What is the main reason why astronomers think that quasars are black holes?
It is postulated that quasars are supermassive black holes that are sources of radiation like x-rays.
Quasars or quasi-stellar radio sources are the most energetic and distant members of a class of objects called active galactic nuclei (AGN). Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared to be similar to stars, rather than extended sources similar to galaxies. Their spectra contain very broad emission lines, unlike any known from stars, hence the name "quasi-stellar." Their luminosity can be 100 times greater than that of the Milky Way. Most quasars were formed approximately 12 billion years ago, and they are normally caused by collisions of galaxies, with the galaxies' central black holes merging to form either a supermassive black hole or a binary black hole system.
Although the true nature of these objects was controversial until the early 1980s, there is now a scientific consensus that a quasar is a compact region in the center of a massive galaxy surrounding a central supermassive black hole Its size is 10–10,000 times the Schwarzschild radius of the enclosed black hole. The energy emitted by a quasar derives from mass falling onto the accretion disc around the black hole.
Quasars show a very high redshift, which is an effect of the metric expansion of space between the quasar and the Earth. When the observed redshift of quasars is interpreted in terms of Hubble's law, it is inferred that quasars are very distant objects. Quasars inhabit the very center of active, young galaxies, and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way, which contains 200–400 billion stars. This radiation is emitted across the electromagnetic spectrum, almost uniformly, from X-rays to the far-infrared with a peak in the ultraviolet-optical bands, with some quasars also being strong sources of radio emission and of gamma-rays.