Astronomers Discovered Five Green Pea Galaxies with Double-peaked Emission Lines

A team of astronomers led by the Shanghai Astronomical Observatory (SHAO), Chinese Academy of Sciences, in collaboration with researchers from the Chinese Academy of Sciences University, University of Science and Technology of China, NASA Goddard Space Flight Center, and the University of Manitoba, found five Green Pea galaxies with double-peaked narrow lines among nearly 1550 Green Pea galaxies. The study suggests that these objects may originate from dual AGN mergers, potentially revealing the co-evolutionary characteristics of a special class of high-mass galaxies and supermassive black holes. This discovery has been published in Monthly Notices of the Royal Astronomical Society on July 19, 2023.
Green Pea galaxies are named for their green and compact optical appearance and are characterized by strong emission lines, particularly [OIII]. They are typically low-redshift galaxies with small mass, low metallicity, 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.
Systematic searches and studies of Green Pea galaxies help us understand the formation and evolution of early galaxies. Researching AGN samples within these galaxies provides insights into the co-evolution of early supermassive black holes and their host galaxies.
Researchers used a Green Pea galaxy sample mainly from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Extra-galactic Survey. It is currently the largest collection of Green Pea galaxy spectra, encompassing nearly 1550 Green Pea galaxy spectra, which is more than double the number of Green Pea galaxies identified in the Sloan Digital Sky Survey (SDSS).
By conducting an analysis of the emission line profiles in the LAMOST and SDSS spectra, researchers only discovered five Green Pea galaxies with double-peaked narrow lines. With multi-band photometry and optical spectral diagnostics, they have reliably confirmed the AGN activity in this sample. Based on the emission line profiles and optical morphology, the physical origin of the double-peaked profiles in these galaxies is more likely attributed to dual AGN mergers rather than outflows or accretion disks.
According to LIN Ruqiu, a 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 to originate from outflows or gas disks." 
"These five Green Pea galaxies exhibit stronger [OIII] equivalent widths (defined as the ratio of line strength to continuum) 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 coevolution of massive galaxies and supermassive black holes." says Dr. ZHENG Zhen-Ya, the corresponding author of the paper and a researcher at the Shanghai Astronomical Observatory.
Figure 1. Column 1: The Pan-STARRS optical gri color images of the double-peaked narrow-line Green Pea galaxies. The image size is 10 arcseconds by 10 arcseconds. Column 2-3: The spectral emission line fitting results of the double-peaked narrow-line Green Pea galaxies. The black, blue, and red line represents the observed spectrum, the fitted components, and the composite model spectrum.
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[2] Liu S., Luo A.-L., Yang H., Shen S.-Y., Wang J.-X., Zhang H.-T., Zheng Z., et al., 2022, ApJ, 927, 57. doi:10.3847/1538-4357/ac4bd9



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