From Sky & Telescope: “Magnetism Rules in the Milky Way’s Core”

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From Sky & Telescope

June 4, 2020
Govert Schilling

A composite image of the central region of our Milky Way galaxy, known as Sagittarius A. SOFIA found that magnetic fields, shown as streamlines, are strong enough to control the material moving around the black hole, even in the presence of enormous gravitational forces.
NASA / SOFIA / L. Proudfit / ESA / Herschel / Hubble Space Telescope

What governs the dynamics of gas close to the center of our Milky Way galaxy? Gravity is the standard answer. After all, there’s a 4 million-solar-mass black hole hiding there.

But new data from NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) reveal that strong magnetic fields may actually dominate, just like they do in the Sun’s corona. The new result may shed light on two outstanding questions about the galactic center.


Into the Galactic Center

SOFIA is a Boeing 747SP turned high-flying observatory. Its instrument, HAWC+, a far-infrared imaging polarimeter, studied the galactic center during flights in May 2017 and July 2018.

NASA SOFIA High-resolution Airborne Wideband Camera-Plus HAWC+ Camera

Measurements of the far-infrared light’s polarization revealed the orientation of dust particles, which rotate to align perpendicular to magnetic field lines.

Charles Dowell (NASA / JPL) led a team in studying the central 15 light-years of the galactic center, deducing an average magnetic field strength of 0.005 gauss. That’s about 100 times weaker than the average magnetic field strength on Earth’s surface. But since the density in the galactic center is so low, only some 10,000 atoms per cubic centimeter, the gas’s magnetic pressure is much higher than its thermal pressure.

As a result, gravity may not be the dominant force that determines the motions of the gas. Instead, “the magnetic field may govern the kinematics and channel the plasma, just like magnetic fields dominate the physics of the solar corona,” says team member Joan Schmelz (Universities Space Research Association), who presented the preliminary results Tuesday at the virtual meeting of the American Astronomical Society.

“The data provide the most detailed look yet at the magnetic fields surrounding our galaxy’s central black hole,” says team member David Chuss (Villanova University).

Explaining Strange Black Hole Behavior

Andrew Fox (Space Telescope Science Institute), who was not involved in the study, says the results are not really surprising. “The galactic center is a very energetic region full of highly ionized plasmas,” he says, “so it makes sense that magnetic fields would dominate other sources of pressure.”

But according to Schmelz, astrophysicists generally tend to avoid including the effects of magnetic fields because they complicate the picture. “We’re now confronted with data that are so compelling that we just can’t ignore magnetic fields anymore,” she says.

If magnetic fields govern gas motions rather than just gravity, this may explain two surprising facts about the core of our Milky Way galaxy: the low birth rate of new stars (despite the presence of huge amounts of gas) and the weak activity of the galaxy’s central black hole. Strong magnetic fields could both suppress star formation and prevent matter from falling into the black hole.

Meanwhile, Schmelz cautions that calculating magnetic field strength from polarization data is not straightforward. “Our next step is to check if our standard techniques do apply in this turbulent environment,” she says. “So far, the observation of Zeeman effects in the Milky Way center [the splitting of spectral lines in the presence of strong magnetic fields] compares favorably with our results.”

See the full article here .


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Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”