Dark Energy Survey identification of a low-mass active galactic nucleus at redshift 0.823 from optical variability
De Vicente, Juan; García-Bellido, Juan; Gaztanaga, Enrique; Miquel, Ramon; Sanchez, Eusebio; Serrano, Santiago; Shen, Yue; Bertin, Emmanuel; Phadke, Kedar A.; Desai, Shantanu; Gruendl, Robert A.; James, David J.; Carollo, Daniela; Guo, Hengxiao; Chen, Yu-Ching; Liu, Xin; Gruen, Daniel; Honscheid, Klaus; Gerdes, David W.; Brooks, David; Doel, Peter; Schubnell, Michael; Gutierrez, Gaston; Soares-Santos, Marcelle; Swanson, Molly E. C.; Möller, Anais; Roodman, Aaron; Tucker, Brad E.; Palmese, Antonella; da Costa, Luiz N.; Maia, Marcio A. G.; Menanteau, Felipe; Burke, Colin J.; Zhang, Kaiwen; Lidman, Christopher; Morganson, Eric; Aguena, Michel; Allam, Sahar; Avila, Santiago; Rosell, Aurelio Carnero; Kind, Matias Carrasco; Costanzi, Matteo; Eifler, Tim F.; Everett, Spencer; Gschwend, Julia; Hinton, Samuel R.; Hollowood, Devon L.; Kuehn, Kyler; Lima, Marcos; Ogando, Ricardo L. C.; Paz-Chinchón, Francisco; Plazas, Andrés A.; Romer, Anita K.; Scarpine, Vic; Smith, Mathew; Sommer, Natalia E.; Suchyta, Eric; Tarle, Gregory; Varga, Tamas N.; (DES Collaboration)
United States, Australia, Brazil, Spain, France, United Kingdom, Italy, India, Germany
Abstract
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.