Dark Energy Survey Identification of A Low-Mass Active Galactic Nucleus at Redshift 0.823 from Optical Variability

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\(\beta\) 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\(\beta\) 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\(\sigma\) statistical uncertainties only), 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 \(M_{\ast}\sim10^{10.5\pm0.3}M_{\odot}\) 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.