Does every galaxy have a black hole at the center? No, not every galaxy hosts a black hole, but the truth is that the vast majority of them actually do.
This statement may be confusing. Why galaxies are supposed to have black holes inside and how these objects are linked to the entire galaxies? And what are the exceptions to this rule? Do the back holes pull on the galactic matter under the event horizon, or just destroy it?
First of all, according to the scientists, most galaxies contain a supermassive black hole at their centers. Distinction between ordinary and supermassive black holes (SMBH in short) is extremely important – the latter are very, very large, on the order of hundreds to billions of solar masses.
Supermassive black holes have some more interesting properties. They have relatively low average density, and the tidal forces – near the event horizon, are relatively weak. SMBH probably form from smaller black holes residing at the centers of galaxies and with time they slowly grow due to the process of accretion of matter from the galaxy itself, or/and by merging with the other small black holes.
Once a supermassive black hole is formed in the galaxy, it starts to interact with its internal and external environment. Some of the ordinary galaxies are not ordinary anymore and they begin a new life of so-called active galaxies, also known as AGN or Active Galactic Nuclei. This class consists of the galaxies with the dominant radio emission originating from the nuclear activity. The optical spectra of these galaxies are not the simple superpositions of the spectra of particular stars in the galaxy, but also show characteristic emission lines due to the high-energy processes taking place in their centers.
Such an object very often has linear, elongated, double structure extending far outside the optical galaxy - so-called radio lobes situated more or less symmetrically on the both sides of the host galaxy, which is, in most cases, elliptical galaxy containing a black hole of mass from ~ 10^6 to 10^9 solar masses. This elongated structure originates from twin jets out flowing in opposite directions from the Active Galactic Nucleus – the region surrounding supermassive black hole, and the radio emission of these lobes results from the electrons ejected at nearly the speed of light through a long jet from the core of the galaxy and deposited in radio lobes. The electrons are trapped by the magnetic field stretching around the galaxy and produce radio waves that we can observe using radio telescopes.
Sometimes the elongated, double radio structure is not observed. It may be due to the weak activity of the galaxy (and, so, expiration of the jet production near the SMBH), or to the geometrical effects. The most common explanation is dealing with such spatial orientation of the entire object that the observer on Earth can only see the galaxy along the axis of the emission of jets, so the double lobes overlap with each other and are indistinguishable from the emission of entire active galactic center.
The activity of galaxies results from the processes of accretion of a matter on the black hole with magnetic field and angular momentum. This activity evidently cannot be infinite - it can be stopped after some time. When the activity ends, the lobes are no longer powered with the inflow of relativistic particles, so as a result, the luminosity of the radio structure tends to cease.
Although the vast majority of galaxies have black holes in their centers, only the AGNs are characterized by a high rate of the accretion of gaseous matter situated close to the central SMBH. Accreted matter forms so-called accretion discs where the rotational energy of the SMBH is transformed into plasma's internal energy. The accelerated material is then slung around to one of the hole poles and expelled as a jet of plasma moving with the velocity near the speed of light and radiating in the wide range of the electromagnetic spectrum in the presence of the magnetic field. That implies that the high fraction of the plasma can be accelerated in the direction defined by the spin of the AGN. But the resulting jets (and, so, double radio structures) are formed only in some fraction of all observed AGNs, probably with a black hole of very strong magnetic field and high spin, and this high spin may result of the previous merging of two SMBH.
So scientists assume that a large black hole exists at the heart of every large galaxy, but not always this galaxy is active due to the factors mentioned above. In fact, most of these black holes are dormant and only a few per cent are active in the sense of drawing material from host galaxy inwards and forming jets and radio structures. Does every type of galaxy host such black hole? Probably yes, talking about most of typical, normal sized galaxies, like elliptical and spiral ones. Our own galaxy, Milky Way, has its own SMBH of four million solar masses which is located about 27,000 light years from Earth, in the direction of the constellation Saggitarius (The Archer). Our galaxy is inactive and this SMBH lives now in quiescence, although this may be temporary. There are some scientific arguments for the theory of restarting activity, and in fact, we are observing some examples of galaxies with restarting jets and radio structures. So Milky Way's black hole may awake and become active in the future.
And what about the exceptions to the rule? For decades astronomers believed that only large galaxies hold black holes in their centers. It is questioned by the recent discoveries of about 100 small, so-called dwarf galaxies with characteristics indicating that they also harbor massive black holes. That suggest the black holes may be even more common than scientists previously thought. What's more, it is even more interesting that little galaxies seem to hold the clues to the origin of the first seeds of supermassive black holes that had occurred in the early universe. Of course there may still exist dwarf galaxies without an internal black hole, but there is no certainty that they cannot form them. There is also a high probability that some of so-called irregular galaxies do not harbor these hungry monsters.
Author: Elzbieta Kuligowska
References & further reading:
Dwarf galaxies give clues to origin of supermassive black holes
Black Holes: By the Numbers
Supermassive black hole (Wikipedia)
Black Holes, Galaxy Formation, and the M_BH-sigma Relation
The relationship between black holes and galaxies