投稿者: KatoRyo

Machine Learning for Strong Lensing: Preparing for Discovery in Next-generation Astronomical Surveys

Speaker: Margherita Grespan (NCBJ, Poland -> Oxford University)

Abstract:

The Rubin Observatory and Euclid mission observe billions of galaxies, with approximately 100,000 expected to be strong gravitational lenses (SGLs). Detecting these rare systems is important for understanding dark matter and galaxy evolution, but requires machine learning to efficiently process vast datasets.
In this seminar, I will present my work on applying transformer encoders to 221 deg² of the Kilo Degree Survey (KiDS) to search for new SGL candidates. Transformers, initially trained on simulated data, are fine-tuned with real data, reducing false positives by 70% and identifying 71 high-confidence SGLs, though precision remains below 1%.
Can we bypass training on simulations altogether? To explore this, I will discuss an active learning approach using the Astronomaly pipeline, which achieves nearly 5% precision with only a few thousand labeled objects.
For next-generation surveys, combining these two methods has the potential to significantly enhance discovery.

Study of the relationship between circumstellar disc evolution and Be donor activities in BeXBs

Speaker: Masafumi Niwano (NAOJ ADC/DoS)

Abstract:

The mechanism of X-ray outbursts in Be X-ray binaries remains a mystery, and understanding their circumstellar discs is crucial for a solution of the mass-transfer problem. In particular, it is important to identify the Be star activities (e.g. pulsations) that cause mass ejection and, hence, disc formation. Therefore, we investigated the relationship between optical flux oscillations and the infrared (IR) excess in a sample of five Be X-ray binaries. Applying the Lomb-Scargle technique to high-cadence optical light curves from the Transiting Exoplanet Survey Satellite (TESS), we detected several significant oscillation modes in the 3-24 h period range for each source. We also measured the IR excess (a proxy for disc growth) of those five sources, using J-band light curves from Palomar Gattini-IR. In four of the five sources, we found anticorrelations between the IR excess and the amplitude of the main flux oscillation modes. This result is inconsistent with the conventional idea that non-radial pulsations drive mass ejections. We propose an alternative scenario where internal temperature variations in the Be star cause transitions between pulsation-active and mass-ejection-active states.

Is Stephen Hawking’s evaporation of black holes wrong? – Black Hole as a new candidate for dark matter –

Speaker: Kazunori Kohri (DoS, NAOJ)

Abstract:

Last year, Prof. Gia Dvali, an expert in quantum gravity theory, criticized Stephen Hawking’s proposal for calculating the evaporation of black holes due to Hawking radiation, insisting that the initial conditions assumed were not appropriate, and if he added the back reaction (the Memory Burden Effect) from the entropy of Hawking radiation emitted by the black hole itself to the calculation, the lifetime of the black hole should be extended even further. Researchers around the world were surprised. My group (Thoss, Burkert and Kohri, MNRAS, 2024, arXiv:2402.17823) was quick to incorporate this correction and pointed out that primordial black holes that have existed since the early Universe may have remained and become dark matter without evaporating. I will explain this topic, which is still the subject of heated debate, in an easy-to-understand way for non-experts in this seminar. https://arxiv.org/abs/2402.17823

Diversity in planetary atmospheres shaped during the magma ocean stage

Speaker: Keiko Hamano (DoS, NAOJ)

Abstract:

Earth, Venus, and Mars are classified as the same planet category, “terrestrial planets”, in the solar system, but their atmospheres and surface environments are currently very different. Geochemical evidence and theoretical studies suggest that Earth, and likely the other terrestrial planets, began in a molten state—a global “magma ocean”—due to the energy released during planetary accretion. As these molten planets formed and solidified, fundamental planetary structures—such as the core, mantle, and atmosphere—were established, providing the initial conditions for subsequent planetary evolution.
In this presentation, the focus will be on the atmosphere that forms above a magma ocean, which is one of the key factors in understanding the diversity of planetary atmospheres. The research explores how basic planetary parameters, such as distance from the Sun (star) and planetary mass, can influence the early atmosphere throughout the magma ocean stage. Additionally, I will discuss the future directions of my research at NAOJ and welcome feedback and comments from the audience to help advance the research efforts.

Magnetic fields in star formation

Speaker: Junhao Liu (DoS, NAOJ)

Abstract:

Star formation is a key research field in modern astrophysics. Magnetic fields are crucial in influencing the key physical processes of star formation in molecular clouds. Here I review research progresses on the important questions regarding the role of magnetic fields in star formation, such as their impact on the star formation rate, cloud formation, cloud collapse, gas fragmentation, angular momentum transfer… Additionally, I will present our ongoing study on the formation mechanism of ~1000 au condensations in clustered massive star formation regions, leveraging a synergy between ALMA dust polarization survey and numerical simulations.

General relativistic radiation MHD simulations of super-Eddington accretion flows around a magnetized neutron star

Speaker: Akihiro Inoue (Osaka University)

Abstract:

Since neutron stars have strong magnetic and gravitational fields, they serve as natural laboratories for extreme plasma physics. When the neutron star is in a binary system, periodic X-ray pulsations are produced by the plasma accretion onto the neutron star. Such objects are called X-ray pulsars and thought to be ideal objects for investigating the extreme plasma physics. However, the magnetic field strength of the neutron star and the accretion rate remain uncertain since they cannot be observed directly.
We perform two-dimensional general relativistic radiation MHD simulations of super-Eddington accretion flows onto a magnetized neutron star, as modeling the X-ray pulsars at the super-Eddington rate. In our simulations, accretion disk, accretion flows along the neutron star’s magnetic field lines, and optically thick outflows driven by the strong radiation force appear. Such outflows can explain the thermal emission with a temperature of 1E+7 K and a blackbody radius of ~100 km detected in the super-Eddington X-ray pulsar, Swift J0243.6+6124. By comparing our models with observations, the magnetic field strength at the neutron star surface in Swift J0243.6+6124 is restricted to (0.3-4)E+12G. In this talk, we also present recent three-dimensional simulations, in which a non-axisymmetric accretion flows form.

Star Formation in Sub-Solar Metallicity Environments: Insights from the outer Galaxy and the Magellanic Clouds

Speaker: Yu Cheng (NAOJ)

Abstract:

The formation of stars in low-metallicity environments is a central problem in galaxy formation and evolution, particularly for understanding the high-redshift universe. However, studies of detailed star formation processes in such environments have been limited due to large distances and the challenge of characterizing low-mass sources. Here, I review recent progress and present observational efforts targeting the outer Galaxy and Magellanic Clouds—the nearest regions with metallicities as low as 0.2–0.5 Z☉. We conducted a multi-wavelength campaign targeting the outer Galaxy source Sh2-284, one of the lowest metallicity star-forming regions in the Milky Way. The synergy of multi-wavelength facilities, including ALMA and JWST, enables simultaneous characterization of dense gas and newly formed stellar populations. I will also present a sample study of massive young stellar objects (YSOs) in the Magellanic Clouds, followed by ALMA long-baseline observations that achieve unprecedented resolution at millimeter wavelengths (~1500 au) to probe fragmentation and kinematics. These results advance our understanding of star formation physics in conditions analogous to the early universe.

UV luminosity function of high-z galaxies using zZ-dependent IMF

Speaker: Keita Fukushima (Seoul National University)

Abstract:

To understand the overall picture of galaxy formation and evolution, it is essential to study the statistical properties of galaxies. The UV luminosity function of galaxies at redshift z>10, observed by the James Webb Space Telescope (JWST), shows that the bright end is more than an order of magnitude higher in number density than expected from the Schechter function. One theoretical model that explains this uses a top-heavy initial mass function (IMF) with an overabundance of massive stars.

In this study, we model an IMF dependent on redshift z and metallicity Z (Chon et al. 2022) and use the spectral synthesis code FSPS (Conroy, Gunn, & White 2009) to create spectral templates for star clusters. The zZ dependent IMF becomes top-heavy compared to the Kroupa IMF at low metallicities and high redshifts. In such cases, the UV luminosity is higher for young star clusters than with the Kroupa IMF but becomes lower at 10 Gyr due to a lower fraction of low-mass stars.

A galaxy formation model was developed by applying a semi-analytic model with metal enrichment to the merger trees obtained from the Shin Uchuu Simulation (Ishiyama et al. 2021). We then applied the spectral templates to this model to derive the UV luminosity function and compared it with observations, discussing the impact of the top-heavy IMF.

Wave dark matter

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.

Recent JWST insights into Little Red Dots and unresolved mysteries

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.