%AGuo, Hengxiao%ABurke, Colin%ALiu, Xin%APhadke, Kedar%AZhang, Kaiwen%AChen, Yu-Ching%AGruendl, Robert%ALidman, Christopher%AShen, Yue%AMorganson, Eric%AAguena, Michel%AAllam, Sahar%AAvila, Santiago%ABertin, Emmanuel%ABrooks, David%ARosell, Aurelio%ACarollo, Daniela%AKind, Matias%ACostanzi, Matteo%Ada Costa, Luiz%ADe Vicente, Juan%ADesai, Shantanu%ADoel, Peter%AEifler, Tim%AEverett, Spencer%AGarcía-Bellido, Juan%AGaztanaga, Enrique%AGerdes, David%AGruen, Daniel%AGschwend, Julia%AGutierrez, Gaston%AHinton, Samuel%AHollowood, Devon%AHonscheid, Klaus%AJames, David%AKuehn, Kyler%ALima, Marcos%AMaia, Marcio%AMenanteau, Felipe%AMiquel, Ramon%AMöller, Anais%AOgando, Ricardo%APalmese, Antonella%APaz-Chinchón, Francisco%APlazas, Andrés%ARomer, Anita%ARoodman, Aaron%ASanchez, Eusebio%AScarpine, Vic%ASchubnell, Michael%ASerrano, Santiago%ASmith, Mathew%ASoares-Santos, Marcelle%ASommer, Natalia%ASuchyta, Eric%ASwanson, Molly%ATarle, Gregory%ATucker, Brad%AVarga, Tamas%Anull Ed.%BJournal Name: Monthly Notices of the Royal Astronomical Society; Journal Volume: 496; Journal Issue: 3 %D2020%I %JJournal Name: Monthly Notices of the Royal Astronomical Society; Journal Volume: 496; Journal Issue: 3 %K %MOSTI ID: 10225452 %PMedium: X %TDark Energy Survey identification of a low-mass active galactic nucleus at redshift 0.823 from optical variability %XABSTRACT We report the identification of a low-mass active galactic nucleus (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 yr. 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 $L_{{\rm 2-12\, keV}}\approx 7.6\pm 0.4\times 10^{43}$ 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 black hole (BH) mass of M• ≈ 106.43–106.72 M⊙ (with the error denoting the systematic uncertainty from different calibrations), 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* ≈ 1010.5 ± 0.3 M⊙ based on modelling the multiwavelength spectral energy distribution. DES J0218−0430 extends the M•–M* relation observed in luminous AGNs at z ∼ 1 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. %0Journal Article