Why Do AGN Lighthouses Switch Off?

Abstract

Abstract. Nearby galactic nuclei are observed to be very much dimmer than active galactic nuclei in distant galaxies. The Chandra X-ray Observatory has provided a definitive explanation for why this is so. With its excellent angular resolution, Chandra has imaged hot X-ray-emitting gas close to the gravitational capture radius of a handful of supermassive black holes, including Sgr A * in the nucleus of our own Galaxy. These observations provide direct and reliable estimates of the Bondi mass accretion ratė M Bondi in these nuclei. It is found thatṀ Bondi is significantly below the Eddington mass accretion rate, but this alone does not explain the dimness of the accretion flows. In all the systems observed so far, the accretion luminosity Lacc ≪ 0.1Ṁ Bondi c 2 , which means that the accretion must occur via a radiatively inefficient mode. This conclusion, which was strongly suspected for many years, is now inescapable. Furthermore, if the accretion in these nuclei occurs via either a Bondi flow or an advection-dominated accretion flow, the accreting plasma must be two-temperature at small radii, and the central mass must have an event horizon. Convection, winds and jets may play a role, but observations do not yet permit definite conclusions.

Bright stars and an optically thick inactive disk in Sgr A*and other dormant galaxy centers

“…We then tested the case of a non-zero value for R in and found that only disks with inner holes as large as 0.1 ∼ 10 16 cm ∼ 10 4 R g yield NIR "echoes" that are weak enough to escape the observational constraints. As such, our results are in complete agreement with the previous results by Falcke & Melia (1997) and Narayan (2002), who also ruled out the existence of an optically thick disk with R in = 0 in Sgr A * .…”

“…In addition, the disk presence may help to explain (Nayakshin 2003;) two major mysteries of Sgr A * : its amazingly low luminosity (e.g., see Baganoff 2003, and reviews by Send offprint requests to: J. Cuadra, e-mail: jcuadra@MPA-Garching.MPG.DE Melia & Falcke 2001;Narayan 2002) and the recently discovered large amplitude X-ray flares (Baganoff et al 2001). Direct observational detection of the disk could be possible if the disk were massive and thus bright enough (e.g., Falcke & Melia 1997;Narayan 2002) but a very dim inactive disk could have eluded such detection . pointed out that the three dimensional orbits of stars such as S2 could be used to test the putative disk hypothesis.…”

Implications for the structure of the relativistic jet from multiwavelength observations of NGC 6251

“…The nonthermal flux can be used to derive an upper limit to the disk luminosity, L disk 10 43 erg s −1 (Ferrarese & Ford 1999;Ho 2009), which in units of Eddington luminosity, L Edd , is L disk /L Edd 10 −4 for an estimated black hole mass M BH = 6 × 10 8 M (Ferrarese & Ford 1999). The sub-Eddington regime could indicate the presence of a radiatively inefficient disk (here we generically refer to the class of models as radiatively inefficient accretion flows, RIAFs, see Narayan 2002, for a review). Ghisellini & Celotti (2001) demonstrated that the FRI-FRII dividing line in the radio luminosity vs optical galaxy luminosity plane (proposed by Ledlow & Owen 1996) can be expressed in terms of a critical accretion rate of 10 −2 −10 −3 , in units of Eddington accretion, and suggested that this might reflect a change in the accretion mode from a standard optically thick geometrically thin efficient Shakura-Sunyaev disk (Shakura & Sunyaev 1973) to a RIAF.…”

Low-luminosity AGNs

“…At low accretion mass rates ofṀ Ṁ Edd , the radiative cooling of the disk becomes inefficient. These accretion flows are generally called radiatively inefficient accretion flows (RIAFs) (Narayan 2002). This class of disk flow also includes ADAFs.…”