Oral Presentation
COSMOS2020: AGN activity of Massive Quiescent Galaxies to z~5 Revealed by X-ray and Radio Stacking
Presenter: Kei Ito (SOKENDAI/NAOJ)
It has been long known that massive early-type galaxies formed in an intense burst of star formation in the early Universe followed by passive evolution. While it remains largely unknown what physical process(es) is responsible for suppressing star formation activities in these galaxies, active galactic nuclei (AGNs) are suspected of playing an important role. In this talk, we report a characterization of the average X-ray and radio properties of massive quiescent galaxies (QGs) to z~5 to discuss the connection between quenching and AGNs. QGs are selected from the COSMOS2020 catalog, including the latest deeper surveys than before. We conduct the stacking analysis of the Chandra COSMOS Legacy Survey images for individually undetected QGs. The stacked X-ray signal is detected up to z~5 for the first time. The X-ray binaries cannot explain the average X-ray luminosity, suggesting that the low-luminosity AGNs exist in QGs. Moreover, the X-ray AGN luminosity of QGs at redshift higher than 1.5 is higher than that of star-forming galaxies. The stacking analysis of the VLA-COSMOS images is also conducted for the identical sample, and the signal for QGs is detected up to z=5. The result is consistent with the X-ray analysis, suggesting the higher radio AGN luminosity in QGs at redshift higher than 1.5. The enhanced AGN activity in QGs indicates its essential role for quenching at high redshift. Their enhanced AGN activity is less evident at redshift lower than 1.5, which can be interpreted as an increasing role of others, such as environmental quenching, at lower redshifts.
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