Supermassive Black Holes and Active Galactic Nuclei
This essay briefly explores supermassive black holes (SMBHs). It will discuss the
tools that astronomers use to find these objects before their link with active galactic nuclei (AGN) is
explored. It will be discovered that SMBHs provide the source of energy that produce AGN. The phenomena that
we observe from these objects are described by a unified model based primarily upon the angle at which they
are observed. Finally the mode by which SMBHs are believed to have formed is discussed.
Finding Supermassive Black Holes
Black holes (BHs) only possess three properties that can be measured (Freedman and
Kaufmann, 2007 p 595-596). These properties are: a small electric charge, angular moment and mass. From a
distance it will only be the effects of BH’s mass that we are able to detect. More precisely it is the
dynamics of stars and gas that provides clues to the presence of BHs.
In a review by Kormedy and Richstone (1995) an account of the search for SMBHs in
galactic nuclei is given. They state that the search for SMBHs is based on the study of stellar and gas
dynamics and eliminating other massive dark objects (MDOs) as being candidates. MDOs, other than BHs, include
clusters of low-mass stars, brown dwarfs or stellar remnants and halo dark matter. The search for SMBHs
therefore must include the exclusion of 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
To determine the mass of a MDO we can
use Newton’s form of Kepler’s third law (Freedman and Kaufmann, 2007 p 618)
if we can observe an orbiting object. As the mass of the central object is much greater than the object we
can assume that 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
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 (Kormedy and Richstone, 1995). The study of gas velocities is more problematic as its
velocity is not only governed by gravitational forces. As such stellar dynamics provide stronger evidence of
Rapid rotation in a galactic nucleus is a good indicator of large masses (Kormedy and
Richstone, 1995). This has been found in M31 which is our best candidate for a SMBH. Based on rotation,
velocity dispersion and mass to light ratio data the nucleus of M31 contains a MDO with a mass of 3 x
The motions of stars in the core of the Milky Way show that our own galaxy contains a
SMBH (Ghez, et al, 1998). Further studies by the group headed by Ghez identified a star orbiting an object
known as Sagittarius A* (Freedman and Kaufmann, 2007 p 627). This star, known as SO-16, was found to be
travelling in excess of 12,000kms-1. Using Newton’s form of Kepler’s third law the mass of the
central object is 3.7 x 106M
ʘ. The star’s orbit brought it to within 45AU of the central object. The implication of
these data is that the central object is a SMBH.
Later studies have added
further evidence the Sagittarius A* is indeed a SMBH. Using Very Long Baseline Interferometry (VLBI) data Shen et al (2005) were able to show that the mass of
this object is contained within about 1AU.
Away from the SMBH clouds of gas are found with velocities of between 1000 and
5000kms-1 leading to 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 spectrum of the BLR can be used to estimate the BLR size and the central mass in a
technique know as reverberation mapping (Peterson and Horne, 2004) which uses high resolution spectroscopy.
This technique does not rely on resolving objects so can be used for targets at greater distances but not as
far as quasars are located (Freedman and Kaufmann, 2007 p 683).
Another tool to determine the mass of a MDO is the Eddington Limit (Freedman and
Kaufmann, 2007 p 682). The luminosity of any accreting compact body is restricted by the balance between the
in-falling material and the outgoing radiation pressure. Using this relationship the minimum mass of the
black hole can be determined based on its luminosity.
Radio masers provide a useful tool to identify SMBH due to their high angular
resolution that they provide (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 rotation of the gas around the central body.
Supermassive Black Holes and Active Galaxy Nuclei
Active galaxies are those that have a small very luminous core that outshines the
combined light of all the stars in the galaxy (NASAweb). Active galaxies are objects such as quasars, Seyfert
galaxies, radio galaxies and blazars. The problem for astronomers was determining the process producing the
range of phenomena observed (See table 1). Salpeter (1964) proposed that quasars were powered by the
accretion of material onto SMBHs.
they are found in
lines in spectrum
Table 1: The phenomena observed from AGN types and their characteristics (Modified
from Freedman and Kaufmann, 2007 p 680).
Quasars are found at great distances from Earth with most beyond a red-shift of 0.3
(Freedman and Kaufmann, 2007 p 671 - 678). The closest one to us is found at 800 million light years. It is
thought the 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). A summary of this is given in Freedman and Kaufmann (2007 p 683 - 685). 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 dependant on temperature. In-falling material
stops at a sharp boundary due to conservation of momentum. As pressure builds not all the material is
accreted onto 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 determines
how we see these objects. This is the basis of what is known as the unified model of AGN as described in
Freedman and Kaufmann (2007 p 685 - 687).
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 become visible with its intense thermal
radiation. The NLZ also becomes visible. Broadening of spectral lines becomes visible due to the Doppler
Effect. The synchrotron radiation from the jets still remains visible. In this situation we see a radio-load
As the angle gets closer to the plane of the accretion disk the torus obscures the
accretion disk. The visible light comes from the hot gas flowing from the accretion disk giving rise to
emission line spectrum. There is no spectral line broadening. Viewed from this angle we see radio
Seyfert galaxies have weak radio emissions but the division between the two types is
still dependant on the angle from which it is viewed. If the BLR is visible we see broad spectral lines. This
would be the situation for Seyfert-1 galaxies. At more acute angles the BLR is hidden behind the torus
leading to narrow emission lines producing a Seyfert-2 galaxy.
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 lead to the formation of SMBHs
(Volonteri, 2010). The low metallicity population III stars are thought to have been large due the lack of
coolants in the early Universe. 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 140M
ʘ. It is thought that these BHs may have formed clusters and combined to form SMBHs in a
short period of time. However, many uncertainties surround the nature of the population III
Volonteri (2010) continues theories of SMBHs formation by reviewing current ideas on
the formation of a SMBH directly from a single gas cloud. The inner core of proto-galaxies provided a good
location for this to occur. It is thought the gas collapsed to a single point due to the low metallicity in
the early Universe. This prevented the cloud from fragmenting to form a number of smaller objects. This model
is dependant 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
Once a 104 to 106M
ʘ compact region had 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,
2010). SMBHs may have formed due to collisions between these stars and the stellar remnants of the population
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 140M
ʘ or greater than 260M
ʘ. Alvarez et al state that at z=6 a stellar-mass BH with an original mass of
ʘ could have grown to 109M
ʘ. 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 (Volonteri, 2010). Primordial BHs could have formed if gravity overcame pressures that build as the
Astronomers have come a long way since the discovery of AGN. Through the use of
different techniques we are confident that the powerhouse of these objects is SMBHs in the cores of these
galaxies. Work is continuing and further evidence is required before we can unequivocally state that our
understanding is correct. However, there is strong circumstantial evidence that out understanding is
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