organized by Division of Science, National Astronomical Observatory of Japan
Speaker: Elisa Ferreira (Kavli IPMU)
Abstract:
In this colloquium, I will consider the prospect that dark matter is composed of ultra-light particles, exhibiting wave-like characteristics in galactic environments. We will delve into the underlying particle physics motivations and the intriguing wave phenomena associated with these candidates. Additionally, we will discuss the potential implications for astronomical observations and the strategies for experimental detection.
Speaker: Kohei Ichikawa (Waseda University)
Abstract:
The James Webb Space Telescope (JWST) has unveiled numerous massive black holes (BHs) in faint, broad-line active galactic nuclei (AGN), even though their survey areas are only ~10^2 arcmin^2, which were previously considered too small for significant number of AGN discoveries before the JWST launch. This discovery highlights the presence of a dust-reddened AGN population, referred to as little red dots (LRDs), which are more abundant than X-ray selected AGNs that are less influenced by obscuration. In this talk, we summarize the recent observational properties of LRDs and discuss what we can learn from them about the growth of supermassive black holes (SMBHs) in the early Universe at z>=5. The large number density of LRDs indicates that the cosmic growth rate of BHs within this population does not decrease but rather increases at higher redshifts beyond z~6. The BH accretion rate density deduced from their luminosity function is significantly higher than that from other AGN surveys in X-ray and infrared bands. To align the cumulative mass density accreted by BHs with the observed BH mass density at z~4-5, as derived from the integration of the BH mass function, the radiative efficiency must be doubled from the canonical 10% value, achieving significance beyond the >2sigma confidence level.
This suggests the presence of rapid spins, with 96% of the maximum limit among these BHs, maintained by prolonged mass accretion instead of chaotic accretion with randomly oriented inflows. Accordingly, we propose a hypothesis that the dense, dust-rch environments within LRDs facilitate the emergence of rapidly spinning and overmassive BH populations during the epoch of reionization. This scenario predicts a potential association between relativistic jets and other high-energy phenomena with overmassive BHs in the early universe. If time allows, we will explore the future prospects of wide-area surveys such as UNIONS and Euclid as well as VLA 3 GHz radio surveys, which could further uncover high-luminosity and/or low-z analogs of LRDs.
Speaker: Wataru Iwakiri (ICEHAP, Chiba University)
Abstract:
Astrophysical high-energy neutrinos (HENs) are valuable tools for tracing the origins of high-energy cosmic rays. In the 2010s, the IceCube Neutrino Observatory, which has 5160 PMT detectors buried in the 1450-2450 m depth of the South Pole, enabled us to detect HENs above 100 TeV, allowing us to observe this new messenger. So far, the IceCube team has identified three objects as HEN sources: the TeV blazar TXS 0506+056, the Seyfert II galaxy NGC 1068, and a galactic plane. However, the primary source of the diffuse flux of astrophysical HENs observed by IceCube remains unclear. Correlation analyses of several years of accumulated data indicate that observed gamma-ray blazars contribute to less than 20% of the diffuse flux at most. Furthermore, observed HENs show no significant correlation with gamma-ray bursts and account for less than 0.4% of the diffuse flux. In this talk, I will discuss potential candidates for astrophysical HEN sources and how to reveal them using multi-messenger approaches.
Speaker: Kento Masuda (Osaka University)
Abstract:
The orbits and physical properties of exoplanets may continue to evolve long after their formation. For instance, it has long been discussed that the shortest-period planets may have orbits unstable to tidal dissipation, ultimately causing them to spiral into their host stars. Additionally, recent transit observations of low-mass planets have uncovered evidence of atmospheric escape, which may shed light on their internal composition. Understanding these evolutionary processes is vital for interpreting the diversity of exoplanetary systems and situating our solar system within a broader context. In this talk, I will present our ongoing efforts to statistically probe these changes by analyzing the dependence of exoplanet occurrence rates on stellar age. Specifically, I will discuss our recent work on deriving the age-dependent occurrence rates of giant planets orbiting Sun-like stars using isochronal ages, and will also mention insights into the rotation-based age estimation method gained from high-resolution spectroscopy of Sun-like stars in twin binaries.
Speaker: Vinicius Placco (NSF NOIRLab)
Abstract:
The lowest metallicity stars in the Milky Way Halo are the fossil records of the earliest star-forming environments in the Universe. Chemo-dynamical studies of such rare objects can address a myriad of open questions, ranging from primordial nucleosynthesis and the mass function of the first stars to the nature of the astrophysical r-process and the early merger history of the Milky Way. The detailed abundance patterns of these stellar relics, which can only be obtained from high-resolution spectroscopy, help us understand the pathways that led to the chemical complexity we observe today. In this talk, I will present the status of near-field cosmology in the era of large-scale surveys. I will also describe recent results on the spectroscopic validation of low-metallicity stars selected from narrow-band photometry and the discovery of chemically peculiar stars in the Milky Way, which present chemical abundance patterns that match the ones from the ejecta of a neutron-star merger event and zero-metallicity supernovae. Combined, these efforts are adding key pieces of information to help stellar archaeologists constrain the chemical evolution of the Universe and solve the intricate chemo-dynamical puzzle of the formation of the Milky Way.
Speaker: Shun Hatano (NAOJ/SOKENDAI)
Abstract:
The origin of infrared emission in ULIRGs—starburst or AGN—remains uncertain. Compactness is key for differentiation, but infrared imaging is limited to resolutions of ~10–100 pc. In this study, we utilize NEOWISE time-domain data to examine near-infrared variability of ULRIGs. We identify multiple ULIRGs showing significant near-infrared variability, despite not being classified as AGN on the BPT diagram.
Speaker: Tomoya Kinugawa (Shinshu University)
Abstract:
We calculated binary evolutions of first stars (Pop III) and showed that the typical chirp mass of Pop III binary black holes is ∼30Msun with a total mass of ∼60Msun and the maximum mass of Pop III BBH is more massive than the limit mass of the pair-instability supernovae.
Our result, which predicted gravitational wave events like GW150914 and GW190521, can explain the origin of massive stellar binary black hole mergers.
In this talk, I will explain Pop III binary evolutions and the properties of Pop III binary black hole mergers.
There is a good chance to check indirectly the existence of Pop III massive stars by gravitational waves.
Speaker: Tommaso Grassi (Max Planck for Extraterrestrial Physics)
Abstract:
Astrophysical numerical models encounter substantial computational challenges when integrating complex, time-dependent chemistry with physical processes. To address these issues, I will present the use of autoencoders for the dimensionality reduction of chemical networks, enabling efficient solutions with standard ODE solvers while preserving key network features. Additionally, I will discuss the application of interpretable machine learning techniques to connect synthetic spectra with model parameters, facilitating the assessment of information retention in observational data.
Speaker: Jessica Santiago (LeCosPa)
Abstract:
Given the exponential growth on the upcoming supernovae data available, the possibilities of rigorously testing the cosmological principle becomes ever more real. One of the ways to do so is by measuring the multipole decomposition of the Hubble and deceleration parameters.
In this presentation, I will discuss the observational-theoretical approach, initially introduced by Kristian & Sachs, which allows for the interpretation of data in non-homogeneous and anisotropic universes. I will also explore the effects introduced by the relative motion between the observer and the matter frame, and show how the induced kinematic dipole can be disentangled from intrinsic anisotropies in the matter distributions. To conclude, I will show that the luminosity distance must be corrected depending on the chosen frame of the observer (z_cmb vs. z_hel).