Supermassive black holes in the centers of the most distant galaxies.
A quasar is a supermassive black hole, with a mass of millions to tens of billions of times the mass of the Sun, that is surrounded by an enormous accretion disk. As gas spirals into the black hole, huge amounts of energy are released across the entire electromagnetic spectrum. This leads to quasars being the most luminous objects. The most powerful ones have luminosities thousands of times greater than their own host galaxy, thus outshining it. This makes these galactic cores the farthest observable objects.
Two relatively nearby quasars with visible host galaxies. The quasars are the bright centers in each galaxy. The left galaxy displays large spiral arms, while the right one appears more fuzzy. Credit:HST
Illustration of a quasar with its accretion disk and a powerful jet coming off its center region. These quasar jets can inject huge amounts of energy into their surroundings and strongly influence the evolution of their host galaxy, but they need to rapidly spin to develop strong jets. However, not all rapidly spinning black holes have jets.