Galaxy number densities around fast radio bursts

John Lo¹, Tetsuya Hashimoto^2*, Tomotsugu Goto³, Yuri Uno², Tzu-Yin Hsu³

¹Senior high school, Kent College Canterbury, Canterbury, UK
²Department of Physics, National Chung Hsing University, Taichung, Taiwan
³Institute of Astronomy, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Tetsuya Hashimoto, email:tetsuya@phys.nchu.edu.tw

Fast radio bursts (FRBs) are mysterious radio flashes with millisecond timescales. Revealing their origins is one of the most important missions in astronomy. The most reliable method has been ‘localization’ which measures the accurate FRB positions in the sky. The localization provides strong evidence that the FRB originated from that source. However, the poor positional accuracy of FRB telescopes such as CHIME has hampered the source identification, except for one Galactic FRB case out of more than 800 FRBs. To overcome this problem, we, for the first time, investigate the number densities of galaxies around FRBs. The number density is correlated with galaxy types, e.g., young small and old massive galaxies at lower and higher number densities, respectively. Therefore, the number densities are expected to constrain FRB progenitors because young (old) FRB progenitors would likely be hosted by young (old) galaxies. Once proven, this could be a strong tool for understanding the origin of FRBs because we can constrain the galaxy type of the majority of FRBs without accurate localization. As the first step, we utilize seven localized FRBs with spectroscopic redshifts to ensure this method gives us a significant result. We found a clear trend that FRBs in higher (lower) galaxy number densities are hosted by old massive (young small) galaxies except for FRB20190714A. Therefore, our result demonstrates the feasibility of our number-density method to constrain the galaxy type of FRB host galaxies. Our method could be applied to most FRB samples, which do not have accurate localization.

Keywords: fast radio bursts, extragalaxies, galaxy environments