Activities of Galactic Nuclei and their Impact on Black Hole's Event Horizon, Host Galaxy, and Beyond

Astrophysical black holes are surprisingly simple physical objects. Their
gravitational field can be fully described by two parameters: mass and
spin. We cannot directly observe black holes as no light escapes from the
event horizon. However, we can detect the light from accreting gas, which
forms a dense disk around the black hole, known as an accretion disk. The
accretion of material by a massive black hole at the center of its host
galaxy forms an active galactic nucleus (AGN), the innermost region of
which emits X-ray radiation. An AGN is energetically efficient for
regulating the growth of galaxies and is crucial for the history of the
Universe we see today. One of the most important tools to probe the
innermost accretion flow is the detection of X-ray reverberation echoes,
where the X-ray photons reflected from the accretion disk are delayed
relative to the primary X-ray source. In this talk, I will first discuss
how detailed measurements of the reflected X-rays from the accretion disk
can be used to probe the innermost regions of accretion flow just outside
the event horizon and determine the fundamental properties of the black
hole, such as its spin, across the complete mass scales from ~ 10^5-10^10
solar masses. Peering into the growth channels of black holes, I will
discuss how we can distinguish accretion vs. merger-dominated black hole
growth and probe the cosmological evolution of black hole spins in the
last 10 billion years of cosmic history. Finally, I will show how
enigmatic relativistic winds or Ultra-Fast Outflows (UFOs) launched from
the AGN accretion disk can be used to probe the feedback mechanism
connecting the central black holes with their host galaxies.