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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.
Abstract: 赤外線天文衛星「あかり」は 2006 年 2 月に打ち上げられ、2011 年 11 月まで 5 年以上にわたって運用されてきた。このうち液体ヘリウムによる冷却期間 (約 1 年半) では、中間 - 遠赤外線 6 波長帯での全天サーベイや 5000 回以上の指 向観測を行った。その後は機械式冷凍機のみの冷却で近赤外線の指向観測を 12000 回以上行った。2010 年 3 月には全天サーベイデータによる点源カタログ の公開を行った。 一方で指向観測データは、データ処理に際して装置に対する知識が必要であった りと、ユーザ側にスキルを要求することから、一般のユーザの利用が進んでいな かった。そこで「あかり」の取得したデータを有効に利用してもらうことを目的 として、処理済みデータの作成、および公開を行った。本講演では「あかり」衛 星、およびアーカイブ活動について紹介する。
AKARI is the first Japanese satellite mission dedicated for the infrared astronomy. AKARI was launched in February 2006 and carried out an all-sky survey and more than five thousands of pointed observations in its liquid helium cooling phase until August 2007. In the following period from June 2008 to February 2010 AKARI made about 13 thousands of pointed observations in the near-IR wavelengths using the mechanical cryocooler. AKARI was turned off in November 24, 2011. The AKARI All-Sky Survey Far- and Mid-Infrared Point Source Catalogues were released in public in March 2010. The data nominate 1.3 million sources in total, and have been used in various fields in astronomy as one of standard database. Point Source Catalogues from the Large Magellanic Cloud survey and the North Ecliptic Pole survey, and Asteroids catalogue have also been provided. In 2013, the AKARI project was reconfigured to continue data reduction and archiving. The team revised the point source catalogues, created All-Sky image maps, and produced science ready processed data of pointed observations. AKARI data have been used in various fields in astronomy from Solar system objects to galaxy formation. In the presentation I will overview the AKARI mission, introduce its science highlights.
Language: Japanese
