Active galactic nuclei

Abstract

Active galactic nuclei are the most powerful, long-lived objects in the Universe. Recent data confirm the theoretical idea that the power source is accretion into a massive black hole. The common occurrence of obscuration and outflows probably means that the contribution of active galactic nuclei to the power density of the Universe has been generally underestimated.

Figure 1

X-ray lightcurve of the powerful, radio-quiet AGN PHL1092 (18). Note the persistent large variability. A remarkable flare on day 8 shows a change in luminosity exceeding 10⁴² erg⋅s⁻². This requires a mass-to-energy conversion efficiency exceeding 0.6 and cannot be associated with stellar processes. Mildly relativistic motions close to a black hole have probably amplified the apparent variability.

Figure 2

The left panel shows the broad iron line seen in the Seyfert I galaxy MCG–6-30-15. The line would be very narrow and centered at 6.35 keV if the emitter were at rest in the galaxy but is clearly highly distorted and skewed to lower energies. The model compared with the data (solid line) assumes that the emitter is orbiting in a disk lying between 6 and 40 GM/c², inclined at 30°. Gravitational redshift due to the deep potential well accounts for part of the skewness. The right panel compares the data taken in 1994 (red dots, also shown in A) with that from 1997. The line profile is robust.

Figure 3

Hubble Space Telescope image of the core of the nearest radio galaxy, Centaurus A (19). A billion M_⊙ black hole in the center of this galaxy squirts jets to the upper left and lower right, which dissipate in large radio lobes (not visible in this optical image). Note the high level of obscuration evident in the image, which is common for radio galaxies and Seyfert II galaxies. The nearest two Seyfert galaxies to us, NGC 4945 and the Circinus galaxy, at about the same distance as Centaurus A but in different directions, also show large levels of obscuration.