This might come as a surprise, but every portrayal ever made of how the Milky Way galaxy looks from afar is more imagined than factual. That's because we sit squarely inside our galaxy's turgid disk, which together with its bloated central bulge make it impossible to see most of what lies on the side opposite the Sun. We're only getting half the picture.
Oh, astronomers have certainly tried to piece together the true shape of its spiral. "For years, people created maps of the whole galaxy based on studying just one section of it, or using only one method," says Robert Benjamin (University of Wisconsin). "Unfortunately, when the models from various groups were compared, they didn't always agree."
A few years ago, Benjamin and others used NASA's Spitzer Space Telescope to deduce that our galaxy really has just two main spiral arms, not four as had been thought. Called the Scutum-Centaurus and Perseus arms, these appear to connect up nicely with the ends of the galaxy's central bar. (Our Sun lies along a minor offshoot, about halfway from the center to the outer edge, known as the Orion Spur.)
But there's been little hard evidence to prove that the galaxy's majestically sweeping arms continue around to its far side — until now. As Thomas Dame (Harvard-Smithsonian Center for Astrophysics) described this week at a meeting of the American Astronomical Society, he and CfA colleague Patrick Thaddeus have identified a distant arm on the disk's outer margin, on the far side, that's roughly 50,000 light-years from its center.
They detected the arm's presence not with some enormous globe-spanning array of radio telescopes, but instead with a very modest 4-foot (1.2-m) dish atop one of the CfA's buildings in Cambridge, Massachusetts. The telescope was tuned to the 115-gigahertz emission of carbon monoxide (CO) in giant molecular clouds, then pointed at hundreds of target fields near the galactic plane.
The telltale carbon-monoxide emission turned up in a string of 10 locations — hardly a comprehensive sampling, but enough to sketch in the distant arm's location and extent. One of the CO-rich clouds turns out to be about 150 light-years across and has a mass of at least 50,000 Suns.
The arm also shows up in prior radio surveys that mapped the abundance of neutral hydrogen throughout the disk. It largely escaped detection, Dame speculates, because it's both displaced from the galactic plane and tilted with respect to it. Also, Dame adds, "The main problem with hydrogen is that there is too much of it." In any case, the CO radio hits match strongest hydrogen concentrations along the distant arm, assuring that it's real.
The new find is an isolated segment that's roughly 60,000 light-years long. Dame and Thaddeus believe it marks the distant end of the Scutum-Centaurus arm, which would mean that the entire arc is more than 200,000 light-years long and that it wraps more than 300° around the galactic center.
The two researchers published their results in the May 10th issue of Astrophysical Journal Letters. To know for certain that the new arm is really an extension of the Scutum-Centaurus Arm, they hope to use their little radio dish to map it more completely in the years ahead.
Even if they don't or can't, it's reassuring to know that the Milky Way really does have the gracefully sweeping arms and beautiful symmetry that befits a grand spiral galaxy.