Fast Inflow Directly Feeding Black Hole Accretion Disk in Quasars
  Quasars are high-luminosity active galactic nuclei believed to be powered by
accretion of interstellar matter onto a super-massive black hole (SMBH)
therein. Most of the observed energy is released in an accretion disk of
inspiralling gas surrounding the SMBH. An enormous amount of fueling material
is expected to be transported inwards. However, basic questions remain
unanswered as to whether and how the accretion disks are supplied with external
gas, since no disk-feeding inflow has hitherto been observed clearly. Here we
report the discovery of highly redshifted broad absorption lines arising from
neutral hydrogen and helium atoms in a small sample of quasars. Their
absorption troughs show a broad range of Doppler velocities from zero extending
continuously inward up to as high as $\sim 5,000$ km s$^{-1}$. We thus see
through streams of cold gas moving with a radially inward velocity component
that spans an immense gradient---a result of gravitational acceleration by the
central SMBH. Extensive photo-ionization modeling for the archetypical object
SDSS J103516.20+142200.6 indicates the inflowing gas to be dense, thick and
moderately ionized, with a characteristic distance to the SMBH of $\sim 1,000$
gravitational radii, possibly overlapping or close to the outer accretion disk.
Our results present the first compelling evidence for the long-sought inflow
directly feeding quasars' accretion disks with external materials, likely
originating from the dusty torus at a parsec scale. Our approach provides a new
tool to probe the bulk of the so far elusive fueling inflows in quasars. Their
studies may help address some of the fundamental questions concerning accretion
physics, the onset and sustainment of quasar activity, and the SMBH growth at
centers of most galaxies.
