Science + Technology

Astronomers Report Unprecedented Double Helix Nebula Near Center of the Milky Way


Astronomers report an unprecedented elongateddouble helix nebula near the center of our Milky Way galaxy, using observationsfrom NASA's Spitzer Space Telescope. The part of the nebula the astronomersobserved stretches 80 light years in length. The research is published March 16in the journal Nature.

"We see twointertwining strands wrapped around each other as in a DNA molecule," said MarkMorris, a UCLA professor of physics and astronomy, and lead author. "Nobody hasever seen anything like that before in the cosmic realm. Most nebulae areeither spiral galaxies full of stars or formless amorphous conglomerations ofdust and gas — space weather. What we see indicates a high degree of order."

The double helixnebula is approximately 300 light years from the enormous black hole at thecenter of the Milky Way. (The Earth is more than 25,000 light years from theblack hole at the galactic center.)

The Spitzer Space Telescope, an infrared telescope,is imaging the sky at unprecedented sensitivity and resolution; Spitzer's sensitivity and spatial resolutionwere required to see the double helix nebula clearly.

"We know thegalactic center has a strong magnetic field that is highly ordered and that themagnetic field lines are oriented perpendicular to the plane of the galaxy,"Morris said. "If you take these magnetic field lines and twist them at theirbase, that sends what is called a torsional wave upthe magnetic field lines.

"You can regardthese magnetic field lines as akin to a taut rubber band," Morris added. "Ifyou twist one end, the twist will travel up the rubber band."

Offering another analogy, he said the wave islike what you see if you take a long loose rope attached at its far end, throwa loop, and watch the loop travel down the rope.

"That's what is being sent down the magneticfield lines of our galaxy," Morris said. "We see this twisting torsional wave propagating out. We don't see it movebecause it takes 100,000 years to move from where we think it was launched towhere we now see it, but it's moving fast — about 1,000 kilometers per second —because the magnetic field is so strong at the galactic center — about 1,000times stronger than where we are in the galaxy's suburbs."

A strong, large-scale magnetic field canaffect the galactic orbits of molecular clouds by exerting a drag on them. Itcan inhibit star formation, and can guide a wind of cosmic rays away from thecentral region; understanding this strong magnetic field is important forunderstanding quasars and violent phenomena in a galactic nucleus. Morris willcontinue to probe the magnetic field at the galactic center in future research.

This magnetic field is strong enough to causeactivity that does not occur elsewhere in the galaxy; the magnetic energy nearthe galactic center is capable of altering the activity of our galactic nucleusand by analogy the nuclei of many galaxies, including quasars, which are amongthe most luminous objects in the universe. All galaxies that have awell-concentrated galactic center may also have a strong magnetic field attheir center, Morris said, but so far, ours is the only galaxy where the viewis good enough to study it.

Morris has argued for many years that themagnetic field at the galactic center is extremely strong; the research publishedin Nature strongly supports that view.

The magnetic field at the galactic center,though 1,000 times weaker than the magnetic field on the sun, occupies such alarge volume that it has vastly more energy than the magnetic field on the sun.It has the energy equivalent of 1,000 supernovae.

What launches the wave, twisting the magneticfield lines near the center of the Milky Way? Morris thinks the answer is notthe monstrous black hole at the galactic center, at least not directly.

Orbiting the black hole like the rings ofSaturn, several light years away, is a massive disk of gas called the circumnuclear disk; Morris hypothesizes that the magneticfield lines are anchored in this disk. The disk orbits the black holeapproximately once every 10,000 years.

"Once every 10,000 years is exactly what weneed to explain the twisting of the magnetic field lines that we see in thedouble helix nebula," Morris said.

Co-authors on the Nature paper are Keven Uchida, a former UCLA graduate student and former memberof Cornell University's Center for Radiophysics and Space Research; and Tuan Do, a UCLAastronomy graduate student. Morris and his UCLA colleagues study the galacticcenter at all wavelengths.

NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the Spitzer SpaceTelescope mission for the agency's Science Mission Directorate. Scienceoperations are conducted at the Spitzer Science Centerat the California Institute of Technology. JPL is a division of Caltech. NASAfunded the research.



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