If I understand correctly, black holes are the centres of galaxies, so what emits light in the centre of galaxies that we see from far away?
In: Physics
As no other answer seems to have covered the actual centre of many galaxies I’ll have a go at it.
The centre of a galaxy is also called a “galactic nucleus” these can have, as you stated, a black hole.
In some instances this very massive black hole, also called a supermassive black hole, turns the “nucleus” into a AGN, an active galactic nucleus.
The most powerful of such AGNs are called “Quasars” and they are literally the most luminous things in the entire observable universe. In fact they can be so luminous that a single quasar is thousands of times brighter than entire galaxies such as our Milky Way.
The black hole in the centre accumulates particles, gas, around it in a so called “accretion disk”. As these particles fall closer to the black hole the temperature of the accretion disk rises due to compression and friction.
As the temperature rises so does the emission of electromagnetic radiation, hence luminosity.
If you think about it, nuclear fusion, the process by which our Sun produces light, converts around 0.67% mass into energy. Now we all agree that the Sun is pretty bright right? The process of accretion however is throught to convert tens of percent up to and even over 40% of mass into energy.
So there you have it, some galaxies have a supermassive black hole in the centre that has a hot glowing accretion disk, which is brighter than the entire galaxy that it is contained in.
Also a small bonus, these AGNs can have “jets” of electromagnetic radiation (like gamma rays) that are directed at us, we call such AGNs “Blazars”. These things, also technically quasars, can be even brighter than a regular quasar.
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