Active galactic nuclei: what’s in a name?

Abstract

Active Galactic Nuclei (AGN) are energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies, and present unique observational signatures that cover the full electromagnetic spectrum over more than twenty orders of magnitude in frequency. The rich phenomenology of AGN has resulted in a large number of different "flavours" in the literature that now comprise a complex and confusing AGN "zoo". It is increasingly clear that these classifications are only partially related to intrin- sic differences between AGN, and primarily reflect variations in a relatively small number of astrophysical parameters as well the method by which each class of AGN is selected. Taken together, observations in different electromagnetic bands as well as variations over time provide complementary windows on the physics of different sub-structures in the AGN. In this review, we present an overview of AGN multi-wavelength properties with the aim of painting their "big picture" through observations in each electromagnetic band from radio to γ-rays as well as AGN variability. We address what we can learn from each observational method, the impact of selection effects, the physics behind the emission at each wavelength, and the potential for future studies. To conclude we use these observations to piece together the basic architecture of AGN, discuss our current understanding of unification models, and highlight some open questions that present opportunities for future observational and theoretical progress.

Composite Spectral Energy Distributions and Infrared–Optical Colors of Type 1 and Type 2 Quasars

“…This contrast suggests a weak connection between the mechanical power of the quasar (as measured by radio luminosity) and the radiative power, with implications for the launching mechanisms of relativistic jets (e.g., Sikora et al 2007;Tadhunter 2016;Padovani et al 2017).…”

Active Galactic Nuclei at All Wavelengths and from All Angles

“…Importantly, it now looks like the dust and the BLR are only present at high powers ( 10 42 erg s −1 )/high Eddington ratios (L/L Edd 0.01; see discussion in Padovani et al, 2017), which implies that dust-driven unification breaks down below these values. Jet-driven, low-power unification (linking BL Lacs with LEG RGs) is unaffected by this.…”

“…Thanks to polarization studies it was in fact realized that the difference between type 1 (broad-lined) and type 2 (narrow-lined) AGN was solely due to orientation with respect to the dust. In the latter objects our view of the accretion disk and the BLR is obscured by the dust and therefore we cannot see the UV bump and the broad lines typical of type 1 AGN but can only detect the narrow lines emitted by slower-moving clouds beyond the dust (but see Elitzur and Netzer, 2016 for a discussion about possible real type 2 AGN where the difference is not caused by dust obscuration; see also section 5.3 of Padovani et al, 2017).…”

“…This "additional" component is now universally accepted to be due to an actively accreting central supermassive ( 10 6 M ⊙ ) black hole (SMBH). This implies a number of fascinating properties, which include very high luminosities (up to L bol ≈ 10 48 erg s −1 ), small emitting regions in most bands (of the order of a milliparsec), strong evolution of the luminosity functions, and broad-band emission covering the entire electromagnetic spectrum (see Padovani et al, 2017, for a review).…”

“…I discuss this next. I refer to Padovani et al (2017) for a more in-depth review of these (and other) topics.…”