A 2 Myr-old Cosmic Burp

There’s some news today about the supermassive black hole in the centre of our Galaxy that some time ago gobbled up so much gas at once that it produced a huge cosmic burp we can still see today. The first evidence of this ancient event was seen about ten years ago in all-sky map obtained in the very high energy light called gamma rays by the Fermi space telescope. These data revealed the presence of symmetrical lobes, extending well above and below the Galactic Centre by about 25,000 lightyears each. (For context, the Galaxy’s disk is about 100,000 lightyears wide – I’ve attached below an illustration of the Fermi lobes superposed over a model of the Galaxy to demonstrate the huge scales involved here.) Such lobes could have been produced by a very energetic event in the distant past when a jet of speedy electrons from the central black hole, also known as Sgr A*, were shot out of the Galaxy with such force they collided with the ambient light above the Galaxy’s disk, kicking it to higher energy itself. Another possibility, however, was that a sudden burst of star formation occurred at or near the central region of our Galaxy, producing a similar effect. In a Nature paper published two days ago, a small team have taken new data of these lobes observed in X-ray light by another space telescope called eRosita, and used them to constrain a very comprehensive simulation of a black hole-driven jet into the gas surrounding the Galaxy.

With their model, the team can reproduce the observed lobes in both gamma rays and X-rays very well, if the outburst event had the whopping luminosity of 6 x 10^44 erg/s, or about 200 billion times the luminosity produced by our Sun. This luminosity estimate, however, depends a lot on assumptions of the gas density in the Galactic Centre and could be lower by a factor of about 10 or more. Nevertheless, the model shows the jet was produced about 2.4 million years ago and lasted about 100,000 years. Fortunately, this incredible event, probably one of several periods of intense activity in the Galactic Centre over the past 10 billion years, didn’t seem to affect life here on Earth, though it would be fascinating to see if there were any traces of it in geological data. (For context, 2.4 million years ago was the time when humanity’s direct evolutionary ancestors, Homo habilis, walked the plains of south and central Africa.) Back to the black hole scenario, the model seems to definitively account for differences in the extents of the gamma- and X-ray observations, which point to their source being the black hole and not that earlier starburst scenario. The question remains, however, what did Sgr A* feast on all those eons ago? Today, Sgr A* is relatively quiescent, with relatively flickers of light seen at infrared and radio wavelengths caused by small changes in the amounts of nearby gas it is currently consuming. Perhaps a passing large cloud of gas from the region was drawn in toward the black hole, producing the relatively short-term activity as Sgr A* was fed by all that mass at once. Amazing stuff! Here’s a link to the Nature paper for those who’d like to learn more.

Credit: NASA Goddard