The properties of LLAGN inferred from high resolution observations

Abstract

Supermassive black holes (SMBHs) gain most of their mass during their brief, 100 Myr, luminous phases as efficiently radiating quasars and Seyfert galaxies. However, the total mechanical energy that is released by a SMBH during its quiescent phase, in which it spends 99 percent of the time, may be comparable to or even greater than the total radiative energy released during luminous phases. As such, the low-luminosity AGNs (LLAGNs) that are found in a large a fraction of all massive galaxies could have a significant role in regulating galaxy growth and establishing the observed relations between galaxies and SMBHs. Almost by definition, however, LLAGNs are difficult to study observationally, with their feeble signals easily lost or confused with other nuclear emission. As a result, much less is known, compared to luminous AGNs, about the phenomenology of low-luminosity activity, let alone its physics. For example, there are widely disparate views about the accretion mode at low-luminosities: do quiescent SMBHs undergo a transition to a geometrically thick RIAF phase that is distinct in its physics and emergent spectrum from those of the thin accretion disks invoked for quasars? Or does non-thermal jet emission become the dominant component at all wavelengths in LLAGNs? Multi-wavelength high-angular-resolution data that isolate the AGN emission are critical for answering these questions. Additional gain is obtained by combining high resolution with the time domain; the variability that seems to be endemic to all AGNs permits isolating weak non-stellar nuclear emission at all wavelengths, even when it is dwarfed by brighter, but constant, non-nuclear emission. I will review recent progress in understanding the properties of LINERs, the most common form of LLAGNs, based on such data. I will argue that there is actually more similarity than contrast between the observed spectral properties of different AGNs, with only a mild and continuous change in the properties as a function of decreasing accretion rate, going from quasars to the faint LINERs in nearby galaxies. This suggests a persistence of thin accretion disks at low accretion rates, in analogy to some recent findings for stellar-mass BHs. At the same time, a relatively larger fraction of the energy emerges in radio bands, suggesting an increasing role for jets/outflows, and their associated mechanical input, at low AGN luminosities.