If 2 galaxies are close to one another perhaps in a galaxy cluster, they are able to merge with each other to make one big galaxy, causing the super massive black holes from each galaxy to interact as well[i]. When galaxies merge together, there are 2 phenomena that could happen to the super massive black holes. Firstly, the two super massive black holes could merge together to make a bigger super massive black hole. However, if the super massive black holes are spinning in the same direction, then one super massive black hole will remain in the centre of the newly formed galaxy and the other will be thrown out of the galaxy[ii].
A diagram of how super massive black holes merge [iii]
Once a galaxy is formed, we know that there is very little star formation in the centre of the galaxy. This is obtained from data of stellar populations, element abundances and the structural properties of the galaxy. This may be due to streams of energy known as jets which get released during AGN. These jets are believed to be able to control star formation in the galaxy; they do this by blowing out all the gas via the high energy produced by the jets[iv]. The situation with energy loss gets worse with in galaxy clusters due to the fact that there is a lower pressure in the atmosphere and the energy seems to be lost much easier. On the other hand, the jets are able to heat up the surrounding gas so that the stars can form a lot faster. However, some evidence shows that the jets produced have absolutely no effect on the star formation rate. Moreover, when there is no AGN there are no jets produced which means that the gas in the galaxy decreases in temperature, which could halt star formation. This is known as the cooling flow problem. The cooling flow problem has also seemed to cause the galaxy to lose mass over time; this is because the mass left from a dying star seems to form into a black hole much easier. Fortunately, if the jets fail to heat up the gas, then the cooling initiates an emergency AGN which heats up the gas, Surrounding supernovas are also able to aid the heating process. Unfortunately, in most circumstances the heat produced may not be enough to heat the gas or the energy produces too much heat which causes the gas to be thrown out of the galaxy, seizing the star formation process completely. This in balance of energy is very common within small galaxies. However, observations from large galaxies show a perfect amount of energy, which means that the gas becomes heated but it does not get thrown out of the galaxy.
A young galaxy contains only hydrogen and helium, and it is poor in heavier elements such as oxygen or lithium. Astrophysicists often consider any elements heavier than helium to be called metals. This young galaxy has a dark matter halo surrounding it which consistently gets bigger and bigger absorbing more surrounding gas as it does so. However, galaxies that have been formed from two parent galaxies merging inherit their content of hydrogen, helium and the high amount of metals, these are called silver spoon galaxies. Due to recent observations the silver spoon galaxies have no dark matter halo to provide more force to rotate the galaxy, this means that the galaxy spins slowly. Moreover, on some occasions the two merging galaxies with small halos make one massive galaxy with a large halo. Most galaxies spin at the same rate at all parts of the galaxy but occasionally the parent galaxy spins at different rates at different points in the galaxy, this indicates our lack of understanding of dark matter and gravity.
When a star runs out of hydrogen it begins to collapse in on itself, the collapse is so intense that the star explodes out into the universe, this is known as a supernova. The death of stars causes the material to be released in to its own galaxy, the metals from the supernova absorb a vast amount of light meaning that the richer the galaxy is in metals the dimmer it is. This is because the heavy elements absorb photons and release infrared radiation which we cannot see with our own eyes. The part of the galaxy that is the hottest is known as the intergalactic medium, this is because these are areas where most of the material that is used to make the galaxy is, this is known as baryonic material[v]. Baryonic material is made up of baryons, like protons and neutrons; this material is used to make gas, planets, comets, stars, neutron stars, and black holes[vi].
When a galaxy is near to the end of its life cycle, all of the star formation gas has been used up and most of it has been transferred into heavy metals like iron via stars collapsing. Dead galaxies have a high mass-to-light ratio due to the collapsed stars which have been turned into white dwarfs and as they cooled they turned into black dwarfs. White dwarfs are very dense and have a surface temperature similar to the temperature that was at the core of the star that collapsed. Black dwarfs are rarer than white dwarfs, because it takes billions of years for the core to cool to the same temperature as the universe, even 13.8 billion years after the big bang there has been no recorded black dwarfs. As a galaxy begins to die it starts to get dimmer and dimmer because the only objects that keeps the galaxy illuminated at this point are long lasting stars[vii]. A galaxy can die through a process called strangulation this is where the galaxy does not have a sufficient amount of hydrogen and helium to continue producing stars. This can be caused by overcrowding clusters of galaxies; this means that surrounding galaxies will attract the nearby gas which leaves the other galaxy without a supply of gas which is used for star formation. This indicates that the gas in a galaxy can not only be removed by its super massive black hole, but can be removed by external forces[viii].
The lifetime of a super massive black hole is 1071 seconds or 1063 years[ix]. Super massive black holes can evaporate via the Hawking process; this is where the black hole emits Hawking radiation. At the event horizon particles and antiparticles are produced, most of the time the particles and antiparticles annihilate when they come into contact with each other but sometimes the particle will be attracted to the black hole and the antiparticle will repel from the black hole, this is because antiparticles are attracted by anti-gravity and not gravity. This occurs because the black hole needs to give off a little bit of mass so that it obeys the first law of thermodynamics which states that energy cannot be created or destroyed, so since a black hole is constantly giving off energy in the form of electromagnetic radiation therefore, it needs to sacrifice some mass. This causes the black hole to lose mass if it is not absorbing matter which is the situation in a dead galaxy as there is no gas or stars for the black hole to feed on. A smaller black hole will radiate its mass a lot faster than a larger black hole. Eventually when all the mass of the black hole has been lost a large gamma ray burst is emitted in the form of jets like we see in AGN[x].
The shapes of galaxies are determined by the way the galaxy forms. Most galaxies are either spiral galaxies like our own galaxy the Milky Way, or they can be elliptical galaxies where most of the stars are concentrated in the centre of the galaxy. However, there are some galaxies that have an irregular shape this is because the galaxy does not have enough gravitational force to make a regular shape[xi]. This is due to the fact that the galaxy is young and it does not yet have a super massive black hole to provide the gravitational force that makes a regular shaped galaxy[xii].
Each galaxy has a different colour; this can be due to many factors. Firstly, different elements will give off different frequencies of light; a different frequency causes a different colour. A young galaxy will have a high amount of hydrogen, but an older galaxy will be a lot richer in metals, this means that the older galaxy will have more variations of colour because it has more types of elements. Dust can affect the colour of a galaxy because high frequency light like blue light can be easily scattered by the dust. On the other hand, low frequency light like red light does not get scattered as easily, so the more dust a galaxy has the redder it will appear[xiii].
The universe is made up of 5% matter, 27% of dark matter and 68% of dark energy. Dark matter does not interact with light or electromagnetic forces, this indicates that dark matter does not absorb, reflect or emit light. Dark matter was first discovered by Swiss Astronomer Fritz Zwicky, he calculated that galaxies are rotating too fast and they should not be able to keep all the stars in the galaxy. This means that the dark matter is preventing the stars from being removed from the galaxy. The principle of dark energy was first developed by Albert Einstein however; he called it the cosmological constant. This showed us that a vacuum just like in space can contain energy. The cosmological constant keeps the universe static and it prevents it from collapsing in on itself. Moreover, American Astronomer Edwin Hubble theorised that the universe is expanding from the red-shift principle. However, what surprised astronomers was the evidence that the expansion of the universe was accelerating. From this evidence we know that there must be a form of energy which continues to the make the expansion of the universe faster and faster[xiv].
[i] https://public.nrao.edu/radioastronomy/galaxy-evolution Accessed on 19/09/2016 (National Radio Astronomy Observatory)
[ii] http://www.universetoday.com/13002/what-happens-when-supermassive-black-holes-collide/ Accessed on 02/09/2016 (Universe Today)
[viii] http://www.bbc.co.uk/news/science-environment-32734978 Accessed on 22/11/2016 (BBC News)
[ix] http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html Accessed on 23/11/2016 (University Of California)
[x] http://www.universetoday.com/40856/hawking-radiation/ Accessed on 23/11/2016 (Universe Today)
[xii] http://coolcosmos.ipac.caltech.edu/ask/219-What-is-an-irregular-galaxy- Accessed on 23/11/2016 (CoolCosmos)
[xiii] http://curious.astro.cornell.edu/physics/98-the-universe/galaxies/observing-galaxies/552-what-do-a-galaxy-s-colors-mean-are-they-its-true-colors-intermediate Accessed on 24/11/2016 (Ask An Astronomer)