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中央研究院天文及天文物理研究所
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Colloquiums and Seminars(2022)

ASIAA Colloquium is usually held on Wednesdays at 2:20-3:20 pm in Room 1203 of the Astronomy-Mathematics Building, NTU. All scientists are welcome to attend. Seminars on more specialized topics are also held on a regular basis.

The ASIAA-NTU joint colloquium series aims to bring to the physics/astronomy/cosmology community in ASIAA/NTU world renown researchers who will talk about the forefront development of physical sciences.

Contact: Colloquium Committee (talks_replace2@_asiaa.sinica.edu.tw)

NEXT Colloquium: 2022-10-12 Wed 14:20~15:20 [R1203]
Speaker:Shutaro Ueda
No. Time/Place Speaker Topic / Abstract
download PDF: download talk PDF file
12022-12-07 Wed
14:20~15:20
R1203
Chin-Fei Lee
[ASIAA]
Colloquium
22022-11-30 Wed
14:20~15:20
R1203
Jesús Alejandro López-Vázquez
[ASIAA]
Colloquium
32022-11-23 Wed
14:20~15:20
R1203
Martin Bureau
[University of Oxford]
Colloquium
42022-11-16 Wed
14:20~15:20
R1203
Can Cui
[Cambridge University]
Colloquium
52022-11-09 Wed
14:20~15:20
R1203
Feng Long
[University of Arizona]
Colloquium
62022-11-02 Wed
09:00~10:00
R1203
Karl Gordon
[STScI]
Colloquium
72022-10-26 Wed
14:20~15:20
R1203
Yen-Chen Pan
[NCU]
Colloquium
82022-10-19 Wed
14:20~15:20
R1203
Andrew Cooper
[NTHU]
Colloquium
92022-10-12 Wed
14:20~15:20
R1203
Shutaro Ueda
[ASIAA]
Colloquium
102022-09-28 Wed
14:20~15:20
R1203
Hsiang-Yi Karen Yang
[NTHU]
Colloquium
Unveiling the Origin of the Fermi/eRosita Bubbles
Abstract

The newly launched eRosita X-ray satellite revealed two gigantic bubbles above and below the Galactic center. The "eRosita bubbles" bare a remarkable resemblance to the Fermi bubbles detected in gamma rays, suggesting a common origin. The physical origin of these giant Galactic bubbles has been hotly debated. Using 3D magnetohydrodynamic simulations including relevant cosmic-ray physics, we show that the multi-wavelength observational data of the gamma- ray/X-ray bubbles as well as the microwave haze could be simultaneously explained by a single event of jet activity of Sgr A* about 2.6 million years ago. I will highlight some of the important constraints derived from our simulations and discuss the implications of the results on galaxy-scale AGN feedback in general.

112022-09-21 Wed
14:20~15:20
R1203
I-Non Chiu
[NCKU]
Colloquium
Cosmological Constraints from Galaxy Clusters and Groups in the eROSITA Final Equatorial Depth Survey
Abstract

We present the first cosmological constraints using the cluster abundance of a sample of eROSITA clusters, which were identified in the eROSITA Final Equatorial Depth Survey (eFEDS). In a joint selection on X-ray and optical observables, the sample contains 455 clusters within a redshift range of 0.1 < z < 1.2, of which 177 systems are covered by the public data from the Hyper Suprime-Cam (HSC) survey that enables a uniform weak-lensing mass calibration. In a framework of empirical modelling and blind analysis, we simultaneously model the cosmology, the X-ray selection, and the observable-to-mass-and-redshift relations with the observables including the X-ray count rate, the optical richness, and the weak-lensing mass. As a result, we deliver cosmological constraints that are in excellent agreement (at a level of < 1 sigma) with the results from the Planck mission, the galaxy-galaxy lensing and clustering analysis of the Dark Energy Survey, and the cluster abundance analysis of the SPT-SZ survey. This work not only presents the first fully self-consistent cosmological constraints obtained in a synergy between wide-field X-ray and weak-lensing surveys, but also demonstrates the success of the empirical modeling in X-ray cluster cosmology studies.

122022-09-14 Wed
14:00~16:00
1st floor Auditorium
Tien-Hsien Chang
[Genomics Research Center, Academia Sinica]
Colloquium
Scientific Misconduct Revisited: Stories and Lessons Learned
132022-09-07 Wed
14:20~15:20
R1203
Shotaro Yamasaki
[NCHU]
Colloquium
Deciphering Multiwavelength Transient Activity of Strongly Magnetized Neutron Stars: From Magnetar Flares to Fast Radio Bursts
Abstract

Strongly magnetized neutron stars occasionally show energetic transient activity in both X-ray and radio bands, such as magnetar flares, faint pulsed radio emission, and bright short-duration radio bursts in reminiscence of cosmological fast radio bursts (FRBs). Magnetar flares are believed to be related to an electron/positron pair plasma produced by a sudden dissipation of magnetic energy in the stellar vicinity. Such a plasma could be either trapped to the stellar surface by strong magnetic fields or launched as a relativistic outflow, which would play important roles in producing/suppressing the simultaneous emission in radio bands. In this talk, I will first present our recent work on (1) how magnetar flare spectra form inside the magnetosphere. Then, I will discuss (2) the potential link between magnetar flares and pulsed radio emission and (3) the plasma properties that are compatible with X-ray and radio bursts co-detected from the Galactic FRB source, a magnetar SGR 1935+2154.

142022-08-31 Wed
14:20~15:20
R1203
Tao-Chung Ching
[NAOC and NRAO]
Colloquium
An early transition to magnetic supercriticality in star formation
Abstract

Magnetic fields have an important role in the evolution of interstellar medium and star formation. As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas. Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the H I narrow self-absorption (HINSA) towards L1544—a well-studied prototypical prestellar core in an early transition between starless and protostellar phases characterized by a high central number density and a low central temperature. A combined analysis of the Zeeman measurements of quasar H I absorption, H I emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding di use, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic fux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the di use CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes.

152022-08-24 Wed
14:20~15:20
R1203
Chien-De Lee
[NCU]
Colloquium
HO Puppis: Not a Be Star, but a Newly Confirmed IW And-type Star
Abstract

HO Puppis (HO Pup) was considered as a Be-star candidate based on its γ Cassiopeiae-type light curve, but lacked spectroscopic confirmation. Using distance measured from Gaia Data Release 2 and the spectral-energy-distribution fit on broadband photometry, the Be-star nature of HO Pup is ruled out. Furthermore, based on the 28,700 photometric data points collected from various time-domain surveys and dedicated intensive-monitoring observations, the light curves of HO Pup closely resemble those of IW And-type stars (as pointed out by Kimura et al.), exhibiting characteristics such as a quasi-standstill phase, brightening, and dips. The light curve of HO Pup displays various variability timescales, including brightening cycles ranging from 23 to 61 days, variations with periods between 3.9 days and 50 minutes during the quasi-standstill phase, and a semiregular ∼14 day period for the dip events. We have also collected time-series spectra (with various spectral resolutions), in which Balmer emission lines and other spectral lines expected for an IW And-type star were detected (even though some of these lines were also expected to be present for Be stars). We detect Bowen fluorescence near the brightening phase, and that can be used to discriminate between IW And-type stars and Be stars. Finally, despite only observing for four nights, the polarization variation was detected, indicating that HO Pup has significant intrinsic polarization.

162022-08-17 Wed
14:00~16:00
1st Floor Auditorium
Cing-Kae Chiao
[IEAS]
Colloquium
Preventing bullying and power harassment
172022-08-10 Wed
15:30~16:30
R1203
Min-Kai Lin
[ASIAA]
Colloquium
Hopes and challenges in modern planet formation
Abstract

With the discovery of over 5000 extra-solar planets to date, the formation and evolution of planets and planetary systems is one of the most rapidly developing fields of astrophysics. In the standard ‘bottom-up’ scenario, planets form from planetesimals — km or larger-sized bodies. Planetesimals form from small, mm-cm size pebbles, which themselves form from micro-sized dust grains immersed in gaseous protoplanetary disks around young stars. I will describe several obstacles, but also new possibilities, on the road from dust to planets from recent theoretical modeling of planetesimal formation in modern models of protoplanetary disks. Meeting Link: https://asiaa.my.webex.com/asiaa.my/j.php?MTID=mb018d9210a80051582617bd55cd3e3ce Meeting number: 2552 863 7090 Password: nF85mT7gpfN

182022-08-03 Wed
14:20~15:20
R1203
Po-Hsun Tseng
[NTU]
Colloquium
CANCELLED
192022-07-27 Wed
14:00~16:00
1st Floor Auditorium
焦興鎧
[中央研究院歐美研究所兼任研究員及財團 法人中華勞資關係研究所所長。]
Colloquium
何謂「職場不法侵害」?-從工作場所霸凌之防範談起
202022-07-20 Wed
14:20~15:20
online
Ken Chen
[ASIAA]
Colloquium
Magnetar-powered Supernovae
Abstract

Massive stars of 30 - 80 solar masses eventually collapse to black holes because the neutrino energy cannot drive a strong shock to overcome the ram pressure of infall, so core bounce fails to produce an explosion. But this picture can change with rapidly rotating stars, in which a neutron star (NS) with a period of a few milliseconds may be born. Rotation can amplify the magnetic field of the NS above 1E15 G, creating a magnetar. In this talk, I will discuss the theoretical models of the exotic explosions powered by the magnetars, and their astrophysical applications. Link to the video call: meet.google.com/qnd-xgva-qyr

212022-07-13 Wed
14:20~15:20
R1203
Chuan-Jui Li
[ASIAA]
Colloquium
From Type Ia Supernova Remnants to Binary Love Stories in Their Previous Life
Abstract

Type Ia supernovae (SNe Ia) have been used as standardizable candles to discover the accelerating expansion of the universe, leading to the revelation of dark energy and the award of 2011 Nobel Prize in Physics to astronomers. Important as they are, SNe Ia are not fully understood. It is not clear whether they originate from (1) white dwarfs accreting from binary companion stars or (2) mergers of two white dwarfs. To probe the nature of SNe Ia, we search for clues to their previous life in the remains of explosions, called supernova (SN) remnants: if a surviving companion or a dense circumstellar medium from companion’s mass loss is detected, the origin of accreting white dwarf can be affirmed. Over the past decades, no surviving companion has been unambiguously confirmed in the Milky Way. We thus decided to push the boundary to other galaxies. Using images and spectra of stars, we identified possible surviving companions outside the Milky Way for the first time. We reported the first detection of dense circumstellar medium within three extragalactic SN Ia remnants. These studies indicate that the origin of accreting white dwarf for SNe Ia could be more prevalent than people previously thought.

222022-07-06 Wed
14:20~15:20
online
Chia-Jung Hsu
[Chalmers U.]
Colloquium
From GMCs to Dense Cores
Abstract

The existence of massive prestellar cores (PSCs) is crucial in turbulent core accretion model. To find the hints of massive prestellar cores, we first performed high resolution simulations of giant molecular cloud (GMC) collisions. We utilise CASA post-processing the surface density maps to generate their ALMA 1.3mm synthetic observation results. We then use dendrogram to identify cores from the simulations and their corresponding synthetic observations. In chemistry aspect, observations have shown that high levels of deuterium fractionation ([N2D+]/[N2H+]>0.1) are often observed in prestellar cores. Therefore, we also carried out chemodynamics simulations of isolated massive prestellar cores. We studied the chemical evolution of species, especially N2H+ and N2D+, in a variety of initial chemical conditions. Our simulation results are compared with observational data, including the core mass functions (CMFs) of clusters, and the N2H+/N2D+ emissions from prestellar cores. We concluded that (1) the clusters do not have strong evidence of collision from the core mass functions, and (2) high depletion factor and high cosmic ray ionisation rate are required for these cores to achieve high level of deuterium fractionation. Video call to this talk: meet.google.com/qnd-xgva-qyr

232022-06-29 Wed
14:20~15:20
R1203
Ryu Makiya
[ASIAA]
Colloquium
Cosmology with the Subaru Prime Focus Spectrograph
Abstract

The Subaru Prime Focus Spectrograph (PFS) is the multiplexed fiber-fed optical/near-IR spectrograph, which will have the first light in early 2024. The PFS collaboration is now planning to perform the cosmological galaxy survey, which will map the large scale structure of the universe in wide redshift range of 0.6 < z < 2.4 via the spectroscopic observation of [OII] emission-line galaxies. The PFS survey will give a significant impact on the fundamental physics, such as the nature of massive neutrinos, dark energy and modified gravity. In this talk I will overview the current status of PFS cosmology project, introduce the simulation suites developed for the PFS, discuss the possible systematics, and forecast the constraining power of PFS. I will also discuss remaining issues (e.g., covariance modeling, likelihood analysis, galaxy distribution) for the analysis of forthcoming real data.

242022-06-22 Wed
14:20~15:20
online
Chia-Yu Hu
[MPE]
Colloquium
Interstellar Chemistry and Dust in Supernova Feedback-resolved Galaxy Simulations
Abstract

Star formation is the driver of galaxy evolution. Understanding the properties of the cold, star-forming gas in the interstellar medium (ISM) is therefore of crucial importance. In this talk, I will present recent developments in high-resolution (sub-parsec) hydrodynamical simulations of the stellar feedback-regulated ISM and their predictions on chemistry properties and line emissions. I will show that steady-state chemistry strongly over-predicts the abundances of H2 but not CO, leading to a reduced conversion factor (X_CO), especially at low metallicities where the H2 formation time becomes much longer than the dynamical time. On parsec scales, X_CO varies by orders of magnitude from place to place, primarily driven by the transition from atomic carbon to CO, and it drops to the Milky Way value once dust shielding kicks in. Finally, I present ongoing simulations that combine ISM chemistry with dust evolution, including SN destruction, AGB yields, and dust growth in dense gas. While dust growth strongly increases the abundance of CO to the observed values via radiation shielding, it has little effect on H2, as the timescales for H2 formation and dust growth are comparable. Video call to this talk: meet.google.com/qnd-xgva-qyr

252022-06-08 Wed
14:20~15:20
online
Jinshi Sai
[ASIAA]
Colloquium
Characterizing Gas Kinematics around Protostars over a Wide Spatial Range from Cores to Disks
Abstract

Revealing gas kinematics around protostars is crucial to understand the process of star and disk formation. Observational studies have characterized gas motions around protostars; protostars are generally surrounded by infalling envelopes (~1000 au) and rotationally supported disks (~100 au). However, more comprehensive picture of the kinematic structure over a wide spatial range from a core scale (~0.05 pc) to a disk scale is not yet obtained. In this talk, I will present observational studies on protostars, in which I investigated the gas kinematics over the wide spatial scale. Velocity structures on different spatial scales are characterized by measuring the radial dependence of the peak velocity of line emission. The peak velocity approximately follows Keplerian rotation and rotation with a constant specific angular momentum, which is suggestive of the infalling envelope, at radii less than a few thousands au, as was also reported in some other protostellar systems. On the other hand, the peak velocity is proportional to r^~0.6 at larger radii, which resembles the velocity structures of the initial dense cores (J/M~r^1.6). These results give rough size scales of the infalling envelopes of ~1400 to 2900 au around individual protostars, and suggest that the spatial scale possibly increases as the system evolves. I introduced the structure function analysis to probe velocity structures at larger radii, implying that the velocity structures at larger scales originate from turbulent motion. Video call link: https://meet.google.com/ydj-unof-qbg

262022-05-25 Wed
14:20~15:20
online
Britt Jeter and Cristina Romero-Canizales
[ASIAA]
Colloquium
Our Supermassive Black Hole: The First Sagittarius A* Results from the Event Horizon Telescope
Abstract

We present the first images of the supermassive black hole at the center of our Milky Way galaxy, Sagittarius A*. These observations were conducted in 2017 using a global array of 8 radio telescopes operating at a wavelength of 1.3 mm, or a frequency of 230 GHz. The EHT resolves a compact emission region with intra-hour variability. A variety of imaging and modelling techniques agree that the data is best represented by a thick ring of emission with an angular diameter of 51.8 +/- 2.3 μas, making this the largest black hole in the sky in terms of angular size. The ring has some level of azimuthal brightness asymmetry, and a dim central depression. When compared against a large number of numerical simulations, these images are consistent with a 4x10^6 solar mass black hole, well described by the Kerr metric. These results are consistent with those produced by observations of stellar orbits, but constrain the enclosed mass to reside in a region 1000 times smaller. Combined with the EHT observations of M87* we presented in 2019, the Sgr A* images demonstrate consistency with general relativity spanning three orders of magnitude in central mass. Video call link: https://meet.google.com/pmi-ajnn-zvx

272022-05-05 Thu
13:00~14:00
R1203
Yun-Ting Cheng
[Caltech]
Seminar
Cosmology and Astrophysics from the Extragalactic Background Light
Abstract

The upcoming cosmological surveys (LSST, Euclid, Roman, SPHEREx, etc) will map the universe with a wide angular and spectral coverage. Most of the studies with large-scale surveys rely on detecting individual sources, and leave the faint, diffuse components in the extragalactic background light (EBL) largely unexplored. To fully exploit the wealth of information from the upcoming cosmological survey datasets, we present two analysis techniques that directly extract cosmological and astrophysical signal from the multi-band large-scale EBL images without individual source detections. First, I will discuss cross correlation for EBL redshift tomography, and show prospects of constraining the EBL by cross-correlating SPHEREx and galaxy surveys. In the second part, I will introduce light cone analysis, a novel technique for analyzing the multi-band EBL images. Without cataloging individual sources, the light cone analysis uses a data-driven approach to simultaneously determine the spectral and spatial distribution of all emitting sources and the underlying large-scale structure traced by them. Our technique does not require any SED assumption and is not subjected to the source confusion as the conventional galaxy redshift surveys. This method will be widely applicable to upcoming cosmological surveys, and can provide complementary information to the galaxy detection approach.

282022-05-03 Tue
14:20~15:20
R1203
Chun-Hao To
[OSU]
Seminar
Challenges and opportunities of multi-probe cluster cosmology
Abstract

Multiple cosmological probes in photometric surveys can measure the cosmic structure. These include cluster abundances and positions, galaxy positions, and weak gravitational lensing shear. In this talk, I will outline a program that combines all these probes to maximize the cosmological science return. Then, using recent results from the Dark Energy Survey as a pathfinder example, I will describe the challenges and opportunities of this program for upcoming large cosmological surveys. I will further present some of the recent progress in tackling these challenges. These include developments of a novel and general sampling scheme that reduces the computational cost of the analysis by more than a factor of 50 and improvements in survey simulations for analysis validation. This talk will be concluded by discussing the prospects of this multi-probe cluster cosmology program in the Rubin Observatory's LSST and CMB-S4 era.

292022-04-27 Wed
14:20~15:20
R1203
Jakub Ripa
[Masaryk University]
Colloquium
Early results from GRBAlpha and VZLUSAT-2 CubeSats with gamma-ray burst detectors
Abstract

Gamma-ray bursts (GRBs) are the brightest explosions in the Universe. Some of them, the short GRBs, are important sources of gravitational waves originating in mergers of neutron stars or possibly also in mergers of neutron stars with black holes. I will present the detector performance and early science results from GRBAlpha, a 1U CubeSat mission, which is a technological pathfinder to a future constellation of nanosatellites monitoring and localazing GRBs. The localization can be achieved by measuring the time difference between the arrival of the signal at different satellites (synchronized by GPS). GRBAlpha was launched in March 2021 and has been operating already about a year on a 550 km altitude sun-synchronous orbit. The onboard gamma-ray burst detector consists of a 75×75×5mm CsI(Tl) scintillator, read out by a dual-channel multi-pixel (SiPM) photon counter (MPPC) setup. It is sensitive in the ~30-900 keV range. The main goal of GRBAlpha is the in-orbit demonstration of the detector concept, verification of the detector's lifetime, and measurement of the background level on low-Earth orbit, including polar regions and in the South Atlantic Anomaly. GRBAlpha has already detected five GRBs and was even able to detect two GRBs within 8 hours, proving that nanosatellites can be used for routine detection of gamma-ray transients. For one GRB, we were able to obtain a high resolution spectrum and compare it with measurements from the Swift satellite. We find that, due to the variable background, about half of the low-Earth polar orbit is suitable for gamma-ray burst detection. One year after launch, the detector performance is good and the degradation of the SiPM photon counters remains at an acceptable level. The same detector system, but double in size, was launched in January 2022 on VZLUSAT-2 (3U CubeSat). The GRB detectors perform as expected and I will present the early measurements from this mission as well. Our ultimate aim is to develop and launch a constellation of nanosatellites monitoring GRBs and these precursor missions help us to converge to this ambitious goal. The talk is given both in R1203 and online. Link to the Google meet: https://meet.google.com/nmv-neii-mry

302022-04-18 Mon
10:00~11:00
online
Atsushi Taruya
[Yukawa Institute for Theoretical Physics, Kyoto University]
Colloquium
Precision cosmology beyond Lambda CDM model
Abstract

Over the past decade, the so-called Lambda Cold Dark Matter model has been fully established as the standard cosmological model. However, there still remain unresolved issues, including the nature of dark energy or the origin of cosmic acceleration. Toward a deeper understanding of the Universe, more systematic observations, combining multiple probes, e.g., large-scale structure, cosmic microwave backgrounds, and gravitational-wave observations as a recently established new probe, are crucial. In doing so, the exploitation of improved methodologies to extract cosmological information as well as the refinement of the theory in confronting with observations are indispensable. In this talk, I present how I tackled these issues based on analytical approaches. The outcomes of our approaches include the constraint on the modified gravity, total mass of neutrinos, cosmic string, primordial gravitational waves.… To further find a clue beyond the standard cosmological model, I summarize what has to be addressed and scrutinized in the coming decades, together with what I want to pursue at ASIAA. The link to this talk: https://meet.google.com/nzn-dkot-qvp

312022-04-13 Wed
14:20~15:20
R1203
Frédéric Deschamps
[IEAS]
Colloquium
The Geographer and the Astronomer
Abstract

One of the main goal of geophysics is to map the structure of the deep Earth, and most particularly of the Earth’s mantle. This region of the Earth extend from about 20 to 2900 km depth and is composed of silicate rocks. In the past few decades seismic tomography models have been able to map lateral changes in seismic velocities within the mantle, providing the best information on the structure of the Earth’s mantle. Global tomographic models have reached a consensus on the large scale structure. The strongest heterogeneities are found in the topmost 300-400 km of the mantle, where they correlate with surface tectonics, and in its lowermost 400-500 km, where the dominant structures are two large low shear-wave velocity provinces (LLSVPs) located beneath Africa and the Pacific. Whether these anomalies originate from purely thermal or a mix of thermal and compositional causes is still debated. Changes in seismic velocity anomalies alone cannot simultaneously resolve the potential thermal and compositional contributions from which they originate, and additional constraints independent from seismic traveltimes are needed to answer these questions. Additional observational constraints may include seismic normal mode, seismic attenuation, the topography of the core-mantle boundary (CMB), tidal tomography, periodic changes in the length-of-the-day, and electromagnetic C-response deduced from long-period variations of the magnetic field. In additional key information can be obtained from experimental and numerical simulations of convection, modelling mantle dynamics. Depending on input parameters, simulations predict possible thermo-chemical structures that can be tested against geophysical observable, in particular seismic tomography maps. In particular, numerical simulations of mantle convection indicate that reservoirs of dense material, modelling LLSVPs, can be maintained for long periods of time provided that the chemical density contrast between these reservoirs and the surrounding mantle is large enough, typically around 1.5-2.0 %. Interestingly, since the laws of physics are invariant in space and time, the tools developed to better understand the Earth’s mantle structure, evolution, and dynamics may be transposed to the study of other bodies. For instance, numerical models of convection may be used to reconstruct the evolution of icy moons and dwarf planets of the solar System and explain structures observed at their surface.

322022-03-16 Wed
14:20~15:20
R1203
C. Y. Cyrus Chu
[IEAS]
Colloquium
U.S. / China trade and S&T conflicts
332022-03-09 Wed
10:00~11:00
Online
Rahul Kannan
[CfA]
Colloquium
Modeling high redshift structure formation and reionization
Abstract

One of the exciting new frontiers in cosmology and structure formation is the Epoch of Reionization (EoR), when the radiation from the early stars and galaxies ionized almost all gas in the Universe. This epoch is an important evolutionary link between the smooth matter distribution at early times and the highly complex structures seen today. Gaining insights into this epoch has been quite challenging because the current generation of telescopes are only able to probe the tail end of this process. Fortunately, a whole slew of instruments that have been specifically designed to study the high-redshift Universe (JWST, ALMA, HERA, SKA, CCAT-p, COMAP, SPHEREx), are about to come online. This will unleash a flood of observational data that will usher the study of EoR into a new, high-precision era. In this talk, I will introduce the THESAN simulation framework that is designed to efficiently leverage current and upcoming high redshift observations to constrain the physics of reionization. The multi-scale nature of the process is tackled by coupling large volume (~100 Mpc) simulations designed to model the large-scale statistical properties of the intergalactic medium (IGM), with high-resolution (~ 10 pc) simulations that zoom-in on single galaxies which are ideal for predicting the resolved properties of the sources responsible for it. I will briefly discuss applications from the first set of papers, including predictions for high redshift galaxy properties, emergence of the Lyman-alpha forest and back reaction of reionization on galaxy formation. I will then highlight the potential for using line intensity mapping of spectral lines originating from the interstellar medium (ISM) of galaxies and the 21 cm emission from the neutral hydrogen gas in the Universe to constrain galaxy formation and cosmology. I will finish by highlighting how this numerical framework, coupled with accurate observational estimates promises important and potentially transformative changes in our understanding of the primitive Universe. Link to this talk: https://meet.google.com/mcb-zrqu-ncp

342022-03-07 Mon
10:00~11:00
online
John Forbes
[Flatiron]
Colloquium
Towards a predictive theory of galaxy formation in the era of big data
Abstract

Galaxy formation is currently faced with immense datasets, both observational and simulated, with much more on the way. As the simulations continue to improve, I argue it is time to start thinking about how to use these realistic-looking galaxies to develop a predictive theory of galaxy formation. A new generation of semi-analytic models and statistical techniques are needed to simultaneously address the strong degeneracies that persist in the problem of galaxy evolution, and to coherently comprehend the vast quantities of extant and forthcoming data. I will focus on a particular set of puzzles around the turbulent driving in galactic disks - is stellar feedback, local gravitational instabilities, the direct impact of cosmological accretion, or something else responsible? link to this talk: https://meet.google.com/yhy-qkdu-yyi

352022-03-01 Tue
10:00~11:00
R1203
Pei-Ying Hsieh
[Joint ALMA Observatory Fellow]
Colloquium
Environments of gas accretion and star formation in the inner 10 pc of the Galactic Center
Abstract

The molecular 2-pc circumnuclear disk (CND) immediately around the Milky Way supermassive black hole (SMBH), SgrA*, resembles the "molecular torus" in AGNs, providing a unique opportunity to study SMBH accretion and nuclear star formation at sub-parsec scales. In recent years, I have been studying the key question of how much of the available gas can actually form stars in the environment around Sgr A*, and how material is being moved around and accreted in this region. The lifetime of the CND has been a long-standing debate over the past decade. The CND can not live longer than 10^5 years if the gas density is under the tidal threshold of SgrA*/nuclear star clusters, thus depleting the source of fuel and star formation. Utilizing the ALMA and various single-dish telescopes, I present CS line maps toward the CND of the Galactic Center. My primary goal is to resolve the compact structures within the CND and the streamers, in order to understand the stability conditions of molecular cores in the vicinity of Sgr A*. My data provide the first homogeneous high-resolution (1.3" = 0.05 pc) observations aiming at resolving density and temperature structures. A stability analysis based on the unmagnetized virial theorem including tidal force shows that 84 (+16/-37) % of the total gas mass (2.5X10^4 Msun) is tidally stable, which accounts for the majority of gas mass. However, turbulence dominates the internal energy and thereby sets the threshold densities 10-100 times higher than the tidal limit at distance >1.5 pc to Sgr A*, and therefore, inhibits the clouds from collapsing to form stars near the SMBH. In this talk, I will also discuss the observed morphology, kinematics of gas, and effects of magnetic fields in the GC. My past, current and future research is a natural extension to 3 main categories fitting the future development of ASIAA - (1) ALMA/JCMT, (2) GLT/EHT, (3) METIS/E-ELT. The Galactic Center is unique and extremely complicated. Therefore to discriminate against different models, eventually we want to utilize multi-wavelength and multi-resolution data to sample the energy of the accretion process at different scales.

362022-02-16 Wed
14:20~15:20
R1203
Chung-Yuan Mou
[National Taiwan University]
Colloquium
Nano-Confined Water
Abstract

Water is a peculiar liquid with many abnormal properties, maximum density at 4 oC is a famous example. A 40-year-old puzzle is about supercooled water. In 1976 C.A. Angell, then at Purdue University, experimented to see how far they could supercool water, and how the liquid would behave at extremely low temperatures. What they saw surprised everybody: As water dipped below −20 °C, its isothermal compressibility began to soar, a sign that its density was fluctuating wildly at the molecular scale. The liquid seemed on the verge of some dramatic transformation. But whatever that transformation was, Angell couldn’t actually see it; it occurred at temperatures below the homogeneous nucleation temperature, where the liquid state was too short-lived for the researchers to measure. In the early 1990s, Gene Stanley came up with a compelling explanation. Stanley’s theory hinged on the concept of critical points, special points in a phase diagram where two thermodynamic phases of matter—say, liquid and gas—meld into one. Water has a well-known critical point at about 374 °C and 218 atm, above which liquid water and water vapor become indistinguishable. Stanley proposed that water has a second critical point, hidden deep in the supercooled regime. At temperatures below that point, there exist two distinct liquid phases of different densities; above that point, the liquid phases merge. In Stanley’s interpretation, the density fluctuations in Angell’s experiment represented a kind of fluctuation between the two phases of water. However, this created a big controversy among theoreticians, two schools fighting each other, David Chandler(Berkeley) was much against the 2nd critical point concept. Then in 2003, Sow-hsin Chen(MIT) and I started a decade-long experimental program(mainly by neutron scattering) to study the supercooled water under nanoconfinement. We can supercool nano-confined water down to 180 K, still maintaining the liquid state. This is because in nanoscale, water cannot freeze. In this talk, I will tell this story of resolving the water controversy, mainly from our own data. Also, an important question of water is to understanding solubility of a hydrophobic molecule under nanoconfinement which impact on several related problems, (a) solubility of methane in water within nanopores of rock under fracking condition, (b) understanding how hydrophobic effect would be changed in confined water, (c) catalysis of gaseous molecule under confinement. Finally, I will speculate on some implications of confined water in several fields: (a) Its role in origin of life, (b) Geological Shale Gas by Fracking, (c) Pulling water out of thin air in desert.

372022-01-27 Thu
14:20~15:20
Online
Junsup Shim
[SNU]
Colloquium
The cosmic evolution of critical point clustering and its cosmological implication
Abstract

Critical points are special objects of a density field where the gradient of the field vanishes. They encode the topological information on the underlying density field and typically are the formation sites of large-scale structures of the cosmic web. Hence, the clustering of critical points provides information on the statistical properties of a given density field and the spatial organization of physical structures anchored to these points. In this talk, I will discuss the cosmic evolution of the clustering properties of peak-, filament-, wall-, void-type critical points focusing on both small separations and baryonic acoustic oscillation scales. A qualitative comparison with the corresponding theory for Gaussian random fields will be also discussed. Finally, I will summarize the cosmological implication of our findings and relate them to the cosmic standard rulers. Link to the talk: http://meet.google.com/ezw-onwu-zns

382022-01-19 Wed
10:00~11:00
online
Dhruba Dutta Chowdhury
[Yale]
Colloquium
Constraining Dark Matter through Gravitational Heating and Cooling Processes
Abstract

Fuzzy Dark Matter (FDM), consisting of ultralight bosons, is an intriguing alternative to Cold Dark Matter (CDM). Unlike in CDM, FDM halos consist of a central solitonic core, surrounded by an envelope of order unity density fluctuations. The envelope density fluctuations also interact with the soliton causing it to wobble and oscillate. Using novel, high-resolution numerical simulations of an FDM halo, corresponding to a particular boson mass, I will demonstrate that the gravitational potential fluctuations associated with the soliton's wobble, its oscillations, and the envelope density fluctuations dynamically heat nuclear objects (e.g., central star clusters and supermassive black holes) and galaxies. As a result, nuclear objects, initially located at rest at the soliton center, migrate outwards over time until the outward motion is counteracted by dynamical friction and an equilibrium is reached. Similarly, a galaxy undergoes significant size expansion and central density reduction over a Hubble time. Generalizing these results for other halo and boson masses and comparing them with observations (such as galaxy size-age relation, measured offsets of supermassive black holes and nuclear star clusters from the centers of their host galaxies) will be able to constrain the boson mass. After discussing FDM, I will also briefly present my work on the peculiar galaxy NGC 1052-DF2 and show what we can learn about its mass distribution from the dynamical friction-induced orbital decay of its globular clusters. Link to the talk: http://meet.google.com/kyg-mjsu-eja

392022-01-12 Wed
14:20~15:20
online
Jeremy Smallwood
[NCTS]
Colloquium
Stellar Flybys Exciting Spiral Arms in Protoplanetary Discs
Abstract

I will give an overview of the mechanics of flyby encounters and how these unbound perturbers can affect the structure of protoplanetary discs. Depending on the density of stellar clusters, stars with gaseous and dusty discs have up to a ~ 30% chance to encounter a flyby event within the first million years. The perturber will excite spiral arms during these flyby events. The spirals have a finite lifetime since the perturber is unbound. I simulate a parabolic encounter interacting with a gaseous protoplanetary disc utilizing three-dimensional smoothed particle hydrodynamical simulations. I find that the spirals can survive even when the perturber has passed periastron and is no longer interacting with the disc. Analyzing the dynamics of these excited spirals can shed light on observations of spirals in protoplanetary discs with no observed companion. *Link to the colloquium: http://meet.google.com/kyg-mjsu-eja

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