Dark Energy Survey identification of a low-mass active galactic nucleus at redshift 0.823 from optical variability

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

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5 Citations (Scopus)


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 Hβ lines, confirming its nature as a broad-line AGN. Archival XMM-Newton X-ray observations suggest an intrinsic hard X-ray luminosity of L2-12 keV ≈ 7.6 ± 0.4 × 1043 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 Hβ from SDSS spectrum, we estimate a virial black hole (BH) mass of M. ≈ 106.43-106.72M⊙ (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 ± 0.3M⊙ 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.

Original languageEnglish
Pages (from-to)3636-3647
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - Aug 2020


  • black hole physics
  • surveys
  • galaxies: active
  • galaxies: high-redshift
  • galaxies: nuclei
  • quasars: general


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