Galaxy interactions are believed to have a great impact on their formation and evolution. Due to the lack of high resolution spatial and spectral information, it still remains an open question whether such impacts vary along the merger sequence. 

Recently, a new study led by graduate student JIN Gaoxiang and Prof. Y. Sophia Dai from the National Astronomical Observatories of Chinese Academy of Sciences (NAOC) has revealed the role of active galactic nuclei (AGNs) during galaxy interactions with the latest integrate field unit (IFU) observations. 

The study was published in The Astrophysical Journal.

Previous single-fiber spectroscopic surveys can only cover the center of nearby galaxies. In the past few years, the state-of-art IFU survey, mapping nearby galaxies at ApachePoint Observatory (MaNGA), offers the spectra in different regions of a galaxy, allowing us to study the spatial distributions of various galaxy properties. 

Galaxy pairs are like 'neighbors' in the universe, and close pairs may affect the evolution of each other, such as their star-formation and the growth of the central supermassive black holes. To accurately identify true neighbors, researchers need to estimate the separations of galaxies in two aspects: the projected distance from their sky positions, and the line-of-sight distance based on their redshift. 

After selecting and visually confirming the galaxy pairs, the researchers classified the galaxies into AGNs, star-forming galaxies and quiescent galaxies. "To our surprise, we find similar AGN fractions among different merger cases and in isolated galaxies," said JIN Gaoxiang, the first author of the study, "their global star-formation rates and central luminosities are also similar." 

Researchers then studied the galaxies' star-formation rate along their radius using the IFU data. "We see clear star-formation increase for star-forming galaxies at all radii within the galaxies, with the strongest enhancement at the galaxy center," said JIN, "however, different from star-forming galaxies, interactions do not clearly affect the star-formation of AGNs and quiescent galaxies, no matter at center or outskirt." 

"Generally speaking, the star-formation condition of AGNs is between the star-forming galaxies and quiescent galaxies, indicating that they are in the middle of quenching but are not fully quenched," said Prof. Dai, the corresponding author of the paper, "the closer to the center, the more quenched these AGN hosts are, suggesting that they follow the 'inside-out' quenching scenario." 

The various stages of galaxies: isolated galaxies (a), gradually attracted to each other (b), and then interact with each other (c) & (d), before they eventually merge into one galaxy (e) & (f). (Image by JIN Gaoxiang)