Colloquiums and Seminars(2023)

The outflow launched by the accretion disk formed after a neutron star merger is an important contributor to the total ejecta from the merger, and hence to the kilonova emission and heavy element yield. The amount of mass ejected and the kinematic, thermodynamic, and compositional properties of the ejecta have many dependencies, making up for a rich landscape of possibilities. I will provide an overview of the problem, and discuss progress in our understanding of this landscape on 3 of its dependencies: the gravitational compactness of the disk, the post-merger magnetic field configuration, and the extent of neutrino flavor transformation by the fast flavor instability.

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.

Bio: Daniel Jaffe is Vice President for Research and Professor of Astronomy at the University of Texas at Austin. As Vice President for Research, Jaffe oversees sponsored projects, major research cores, and research compliance. He oversees several major interdisciplinary research institutes, including the Texas Advanced Computing Center, and the University’s Bridging Barriers grand challenge research initiative. His unit plays a major role in developing large center grant proposals. Jaffe leads a research group in astronomical spectroscopy, astronomical instrumentation, and development of dispersive optics using precision nanolithography. His team studies the formation of stars and planetary systems and the effects of stellar radiation on the dense interstellar medium using infrared spectroscopy. They also study the physics of protoplanetary disks and use radial velocity techniques to search for planets around very young stars. He is PI for a Korea-Texas collaboration to build high-resolution near-IR spectrometers including IGRINS, which has been a productive instrument on the McDonald Observatory 2.7m, the 4.2m Lowell Discovery and the 8m Gemini South telescopes. The team is constructing a revolutionary 1-5 micron high-resolution spectrometer for the Giant Magellan Telescope. Jaffe's group develops novel diffractive devices for infrared spectroscopy using Si nanolithography. They have built the mid-IR grisms for the NIRCam instrument on the James Webb Space telescope and have fabricated near-IR immersion gratings for IGRINS and for the ISHELL spectrograph on the NASA IRTF while developing new production techniques for space astronomy and earth science applications.

Jets and outflows are integral parts of the physical processes that form the protostellar systems. We review the characteristics of these enigmatic powerful phenomena that constitute telltale signs of the underlying fundamental physics revealed by generations of radio and optical telescopes. We highlight the breakthrough advances in theoretical understanding of the formation, thanks to the unprecedented revelation of the fine delicate structures. Kinematic and morphological theoretically predicted features of jets, winds, and outflows are extracted and favorably compatible with observational data down to sub-arcsec resolutions. The systematics of coupled nested velocity and emission components spans from Class 0 to II jet–outflow systems in molecular and atomic lines, whose ubiquitousness is naturally explained.

Several ongoing surveys, notably by TESS and Gaia missions, are likely to discover star-black hole binaries in our Galaxy in the near future. A fraction of them may be triple systems comprising an inner binary, instead of a single black hole. They may be progenitors of the BBHs discovered from the gravitational wave. If such a star-binary black-hole system exists in our Galaxy, its outer tertiary star should exhibit a radial velocity modulation reflecting the nature of the inner binary. Combining an analytic approximation based the quadruple perturbation theory and N-body simulations of such triples, we examined the detectability of wide-separation inner BBHs in the Galaxy from the characteristic signals of the radial velocity of the tertiary star. The resulting radial velocities consist of two different types of modulations, short-term radial-velocity variations of roughly twice the orbital frequency of the inner BBH, and long-term modulations in significantly inclined triples. The latter is due to the precession of the inner and outer orbits over roughly the von Zeipel-Kozai-Lidov oscillation timescale. We conclude that it is quite feasible to detect such radial velocity modulations if those triples exist in our Galaxy.

A significant fraction of exoplanetary systems is known to exhibit spin-orbit misalignments. This surprising fact has been mainly revealed by a spectroscopic method, known as the Rossiter-McLaughlin effect for transiting planetary systems. This method measures the projected angle between the stellar spin and the planetary orbital axes, but is insensitive to the obliquity of the stellar spin with respect to the observer. Asteroseismology offers a unique method to infer the stellar obliquity in a complementary fashion. In this talk, I will first review the current statistics of the observed spin-orbit angles and proposed models for the origin of the misalignment. Then I will show our recent work on the spin-orbit architecture of transiting planetary systems using asteroseismology, and discuss its implications.

I will present our our new state-of-the-art Messenger Monte Carlo MAPPINGS V code (M³). The turbulent ISM causes inhomogeneity of electron temperature and density within the nebula, which is most effectively modeled through Monte Carlo ray tracing methods. We analyze the dependence of different optical emission lines on the complexity of nebular geometry, finding that the emission lines residing on the nebular boundary are highly sensitive to the complexity of nebular geometry, while the emission lines produced throughout the nebula are sensitive to the density distribution of the ISM within the nebula. Our fractal photoionizationmodel demonstrates that a complex nebular geometry is required for the accurate modeling of H II regions and emission-line galaxies. Finally, I will discuss the opportunities for these types of models for understanding galaxy formation and evolution with the current and next generation telescopes including JWST and the GMT.

Pulsars and Fast Radio Bursts (FRB) are some of the most energetic and "fast" astronomical objects. They are both possibly from "neutron stars". In this talk, I will introduce some of my group and collaborators' pulsars and FRB research using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). I will talk about the discovery and timing of new pulsars, testing theories of gravitation using pulsar timing, and measuring pulsar emission and geometry. I will also talk about some of the FAST FRB key science project results, including the large sample repeating FRB studies and the radio follow-up of the Galactic FRB source -- SGR J1935+2154.

AGN feedback plays an important role in regulating star formation activity and nearby interstellar medium (ISM) through outflowing winds and ionizing radiation. However, in the past, the lack of high spatial resolution in the infrared has limited our ability to closely examine the star-forming ISM in the vicinity of an AGN. The advent of integral-field observations through the James Webb Space Telescope (JWST) has opened up a tremendous opportunity to study the starburst-AGN connection in the sub-kpc scale. In this presentation, I will present the MIRI IFU observation of a type 1 Seyfert galaxy NGC7469, which hosts both a rapidly accreting black hole and a circumnuclear starburst ring with a radius of 500 pc. The high spatial and spectral resolution of MIRI has enabled us to isolate the starbursting event surrounding the AGN and study the dust and warm molecular gas on a ~100 pc scale. Our findings show that the starburst ring exhibits prominent Polycyclic Aromatic Hydrocarbon (PAH) emission, with grain sizes and ionization states varying by only ∼30%. A suite of H2 pure rotational lines is also detected throughout the ring, enabling us to estimate the warm molecular gas mass and temperature. Our study demonstrates that with JWST, we can finally study the resolved properties of the near nuclear ISM in great detail, even in the dustiest galaxies, on the scales of individual star-forming regions.

Dwarf galaxies are little fuzzy galaxies that contain much fewer stars than Milky Way-mass galaxies. The observation of these little galaxies can provide constraints on the physical properties of dark matter, and enhance our understanding of the galaxy formation and quenching mechanism. Finding these dwarf galaxies is, however, not an easy task as they are faint and dim from our perspective. I will describe our recent efforts on the search for nearby dwarf galaxies. I will start with the Satellites Around Galactic Analogs (SAGA) Survey, a spectroscopic survey that identifies satellite galaxies around more than 100 Milky Way-like analogs at 25-40 Mpc, putting our Milky Way in a cosmological context. I will highlight some new SAGA results and then discuss how SAGA data can be applied to other dwarf galaxy searches, which include an ongoing secondary target program with the Dark Energy Spectroscopic Instrument (DESI) and a plan for the Rubin Observatory Legacy Survey of Space and Time (LSST). Both programs will make a much larger population of dwarf galaxies available for the study of dark matter in coming years. Finally, I will turn to our searches for much closer and much fainter dwarf galaxies with resolved stars in the Milky Way’s backyard (within 1 Mpc), and discuss how these ultra-faint dwarf galaxies push our understanding of the galaxy-halo connection to a much lower mass regime.

Decades ago, astronomers viewed the warm ionized interstellar medium (WIM) as a turbulent, space-filling plasma with a mean density of about 0.03 electrons/cm^3 in the Galactic plane. How accurate is this picture? In particular, is there evidence for a more bubble-like (porous) distribution of the WIM? Techniques developed over the last twenty years, broadly referred to as scintillometry, allow us to make new progress on this question. By giving us the ability to locate scattering centers along the line of sight to pulsars, evidence is emerging that a bubble-like topology is appropriate for the WIM. I will present results of a survey of 22 relatively nearby pulsars, observed with the Green Bank Telescope and the Arecibo Telescope. Most show one dominant scattering screen along the line of sight, but higher sensitivity observations reveal larger numbers of scattering screens. What view of WIM topology is implied by these observations?

The Universe evolves as it expands. It was once generally smooth and filled with neutral gas, as revealed by the cosmic microwave background observations. Today’s Universe is predominantly filled with ionised plasmas threaded by magnetic fields. How did the transition happen? How do we probe the cosmic evolutionary history from the Earth? In this colloquium, I will discuss how astrophysicists explore the evolving Universe, with foci on two research frontiers: large-scale cosmic magnetic fields and gas reionisation. These two science themes have formed several key science projects for the forthcoming unprecedentedly powerful radio telescopes, the Square Kilometre Array (SKA), its precursors (e.g. ASKAP, MeerKAT, MWA) and pathfinders (e.g. LOFAR), among many other experiments. I will highlight, with demonstrations, the essential ingredients to optimise the scientific gains from these observational experiments by studying how information is encoded into the radiation we receive (i.e., cosmological radiative transfer). Finally, I will conclude this talk with a summary of the key findings from my theoretical calculations and an outlook on the exciting developments in our understanding of the ever-changing cosmos.

Cosmological large-scale structures are shaped by nonlinear processes mainly driven by gravity. Previous methods of analyzing this rely on theoretical templates that are based on perturbative expansion about the linear solution, restricting the extraction of information to large, mildly nonlinear, scales. On the other hand, N-body simulations can uncover structures on smaller scales, until non-gravitational effects such as gas cooling and feedback eventually become a factor. However, their high computational cost hinders their direct use in statistical inference. In this talk, I will discuss the emulator approach as a potential solution. In particular, I will present our Dark Quest simulation project and its applications to the SDSS and HSC datasets. I will briefly touch on the future direction of the whole analysis framework, which involves coupling simulators and observations for automated knowledge acquisition.

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.