Seeing the Far Side of the Milky Way

The detection of a star-forming region 66,500 light-years from Earth, on the other side of our galaxy’s center, lends weight to the existence of an extended arm of the Milky Way.

Location of water maser on far side of Milky Way

This artist’s view of the Milky Way maps the location of the Sun on one side and the star-forming region (water maser source G007.47+00.05) on the opposite side in the Scutum-Centaurus spiral arm.
(c) Bill Saxton / NRAO / AUI / NSF / Robert Hurt / NASA

It’s tempting to look at the Andromeda galaxy, our nearest galactic sibling, as if we’re looking in a mirror. After all, it too is a spiral galaxy, it’s just that it has two spiral arms while the Milky Way has four (and a central bar). No wait, infrared observations showed the Milky Way actually has only two spiral arms. Wait again: Radio observations reveal that all four arms really do exist.

Characterizing the Milky Way’s basic structure has proven a difficult task because, unlike in the case of Andromeda, we can’t just look at it. For one, dust lanes block our visible-light view, so astronomers resort to longer wavelengths to pierce the veils of dust and gas. For another, we sit amongst the clouds and stars that whirl around the galactic center, a position that challenges our ability to discern their distances.

Now Alberto Sanna (Max Planck Institute for Radio Astronomy, Germany) and colleagues have published a test-of-concept in the October 13th Science that uses a purely geometric method to map out the farthest reaches of the Milky Way.

Mapping the Milky Way

Most distances in the galaxies are determined via objects’ motions. Gas clouds and stars alike revolve around the galaxy’s center, so if we measure their spectra, we’ll capture spectral lines that have shifted blueward or redward, depending on whether the object is moving toward or away from us. But this technique has its share of problems. Assuming circular orbits isn’t valid near the galaxy’s center, where stars’ orbits change due to the presence of the central bar. Another problem is that the distance to an object that’s circling the galactic center inside the Sun’s own orbit will be ambiguous. A star’s measured velocity could correspond to two possible distances, one nearer and one that’s farther away.

So when Thomas Dame and Patrick Thaddeus (both at the Harvard-Smithsonian Center for Astrophysics) discovered a segment of spiral arm on the far side of the Milky Way, dubbed the Outer Scutum-Centaurus Arm, they wanted confirmation. Dame and Thaddeus had observed the radio-wave sky in 2011, pinpointing 115-gigahertz emission from star-forming clouds some 68,000 light-years away on the far side of the galactic center, but those distances came by way of measuring the clouds’ velocities.

Later observations of HII regions, cavities carved out by the ultraviolet radiation from newborn stars, also indicated that stars were forming in the outer fringes of the Milky Way. Their distances agreed with the first measurements, but again, they were derived from the source’s velocities.

Now, a single new detection is lending more weight to the outer arm’s existence than dozens of previous detections. That’s because, rather than measuring velocity, Sanna and colleagues determined the source’s distance based on parallax, a purely geometric and extremely precise measure of distance.

The source is a water maser, a pocket of water molecules that has absorbed ultraviolet radiation from newborn stars and is now pumping out microwaves at 22.2 gigahertz. The international team of astronomers used the Very Long Baseline Array (VLBA) to monitor the water maser’s exact location on the sky relative to background sources. That’s equivalent to monitoring the location of a finger held in front of your face. Relative to background objects, your finger will appear to move based on your viewing angle (e.g., left eye or right eye). The farther your finger, the less it will shift as you change your viewing angle. (In the case of the VLBA, viewing angle changes throughout the year as Earth orbits the Sun.)

Parallax determines water maser's distance

The astronomers determined the water maser's distance by measuring the angle of the object's apparent shift in position, as seen from opposite sides of Earth's orbit around the Sun. The technique is known as trigonometric parallax.
(c) Bill Saxton / NRAO / AUI / NSF / Robert Hurt / NASA

The observations yield a parallax angle of 0.049 milli-arcseconds (that’s 0.000049 arcseconds!), which corresponds to a distance of 66,500 light-years. That’s one incredibly precise and unambiguous measurement, and it serves as a proof of concept. Over the coming years, the VLBA will continue to map parallaxes and on-the-sky motions of distant sources such as water masers on a quest to map out the entire Milky Way’s structure.

“Within the next 10 years, we should have a fairly complete picture,” predicts coauthor Mark Reid (Harvard-Smithsonian Center for Astrophysics).

Surprises in Store?

Those future results may contain surprises, as previous studies have already hinted.

One study, by Michael Feast (University of Cape Town, South Africa), used an entirely different method to chart the far side of the Milky Way: Cepheid variable stars. Henrietta Leavitt discovered in 1908 that these stars pulsate in proportion to their luminosity, making them an ideal cosmic ruler. (They’re young stars so, like water masers and HII regions, they’re also associated with star-forming regions.)

Feast discovered Cepheids on the far side of the Milky Way — but they lie even farther away than the water maser, up to 72,000 light-years away from Earth.

Five Cepheids in Milky Way's flared disk

The five Cepheid variable stars discovered by Michael Feast and colleagues lie above and below our galaxy's plane as pictured here. The authors suggested they could be part the Outer Scutum-Centaurus (OSC) arm, which flares on the very outer edge of the disk, but with new measurements in hand, it appears the stars lie far beyond the OSC arm. The Sun is the yellow circle on the right side, surrounded by local Cepheid variables known at the time.
Robin Catchpole (University of Cambridge, UK)

Dame, the discoverer of the Outer Scutum-Centaurus (OSC) Arm and a coauthor on the parallax paper, notes that when the Cepheid results first came out, he thought they might mean his initial distance estimate to the OSC arm was too low. But the new parallax result is perfectly in line with his initial estimate, which puts the young Cepheid stars well beyond the OSC arm.

“So I don’t know what to say about the Feast Cepheids at this point,” Dame notes. “Perhaps they are in an arm even farther out than OSC, but there is not much other evidence for such an arm at this point.”

As the VLBA continues its painstaking survey of the Milky Way’s spiral structure, it will be interesting to see what else it finds.

Regardless, this new detection demonstrates that the technique is viable, albeit painstaking and time-consuming – map out the far side of the Milky Way (clear out ambiguity) over the next 10 years.

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