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 \sim 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 _ { { 2 - 12 keV } } \sim 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 \beta from SDSS spectrum , we estimate a virial BH mass of M _ { \bullet } \sim 10 ^ { 6.4 \pm 0.1 } 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 } \sim 10 ^ { 10.5 \pm 0.3 } 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 \sim 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 .