The Redshifted Hydrogen Balmer and Metastable He I Absorption Line
  System in Mini-FeLoBAL Quasar SDSS J112526.12+002901.3: A Parsec Scale
  Accretion Inflow?
  The accretion of interstellar medium onto the central super massive black
holes is widely accepted as the source of the gigantic energy released by the
active galactic nuclei. But few pieces of observational evidence have been
confirmed directly demonstrating the existence of the inflows. The absorption
line system in the spectra of quasar SDSS J112526.12+002901.3 presents an
interesting example, in which the rarely detected hydrogen Balmer and
metastable He I absorption lines are found redshifted to the quasar's rest
frame along with the low-ionization metal absorption lines Mg II, Fe II, etc.
The repeated SDSS spectroscopic observations suggest a transverse velocity
smaller than the radial velocity. The motion of the absorbing medium is thus
dominated by infall. The He I* lines present a powerful probe to the strength
of ionizing flux, while the Balmer lines imply a dense environment. With the
help of photoionization simulations, we find the absorbing medium is exposed to
the radiation with ionization parameter $U\approx 10^{-1.8}$, and the density
is $n(\mathrm{H})\approx 10^9\ \mathrm{cm}^{-3}$. Thus the absorbing medium is
located $\sim 4\ \mathrm{pc}$ away from the central engine. According to the
similarity in the distance and physical conditions between the absorbing medium
and the torus, we strongly propose the absorption line system as a candidate
for the accretion inflow which originates from the inner surface of the torus.
