Posted on Oct 13, 2017
“How important is it, really, for us to be able to see clear across to the other side of our own galaxy?” asks Tom Bania, a radio astronomer at Boston University. “It is the most important thing in all of astrophysics. It took humankind thousands of years to map the Earth accurately; a map of the galaxy will constrain about a dozen or so models of the structure and evolution of the Milky Way. To me, perhaps the ‘Holy Grail’ of astronomy is to provide a clear perspective of our relationship to the physical universe. The map of our galaxy is a part of that, and that map is still incomplete.”
Our cosmic home, the Milky Way Galaxy is a vast, star-filled spiral in excess of 100,000 light-years wide, with our solar system drifting between two spiral arms at its outer edges, some 27,000 light-years from its center. Beyond that, like Earth's terra incognita of the ancient sea-faring mariners, no space craft has ever traveled beyond its opaque central disk the Milky Way to turn back and take its picture.
“Optically, it’s like trying to look through a velvet cloth—black as black can be,” says Thomas Dame, Director of the Radio Telescope Data Center at the Harvard-Smithsonian Center for Astrophysics and Senior Radio Astronomer at the Smithsonian Astrophysical Observatory. “In terms of tracing and understanding the spiral structure, essentially half of the Milky Way is terra incognita.”
For astronomers trying to map it, suggests Scientific American, "the effort is a bit like learning the anatomy of a human body from the perspective of a single skin cell somewhere on a forearm. How many spiral arms does the Milky Way have, and how do those spiral arms branch and curl around the galaxy? How many stars does the Milky Way really contain? How much does it weigh? What does our cosmic home actually look like, viewed from another nearby galaxy?"
Using the National Science Foundation's Very Long Baseline Array (VLBA), an interlinked system of 10 radio telescopes stretching across Hawaii, North America and the Caribbean, an international team of astronomers have directly measured the distance to an object called G007.47+00.05, a star-forming region located on the opposite side of the galaxy from our solar system by looking for radio emissions from molecular gas clouds and massive, young stars, both of which typically reside in spiral arms. The measurement showed the region to be some 66,000 light-years away—nearly 40,000 light-years beyond the galactic center, and roughly double the distance of the previous record-holding direct measurement of distance in the Milky Way.
The VLBA’s measurement is “equivalent to seeing a baseball on the surface of the moon,” says lead author Alberto Sanna, a postdoctoral researcher at the Max Planck Institute for Radio Astronomy in Germany. The feat, Sanna says, shows “we can measure the whole extent of our galaxy, to accurately number and map the Milky Way’s spiral arms and know their true shapes, so that we can learn what the Milky Way really looks like.”
Distance measurements are crucial for understanding the structure of the Milky Way. Most of our Galaxy's material, consisting principally of stars, gas, and dust, lies within a flattened disk, in which our Solar System is embedded. Because we can't see our Galaxy face-on, its structure, including the shape of its spiral arms, can only be mapped by measuring distances to objects elsewhere in the Galaxy.
The astronomers used a technique called trigonometric parallax, first used in 1838 to measure the distance to a star. This technique measures the apparent shift in the sky position of a celestial object as seen from opposite sides of the Earth's orbit around the Sun. This effect can be demonstrated by holding a finger in front of one's nose and alternately closing each eye—the finger appears to jump from side to side.
Measuring the angle of an object's apparent shift in position this way allows astronomers to use simple trigonometry to directly calculate the distance to that object. The smaller the angle, the greater the distance.
The VLBA, a continent-wide radio telescope system with ten dish antennas distributed across North America, Hawaii, and the Caribbean, can measure the minuscule angles associated with great distances. In this case, the measurement was roughly equal to the angular size of a baseball on the Moon.
The new VLBA observations, made in 2014 and 2015, measured a distance of more than 66,000 light-years to a star-forming region called G007.47+00.05 on the opposite side of the Milky Way from the Sun, well past the Galaxy's center, some 27,000 light-years distant. The previous record for a parallax measurement was about 36,000 light-years.
"Most of the stars and gas in our Galaxy are within this newly-measured distance from the Sun. With the VLBA, we now have the capability to measure enough distances to accurately trace the Galaxy's spiral arms and learn their true shapes," Sanna said.
The VLBA observations measured the distance to a region where new stars are being formed. Such regions include areas where molecules of water and methanol act as natural amplifiers of radio signals—masers, the radio-wave equivalent of lasers for light waves. This effect makes the radio signals bright and readily observable with radio telescopes.
"The Milky Way has hundreds of such star-forming regions that include masers, so we have plenty of 'mileposts' to use for our mapping project, but this one is special. We're looking all the way through the Milky Way, past its center, way out into the other side," said the MPIfR's Karl Menten.
The astronomers' goal is to finally reveal what our own Galaxy looks like if we could leave it, travel outward perhaps a million light-years, and view it face-on, rather than along the plane of its disk. This task will require many more observations and much painstaking work, but, the scientists say, the tools for the job now are in hand. How long will it take?
"Within the next 10 years, we should have a fairly complete picture," Mark Reid of the Harvard-Smithsonian Center for Astrophysics (CFA) predicted.
Sanna, Menten, and Reid worked with Dame of the CfA and Andreas Brunthaler of MPIfR. The team reported their findings in the 13 October issue of the journal Science.
The Daily Galaxy via National Radio Astronomy Observatory and Scientific American