Dark Energy Survey identification of a low-mass active galactic nucleus at redshift 0.823 from optical variability. Issue 3 (23rd June 2020)

• Record Type: Journal Article
• Authors: Guo, Hengxiao; Burke, Colin J; Liu, Xin; Phadke, Kedar A; Zhang, Kaiwen; Chen, Yu-Ching; Gruendl, Robert A; Lidman, Christopher; Shen, Yue; Morganson, Eric; Aguena, Michel; Allam, Sahar; Avila, Santiago; Bertin, Emmanuel; Brooks, David; Rosell, Aurelio Carnero; Carollo, Daniela; Kind, Matias Carrasco; Costanzi, Matteo; da Costa, Luiz N; De Vicente, Juan; Desai, Shantanu; Doel, Peter; Eifler, Tim F; Everett, Spencer; García-Bellido, Juan; Gaztanaga, Enrique; Gerdes, David W; Gruen, Daniel; Gschwend, Julia; Gutierrez, Gaston; Hinton, Samuel R; Hollowood, Devon L; Honscheid, Klaus; James, David J; Kuehn, Kyler; Lima, Marcos; Maia, Marcio A G; Menanteau, Felipe; Miquel, Ramon; Möller, Anais; Ogando, Ricardo L C; Palmese, Antonella; Paz-Chinchón, Francisco; Plazas, Andrés A; Romer, Anita K; Roodman, Aaron; Sanchez, Eusebio; Scarpine, Vic; Schubnell, Michael; Serrano, Santiago; Smith, Mathew; Soares-Santos, Marcelle; Sommer, Natalia E; Suchyta, Eric; Swanson, Molly E C; Tarle, Gregory; Tucker, Brad E; Varga, Tamas N;
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 496:Issue 3(2020)
• Page Start: 3636
• Page End: 3647
• Publication Date: 2020-06-23
• Subjects: black hole physics -- surveys -- galaxies: active -- galaxies: high-redshift -- galaxies: nuclei -- quasars: general; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa1803
• Languages: English
• ISSNs: 0035-8711

Abstract

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 ≈ 10 6.43 –10 6.72 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 * ≈ 10 10.5 ± 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.