Dark Energy Survey Identification of A Low-Mass ActiveGalactic Nucleus at Redshift 0.823 from Optical Variability. (arXiv:2003.10457v1 [astro-ph.GA])

Dark Energy Survey Identification of A Low-Mass ActiveGalactic Nucleus at Redshift 0.823 from Optical Variability. (arXiv:2003.10457v1 [astro-ph.GA])
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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 $L_{{rm
2-12,keV}}sim7.6pm0.4times10^{43}$ 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 $M_{bullet}sim10^{6.4pm0.1}M_{odot}$ (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
$M_{ast}sim10^{10.5pm0.3}M_{odot}$ based on modeling the multi-wavelength
spectral energy distribution. DES J0218$-$0430 extends the
$M_{bullet}$-$M_{ast}$ relation observed in luminous AGNs at $zsim1$ 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.

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 $L_{{rm
2-12,keV}}sim7.6pm0.4times10^{43}$ 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 $M_{bullet}sim10^{6.4pm0.1}M_{odot}$ (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
$M_{ast}sim10^{10.5pm0.3}M_{odot}$ based on modeling the multi-wavelength
spectral energy distribution. DES J0218$-$0430 extends the
$M_{bullet}$-$M_{ast}$ relation observed in luminous AGNs at $zsim1$ 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.

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