Astronomers investigate the origin of optical variability in emission line galaxies

(ORDO NEWS) — A new Chinese study suggests that the optical variability in emission line galaxies (ELGs) is likely caused by star formation activity rather than the activity of supermassive black holes.

The study, conducted by astronomers from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences (CAS), in collaboration with scientists from the University of Science and Technology of China, the Yunnan Observatories and the China Polar Research Institute, was published in the Astrophysical Journal Nov. 17.

The scientists used ELG narrowband (NB) images to study the optical variability of galaxies. The images were taken at intervals of 12~15 years.

ELGs are relatively low-mass star-forming galaxies with less dust. They are widely used to study the history of star formation in the universe.

At the same time, there were no systematic studies of the optical variability of such galaxies. As a result, it was not clear to scientists what percentage of ELG exhibited optical variability, and what caused it.

To answer these questions, the scientists examined the optical variability of a sample from 181 ELGs.

They used NB imaging data from the Subaru ground-based 8-meter telescope to test for photometric changes in the sample’s NB.

After eliminating the effect of the shape of the various NB filters, the scientists found that less than 3% of the sample showed significant optical variability.

The researchers then evaluated the optically variable ELG X-ray brightness, mid-infrared activity, radioactivity, morphology, and other data to determine the cause of the variability.

Three sources of such variability are known: an active galactic nucleus (AGN), a supernova explosion during active star formation, or a tidal disruption event.

According to Lin Rukyu, the study’s first author, AGNs were not the main factor influencing optical variability in these emission line galaxies.

Lin noted that high-resolution images from the Hubble Space Telescope showed the morphology of the ELG merger, supporting the idea that supernovae are behind the ELG’s optical variability.

“Star formation is enhanced by galaxy mergers, and thus we could see more star formation activity such as supernova explosions,” Lin said.

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