14 DAY TRIAL // As opposed to other spiral galaxies of comparable size and mass, the Milky Way has a relatively tame core, which does not display too many signs of activity. But a new study found that things weren't always so quiet in the region surrounding our supermassive black hole.
Ghostly gamma-ray beams detected by the NASA Fermi Space Telescope suggest that the galactic core was once just as active as any we see in other galaxies today. The reasons why activity was subdued are still a matter of debate in the astronomical community.
In other galaxies, active supermassive black holes consume a lot of material from their accretion disks, producing bright jets at radio wavelengths that spread out opposite each other, as evidenced by the image to the left.
What Fermi discovered were signs of similar radio jets at the “poles” of the Milky Way, extending up to 27,000 light-years in both directions. For comparison, the galaxy is some 100,000 light-years across.
These jets also contain signs of gamma rays, as well as photons at other energy levels, and are the only structures of this kind located close enough to Earth for Fermi to see. Ghostly jets are far more difficult to identify than active ones.
“These faint jets are a ghost or after-image of what existed a million years ago. They strengthen the case for an active galactic nucleus in the Milky Way's relatively recent past,” expert Meng Su says.
The investigator, who holds an appointment as an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), was the lead author of a new paper detailing the findings. The work appears in the latest issue of the esteemed Astrophysical Journal.
One of the most interesting aspects of the ghostly gamma-ray emissions is that they are angled by around 15 degrees in respect to the galactic plane. This tilt may be a reflection of the fact that the accretion disk surrounding the supermassive black hole at the galactic core has a similar inclination.
“The central accretion disk can warp as it spirals in toward the black hole, under the influence of the black hole's spin,” argues CfA astrophysicist and paper coauthor, Douglas Finkbeiner.
“The magnetic field embedded in the disk therefore accelerates the jet material along the spin axis of the black hole, which may not be aligned with the Milky Way,” the expert goes on to say.
The research team is quick to point out that the gamma-ray radiation and the radio plumes are two different structures. While the former are produced by the black hole, the latter were formed by plasma being ejected from the galactic core.
“These jets probably flickered on and off as the supermassive black hole alternately gulped and sipped material,” Finkbeiner concludes.