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In the current Universe, cosmic rays (CRs) and magnetic fields are ubiquitous and have important roles at various scales from the Earth to clusters of galaxies. However, it has not been understood when, where, and how the first population of CRs and magnetic fields are generated since the big bang. These questions are, in other words, relevant for us to reveal the history of the nonthermal Universe and their roles in high-redshift environments. In the standard picture, a large-scale magnetic field is generated before the first CRs are accelerated. We recently proposed a new scenario that the first CRs are accelerated by the first supernova remnant of the first stars before the generation of large-scale magnetic fields. After the acceleration of the first CRs, the first CRs propagate in the intergalactic medium. Then, the propagating CRs generate the large-scale magnetic field by some mechanisms. In this talk, I will explain how the first CRs are accelerated without a large-scale magnetic field, and how the first CRs generate the large-scale magnetic field in the intergalactic medium in the early universe. In addition, I would like to take about the resistive heating by the streaming CRs in the early universe.
Zoom Application (online)
Accreting black holes launch powerful outflows: uncollimated disk winds and highly collimated relativistic jets, in some conditions. Studying these outflows are important to understand the accretion processes of black holes and the feedback to their environment. Galactic X-ray binaries are ideal objects for the studies of black hole outflows. Because of their proximity to us, they can be very bright in X-rays and hence provide opportunities for sensitive observations of the vicinity of black holes. In addition, their strong variability and the shortness of its time scales enable us to probe the evolution of the outflows at a wide range of mass accretion rates. In this talk, I will review the X-ray studies of outflows from Galactic X-ray binaries, and discuss how XRISM, the 7th Japanese X-ray astronomical satellite scheduled to launch in fiscal year 2023, can contribute to the understanding of black hole outflows through high resolution X-ray spectroscopy.
F608 & Zoom (hybrid)
Active galactic nuclei (AGN) feedback plays an important role in galaxy evolution that quenches the growth of massive galaxies and regulate the gas in circumgalactic environments. In this work, we present results from hydrodynamical tests of AGN jets in idealized initial conditions and isolated galaxy with gas halo. We are comparing our results with theoretical predictions and other works from different codes. Furthermore, we are trying to study the evolution of AGN inflated bubbles and their interactions with the ambient intracluster medium (ICM). Most of the energy is transferred to the ICM quickly, especially for jet kinetic feedback which thermalize gas halo and push the gas away from galaxy disk. Comparing the jet feedback with thermal or quasar feedback, it is found that any modes of AGN feedback results in reduced star formation rate. The jet has greater impact on the hot halo gas and let denser star forming phase in place, while quasar feedback acts more locally on dense star forming phase gas. In all cases, we observe gas and metals blown away from gas rich disk, reducing the metal content in the gas that slows down the cooling, resulting in long-term star formation quenching in the galaxy.
F608 & Zoom (hybrid)
Cold streams of gas (T ~ 10^4 K) are the dominant channels of gas accretion onto high-redshift galaxies and the main explanation of the cosmic star-formation history peak around redshift 2. Recent studies aim to theorize the survival conditions of cold streams surrounded by the hotter (~ 10^6 K) circum-galactic medium (CGM) gas. They provided evidence of the impact of Kelvin Helmholtz instabilities (KHI) on both stream dynamics and detectability in Lyα emission. KHI are not solved yet by the current resolution of cosmological simulation in the CGM, thus the need of idealized high-resolution simulation. Here we study the impact of magnetic field and anisotropic thermal conduction (TC) on radiatively cooling streams with a suite of magnetohydrodynamical two-dimensional simulations. We used the Athena++ code in which we implemented an anisotropic TC extension. Overall, the magnetic field and the TC reduce the stream mass growth and loss, respectively for strong and weak cooling regimes and can even prevent the disruption of the stream. The magnetic field angle and the TC reduce the cooling emission in most simulations up to 1 order of magnitude. While assuming an initially weak magnetic field with β = 10^5 (ratio of thermal pressure over magnetic pressure), we also find that the condensation of CGM gas onto the stream greatly enhances the magnetic field to the order of β ~ 1. Extrapolating our results to the cosmological context could imply that cold streams may fuel galaxies with cold turbulent magnetized gas, and they would less likely be disrupted by KHI.
F608 & Zoom (hybrid)
Extreme ultraviolet imaging spectroscopic observations often show an increase in line width around the loop-top or above-loop-top (ALT) region of solar flares, suggestive of turbulence. Although turbulence on ALT region could affect electron acceleration significantly, the origin of the turbulence is unclear. We performed three-dimensional magnetohydrodynamic (MHD) simulations to investigate the mechanism of generating turbulence on ALT region. We found a rapid growth of MHD instabilities around the upper parts of the ALT region (arms of the magnetic tuning fork). The instabilities grow more rapidly than the magnetic Rayleigh-Taylor-type instabilities at the density interface beneath the reconnecting current sheet. Eventually, the ALT region is filled with turbulent flows. The arms of the magnetic tuning fork have bad-curvature and transonic flows. Therefore, we consider that the rapidly growing instabilities are combinations of pressure-driven and centrifugally driven Rayleigh-Taylor-type instabilities. We will also discuss the possible effects of turbulence for electron acceleration and suggestion for observation.
F608 & Zoom (hybrid)
The evolution of the protoplanetary disk is dominated by the central star energetically. Many earlier studies consider that the central star regulates the disk evolution as light (e.g., Photoevaporation) and gravitational source. On the other hand, the star has supersonic plasma flow, "stellar wind". Recent studies (e.g., Cranmer et al. 2017) suggest that young stars are magnetically active and have mass-loss rates substantially larger than the present Sun. Its kinetic luminosity can be comparable to stellar ultraviolet luminosity. Therefore, the stellar wind is energetically important in the disk evolution and we newly discuss its effect. We consider the situation that the stellar wind flows from central stars toward the inner edge of the transition disk which is the evolved protoplanetary disk with an inner hole. We modeled the transition disk and stellar wind using 2D MHD simulation. We find that the stellar wind forms a bow shock at the inner edge and the post-shocked hot gas can emit ionizing photons (EUV: 13.6 - 100 eV). The photons can ionize and disperse the disk gas (photoevaporation). In this talk, we discuss the importance of stellar wind as a EUV source in disk evolution.
F608 & Zoom (hybrid)
Extremely metal-poor dwarf galaxies (EMPGs) with Z/Zsun < 0.1 in the local universe are of particular interest in terms of galaxy formation. Their low mass and shallow potentials are sensitive to feedback processes such as supernovae, and their low metallicity may have individually distinguishable star formation records. For these reasons, EMPGs have been actively studied both observationally and theoretically in recent years. In this talk, I will present the results of simulations of such extremely metal-poor galaxies and discuss their statistical properties. I will also talk about plans for my master’s thesis.
F608 & Zoom (hybrid)
Since primordial black holes are one of the candidates for dark matter, various constraints have been given to their abundance. In particular, for PBHs in the low mass range below 10^17 g, there are some constraints from Hawking radiation in which gamma-ray emission or electron-positron pair annihilation has been used. In my talk, we will discuss the possibility of constraint using MeV neutrinos based on the existing constraint from the extragalactic gamma-ray background. Here, I assumed the mass of PBHs is monochromatic and calculated the time-integrated photon/neutrino emission rate from high z to the present epoch. By setting the expected flux of the photon/neutrino not over the observational flux, we found that it is possible to constrain the abundance of PBH in the mass range around 10^15 g.
F608 & Zoom (hybrid)
The observations of the dark matter-deficient galaxies, NGC 1052-DF2 and DF4 (DF2, DF4 in short, respectively), triggered a controversy over their formation scenario. Previous studies showed that strong tidal effect can turn a normal galaxy into the galaxy which satisfies some of the properties of DF2 by N-body simulation. However, it didn't consider dynamical friction because it implemented its DM halo as an external potential. In my study, I conducted a self-consistent N-body simulation to reproduce a DF2-like galaxy and indicated that dynamical friction weakens the tidal effect. This result suggests that the tidal scenario needs more extreme situation. As a current work, I'm doing parameter search to create a DF2-like galaxy and measuring how often we can observe such objects.
F608 & Zoom (hybrid)
The global magnetic fields threading black holes and accretion disks play an important role in driving jets and disk winds. However, the mechanism that determines the global fields are not fully understood. One of the well-known key processes is magnetic field transport which is governed by both inward advection and outward diffusion in an accretion disk. Previous work shows that geometrically thin disks such as standard disks cannot transport the field inward efficiently because outward diffusion is dominant. This is a long-standing issue in flux transport. In this talk, I will present the results that we analytically and numerically investigate magnetic field transport in a geometrically thick disk which may be formed around a black hole: radiatively inefficient accretion flows (RIAFs) and super-Eddington accretion flows. I will also discuss a condition for the external field strength that RIAF can be a magnetically arrested disk. Then, I will talk about my master's thesis plan and future work.
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The importance of star formation “feedback” to the energetics of the interstellar medium (ISM) has been appreciated throughout the modern history of astronomy. Star formation is inefficient in gas consumption because feedback efficiently maintains the pressure support against gravity, which is otherwise rapidly lost via cooling and turbulence dissipation. At the same time, collective actions of feedback drive galactic-scale outflows, controlling the baryonic cycle in galaxy halos. In this talk, I will introduce the TIGRESS framework and its non-equilibrium cooling and radiation (NCR) extension. We solve magneto-hydrodynamics equations in a local shearing box representing a patch of galactic disks to take advantage of limited outer dimensions (~kpc) to achieve uniformly high resolution (~pc). The TIGRESS-NCR framework synthesizes our current best knowledge on governing physics of the star-forming ISM, including supernova and UV radiation feedback as well as photochemical reactions associated with UV (and cosmic rays) to set radiative heating rates and abundances for major coolants self-consistently. I will present the first results from a suite of simulations using the TIGRESS-NCR framework and explain the co-regulation of SFRs and the ISM. Specifically, I will delineate the self-regulation of SFRs in the context of pressure-regulated, feedback-modulated star formation theory and ISM phase structure and energetics with detailed breakdowns into energy source/sink from different processes and in different phases. Finally, I emphasize that having a numerical framework synthesizing the current best knowledge is a departure point to numerical experiments with other possibilities, including spiral arms and exploring a range of parameter space.
F608 & Zoom (hybrid)
Following the first discovery around 60 years ago, quasars have been studied extensively from both theories and observations sides. Quasars are used as a unique tracer of supermassive black holes and also as a beacon to illuminate/probe the distant universe. In addition, recent observations suggest that quasars may exercise a critical impact on galaxy evolution, necessitating a detailed understanding of the associated physics. This talk aims to bring a broad view of quasar observations, covering both observations of known quasars (in the relatively nearby universe) and hunt for new quasars (in the distant universe). The first part will touch on the measurements of Fe/Mg abundance ratios in quasars, expected to work as a "cosmic clock" to infer the epoch when the cosmic star formation has started. This is a slowly-evolving field, and I'll describe how the progress is being made to reach conclusions. The latter part will focus on our SHELLQs project, looking for low-luminosity quasars at z > 6 based on the Subaru Hyper Suprime-Cam SSP survey. By now we have discovered nearly 200 quasars, used to establish the quasar luminosity functions at z ~ 6 and z ~ 7 (the latter being a preliminary result). Follow-up observations with near-IR spectrographs, ALMA, Chandra, and other instruments are ongoing, revealing the nature of high-z quasars with unprecedentedly low luminosities. In particular we have a 50-hour mid-sized JWST program approved, and the very first observations will take place in a week from now; hopefully I could present the first set of data and results.
F608 & Zoom (hybrid)
With the advent of the Gaia data we have uncovered the fine details of the velocity distribution of the stars in the Galactic disc, and precisely characterised its substructures throughout a significant portion of the Milky Way disc(s). In doing so, we are now able to quantify the properties and morphology of each individual kinematic substructure, a first step towards a deeper dynamical modeling of the Galaxy, beyond the simple usage of the first and second order moments (mean velocity and its dispersion). I will present the latest advances in this topic, the new structures that have been detected, and discuss its implications on the global properties of the Galaxy and its history. In particular, with regards to the Sagittarius dwarf galaxy, a most interesting system that has been giving us trouble since its discovery 28 years ago.
F608 & Zoom (hybrid)
Dust is not only the building block of the planet, but its thermal radiation is a fundamental tool in understanding the structure of the disk. In addition, dust affects the ionization degree of the gas phase and the coupling between the gas and the magnetic field by adsorbing charged particles in the gas phase. Dust growth alters the degree of coupling between dusts and gases and changes dust dynamics, as well as the above-mentioned roles of dust, leading to a variety of novel phenomena. However, in many previous studies, dust growth and associated changes in dust dynamics have been studied using one-dimensional, simplified simulations. Therefore, many aspects of dust growth and dynamics in multidimensional gas dynamics such as outflows and disk non-axisymmetric structures observed around protostars, remain unexplored. In this talk, I will explain various novel phenomena discovered by our recent world-first 3D dust-gas two-fluid magnetohydrodynamic simulation considering dust growth.
F608 & Zoom (hybrid)
Gas clouds are observed as a component of the circum-galactic medium (CGM) and also detached from galaxies within the intra-cluster medium (ICM). Their observational detection differs according to their appearence. Infalling high-velocity clouds (HVCs) were at first directly observed in HI surveys of our Milky Way. Surprisingly, they contain "normal" gas phases like the interstellar medium (ISM), but are incapable to form stars., though numerical studies of HVCs with different masses passing through hot CGM with various relative velocities showed that very high, but realistic speed leads to a sufficient compression of the clouds' centers so that Jeans mass can be exceeded. An important property is that HVCs are self-gravitating so that they overcome disruptive instabilities and should reach the galactic gas disk. In contrast, in galaxy clusters, gas clouds are observed in H-alpha departing from gas-rich galaxies to one side only, so that ram-pressure stripping (RPS) caused by the motion of the galaxy through the ICM is the plausible mechanism. From the H-alpha emission first interpretations assumed newly formed stars as the source of ionization. However, we will report from a deep H-alpha survey of the Virgo cluster (VESTIGE) that star formation can be verified only by additional coincidences with GALEX locations. Since the star-formation rates (SFRs) in such clouds are too low to fill the stellar mass distribution according to the global initial-mass function (IMF), one has to explore how uncertain the SFRs derived from H-alpha are. For numerical simulations of the evolution of dwarf galaxies the simple assumption of a filled IMF at low SFRs has significant artefacts for the SF self-regulation, the gas dynamics, and the element enrichment.
Zoom Application (online)
Detailed observations of massive elliptical galaxies in the local Universe have shown that these galaxies formed in an intense burst of star formation in the early Universe followed by passive evolution. Despite the enormous amount of work on massive galaxies in the literature, the physical drivers of the intense starburst and suppression of subsequent star formation ('quenching') remain unidentified. This is one of the outstanding issues in the field of galaxy formation and evolution. Recent advent of near-IR facilities on large-aperture telescope has made it possible to identify progenitors of nearby elliptical galaxies in the early Universe. The first quiescent galaxies in the Universe may hold a key to understanding the physical processes mentioned above. We have used MOSFIRE on Keck to confirm massive quiescent galaxies at z~4 in deep fields such as COSMOS and SXDS and successfully confirmed the most distant quiescent galaxy known to date at z=4.01. Star formation histories of this and other galaxies at similar redshifts show that they indeed experienced an intense starburst in a recent past, suggesting that we are actually very close to the main formation epoch of these galaxies. We summarize our recent work on their physical sizes, AGN activities, dynamical properties, and environments, and conclude with future prospects with JWST.
F608 & Zoom (hybrid)
Almost every galaxy hosts a massive black hole (MBH) in the central region. The central MBHs become bright as Active Galactic Nuclei (AGN) by releasing the gravitational energy of accreting gas. Galaxy collisions have been so far considered to enhance the AGN activity by triggering mass fueling to the central MBH due to angular momentum transfer (Sanders et al. 1988 etc.). On the other hand, galaxy collisions may suppress AGN activity. If the mass reservoir of the central MBH is swept away by galaxy collisions, then the AGN activity must be turned off due to a lack of fueling sources. We have examined the possibility of the shutdown process of the AGN by using an analytic model and three-dimensional hydrodynamic simulations, which model a past head-on galaxy collision in the Andromeda galaxy. The analytic model shows that the torus-shaped gas surrounding the MBH can be totally stripped via momentum transfer between the torus gas and the gas of an infalling satellite galaxy. The three-dimensional hydrodynamic simulations derive the condition for switching off the AGN activity: the column density of the gas in the infalling galaxy is greater than that of the torus. The derived condition suggests that a substantial fraction of current AGN can become inactive; therefore, galaxy collisions would be responsible for both switching off and turning on the AGN activity, depending on the collision orbit (head-on or off-center).
F608 & Zoom (hybrid)
I present results from our numerical-relativity simulations on how to possibly discriminate neutron-star equations of state (EOSs) with a quark-hadron crossover (QHC) with respect to EOSs with purely hadronic matter or with a first-order quark-hadron transition through gravitational waves emitted in binary neutron star mergers. In contrast to other EOSs, QHC EOSs show a peak in sound speed, and thus a stiffening, in the transition region. This has possibly measurable effects on the frequency of the main peak of the post-merger gravitational-wave spectrum. We find that in some cases QHC EOSs may be comparatively easily distinguished from other EOSs, giving important clues on quark dynamics in the high-density end of neutron-star EOSs.
Zoom Application (online)
Understanding properties of galaxies in the epoch of reionization (EoR) is a frontier in the modern astronomy. With the advent of ALMA, it has become possible to detect far-infrared fine structure lines (e.g. [CII] 158 μm and [OIII] 88 μm) and dust continuum emission in star-forming galaxies in the EoR. In my talk, I will first show the results of our ALMA observations that have demonstrated that i) some [OIII] 88 μm emitters have matured stellar populations even at z > 6, and ii) high-z galaxies typically have very high [OIII]-to-[CII] luminosity ratio ranging from 3 to 12 or higher, which has implications for interstellar medium of high-z galaxies. Then, I will introduce our approved medium-sized JWST GO1 program that will target a sample of 13 z ~ 6-8 ALMA [OIII] emitters with NIRCam and NIRSPec IFU modes (PIs: J. Alvarez-Marquez & T. Hashimoto).
Zoom Application (online)
Black hole-neutron star mergers are one of the main targets of ground-based gravitational wave detectors, and two events were detected in 2020. No electromagnetic counterpart was detected in these events, but depending on the parameters of the binary, electromagnetic counterparts such as kilonova and gamma-ray bursts are expected to accompany the mergers. In order to obtain a self-consistent time evolution of the black hole-neutron star merger and the subsequent black hole accretion disk, we performed numerical simulations for up to two seconds, incorporating the effects of general relativity, neutrino radiation transport, and magnetohydrodynamics. As a result, we found the following facts. 1) The neutron star experiences tidal disruption and dynamical mass ejection takes place over ≲10 ms, together with the formation of a massive accretion disk 2) The magnetic field in the disk is amplified by magnetohydrodynamic instabilities, which generate turbulent viscosity and drives the post-merger mass ejection. 3) A magnetosphere is developed near the rotational axis of the black hole, and a high-intensity Poynting flux is generated, consistent with a short-hard gamma-ray burst.
F608 & Zoom Application (hybrid)
Dust extinction (absorption and scattering of stellar light) and emission are fundamental processes in characterizing the galaxy spectral energy distributions (SEDs) and in interpreting multi-band galaxy survey data. However, the evolution of dust properties (especially, grain size distribution and grain composition) affecting these processes has not been well understood. In this presentation, we first explain basic dust evolution processes that affect the grain size distribution. Next we introduce our recent modeling efforts toward the understanding of grain size distribution and grain composition. In these studies, we confirmed that the extinction curves and dust emission SEDs in nearby galaxies are fairly reproduced by our models. The models are combined with a cosmological simulation, and are successful in reproducing the Milky Way extinction curve and dust emission SED. Based on the above success, we discuss what our model predicts for high-redshift galaxies, especially based on our recent efforts of radiative transfer calculations that directly link our predictions to observed dust properties at high redshift.
Zoom Application (online)
Gravitational lensing is a strong tool to probe invisible objects in the Universe, like dark matter. There are mainly three classes of gravitational lensing depending on the strength of lensing effect: strong, micro, and weak lensing. In this talk, I will present two independent projects utilizing weak and micro lensing aimed at revealing the nature of dark matter. (1) Subaru Hyper Suprime-Cam (HSC) weak lensing cosmology: Weak lensing is the coherent distortion of the distant galaxy shape due to the forground gravitational potential of the large scale structure, and it is detectable when statistically combining millions of galaxy shapes. Especially, the cross correlation of the weak lensing and the foreground galaxy distirbution (galaxy-galaxy lensing) is useful to break the degeneracy of galaxy bias and cosmological parameters in galaxy autocorrelation (galaxy clustering). In this talk, I will talk about the validation of theory against the mock data and the blind analysis of HSC first year data, which is getting popular in the precision cosmology era. (2) Primordial Black Hole(PBH) search by microlensing observation with Subaru HSC: PBH is one of the viable candidates of dark matter, which may have formed in the early universe. PBH is a heavy compact object, and hence can be detectable by the microlensing observation if really exists. To test the PBH scenario of dark matter, we carried out microlensing observations in M31 stars with Subaru HSC. I will present several examples of PBH scenarios and the constraints on them. I will also present new numerical/statistical techniques to improve the constraint with existing microlensing dataset.
Zoom Application (online)
Astrometric satellite Gaia is supposed to detect non-interacting black hole-luminous companion (BH-LC) binaries by observing the sinusoidal motion of LCs. Since orbital period of the detctable BH-LC binaries with Gaia is longer than that of BH X-ray binaries in the Milky Way (MW) and binary BHs in extra-galactic distances, more and more people become interested in the satellite. More than ten papers theoretically investigated Gaia's detectability and what properties of BH binaries can be revealed by the observations. In this talk, I will summarize the previous work and show our results (Shikauchi+2020, 2022). We estimated that several to tens of BH-LC binaries can be detected with Gaia in the five-year observation. Then, I will also indicate some important implications on their formation process and uncertainties of binary evolution models. First, follow-up observation may reveal the formation process of BH binaries, whether they evolved with/without dynamical interaction. Second, we may be able to give a constraint on binary evolution models such as supernova model and common envelope evolution model. Though the recent data release (Gaia DR3) first includes the information of ``non-single stars'', e.g. binaries, no BH binaries were reported. This is probably due to a strict constraint on the selection of candidates. It is worth noting that the future data releases and follow-up observations may be able to reveal new BH candidates and their properties.
F608 & Zoom Application (hybrid)
Polarised light of the cosmic microwave background, the remnant light of the Big Bang, is sensitive to parity-violating physics, cosmic birefringence. In this presentation we report on a new measurement of cosmic birefringence from polarisation data of the European Space Agency (ESA)’s Planck satellite released in 2018. The statistical significance of the measured signal is 2.4 sigma. Recently, we found a signal with 3.3 sigma statistical significance when we use the latest Planck data and consider an effect of polarised foreground emission. If confirmed with higher statistical significance in future, it would have important implications for the elusive nature of dark matter and dark energy.
Zoom Application (online)
The accelerating expansion of the universe is one of the most mysterious phenomena. The cosmic acceleration implies the existence of dark energy or the breakdown of Einstein’s general relativity. Either way, revealing the source of cosmic acceleration can result in a paradigm shift in the field of modern physics. Weak gravitational lensing is a small, coherent distortion of distant galaxy images due to gravitational potential, which allows the direct measurement of dark matter spatial distribution. Weak lensing is one of the most powerful cosmological probes because of its capability to measure the nature of cosmic acceleration through the evolution of the large-scale structure of the universe. Hyper Suprime-Cam (HSC), a newly developed prime focus camera at Subaru Telescope, started a wide, deep galaxy imaging survey in 2014, covering 1,400 sq. degrees of the sky down to the i-band limiting magnitude of 26. The wide field of view, light-gathering power, and superb image quality of HSC make it possible to measure the weak lensing distortion with unprecedented precision. In this talk, I will present basics of weak lensing cosmology and cosmology results from the Subaru Hyper Suprime-Cam Survey first-year data, including the overview of the HSC instrument, and cosmological constraints from cosmic shear and the combination of galaxy-galaxy lensing and clustering.
Zoom Application (online)
Cold dark matter numerical simulations predict steep, `cuspy' density profiles for dark matter halos, while observations favour shallower `cores'. The introduction of baryonic physics in simulations alleviates this discrepancy, notably as feedback-driven outflow episodes contribute to expanding the dark matter distribution. I will present different theoretical models describing core formation in dark matter haloes. In the first one, small stochastic density fluctuations induced by stellar feedback in the interstellar medium dynamically heat up the halo, leading to the formation of a core. In the second one, sudden bulk outflows reorganise the halo mass distribution while it relaxes to a new equilibrium. In the third one, the combination of dynamical friction from incoming satellites with outflows speeds up core formation and enables the presence of cores early in the history of the universe.
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The period from Dark Ages to Epoch of Reionization (EoR) is milestone in the cosmic evolutionary history, which is crucial to understand the formation and evolution of the first population of stars, black holes, and galaxies. Cosmic 21cm line signal emitted from neutral hydrogen atom is key observable during the EoR. Recently the 21cm line cosmology has been developing both theoretically and observationally. The SKA telescope is scheduled to start observation in 2027. In my talk, I will briefly summarize the basics of the 21cm line physics , current theoretical and observational progresses, and SKA project. I also introduce our approach to analyze the 21cm line signal with artificial neural networks (ANNs), which is a machine learning technique.
Zoom Application (online)
Cosmic rays go hand-in-hand with violent and energetic astrophysical conditions, and can produce observable signatures across the electromagnetic spectrum. They also play a role as an active agent in shaping the evolution of their local environment, and their effects in modifying astrophysical processes over a broad range of length-scales can be substantial. In this talk, I will present an overview of some of my recent work on modeling the effects and signatures of cosmic rays in different environments - from star-forming clouds in our cosmic backyard, to circum-galactic structures and populations in the furthest reaches of the observable Universe. I will outline the impacts that cosmic rays can have on the evolution and dynamics their host system, how their astrophysical effects can manifest themselves on sub-galactic, galactic, and super-galactic scales, and how the microphysics of cosmic ray feedback may operate within interstellar and circum-galactic settings.
Zoom Application (online)
High-resolution cosmological hydrodynamic simulations are currently limited to relatively small volumes due to their computational expense. However, much larger volumes are required to probe rare, overdense environments, and measure clustering statistics of the large-scale structure. This is particularly important for comparing models to wide-field surveys from upcoming observational facilities, such as Euclid and Rubin. Zoom simulations can be used to probe rare environments, but periodic simulations are required to model the large scale clustering of galaxies. In this talk I will detail two projects that aim to overcome some of the limitations of both approaches, utilising large suites of zoom simulations as well as machine learning approaches.
Zoom Application (online)
For the origin of the first supermassive black holes in the Universe, different scenarios have been proposed, ranging from the direct collapse of massive gas clouds into one single object, run-away collisions in very dense stellar clusters, as well as accretion of initially stellar-mass black holes. In this talk, I will argue that realistic scenarios will have to combine these approaches to allow significant growth both via stellar collisions and gas accretion. The combination of the two processes provides additional means of dissipation, and thereby makes collisions more likely and more efficient, while alleviating the fragmentation problem in the context of the gas. I will present results both from numerical simulations that take both the hydrodynamics and the stellar dynamics into account, as well as complementary analytical work to show how the combination of both processes leads to the formation of supermassive black holes.
Zoom Application (online)
Star and planet formation is one of the most fundamental structure-formation processes in the universe. Physical processes of star and planet formation have widely been investigated as one of the major targets of astronomy and astrophysics by observations in all the wavelength region from radio to X-ray during the last few decades. Although a rough outline of these processes has been presented, there remain many unknowns and missing links. On the other hand, star and planet formation is a process where interstellar matter is evolved into planets. Hence exploring chemical evolution is of fundamental importance in understanding an origin of the solar system, and eventually an origin of life on the Earth. We are thus studying the chemical evolution by collaborating with people in molecular science field. However, such chemical approach also tells us novel information on physical processes of star and planet formation. In this seminar, I will introduce efforts on such studies.
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Accreting young stars such as protostars and pre-main-sequence stars show many fundamental processes of the formation of astrophysical objects. The systems are great laboratories to study how gas accretion and ejection are driven. In this talk, I will summarize and discuss the basic processes of mass accretion and ejection around protostars and pre-main-sequence stars. I will also discuss some applications to other systems such as proto-giant planets and supermassive black holes.
Zoom Application (online)
This talk will summarize the research topics at the frontier for the next decade in cosmological structure formation. In particular, the advent of JWST will soon bring us a new view of the high-redshift universe and reionization. Sources of ionizing photons will be discovered (first galaxies), and the escape fractions of first galaxies will be estimated at z>6, trying to distinguish the `early’ vs. `late’ reionization scenario. The Japanese proposal of GREX-PLUS will refine galaxy statistics at z>6. At slightly lower redshift, the co-evolution of SMBH and galaxies will be tested at the lower mass-end of the Mbh—sigma relationship by JWST/ELT/TMT. Concurrently to these observational campaigns, theoretical efforts using radiation hydrodynamic simulation with higher resolution cosmological simulations are also progressing.
Zoom Application (online)
Relativistic jets launched by supermassive black holes, so-called active galactic nuclei (AGNs), are known as the most energetic particle accelerators in the universe. Among AGN populations, blazars whose jets point to the Earth are one of the most identical objects to investigate the nature of these powerful particle accelerators, since we can see their gamma-ray emission due to the beaming effect. In this talk, I will review the latest results of gamma-ray studies of blazars. Then, I will briefly discuss some issues in current modelings of blazar gamma-ray emission.