Five Green Pea (GP) galaxies with double-peaked thin lines were discovered by astronomers from Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences and their collaborators out of roughly 1,550 GP galaxies.
These objects might result from the merging of two active galactic nuclei (AGN), which could disclose the co-evolutionary traits of a particular class of high-mass galaxies and supermassive black holes.
The study was published in Monthly Notices of the Royal Astronomical Society on July 19, 2023.
Green Pea Galaxies are a class of compact, star-forming galaxies that were first discovered by citizen scientists as part of the Galaxy Zoo project. These galaxies are characterized by their small size, intense star formation activity, and strong emission lines in their spectra, particularly in the green wavelength range, which gives them their name.
In addition to having a distinctive optical appearance that is green and compact, GP galaxies exhibit strong emission lines, most notably the doubly ionized oxygen [OIII] emission line.
Typically, they are low-redshift galaxies with low metallicities, modest masses, and active star formation. They are considered analogs of early galaxies in the nearby universe. Some show signs of AGN activity, indicating the actively accreting supermassive black holes in their nuclei.
We could learn more about the origin and evolution of early galaxies by conducting systematic searches and investigations of GP galaxies. These galaxies’ AGN samples shed light on how early supermassive black holes and the host galaxies they were housed in co-evolved.
The researchers used a GP galaxy sample mainly from Extra-galactic Survey of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). It is currently the largest collection of GP galaxy spectra, encompassing nearly 1,550 GP galaxy spectra, which is more than double the number of GP galaxies identified in the Sloan Digital Sky Survey (SDSS).
These five GP galaxies exhibit stronger [OIII] equivalent widths compared to double-peaked emission line galaxies in Type 2 AGN. The underlying cause of this phenomenon may be related to galaxy mergers in the early universe. The upcoming new phase of the LAMOST Extra-galactic Survey holds promise for providing us with more samples of such unique galaxies, further unraveling the co-evolution of massive galaxies and supermassive black holes.
Professor Zheng Zhenya
The researchers found five GP galaxies with double-peaked thin lines by analyzing the emission line profiles of the LAMOST and SDSS spectra. They successfully verified the AGN activity in this sample using multi-band photometry and optical spectrum diagnostics.
The physical cause of the double-peaked profiles in these galaxies is most likely attributable to twin AGN mergers rather than outflows or accretion disks based on emission line profiles and optical morphologies.
According to Lin Ruqiu, Ph.D. student at SHAO and first author of the study, “These five galaxies have a narrow line width for each component and lack obvious inclination. Therefore, the double-peaked emission lines are less likely originated from outflows or gas disks.”
“These five GP galaxies exhibit stronger [OIII] equivalent widths compared to double-peaked emission line galaxies in Type 2 AGN. The underlying cause of this phenomenon may be related to galaxy mergers in the early universe,” said Prof. Zheng Zhenya from SHAO, corresponding author of the study.
“The upcoming new phase of the LAMOST Extra-galactic Survey holds promise for providing us with more samples of such unique galaxies, further unraveling the co-evolution of massive galaxies and supermassive black holes.”
