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This paper talks about Supermassive Black Hole and how it plays a part in Active Galactic Nuclei. It will go on to introduce the types of AGNs and observational techniques that were used to find SMBH, its findings and also future observations.
Active galaxy nuclei (aka AGN) are ultra-luminous galaxies that give out synchrotron radiation. This radiation is sourced by a black hole and becomes super luminous when it indulges in material such as hydrogen clouds, stars and even dust. There are a few types of AGNs, such as:
Quasar: ultra-luminous, extremely active high-redshift galaxy, they produce radio and optical emission lines. These galaxies were so bright in the horizon that they resembled star-like objects hence the name 'quasi-stellar radio source' (QSR) or 'quasi-stellar object' QSO (Ho 1999) Quasars play a very important role in the history of galaxy and star formation.
Seyfert galaxy: produce strong emission lines at wavelengths at far red infrared. There are two types of Seyfert galaxies: Type I and Type II. Type I Seyfert has small nuclei and produce narrow and broad emission lines. As for Type II Seyfert, it has large nuclei and produces narrow emission lines. (OpenWeb)
Radio galaxies: produces radio emission and jets. To add on, radio galaxies can be classified into BLR (Broad Line Radio) or NLR (Narrow Line Radio) galaxies. (OpenWeb)
BL Lac objects: Also known as Blazars, this galaxy is observed at >100MeV. It is detected in electromagnetic waves such as radio and gamma. (NasaWeb)
Black Hole model
The area in space that devours masses and prohibits light from escaping its region is called a black hole. Only a Supermassive Black Hole (SMBH) can be the driving force of AGNs. SMBH have an event horizon that is very small in radius and it is here that the escape speed is equivalent to the speed of light; e = mc2. If we look further, we will learn that the event horizon's radius is known as Schwarzschild radius and the escape velocity is calculated as (OpenWeb):
Rs = 2GM/c2
Diagram 1: AGN standard model (StrwWeb)
Located 0.01 - 1pc from the BH, the Broad Line Region (BLR) produces broad emission Doppler lines. The Narrow Line Region (NLR), located far from the disk region produces narrow lines and the gasses in this area move at a much slower pace. The obscuring Torus is a ring shaped cloud of dust and gas that prohibits the view of a portion of the BLR. If both the BLR and NLR areas are seen, it is considered a Type I AGN. However, if we are only able to view the NLR then it is considered Type II AGN. (CaltechWeb)
By changing the direction of the accretion disk we get to have a different look of the galaxy. If we are looking at the jets at a 90o angle, it's a Blazar. If we were to look into the BLR and NLR regions, it is then a Type I Seyfert or Quasars. Interestingly enough if we adjust our view towards the NLR, it is then a Type II Seyfert. Finally, if we look into regions that are obscured by the torus, we then view Radio galaxies (SAOWeb)
Diagram 2: Angle of AGNs via the Unified Model (OpenModWeb)
Since we can't view SMBH directly, the only way for us to observe is via its' accretion disk. The factor of it being inactive or active should be considered.
Stellar kinematics: used to study most galaxies as stellar activities are determined by their gravitational force. This method detects the gravitational regions around the black hole which lies in absorption lines of the stars LOSVD (line-of-sight velocity distribution) - referred to as Gaussians. [(Baes 2008) and (Ho(1999)] It also allows the calculation of M/L ratio, e black holes Keplerian orbit and mass. However, it takes a lot of time and modelling to actually study the kinematics.
Water masers: radio spectroscopy is used to study water masers located in NGC 4258. (Ho 1999) Very Large Base Interferometry (VLBI) has detected emissions at 22 GHz and a mass precision of 0.5.
Gas dynamics: ionised gases have certain amount of interstellar dust that affect the study of the rotation curve and mass of the black hole. [Note: Dust particles absorb and scatter HÎ± emission] (Baes, 2008)
STIS has shown insights on the motion of gas that controls the region around the SMBH, given that it has a resolution of ~0.1. (Bender & Saglia 2007) As mentioned above, by observing gas kinematics, the mass of the black hole, bulge luminosity and stellar velocity distribution can be concluded. (Walsh et el, 2008)
Reverberation mapping: an indirect method is used to study the size and mass of BLR's in AGN and quasars. (Baes 2008) Spectroscopy is used to determine the emission spectrum and locate objets located at far off places(AstrophWeb). Virial theorem below is used to determine the total mass (Nelson et el, 2004):
MBH = Î·*(v2rBLR/G)
Data provided by Ho & Kormendy (2000) have shown that by using reverberation mapping they have concluded that the heart of Seyfert galaxies are influenced by black hole masses of MBH~10^7 - 10^8 MSun and quasar engines have shown figures of MBH ~ 10^8 - 10^9MSun. As quasars are also found in galaxies that are more massive, this supports the relationship between MBh - MElliptical.
Iron KÎ± emission: indirect method (Ho 1999) that gives out 6.44 KeV, broad iron KÎ± line. It is well known to be a feature of Seyfert 1 galaxies at high redshifts (Baes 2008) and caused by X-ray emissions given out by the SMBH's accretion disk. Unfortunately, this method does not assist in determining the black holes mass.
Milky Way: Stellar kinematics show the mass of the BH is ~ 2 X 106 MSun. By using VLBI, X-ray emissions have been studied from Sgr A*(DailyWeb). Our galaxy is the closest candidate studied and has a size of 1AU (Shen et el,2005).. At present, Sgr A* is only consuming ~0.01% of stellar winds and is close to the brink of starvation. (SpaceBWeb)
M31 (Andromeda): This unique galaxy has three nucleui: P1, P2 and P3. The nucleus of M31 indicates a high velocity dispersion rotation. Red stars with the same composition infest P1 & P2 and are also a part of Andromeda's bulge. However, P3 is ruled by A-list stars formed around 200 Myrs ago. Absorption lines in this area are at 977 +/- 106 km/s and has a velocity of ~1700 km/s Bender & Saglia 2007)
Diagram 3: M31 and its nucleus. (Bender & Saglia 2007)
NGC 4258: located in the thin, gaseous annulus and is 0.14 - 0.18pc from the galactic centre. Studies have shown that is has a Keplerian rotation curve and mass of 3.9 X 107MSun. The distance of 7.2 +/- 0.5 Mpc tells us that it has a density, Ï>4Ã-109 MSun pc-3. (Baes, 2008)
In section 4(b), the study of masers has led us to conclude that NGC 4258 has a MDO 4 X 107 MSun and a rotation curve that is Keplerian. It also shows weak molecular gaseous and perpendicular disk-jets. It is extremely likely that a black hole is residing within the heart of NGC 4258. (Kormendy & Richstone, 1995)
M87 (aka NGC4486): As discussed in section 4(c), gas dynamics is used to study M87- located 60 mil ly in the Virgo Cluster (ChandraWeb). An MDO of ~ 3 X 109 MSun was discovered(Kormendy & Richstone 1995) and results show signs of Keplerian orbit, perpendicular disk-jets, emission lines of â‰ˆ1700km/s and disk shearing in its spiral structure.(Kormendy 1995)
Diagram 4: M87's Doppler shift measurement. (CSepWeb)
Sightings of Massive Dark Objects (MDO's)
An MDO can be anything ranging from bright stars, brown dwarfs, black holes and even dark matter halo. Before we can hypothesize that an MDO is a SMBH we will definitely need indirect/direct prove. One of it is by finally capturing an image of the event horizon or by indirect methods as below:
determining iron KÎ± lines, mentioned in 4(e),
studying matter that falls into the black hole's event horizon
understanding the Î›CDM model of the galaxy formation. (Ho 1999)
However, galaxies such as our Milky Way and NGC 4258 have shown precisely Keplerian curves with an extremely small capacity and thus it's safe to say that these galaxies contain SMBH. (Ho & Kormendy 2000)
Formation of SMBH
Looking into the beginning of the Universe, baryonic clouds masses were at 105 - 106 (Silk & Rees 1997) and led to the birth of high-redshift quasars in the darkest regions. By understanding quasars we understand the formation of the earliest galaxies.
There is a distinct relationship between SMBH and stars and gas at the centre of the galaxy's bulge and that is SMBH are only found in galaxies that have bulges (i.e: Sgr A*) or bulges alone. The MBH - Mbulge relationship is that MBH is ~0.2% Mbulge (Kormendy, 1995), related to high redshifts. However there is a variation of (1 + z)3/2 (Wyithe & Padmanabhan 2006) Referring to Bender & Saglia (2007) it mentions that the galaxy and black hole stops growing when the ratio reaches 0.2%
The most common theory is that Pop III stars collapse to produce black holes. (Ohkubo et el. 2009) Over a period of 106~7 year, the Eddington accretion transforms the black hole to the size of 105âˆ¼6MSun. (Wang et el, 2005) or that black holes first existed in a mass of 0.2% (KormendyWeb)
The favoured theory is the Î›CDM model where Dark Matter halo is said to govern the formation of galaxies and quasars via hierarchical assembly - objects collapse and then merge. Referring to Li et el (2007), it is mentioned that SFR â‰ˆ 103 M_Sun/yr happens at z â‰¥ 10. This shows us that before SMBH are formed and experience strong quasar activity, they first have to form bulges.
Diagram 5: Relationship between black hole mass and bulge (DomondWeb)
Future observation and discussion
We hope to look at future observations to help enhance the findings of SMBH. One such development is the enhancement of Near-Infrared (NIR) spectrographs. This is because NIR is least affected by dust and therefore enhances the search for these monsters. Another enhancement is by using GRAVITY: Adaptive Optics (AO) equipment that was designed for the VLT interferometer, used to study stellar kinematics. (Bender & Saglia 2007) To add on, future observations mention X-ray Multi-Mirror Mission (XMM) to use X-ray spectroscopy in the study of SMBH mass. (Ho & Kormendy 2000) We look forward to one unveiling the face of the black holes event horizon. Observational techniques and methods are just a stepping stone.