Far from being rare, exotic entities that inhabit only a tiny fraction of galaxies, central black holes (BHs) are now believed to be basic constituents of most, if not all, massive galaxies (Magorrian et al. 1998; Kormendy 2004). Although less common in low-mass systems, central BHs also have been identified in some late-type, even dwarf, galaxies (Filippenko & Ho 2003; Barth et al. 2004; Greene & Ho 2004, 2007b; Dong et al. 2007; Greene, Ho & Barth 2008). The realization that BH mass correlates strongly with the properties of the host galaxy (Kormendy 1993; Kormendy & Richstone 1995; Magorrian et al. 1998; Gebhardt et al. 2000; Ferrarese & Merritt 2000; Barth, Greene & Ho 2005; Greene & Ho 2006) has generated intense interest in linking BH growth with galaxy formation, as attested by the increasing number of conferences focusing on this theme (e.g., Schmitt, Kinney & Ho 1999; Ho 2004a; Storchi-Bergmann, Ho & Schmitt 2004; Merloni, Nayakshin & Sunyaev 2005; Fiore 2006). As a direct manifestation of BH accretion, and therefore BH growth, active galactic nuclei (AGNs) and the consequences of their energy feedback have figured prominently in most current ideas of structure formation (e.g., Granato et al. 2004; Springel, Di Matteo & Hernquist 2005; Hopkins et al. 2006). At the same time, the community's heightened awareness of the importance of BHs has galvanized broad interest in the study of the AGN phenomenon itself. With the BH mass known - arguably the most fundamental parameter of the system - what once rested on phenomenological analysis can now be put on a more secure physical basis.
This review focuses on nuclear activity in nearby galaxies. By selection, most of the objects occupy the faintest end of the AGN luminosity function and have very low accretion rates. While energetically unimpressive, low-luminosity AGNs (LLAGNs) deserve scrutiny for several reasons. By virtue of the short duty cycle of BH accretion (~ 10-2; Greene & Ho 2007a), most AGNs spend their lives in a low state, such that the bulk of the population has relatively modest luminosities. Over the past several decades, this attribute has led to considerable controversy regarding the physical origin of LLAGNs. As absolute luminosity can no longer be used as a defining metric of nonstellar activity, many alternative excitation mechanisms have been proposed to explain LLAGNs. Fortunately, the advent of new telescopes and new analysis techniques have yielded many fresh insights into this thorny old problem. A major goal of this review is to summarize these recent developments. Along the way, I will emphasize how the collective properties of LLAGNs can shed light on a poorly understood regime of the central engine, namely that governed by low mass accretion rate. A key point I will stress is that LLAGNs are not simply scaled-down versions of their more familiar cousins, the classical Seyfert galaxies and quasars.
Despite the impressive progress made in the direct detection of central BHs in nearby inactive galaxies, our knowledge of the demographics of BHs remains highly incomplete. Direct measurements of BH masses based on resolved gas or stellar kinematics are still far from routine and are available only for about three dozen galaxies. Certainly nothing approaching a "complete" sample exists yet. More importantly, it is not obvious that the current statistics are unbiased. As discussed by Barth (2004), most nearby galaxies possess chaotic gas velocity fields that defy simple analysis. Stellar kinematics provide a powerful alternative, but in practice this technique has been limited to relatively dust-free systems and, for practical reasons, to galaxies of relatively high central surface brightness. The latter restriction selects against the most luminous, giant ellipticals. Present surveys also severely underrepresent disk-dominated galaxies, because the bulge component in these systems is inconspicuous and star formation tends to perturb the velocity field of the gas. Finally, apart from galaxies within the Local Group, even the highest angular resolution currently achieved is inadequate to directly detect BHs with masses 106 M. Consequently we are nearly completely ignorant about the low end of the BH mass function. Given the above limitations, it is desirable to consider alternative constraints on BH demography. The commonly held and now well-substantiated premise that AGNs derive their energy output from BH accretion implies that an AGN signifies the presence of a central BH in a galaxy. The AGN signature in and of itself provides little direct information on BH masses, but AGN statistics can inform us, effectively and efficiently, of some key aspects of BH demography. For example, what fraction of all galaxies contain BHs? Do BHs exist preferentially in galaxies of certain types? Does environment matter? I will discuss how studies of nearby AGNs have begun to answer some of these important questions.
This review is structured as follows. I begin with an overview of the basic methodology of the spectral classification of emission-line nuclei (Section 2) by describing the currently adopted system, its physical motivation, the complications of starlight subtraction, and some practical examples. Section 3 summarizes past and current spectroscopic surveys and introduces the Palomar survey, covering detection rates, measurement of weak broad emission lines, and issues of robustness and completeless. Host galaxy properties are the subject of Section 4, where in addition to global and environmental effects I also cover results on nuclear stellar populations. In Section 5, I devote considerable attention to the nuclear properties of LLAGNs in general and LINERs in particular, focusing on modern results obtained from high-resolution, multiwavelength observations from radio to hard-X-ray energies. I use these data to draw inferences concerning the broad-line region (BLR), torus, narrow-line region (NLR), spectral energy distribution (SED), luminosity function, bolometric luminosities, and Eddington ratios. This section contains many technical details, but these will be essential ingredients for formulating the big picture at the end. Section 6 covers the controversial subject of the excitation mechanism of LINERs, the growing puzzle concerning the energy budget in these systems, and the nature of narrow-line nuclei. The implications of LLAGNs for BH demographics are discussed in Section 7. Section 8 attempts to synthesize the disparate lines of evidence into a coherent physical framework for LLAGNs and their relation to other classes of objects. Finally, Section 9 concludes with some personal perspectives and suggestions for future directions.