談話会
Colloquium

Chengchao Yuan (Penn State)
Galaxy and SMBH mergers in the era of multi-messenger astrophysics
Recently, the coincident detection of gravitational waves (GWs) and electromagnetic (EM) counterpart from binary neutron star mergers heralds a new era of multi-messenger astronomy. Beginning in 2012-2013 by the discovery of the cosmic high-energy neutrino background, neutrino astrophysics also plays an important and indispensable part in the future multi-messenger analyses. In this talk, I’ll focus on the neutrino and EM emissions from two important candidates, galaxy mergers and super-massive black hole mergers, for the next generation neutrino and GW detectors, e.g. IceCube-Gen2 and LISA. I’ll show that our results together with future detections can be used to elucidate the evolution and the physical mechanism of these sources in greater details.

Yutaka Hirai (RIKEN)
Enrichment of Heavy Elements in Dwarf Galaxies and the Milky Way
Abundances of heavy elements in metal-poor stars help us understand their astrophysical sites and evolutionary histories of galaxies. High-dispersion spectroscopic observations have identified that abundances of neutron-capture elements such as Sr, Ba, and Eu show star-to-star scatters in extremely metal-poor stars in the Milky Way while there is an increasing trend toward lower metallicity in the abundance of Zn. However, their astrophysical sites and the enrichment in galaxies are not well understood. Here I will show the enrichment of heavy elements of dwarf galaxies and the Milky Way using N-body/smoothed particle hydrodynamics simulations. The results suggest that binary neutron star mergers can contribute to the enrichment of Sr, Ba, and Eu for [Fe/H] 􏰀 < −2. Likewise, this study finds that ejecta from supernovae by low-mass progenitors can form stars with high [Zn/Fe] and [Sr/Ba] ratios. In this talk, I will also discuss relationships among abundances of elements, kinematics of stars, and the Milky Way formation.

Kohei Hayashi (Tohoku U.)
The Galactic dwarf galaxy as a constraint on the nature of dark matter
Dark matter is now known to be the vital ingredient for the growth of structure in the Universe, while its nature remains a mystery. The dwarf galaxies are excellent laboratories to study the nature of dark matter as they are largely dark-matter dominated systems. In particular, revealing their dark matter distributions is of importance in testing dark matter models. This is because different dark matter models predict different dark matter density profiles. In this work, we estimate the dark matter distributions in the eight luminous dwarf satellites through our dynamical analysis of their stellar-kinematic data. Then we find the diversity of the inner slopes in their dark matter halos. Interestingly, this diversity can be explained if we consider the impact of baryonic feedback on the central dark matter densities, which depends largely on the ratio of stellar-to-halo mass as predicted by recent Lambda cold dark matter and hydrodynamical simulations. Thus, to set constraint on the nature of dark matter, it is necessary to look at much fainter dwarfs, namely ultra-faint dwarfs, which are believed to have held original dark matter density profiles.
In this talk, I will present our effort to characterize the dark matter distributions of dwarf galaxies with current spectrographs. Then, I will discuss the feasibility and future prospects for this dark matter study with Subaru-PFS survey.