Free Astronomy Magazine March-April 2022

MARCH-APRIL 2022 T his schematic is based on multiwavelength observations of a suspected jet from the massive black hole at the center of our Milky Way galaxy. The wide view shows our galaxy edge-on, with two huge bubbles of plasma glowing in gamma-rays and X-rays. These are evidence for an explosive outburst from the black hole about 2 million years ago. Probing deep into the galaxy’s core (inset), astronomers using the Hubble Space Telescope have captured a glowing cloud of hydrogen near the black hole. The interpretation is that the cloud is being hit by a narrow, columnated jet of material that was blasted out of the black hole merely 2000 years ago. The black hole is still active, but on a smaller scale of energy out- put than previously known outbursts. When the jet slams into the hydrogen knot the outflow scatters into octopus-like tendrils that continue along a trajectory out of our galaxy. [NASA, ESA, Gerald Cecil (UNC-Chapel Hill), Dani Player (STScI)] Wagner’s conclusion: “Our central black hole clearly surged in luminos- ity at least 1 millionfold in the last million years. That sufficed for a jet to punch into the Galactic halo.” Previous observations by Hubble and other telescopes found evi- dence that the Milky Way’s black hole had an outburst about 2-4 mil- lion years ago. That was energetic enough to create an immense pair of bubbles towering above our galaxy that glow in gamma-rays. They were first discovered by NASA’s Fermi Gamma-ray Space Telescope in 2010 and are surrounded by X-ray bubbles that were discovered in 2003 by the ROSAT satellite and mapped fully in 2020 by the eROSITA satellite. Hubble ultravio- let-light spectra have been used to measure the expansion velocity and composition of the ballooning lobes. Hubble spectra later found that the burst was so powerful that it lit up a gaseous structure, called the Mag- ellanic stream, at about 200,000 light-years from the galactic center. Gas is glowing from that event even today. To get a better idea of what’s going on, Cecil looked at Hubble and radio images of another galaxy with a black hole outflow. Located 47 million light-years away, the ac- tive spiral galaxy NGC 1068 has a string of bubble features aligned along an outflow from the very ac- tive black hole at its center. Cecil found that the scales of the radio and X-ray structures emerging from both NGC 1068 and our Milky Way are very similar. “A bow shock bubble at the top of the NGC 1068 outflow coincides with the scale of the Fermi bubble start in the Milky Way. NGC 1068 may be showing us what the Milky Way was doing dur- ing its major power surge several million years ago.” The residual jet feature is close enough to the Milky Way’s black hole that it would be- come much more prominent only a few decades after the black hole powers up again. Cecil notes that “the black hole need only increase its luminosity by a hundredfold over that time to refill the jet channel with emitting particles. It would be cool to see how far the jet gets in that outburst. To reach into the Fermi gamma-ray bubbles would re- quire that the jet sustain for hun- dreds of thousands of years because those bubbles are each 50,000 light years across!” The anticipated im- ages of the black hole’s shadow made with the National Science Foundation’s Event Horizon Tele- scope may reveal where and how the jet is launched. !