Understanding Broad Mg II Variability in Quasars with Photoionization:
  Implications for Reverberation Mapping and Changing-Look Quasars
  The broad Mg II line in quasars has distinct variability properties compared
with broad Balmer lines: it is less variable, and usually does not display a
"breathing" mode, the increase in the average cloud distance when luminosity
increases. We demonstrate that these variability properties of Mg II can be
reasonably well explained by simple Locally Optimally Emitting Cloud (LOC)
photoionization models, confirming earlier photoionization results. In the
fiducial LOC model, the Mg II-emitting gas is on average more distant from the
ionizing source than the H$\alpha$/H$\beta$ gas, and responds with a lower
amplitude to continuum variations. If the broad-line region (BLR) is truncated
at a physical radius of $\sim 0.3$ pc (for a $10^{8.5}M_{\odot}$ BH accreting
at Eddington ratio of 0.1), most of the Mg II flux will always be emitted near
this outer boundary and hence will not display breathing. These results
indicate that reverberation mapping results on broad Mg II, while generally
more difficult to obtain due to the lower line responsivity, can still be used
to infer the Mg II BLR size and hence black hole mass. But it is possible that
Mg II does not have a well defined intrinsic BLR size-luminosity relation for
individual quasars, even though a global one for the general population may
still exist. The dramatic changes in broad H$\alpha$/H$\beta$ emission in the
observationally-rare changing-look quasars are fully consistent with
photoionization responses to extreme continuum variability, and the LOC model
provides natural explanations for the persistence of broad Mg II in
changing-look quasars defined on H$\alpha$/H$\beta$, and the rare population of
broad Mg II emitters in the spectra of massive inactive galaxies.
