學術演講及專題研討 (2023)

Source count --- the number density of sources as a function of flux density --- is one of the most fundamental statistics of imaging observations. One of the advantages is its simplicity, i.e., compared with more complicated and advanced analyses such as luminosity/mass functions, there is little room for analysis errors to distort results, yet the source counts still carry important information on galaxy evolution. We present these fundamental statistics for the newly advent James Webb Space Telescope (JWST) MIRI instrument in the six mid-infrared bands, i.e., 7.7, 10, 12.8, 15, 18 and 21 ~m. The resulting IR populations of galaxy source counts are up to ~100 times deeper than previous works, reflecting the superb sensitivity of the JWST.

Furthermore, we physically interpret these JWST number counts to constrain cosmic star-formation history (CSFH), and black hole accretion history (BHAH). Following Gruppioni et al. (2011), we parameterize IR luminosity functions (LFs) and their evolutions for five different populations of galaxies (star-forming galaxies, starbursts, composite, Seyfert 1 and 2). By simultaneously fitting the model to the six mid-infrared number counts, together with the previous results, we determine the best-fit evolutions of MIR LFs for each of the five types of galaxies, and thereby, CSFH and BHAH. The obtained CSFH and BHAH are consistent with the previous estimates, but thanks to JWST, our estimates are based on tens to hundreds of times fainter MIR sources, whose existence was merely an extrapolation in previous studies.

Neutrinos are massless in the standard model, but neutrino oscillation experiments have confirmed that at least two out of the three active neutrino species have mass. Cosmological data can be an important probe for neutrino properties, like mass, energy density, and non-standard interactions. In this talk, I shall first discuss the bounds on the neutrino mass sum and the mass hierarchy from cosmological data, and how cosmological data cannot differentiate between the normal and inverted hierarchy well. Next, using Bayesian evidence and KL Divergence calculations, we shall see that there is no conclusive evidence for normal neutrino mass hierarchy from the combined power of the latest neutrino oscillations, neutrinoless double beta decay, and cosmological data, when we consider mass hierarchy agnostic priors. Finally, we shall look at constraints from cosmological data, on the possible neutrino non-standard self-interactions mediated by a heavy scalar, its role as a potential solution to the Hubble tension, and how this self-interaction model can help reconcile two inflationary models: Natural Inflation and Coleman-Weinberg Inflation, with cosmological data, even though these inflationary models are ruled out at more than 2-sigma in the Lambda-CDM model.

Ice plays a critical role in chemical evolution during star formation. Complex organic molecules, which have become frequently detected in Class 0/I protostars, form on ice mantles and desorb into gas-phase when the temperature increases. However, the formation pathways of COMs and whether most protostars undergo similar chemical evolution remain open questions with little observational constraints. Most COMs form in the ice mantles covering dust grains. While ALMA provides sub-100 au resolution for studying gaseous COMs in nearby embedded protostars, measurements of the chemical composition in ices had been limited by low-resolution and limited sensitivity until JWST, which can probe ices at a spatial scale comparable to that by ALMA with unprecedented sensitivity. In this talk, I will overview the role of ice in the chemical evolution of star formation as well as the formation pathways of COMs. I will discuss the recent JWST results on ice in protostellar environments, especially focusing on the latest results of the CORINOS and IceAge program. I will also discuss the prospects of ice chemistry in the era of JWST.

The astronomers started to explore the universe with the multi-messenger since the 21st century. Studies of the gravitational wave (GW) play an important role and became popular in multi-messenger astronomy. In this talk, I will simply introduce the role of GW in multi-messenger astronomy and then introduce the basic contents of GW including the design to detect it, the projects of the ground-based observatories, the Taiwanese team involved in these projects and my cooperative studies related to these projects. The novel timing algorithm is the key to resolve the GW signal from the noisy data so I will also introduce these timing methods. In addition, searches of GW do not only depend on a good timing algorithm but also on an efficient pipeline, which is based on the artificial intelligence/deep learning. I will also demonstrate some preliminary results obtained from my cooperative studies. Now the open science center also provides lots of archival data obtained from LIGO-Virgo and as well as the KAGRA, and more manpower and computing resources are required to update our understanding of the universe to the next stage. Please join us!

In this talk I will describe recent results on modeling plasmas accreting onto supermassive black holes SgrA* and M87*, the prime targets of the Event Horizon Telescope. Specifically, I will present recent results from largest-to-date 3D GRMHD simulation and 2D kinetic simulations of pair production discharges, magnetic reconnection and global accretion flows. I will argue that flares from SMBHs are powered by relativistic magnetic reconnection during flux eruption events in magnetically arrested disks. Additionally, I will show that collisionless physics of accreting plasmas can produce significant changes in the large-scale flow dynamics, via the enhanced rate of magnetic reconnection and dynamically important heat flux, compared to commonly employed GRMHD models.

Over the past two decades, the realm of dusty, star-forming galaxies (DSFGs) has changed our understanding of the cosmic star formation embedded in the dust, thanks to the combination of space and ground-based IR and millimetre telescopes. Meanwhile, with the new advance of JWST, we have come to another new era to study statistical properties and internal structures of DSFGs. During my talk, I will go through the observations of different scales of DSFGs and discuss the new opportunities to explore the early Universe with DSFGs. I will start with the large sky surveys for DSFGs, particularly focused on the ALMACAL sky survey, which has turned ALMA into a survey machine by utilising all the calibration observations. I will present our recent efforts on the multi-band survey for DSFGs and ongoing projects. After that, I will introduce several new opportunities to understand the interstellar medium of the DSFGs, especially on the initial mass function and the magnetic fields. In the last part, I will discuss the clustering properties of DSFGs and their connections to the cosmic web and present-day galaxy clusters. I will also discuss possible collaborations with local experts from ASIAA.

I will give a summary of my recent works on the dynamics of well-coupled dust grains in protoplanetary disks under radiation pressure and planet-disk interactions. In the pre-planet-formation study, I will show that the stellar radiation on the dust grains could trigger an instability, leading to the formation of dust clumps at the inner disk edge that breaks the commonly assumed axisymmetry. The clumps effectively reduce the extinction level in the disk, sustaining the disk edge recession due to radiation pressure, and shedding light on the formation of large dust cavities in the observed transitional disks. Moreover, the dust clumps may be the seed of planetesimal formation. In the post-planet-formation study, I will show that the planet-disk interactions will produce morphological features on the dust ring trapped at the pressure maximum outside the planet-opened gap. Vertically, the dust on the gap edges will be carried to high disk elevations by the planet-induced meridional gas flows. And radially, the dust trapped at the planet-induced pressure bump will be additionally perturbed by the planetary wakes in the gas, resulting in a widened dust ring. These findings may help to explain the radially extended dust rings in the planet-hosting disk PDS 70 and AB Aur, and the nontrivial pattern of the deprojected dust ring in the GM Aur disk.

Time-resolved infrared spectroscopy, offering both temporal and spectral resolution in the experimental measurements, plays an important role in atmospheric, biological, and chemical physics studies. Dual-comb spectroscopy, a multi-heterodyne Fourier transform spectroscopy based on two frequency combs at slightly different repetition frequencies, enables broadband molecular fingerprinting with high-resolution and fast spectral acquisition. Here, a new approach for high-resolution time-resolved spectroscopy using mid-infrared dual-comb spectrometers will be reported. The time-resolved dual-comb spectra under different experimental conditions can be measured with Doppler-limited resolution at microsecond time resolution. Moreover, employing the dual-comb spectrometers coupled with a flash photolysis cell, multiple species, including the free radicals and reaction intermediates can be simultaneously and quantitatively detected, thus enabling to explore complex reaction processes and mechanisms. Our recent works on the study of the yields and formation mechanisms of OH and HO2 radicals in the reactions involving the Criegee intermediates, short-lived species involved in many key atmospheric reactions, will be presented in this talk. The approach with time-resolved dual-comb spectroscopy holds promise not only in exploring the issues of chemical physics but also in discovering transient processes of light-matter interactions in different fields.