Kavli Affiliate: John D. Silverman
| First 5 Authors: Jia-Lai Kang, Chris Done, Scott Hagen, Matthew J. Temple, John D. Silverman
| Summary:
A recent study on the spectral energy distribution (SED) of AGN combined
unobscured X-ray sources from the eROSITA eFEDS Survey with high quality
optical imaging from Subaru’s Hyper Suprime-Cam (HSC). The HSC data enabled
accurate host galaxy subtraction as well as giving a uniform black hole mass
estimator from the stellar mass. The resulting stacked optical/X-ray SEDs for
black holes at fixed mass show a dramatic transition, where the dominating disc
component in bright AGN evaporates into an X-ray hot plasma below $L/L_{rm
Edd}sim 0.01$. The models fit to these datasets predicted the largest change
in SED in the rest frame UV ($< 3000,r{A}$), but this waveband was not
included in the original study. Here we use archival $u$-band and UV photometry
to extend the SEDs into this range, and confirm the UV is indeed intrinsically
faint in AGN below $L/L_{rm Edd}sim 0.01$ as predicted. This dramatic drop in
UV photo-ionising flux is also seen from its effect on the broad emission
lines. We stack the recently released SDSS DR18 optical spectra for this
sample, and show that the broad H$beta$ line disappears along with the UV
bright component at $L/L_{rm Edd}sim 0.01$. This shows that there is a
population of unobscured, X-ray bright, UV faint AGN which lack broad emission
lines (true type 2 Seyferts).
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