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Abstract: One of the big unsolved questions in galaxy evolution is how the star-formation activity in galaxies stop and the transformation of morphologies, and how this process and governing mechanisms evolve in cosmic time.
This talk will focus on z=1 and later, the period that the cosmic star-formation rate density drops rapidly. I will introduce our recent large spectroscopic campaign, the LEGA-C survey, which obtains 3000 ultra-deep optical spectra of galaxies z~1 using 100 nights of the Very Large Telescope. The high-S/N spectra probe the ages and the assembly histories of galaxies through stellar continua. Combing the structural parameters from HST images, I will demonstrate that there is no one simple mechanism that can explain the complex correlation between the formation histories and structures of galaxies. The majority of galaxies likely went through a gentle process that shuts down star-formation slowly without altering the structure, while some galaxies are experiencing rapid change in both star-formation rate and structural at the same time.
I will further show follow-up studies on post-starburst galaxies, those undergo the rapid decline of star-formation rates, to investigate the driving mechanisms. Various pieces of evidence suggest that post-starburst galaxies had a centrally-concentrated starburst event before the star-formation activity halts.
Abstract: Solar flares are explosive phenomena on the solar surface. They often produce high XUV radiations, high energy particles, and mass ejections which have potential risks to damage human technologies. On other stars, such as M dwarfs and young stars, larger ‘superflares’ (more than ten times larger than the most energetic solar flares) are known to frequently occur, severely affecting the exoplanet habitability. Recently, there is an increasing interest in the question “How do the stellar magnetic activities on central stars affect the planetary environment?”, and therefore the properties of stellar superflares become more and more important. Optical spectroscopic observations have a potential to reveal the properties of superflares, but the detections of ssuperflares have been very rare due to its low occurrence frequency. We have conducted optical spectroscopic observations of stellar superflares on M-/G-dwarfs with the 3.8-m Seimei telescope since 2019. We have succeeded in detecting optical spectra of the rare stellar superflares with very high time-cadence and high quality. We found a very dynamic change in optical spectra during a superflare as evidence of strong injections of high-energy particles to the stellar atmosphere (Namekata et al. 2020b). Moreover, we detected the first, conclusive evidence of stellar mass ejection, which revealed the mechanism of mass ejections and would enable us to estimate its direct impact on exoplanets (Namekata et al. submitted). In this talk, I will talk about the recent results by the Seimei telescope.
