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])
<a href="http://arxiv.org/find/astro-ph/1/au:+Guo_H/0/1/0/all/0/1">H. Guo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Burke_C/0/1/0/all/0/1">C. J. Burke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Liu_X/0/1/0/all/0/1">X. Liu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Phadke_K/0/1/0/all/0/1">K. A. Phadke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_K/0/1/0/all/0/1">K. Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chen_Y/0/1/0/all/0/1">Y. -C. Chen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lidman_C/0/1/0/all/0/1">C. Lidman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shen_Y/0/1/0/all/0/1">Y. Shen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Morganson_E/0/1/0/all/0/1">E. Morganson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aguena_M/0/1/0/all/0/1">M. Aguena</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Allam_S/0/1/0/all/0/1">S. Allam</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Avila_S/0/1/0/all/0/1">S. 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Swanson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tarle_G/0/1/0/all/0/1">G. Tarle</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tucker_B/0/1/0/all/0/1">B. E. Tucker</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Varga_T/0/1/0/all/0/1">T. N. Varga</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Collaboration_DES/0/1/0/all/0/1">DES Collaboration</a>

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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