A team of astronomers has obtained the first direct measurement of a young galaxy’s magnetic field, and were surprised at their findings. Using the Robert C. Byrd Green Bank Telescope, the world’s largest fully steerable radio telescope, the researchers were able to peer back in time, to gauge the nascent galaxy’s magnetic field as it appeared 6.5 billion years ago. Surprisingly, the magnetic field of this distant “protogalaxy” is at least 10 times greater than the average value in the Milky Way. “This was a complete surprise,” said Arthur Wolfe, a professor of physics at UC San Diego’s Center for Astrophysics and Space Sciences who headed the team. “The magnetic field we measured is at least an order of magnitude larger than the average value of the magnetic field detected in our own galaxy.” So what does this mean for the “dynamo effect” theory of galactic magnetic fields?
Astronomers have believed the magnetic fields within our own Milky Way and other nearby galaxies—which control the rate of star formation and the dynamics of interstellar gas–arose from a slow “dynamo effect.” In this process, slowly rotating galaxies are thought to have generated magnetic fields that grew very gradually as they evolved over 5 billion to 10 billion years to their current levels.
Until recently, astronomers knew very little about magnetic fields outside our own galaxy, having directly measured the magnetic field in only one nearby galaxy.
But in July of this year, a team of Swiss and American astronomers reported that an indirect measurement of the magnetic fields of 20 distant galaxies, using the bright light from quasars, suggests that the magnetic fields of young galaxies were as strong when the universe was only a third of its current age as they are in the mature galaxies today.
And now, this most recent direct detection of a galactic magnetic field seems to cast doubt on the dynamo effect as well. Astronomers from the University of California campuses at Berkeley probed a young protogalaxy DLA-3C286, located in a region of the northern sky that is directly overhead during the spring.
However, Wolfe said those indirect measurements and his team’s latest direct measurement of a distant galaxy’s magnetic field “do not necessarily cast doubt on the leading theory of magnetic field generation, the mean-field-dynamo model, which predicts that the magnetic field strengths should be much weaker in galaxies in the cosmological past.”
“Our results present a challenge to the dynamo model, but they do not rule it out,” he added. “Rather the strong field that we detect is in gas with little if no star formation, and an interesting implication is that the presence of the magnetic fields is an important reason why star formation is very weak in these types of protogalaxies.”
Wolfe said his team has two other plausible explanations for what they observed. “We speculate that either we are seeing a field toward the central regions of a massive galaxy, since magnetic fields are known to be larger towards the centers of nearby galaxies. It is also possible that the field we detect has been amplified by a shock wave generated by the collision between two galaxies.”
“In either case,” he added, “our detection indicates that magnetic fields may be important factors in the evolution of galaxies, and in particular may be responsible for the low star formation rates detected throughout the gaseous progenitors of young galaxies in the early universe.”
“The challenge now,” said J. Xavier Prochaska, another member of the team who is a professor of astronomy at UC Santa Cruz, “is to perform observations like these on galaxies throughout the universe.”
The giant radio telescope, can be pointed with an accuracy of one arcsecond–equivalent to the width of a single human hair seen six feet away–enabled the astronomers to measure the magnetic field of a single galaxy.
Source: UC San Diego