The Fueling and Evolution of AGN: Internal and External Triggers

The quest for a coherent picture of nuclear activity has witnessed giant leaps in the last decades. Four decades ago, the idea was put forward that accretion of matter onto a massive compact object or a supermassive black hole (SMBH) of mass >10 M could power very luminous active galactic nuclei (AGN), in particular, quasi-stellar objects (QSOs) (Lynden-Bell 1969; Soltan 1982; Rees 1984). In the last decade, dynamical evidence increasingly suggests that SMBH pervade the centers of most massive galaxies (Sect. 2 and references therein). The challenge has now shifted towards probing the fueling and evolution of AGN over a wide range of cosmic lookback times, and elucidating how they relate to their host galaxies in both the local and cosmological context. In this review, I will focus on the fueling and evolution of AGN under the influence of internal and external triggers. In the nature versus nurture paradigm, I use the term internal triggers to refer to intrinsic properties of host galaxies (e.g., morphological or Hubble type, color, and non-axisymmetric features such as large-scale bars and nuclear bars) while external triggers refer to factors such as environment and interactions. The distinction is an over-simplification as many of the so called intrinsic properties of galaxies can be induced or dissolved under the influence of external triggers. Connections will be explored between the nuclear and larger-scale properties of AGN, both locally and at intermediate redshifts. One of the driving objectives is to understand why not all relatively massive galaxies show signs of AGN activity (via high-excitation optical lines or X-ray emission) despite mounting dynamical evidence that they harbor SMBHs. The most daunting challenge in fueling AGN is arguably the angular momentum problem (Subsect. 3.2). Even matter located at a radius of a few hundred pc must lose more than 99.99% of its specific angular momentum before it is fit for consumption by a BH. The sequence of this review is as follows. Section 2 briefly addresses BH demographics and the BH-bulge-halo correlations. Section 3 sets the stage for the rest of this paper by providing an overview of central issues in the