Credit: EHT Collaboration
The Milky Way's Magnetic Field, From the Center Out
At the heart of the Milky Way, 27,000 light-years from earth in the dead center of our Galaxy, lies a four-million solar mass black hole known as Sgr A*. The black hole itself is invisible, but we see the clear effects of its gravity on stars that twirl and dance around it. In 2022 images from the Event Horizon Telescope revealed, for the first time ever, an image of the shadow of the black hole on the gravitationally-distorted light from glowing gas near it.
The EHT however did not just image the glowing gas Sgr A*; it also measured the polarization of the radio emission from it. Polarization of an electromagnetic wave like light, or the radio waves observed by EHT, is a measure of the tilt of the plane of oscillation of the electomagnetic wave. Because the radio waves from the gas near the black hole are produced by electrons spiraling around the magnetic field which threads through the gas, the polarization of the radio waves indirectly measures the direction of the magnetic field around the black hole.
The image above left shows the false-color image of the hot, glowing gas around Sgr A* as observed by the Event Horizon Telescope in orange, with the black hole's shadow at the center. Superimposed on this image are lines which represent the polarization of radio waves from this glowing gas seen by EHT, providing the orientation of the twisted magnetic field around the supermassive black hole.
Zooming out, the image shows the magnetic field from a much larger region of the Galactic center, determined by polarization measurements taken by the SOFIA infrared observatory. It's thought that the direction of the magnetic field near the center of the Galaxy may play a key role in restricting the amount of material that falls into Sgr A*, which might explain why the black hole is not as active as the rapidly-feeding supermassive black holes at the centers of quasars, where light from the galaxy is dominated by the emission of the actively feeding black hole at the quasar's center. On an even larger scale, the image above shows the lines of magnetic force threading the entire Milky Way, as seen by ESA's Planck infrared space telescope.
Published: April 1, 2024
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Page Author: Dr. Michael F. Corcoran
Last modified Monday, 08-Apr-2024 11:52:40 EDT