Dark Energy Survey Identification of A Low-Mass Active Galactic Nucleus at Redshift 0.823 from Optical Variability
Creators: name: Guo, Hengxiao
name: Burke, Colin J.
name: Liu, Xin
name: Phadke, Kedar A.
name: Zhang, Kaiwen
name: Chen, Yu-Ching
name: Gruendl, Robert A.
name: Lidman, Christopher
name: Shen, Yue
name: Morganson, Eric
name: Aguena, Michel
name: Allam, Sahar
name: Avila, Santiago
name: Bertin, Emmanuel
name: Brooks, David
name: Rosell, Aurelio Carnero
name: Carollo, Daniela
name: Kind, Matias Carrasco
name: Costanzi, Matteo
name: da Costa, Luiz N.
name: De Vicente, Juan
name: Desai, Shantanu
name: Doel, Peter
name: Eifler, Tim F.
name: Everett, Spencer
name: García-Bellido, Juan
name: Gaztanaga, Enrique
name: Gerdes, David W.
name: Gruen, Daniel
name: Gschwend, Julia
name: Gutierrez, Gaston
name: Hinton, Samuel R.
name: Hollowood, Devon L.
name: Honscheid, Klaus
name: James, David J.
name: Kuehn, Kyler
name: Lima, Marcos
name: Maia, Marcio A. G.
name: Menanteau, Felipe
name: Miquel, Ramon
name: Möller, Anais
name: Ogando, Ricardo L. C.
name: Palmese, Antonella
name: Paz-Chinchón, Francisco
name: Plazas, Andrés A.
name: Romer, Anita K.
name: Roodman, Aaron
name: Sanchez, Eusebio
name: Scarpine, Vic
name: Schubnell, Michael
name: Serrano, Santiago
name: Smith, Mathew
name: Soares-Santos, Marcelle
name: Sommer, Natalia E.
name: Suchyta, Eric
name: Swanson, Molly E. C.
name: Tarle, Gregory
name: Tucker, Brad E.
name: Varga, Tamas N.
Publisher: name: arXiv
Year: 2020
Subjects: subject: Astrophysics of Galaxies (astro-ph.GA)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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$-$0430 as an AGN candidate by characterizing its long-term optical variability alone based on DES optical broad-band light curves spanning over 6 years. An archival optical spectrum from the fourth phase of the Sloan Digital Sky Survey shows both broad Mg II and broad H$β$ 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$β$ 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$σ$ statistical uncertainties only), consistent with the estimation from OzDES, making it the lowest mass AGN with redshift $&gt;$ 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.
description: 13 pages, 8 figures, 1 table, accepted to MNRAS