Active galactic nuclei

The unification model

The paradigm that a super massive black hole in the centre of the galaxy accreting matter is the driver of the radio source, was developed in the 1980s and was the seed for the

’Orientation unification’ model (see Fig. 1.6). The model describes the active nucleus as

Figure 1.6: A sketch of the unification model, showing that the observed properties are highly dependent on the viewing angle (reproduced from Torres 2003).

an axial structure where the central black hole is accreting material from a surrounding disk of gas and dust extending ∼ 100 AU from the centre. The gravitational potential of the material is transferred into radiation via viscous dissipation and results in a source radiating across the full electromagnetic spectrum. The inner part of the accretion disk, a few AUs, is believed to be responsible for the observed X-ray emission.

Just surrounding the X-ray emitting region, is the broad line region (BLR). The broad emission lines emitted from this region of the accretion disk, play an important role in our understanding of AGN. The lines portray the bulk motions in the BLR, which are almost certainly controlled by the central source. Additionally, the BLR reprocesses ionising UV continuum photons, which provide indirect information about this part of the continuum.

This means that brightness variations in the emitted BLR lines, reflect the changes in the underlying continuum. The line emitted from the BLR is Doppler-broadened and even a single spectrum can show emission lines of different widths. The broadening of the lines, however, can become a disadvantage as the lines can blend together and make a complete de-blending impossible.

At a distance of ∼ 10⁴AU from the centre the emission from the accretion disk is believed to be dominated by IR radiation. At these distances we also find the narrow line region (NLR). This is low density gas, whose motions are dominated by gravity. The NLR is the largest spatial scale region, where the ionising radiation from the central source dominates. It is the only AGN component, which can be spatially resolved in the optical.

This gives important information about how the NLR is illuminated by the central source

1.3 High-z Radio Galaxies 13

Radio properties Orientation Face-on Edge-on Radio quiet Seyfert I Seyfert II

QSO

Radio loud BL Lac FR I

BLRG NLRG

Quasar/OVV FR II

Table 1.1: Overview of the different AGN types, classified by their orientation and if they are radio quite or loud.

in a non-isotropic way, and can tell us how the central source is fuelled. The spatial extent of the region means that the physical and kinematic distribution of the gas, to some extent, can be mapped directly. The low density of the gas in the NLR allow forbidden transitions to not be collisionally suppressed. The intensity ratios of certain pairs of forbidden lines, allow us to measure the electron density and temperature of the NLR gas (Peterson, 1997).

In contrast to the broad lines from the BLR, the forbidden lines are isotropic since self-absorption in the narrow lines is negligible, and they therefore do not reflect variations in the underlying continuum.

Evidence of gas-jet interaction in the form of shock-heating and outflows has been found at the NLR location in the disk. The unresolved AGN-environment is further surrounded by an optically thick obscuring torus, limiting the radiation from the AGN to escape only along the torus axis. In radio loud sources the radio axis are aligned with the torus axis, but these do not show a preferred orientation with the rotation axis of the host galaxies (Drouart et al., 2012).

The unification model assumes that the intrinsic diversity of AGN is less than what is observed. The wide variety of observed AGN is a combination of actual variations in the physical parameters, and apparent variations, depending on the viewing angle. The dominant factor in the classification of an AGN in this scheme is the orientation of the system. The classification therefore becomes a function (mostly) of the viewing angle. If the AGN is observed face-on (i.e. the line of sight is parallel to the axis of the torus), then the central accretion disk and the BLR are un-obscured and the broad lines can be detected, this is known as a Seyfert I or QSO for the radio quiet galaxies, and a BL Lac or OVV for the radio loud galaxies. If the system is viewed more edge-on, then the central part is not visible and the broad lines are not detected, this is know as a Seyfert II galaxies for the radio quiet, and a FR I or FR II for the radio loud galaxies (see Fig. 1.6 and Table 1.1).

There are two main differences between Seyfert I and II:

1) broad emission lines are observed for Seyfert I but not for Seyfert II and

2) the AGN continuum to the stellar continuum ratio of Seyfert IIs are about a magnitude weaker than of Seyfert Is.

The most obvious candidate causing these differences is dust obscuration. However, this hypothesis does have problems:

i) The featureless continua observed for Seyfert IIs look like power laws, although a reddened power law should no longer look like a power law (Peterson, 1997).

ii) The continua of Seyfert IIs are only about one magnitude weaker than for Seyfert Is, which is surprising as the broad lines are completely extinguished.

To explain the still visible continua for Seyfert IIs, Osterbrock (1978) introduced an additional component, the ‘scattering medium’. This medium lies above the hole in the torus and scatters the continuum emission, such that it is visible to the observer viewing the system edge-on. The suggestion of an additional scattering medium, is supported by the very weak and broad emission lines in the linear polarisation spectrum observed for the nearby Seyfert II galaxy NCG1068 (e.g. Antonucci & Miller, 1985). Scattering or reflection of the AGN continuum emission by either dust or electrons can result in a polarised spectrum. The continuum for NCG1068 is polarised by 16% and the polarised spectrum is wavelength independent far into the UV (∼1500˚A) (Code et al., 1993), which suggests that electrons are the scattering particles. The narrow emission lines are only polarised by 1%, indicating that they are observed directly and not after scattering (Vernet et al., 2001).

The simplicity of the unification model appeals to our belief that any description of nature should be as simple as possible, if there is no evidence for the contrary, also known as Occam’s razor. However, it is still unclear what the fundamental parameters of the unification model are. Intrinsic luminosity and orientation provide good starting points for describing the systems, but there might be other important factors such as the morphology and gas to dust content, which influence the appearance of the system. Other model such as an unification model by evolution has been suggested. Nevertheless, the unification model has become a popular interpretation scheme of the different types of AGN.