New upper limits of extra-galactic civilizations placed by the past radio SETI obsesrvations

Search for Extra-Terrestrial Intelligence (SETI) has been conducted for more than 60 years, and no technosignatures have yet been detected. Very few searches have been attempted in the extra-galactic Universe, where a much larger volume is available for extragalactic civilizations. According to the Kardashev scale, a civilization that can handle huge energy harvested from a star or a galaxy is called KII or KIII civilization. Hypothetical technosignatures from such advanced civilizations would also be extremely bright, which can be detected by the current radio telescopes even for distant galaxies. However, only a few previous studies have focused on extragalactic civilizations. To explore a frontier of extragalactic SETI, we study the fraction of KII or KIII civilizations possessing radio transmitters on an extragalactic scale. We utilize the data from the past radio SETI observations and a galaxy mass function to statistically constrain the stellar transmitter rate of KII or KIII civilizations in extra galaxies. We set a new upper limit on the galactic transmitter rate, which scales inversely with the number of galaxies in a surveyed volume. The detailed results and discussions will be presented in my poster.