Dark Energy Survey Identification of A Low-Mass Active Galactic Nucleus at Redshift 0.823 from Optical Variability

15 Jun 2020  ·  Guo Hengxiao DES Collaboration, Burke Colin J. DES Collaboration, Liu Xin DES Collaboration, Phadke Kedar A. DES Collaboration, Zhang Kaiwen DES Collaboration, Chen Yu-Ching DES Collaboration, Gruendl Robert A. DES Collaboration, Lidman Christopher DES Collaboration, Shen Yue DES Collaboration, Morganson Eric DES Collaboration, Aguena Michel DES Collaboration, Allam Sahar DES Collaboration, Avila Santiago DES Collaboration, Bertin Emmanuel DES Collaboration, Brooks David DES Collaboration, Rosell Aurelio Carnero DES Collaboration, Carollo Daniela DES Collaboration, Kind Matias Carrasco DES Collaboration, Costanzi Matteo DES Collaboration, da Costa Luiz N. DES Collaboration, De Vicente Juan DES Collaboration, Desai Shantanu DES Collaboration, Doel Peter DES Collaboration, Eifler Tim F. DES Collaboration, Everett Spencer DES Collaboration, García-Bellido Juan DES Collaboration, Gaztanaga Enrique DES Collaboration, Gerdes David W. DES Collaboration, Gruen Daniel DES Collaboration, Gschwend Julia DES Collaboration, Gutierrez Gaston DES Collaboration, Hinton Samuel R. DES Collaboration, Hollowood Devon L. DES Collaboration, Honscheid Klaus DES Collaboration, James David J. DES Collaboration, Kuehn Kyler DES Collaboration, Lima Marcos DES Collaboration, Maia Marcio A. G. DES Collaboration, Menanteau Felipe DES Collaboration, Miquel Ramon DES Collaboration, Möller Anais DES Collaboration, Ogando Ricardo L. C. DES Collaboration, Palmese Antonella DES Collaboration, Paz-Chinchón Francisco DES Collaboration, Plazas Andrés A. DES Collaboration, Romer Anita K. DES Collaboration, Roodman Aaron DES Collaboration, Sanchez Eusebio DES Collaboration, Scarpine Vic DES Collaboration, Schubnell Michael DES Collaboration, Serrano Santiago DES Collaboration, Smith Mathew DES Collaboration, Soares-Santos Marcelle DES Collaboration, Sommer Natalia E. DES Collaboration, Suchyta Eric DES Collaboration, Swanson Molly E. C. DES Collaboration, Tarle Gregory DES Collaboration, Tucker Brad E. DES Collaboration, Varga Tamas N. DES Collaboration ·

We report the identification of a low-mass AGN, DES J0218$-$0430, in a redshift $z = 0.823$ galaxy in the Dark Energy Survey (DES) Supernova field. We select DES J0218$-$0430 as an AGN candidate by characterizing its long-term optical variability alone based on DES optical broad-band light curves spanning over 6 years. An archival optical spectrum from the fourth phase of the Sloan Digital Sky Survey shows both broad Mg II and broad H$\beta$ lines, confirming its nature as a broad-line AGN. Archival XMM-Newton X-ray observations suggest an intrinsic hard X-ray luminosity of $L_{{\rm 2-12\,keV}}\sim7.6\pm0.4\times10^{43}$ erg s$^{-1}$, which exceeds those of the most X-ray luminous starburst galaxies, in support of an AGN driving the optical variability. Based on the broad H$\beta$ from SDSS spectrum, we estimate a virial BH mass of $M_{\bullet}\approx10^{6.43}$-$10^{6.72}M_{\odot}$ (with the error denoting 1$\sigma$ statistical uncertainties only), consistent with the estimation from OzDES, making it the lowest mass AGN with redshift $>$ 0.4 detected in optical. We estimate the host galaxy stellar mass to be $M_{\ast}\sim10^{10.5\pm0.3}M_{\odot}$ based on modeling the multi-wavelength spectral energy distribution. DES J0218$-$0430 extends the $M_{\bullet}$-$M_{\ast}$ relation observed in luminous AGNs at $z\sim1$ to masses lower than being probed by previous work. Our work demonstrates the feasibility of using optical variability to identify low-mass AGNs at higher redshift in deeper synoptic surveys with direct implications for the upcoming Legacy Survey of Space and Time at Vera C. Rubin Observatory.

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Astrophysics of Galaxies