Scientists Peek Across the Galaxy to the “Dark Side” of the Milky Way

The far side of the Moon is often called the "dark side." It's not because it never receives light but because from Earth, humans never see it. Similarly, our home galaxy, the Milky Way, has a "dark side" that has remained an enigma until now, reports Lee Billings for Scientific American.

If you gaze up at night in areas with low light pollution, you can often see the bright band of the Milky Way smeared across the sky. But only a portion of the galaxy is visible. Through measurements and observations scientists have discerned that the Milky Way is a spiral galaxy and that our solar system is embraced by two major arms that extend from the galaxy's center. But we have yet to send a space probe or telescope far enough away to take a portrait of our corner of the universe. Without that perspective, the opposing side of the galaxy remains obscured from our view by the dust-filled galactic center.

"Optically, it’s like trying to look through a velvet cloth—black as black can be," Thomas Dame, an astronomer at Harvard–Smithsonian Center for Astrophysics (CfA) tells Scientific American. "In terms of tracing and understanding the spiral structure, essentially half of the Milky Way is terra incognita."

And that unknown means that some basic questions about the galaxy— how many stars it contains, for instance — remain unanswered. "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," says Tom Bania, an astronomer at Boston University. "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.”

In a new study, Dame and colleagues are starting to fill in the gaps that loom large in the map of the Milky Way.

The team used a system of ten identical antennae that span locations from Hawaii to the Virgin Islands to Washington State, a setup called the Very Long Baseline Array (VLBA). With such a massive distance covered, the array can "see" using radio waves, which are much larger than those in visible light. That helped the team peer beyond the galactic center.

The VLBA was able to measure the distance to a region where new stars are born, where clouds of water and methanol amplify radio signals, writes Deborah Byrd for EarthSky.org.

The researchers used an old method called trigonometric parallax, which calculates distance by tracking a single celestial object's apparent shift in position as seen from Earth's orbit in two separate places, months apart. An easy way to imagine how this works is to hold a finger close to your face and alternate closing one eye then the other. The finger appears to move. Measuring the angle of the shift allows astronomers to calculate the distance of the object with trigonometry, explains a press release from the Max Plank Society.

The VLBA gave the researchers the ability to measure the very small parallax motion seen across more than 66,000 light-years to the star-forming region on the galaxy's far side.

“It really is excellent work—I believe this is the smallest parallax ever obtained, and it is certainly a milestone in modern observational astronomy,” Mareki Honma, an astronomer at the National Astronomical Observatory of Japan who was not involved in the new work, tells Scientific American.

The researchers published their results in Science. It is just part of an ongoing effort by many groups and observatories to properly map the entire Milky Way. Eventually, researchers hope to be able to quantify the distribution of luminous and dark matter across the galaxy, the VLBA's website notes. With such an unprecedented view, scientists hope they will be able to answer longstanding questions about the galaxy's birth and evolution, including perhaps, how we came to be.

Maris Fessenden | | READ MORE

Maris Fessenden is a freelance science writer and artist who appreciates small things and wide open spaces.