Astronomers at the University of Rome, Tor Vergata, have concluded that the Milky Way had an active core several billion years ago. The radiation emitted by the galaxy could kill all multicellular life that existed several tens of thousands of light-years from the center. This is reported in a preprint published in the arXiv.org repository.
Sagittarius A is a radio source located in the center of the Milky Way, presumably consisting of a supermassive black hole, fragments of a supernova and ionized gas and dust clouds. According to scientists, Sagittarius A was an active galactic nucleus (AGN) about eight billion years ago, emitting X-rays and deep ultraviolet light for tens or hundreds of millions of years.
Scientists have assessed the impact of AGNs on potentially habitable planets in our galaxy. Deep ultraviolet rays should "rip off" the atmosphere on objects that are close enough to the center of the Milky Way. Even if the radiation was greatly attenuated by the gas and dust surrounding the galactic core, then all terrestrial planets that were closer than a thousand parsecs (3.2 thousand light years) to Sagittarius A completely lost their gas envelopes by the end of the core activity phase.
According to the findings of the researchers, the radiation emanating from the center of the Milky Way was lethal for complex multicellular organisms that lived on planets 2-8 thousand parsecs away from Sagittarius A. Unicellular organisms could withstand this radiation, but died at 0.25-0.81 kiloparsecs from the galactic nucleus. These values were calculated for an AGN luminosity of 10-100 percent of the critical.
The critical (Eddington) luminosity is called the maximum power of electromagnetic radiation at which the forces of gravity and the radiation pressure are balanced. When this limit is exceeded, a strong stellar wind occurs when the object's matter begins to flow into interstellar space.