Tuesday, 11 December 2018

What Are Fermi Bubbles?


     
        In 2010, data gathered by the Fermi Gamma-Ray   Space   Telescope revealed a new discovery. Scientists were surprised to find two enormous, bubble-like clouds that extend 50,000 light-years across the center of our galaxy, the Milky Way.
   
    The  two  gamma-ray-emitting  bubbles stretch across more than half of the visible sky and may be millions of years old. (Gamma rays are electromagnetic radiation at  the highest-energy,  or  shortest- wavelength, end of the electromagnetic spectrum.) The origin of these previously unseen structures, however, remains a truly baffling mystery.  
       
   A research paper appearing in the Astrophysical Journal in 2014 described some features of the aptly dubbed "Fermi bubbles."  First,  the  outlines  of  the structures are very sharp and well defined, and the bubbles glow evenly across their  enormous surfaces. The most distant areas of the bubbles feature extremely high- energy gamma rays,  yet there is no apparent cause for them that far from the  galactic center. Lastly, the parts of the  Fermi bubbles nearest the nucleus of the  Milky Way contain both gamma rays and  microwaves, but as the bubbles extend  farther out, only the gamma rays are  detectable.
      
       Theorists   have   offered   several explanations for the unusual structures. The two most predominant theories both suggest the bubbles were formed by a large, rapid energy release.  
    
        One possibility claims that enormous streams or jets of accelerated particles originating  and  blasting  out  of  the supermassive black hole at the center of the Milky Way created the Fermi bubbles. Astronomers  have  observed  such  a phenomenon in other galaxies, and while it is unknown if the Milky Way black hole has an active jet today, it may have had one millions of years ago.

       Another commonly held theory argues that the Fermi bubbles were created during star formations over a period of millions or even billions of years. The gas ejections created from bursts of star formations, similar to the ones that produced huge star clusters in the Milky Way, theoretically rode massive galactic winds out to far-off distances and are held there by powerful magnetic forces.
 
       Scientists are eager to unravel the mystery of the Fermi bubbles' origin. "Whatever the energy source behind these huge bubbles may be," says David N. Spergel, a theoretical astrophysicist at Princeton University, "it is connected to the many deep questions in astrophysics."
   
    

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