Why are there black holes at the center of some galaxies?

[NeutrinoWave]

You might find this interesting

BBC News - Supermassive black hole will 'eat' gas cloud

"Researchers have spotted a giant gas cloud spiralling into the supermassive black hole at our galaxy's centre."

[jimmyjamjoejoe]

This is a little off the current topic of the thread, and forgive my ignorance, but this occurred to me the other day - If these black holes sit at the dense centers of galaxies, consuming huge amounts of mass, surely their mass would constantly increase? Or is that mass converted to energy and ejected etc?

[JulianO]

This is a little off the current topic of the thread, and forgive my ignorance, but this occurred to me the other day - If these black holes sit at the dense centers of galaxies, consuming huge amounts of mass, surely their mass would constantly increase? Or is that mass converted to energy and ejected etc?

There mass is constantly increasing, although they can lose mass/energy too by Hawking radiation, that isn't very significant for large black holes.

However black holes like the one in the centre of our galaxy eat very little. They are very quiet - especially compared to others. Just snacking on stuff now and then. Others consume vast quantities of fuel and shine extremely brightly producing vast jets of output. There is a limit to how fast even these active black holes can consume matter though. Above a certain limit they sort of choke and can't take in stuff at a faster rate.