion and dark energy from the first year of the Dark Energy Spectroscopic Instrument (DESI). DESI is mapping the sky with a 5000-fiber robotic focal plane and 10 optical spectrographs. I will describe the design of the instrument, the survey, and the analysis of the first 5 million galaxies and quasars.

The Dark Energy Sepctroscopic Instrument (DESI) is mapping 40 million galaxies and quasars to precisely measure the expansion history of the universe. I will present results from the DESI Year 1 data. I will describe the science goals and survey design for the second-generation survey of the Dark Eenergy Spectroscopic Instrument (DESI-2) and the successor Spec-S5. These surveys are designed to improve constraints on dark energy and to probe the inflationary epoch. High-redshift (z>2) galaxies will be selected using a combination of broad-band and medium-band imaging. The more ambitious scope of Spec-S5 relies upon significant upgrades to the telescopes, fiber robots and spectrographs.

High-precision data obtained by new-generation observational instruments such as JWST and EUCLID are expected to bring new developments to dark energy research. As these datasets surpass conventional data both in quantity and quality, there arises a need for more accurate analytical methods. Weak gravitational lensing stands out as a valuable tool for probing dark matter and dark energy. Traditionally, gravitational shear, a combination of the second derivative of the lens potential, has been measured to reconstruct mass distribution. However high-quality space imaging data are expected to contain more detailed information of the lensing potential. We have developed a new method to measure the gravitational flexion, which corresponds to certain combinations of the third derivatives of the lens potential. Measuring flexion enables a more accurate mass reconstruction. In this talk, I will introduce our method of measuring flexion and present the results of flexion measurement for the JWST cluster SMACS0723.3-7327.