Kavli Affiliate: Claudio Ricci
| First 5 Authors: Grisha Zeltyn, Benny Trakhtenbrot, Michael Eracleous, Qian Yang, Paul Green
| Summary:
"Changing-look" Active Galactic Nuclei (CL-AGNs) are challenging our basic
ideas about the physics of accretion flows and of circumnuclear gas around
supermassive black holes (SMBHs). Using first year Sloan Digital Sky Survey V
(SDSS-V) repeated spectroscopy of nearly 29,000 previously-known AGNs, combined
with dedicated follow-up spectroscopic observations, and publicly available
optical light curves, we have identified 116 CL-AGNs where (at least) one broad
emission line has essentially (dis-)appeared, as well as 88 other extremely
variable systems. Our CL-AGN sample, with 107 newly identified cases, is among
the largest reported to date, and includes $sim$0.4% of the AGNs re-observed
in the first year of SDSS-V operations. Among our CL-AGNs, 67% exhibit dimming
while 33% exhibit brightening. Our data and sample probe extreme AGN spectral
variability on timescales of months to decades, including some cases of
recurring transitions on surprisingly short timescales ($lesssim$ 2 months in
the rest frame). We find that CL events are preferentially found in lower
Eddington ratio ($f_{Edd}$) systems: Our CL-AGNs have a $f_{Edd}$ distribution
that significantly differs from that of a redshift- and a carefully
constructed, luminosity-matched control sample ($p_{KS}$ $lesssim$ 2 $times$
$10^{-4}$ ; median $f_{Edd}$ $approx$ 0.025 vs. 0.043). This preference for
low $f_{Edd}$ strengthens previous findings of higher CL-AGN incidence at lower
Eddington ratios, found in much smaller samples of spectroscopically confirmed
CL-AGNs. Finally, we show that the broad MgII emission line in our CL-AGN
sample tends to vary significantly less than the broad H$beta$ emission line.
Our large CL-AGN sample demonstrates the advantages and challenges in using
multi-epoch spectroscopy from large surveys to study extreme AGN variability,
SMBH fueling, and AGN physics.
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