Supermassive Black Holes and Active Galactic Nuclei

This post explores supermassive black holes (SMBHs). It will discuss the tools that astronomers use to find these objects before exploring their link with active galactic nuclei (AGN). It will be discovered that SMBHs provide the energy source that produces AGN. The phenomena we observe from these objects are described by a unified model based primarily on the angle at which they are observed. Finally, the mode by which SMBHs are believed to have formed is discussed.

Supermassive Black Holes and Active Galactic Nuclei
Supermassive Black Holes and Active Galactic Nuclei (Credit: ESA/NASA/AVO/Paolo Padovani)

Finding Supermassive Black Holes

Black holes (BHs) only possess three properties that can be measured (Freedman and Kaufmann). These properties are: a small electric charge, angular moment and mass. We can only detect the effects of BH’s mass from a distance. More precisely, the dynamics of stars and gas provide clues to the presence of BHs.

A review by Kormedy and Richstone (1995) provides an account of the search for SMBHs in galactic nuclei. They state that the search for SMBHs is based on studying stellar and gas dynamics and eliminating other massive dark objects (MDOs) as candidates. MDOs, other than BHs, include clusters of low-mass stars, brown dwarfs or stellar remnants and halo dark matter. Therefore, the search for SMBHs must exclude these objects as a possible mechanism for the observations. With current survey resolutions, this is problematic, but there is strong circumstantial evidence for SMBHs in galactic centres.

To determine the mass of an MDO, we can use Newton’s form of Kepler’s third law if we can observe an orbiting object. As the mass of the central object is much greater than the object, we can assume that the central mass is essentially the same as the combined mass. We are left with the mass of the MDO being proportional to the radius of orbit and velocity and inversely proportional to the gravitational constant:

M= rv2/G

This equation allows the determination of the central mass, knowing only the orbit radius of an orbiting body and its velocity. Objects useful for this purpose are stars and gas clouds in Keplerian orbits. The study of gas velocities is more problematic as their velocity is not only governed by gravitational forces. As such, stellar dynamics provide stronger evidence of MDOs.

Rapid rotation in a galactic nucleus indicates large masses (Kormedy and Richstone 1995). This has been found in M31, our best candidate for an SMBH. Based on rotation, velocity dispersion, and mass-to-light ratio data, the nucleus of M31 contains an MDO with a mass of 3 x 107 Mʘ (Mʘ = solar masses).

The motions of stars in the core of the Milky Way show that our galaxy contains an SMBH (Ghez, et al, 1998). Further studies by the group headed by Ghez identified a star orbiting an object known as Sagittarius A*. This star, known as SO-16, was found to be travelling more than 12,000kms-1. Using Newton’s form of Kepler’s third law, the mass of the central object is 3.7 x 106Mʘ. The star’s orbit brought it to within 45AU of the central object. These data imply that the central object is an SMBH.

Later studies have added further evidence that Sagittarius A* is an SMBH. Using Very Long Baseline Interferometry (VLBI) data, Shen et al. (2005) showed that the mass of this object is contained within about 1AU.

Away from the SMBH, gas clouds with velocities between 1000 and 5000kms-1 are found, leading to the broadening of emission lines (COSweb). If these clouds are in Keplerian rotation around the MDO, their velocities can be used to determine the mass of the central object (Ho and Kormendy, 2002). This broad line region (BLR) extends from 0.01 to 1pc from the central body.

The BLR spectrum can be used to estimate the BLR size and the central mass in a technique known as reverberation mapping (Peterson and Horne, 2004), which uses high-resolution spectroscopy. This technique does not rely on resolving objects, so it can be used for targets at greater distances but not as far as quasars (Freedman and Kaufmann).

Another tool for determining the mass of an MDO is the Eddington Limit. The balance between the in-falling material and the outgoing radiation pressure restricts the luminosity of any accreting compact body. Using this relationship, the minimum mass of the black hole can be determined based on its luminosity.

Radio masers are useful tools for identifying SMBH due to their high angular resolution (Ho and Kormendy, 2002). When a disk is viewed edge-on, this technique provides a resolution greater than the Hubble Space Telescope. This technique provides accurate mass determinations and details about the nature of the gas rotation around the central body.

Supermassive Black Holes and Active Galaxy Nuclei

Active galaxies have a small, very luminous core that outshines the combined light of all the stars in the galaxy (NASAweb). These galaxies include quasars, Seyfert galaxies, radio galaxies and blazars. The problem for astronomers was determining the process of producing the range of phenomena observed (See Table 1). Salpeter (1964) proposed that quasars were powered by the accretion of material onto SMBHs.

TABLE

Object  Host Galaxy TypeRadio Emission strength Emission Lines in Spectrum 
Blazar Elliptical Strong None 
Radio-load quasar Elliptical Strong Broad 
Radio galaxy Elliptical Strong Narrow 
Radio-quiet quasar Spiral or Elliptical Weak Broad 
Seyfert 1 Spiral Weak Broad 
Seyfert 2 Spiral Weak Narrow 

Table 1: The phenomena observed from AGN types and their characteristics.

Active Galaxy Nuclei (AGN)

Quasars are found at great distances from Earth, most beyond a redshift of 0.3. The closest one to us is found at 800 million light years. It is thought that many SMBHs have exhausted the available material for accretion and are now dormant. Seyfert galaxies fill the void between the present time and when quasars were more common. The brightest Seyfert galaxies are as bright as the least luminous quasars.

Before discussing a model for AGN, it is worth describing the structure of these objects (ref diagram 1). Gravity is the source of energy driving AGN. Computer simulations have shown that an accretion disk forms, and with friction, this material emits radiation at various frequencies depending on temperature. In-falling material stops at a sharp boundary due to the conservation of momentum. As pressure builds, not all the material is accreted to the SMBH. Instead material is ejected at right angles to the accretion disk to form opposing jets. This material is ejected within a magnetic field twisted by the plasma in the accretion disk. Observations have found that a dusty ring (torus) forms in these systems. At a distance of about 1000 light-years, low-density gas clouds are present. With lower velocities, these clouds produce narrow emission lines. This is the narrow line zone (NLZ, COSweb).

The structure and features of an AGN and the angle at which it is observed determine how we see these objects. This is the basis of what is known as the unified model of AGN.

Active Galaxy Nuclei Classification

If the AGN is viewed from about 90o from the accretion disk plane, the object is seen straight down one of the jets. At this angle, we see a featureless spectrum produced by synchrotron radiation. This is the situation for a blazer.

At a more oblique angle, the accretion disk, with its intense thermal radiation, becomes visible. The NLZ also becomes visible. Due to the Doppler Effect, the broadening of spectral lines becomes visible. The synchrotron radiation from the jets remains visible. In this situation, we see a radio-load quasar.

As the angle gets closer to the plane of the accretion disk, the torus obscures it. The visible light comes from the hot gas flowing from the accretion disk, giving rise to an emission line spectrum. There is no spectral line broadening. Viewed from this angle, we see radio galaxies.

Seyfert galaxies have weak radio emissions, but the division between the two types still depends on the angle from which they are viewed. If the BLR is visible, we see broad spectral lines. This would be the situation for Seyfert-1 galaxies. The BLR is hidden behind the torus at more acute angles, leading to narrow emission lines and producing a Seyfert-2 galaxy.

Supermassive Black Holes and Active Galactic Nuclei diagram

Figure 1: Illustration of how the unified model can be used to explain the phenomena from various AGN types (adapted from CALweb)

Formation of Supermassive Black Holes

The first formed stars in the Universe may have led to the formation of SMBHs (Volonteri, 2010). The low metallicity population III stars are mainly considered due to the early Universe’s lack of coolants. If the stars held onto most of their mass before death, intermediate-sized black holes (BHs) may have resulted from stars sized from 25 to 140Mʘ. It is thought that these BHs may have formed clusters and combined to form SMBHs in a short period. However, many uncertainties surround the nature of the population III stars.

Volonteri continues theories of SMBH formation by reviewing current ideas on forming an SMBH directly from a single gas cloud. The inner core of proto-galaxies provided a good location for this to occur. The gas collapsed to a single point due to the low metallicity in the early Universe. This prevented the cloud from fragmenting and forming several smaller objects. This model depends on a mechanism for the dispersion of angular moment that would prevent the formation of a dense central object. Simulations have shown that a bar structure or turbulent motion may have been the mechanism. In the later case simulations have shown that as much as 90% of the angular moment can be shed from the central region.

Once a 104 to 106Mʘ compact region has formed, the efficiency and speed of mass accumulation will determine if a massive star or BH is formed. The correct conditions needed to be present to prevent the formation of a massive star rather than a BH, and this is problematic. Mayer et al. (2010) state that the correct conditions are produced for a BH to form directly from gas clouds during galaxy mergers. The newly formed BHs grow further by the accretion of material from an accretion disk.

If the accretion of matter in the single gas cloud scenario is not quick enough, earlier-formed population III stars completing their cycle may increase the metallicity of the gas cloud. This would increase the cooling efficiency of the gas, leading to the formation of low-mass stars (Volonteri). SMBHs may have formed due to collisions between these stars and the stellar remnants of the population III stars.

SMBHs may have resulted from the accretion of material onto BHs formed at the end of the lives of the population III stars (Alvarez, Wise and Abel, 2008). Modelling has shown that a BH may form from these stars if their mass is less than 140Mʘ or greater than 260Mʘ. Alvarez et al. state that at z=6, a stellar-mass BH with an original mass of 100Mʘ could have grown to 109Mʘ. This conclusion makes this a viable mechanism for SMBH formation at distances greater than about z>6.

It is also possible that SMBHs formed due to density fluctuations in the early universe. Primordial BHs could have formed if gravity had overcome the pressures built up as the material collapsed.

Conclusion

Astronomers have come a long way since discovering Supermassive Black Holes and Active Galactic Nuclei. Using different techniques, we are confident that the powerhouse of these objects is SMBHs in the cores of these galaxies. Work continues, and further evidence is required before we can unequivocally state that our understanding is correct. However, there is strong circumstantial evidence that our understanding is correct.

If you want to learn about SMBH’s smaller cousins, you should read our post on stellar-mass black holes.

Robert Findlay

Recent Posts