Have we found the Milky Way’s twin?

Our galactic home in the cosmos — the Milky Way — is merely one among the observable Universe’s two trillion.

From our own perspective on Earth, we’ve identified the presence of spiral arms.

By viewing the Milky Way in infrared wavelengths of light, we can see through the galactic dust and view the distribution of stars and star-forming regions behind them. As revealed by the 2 micron all-sky survey (2MASS), the densest collections of galactic dust can be seen tracing out our spiral arms.

However, being stuck within the Milky Way itself, we exclusively view it edge-on.

The European Space Agency’s space-based Gaia mission has mapped out the three-dimensional positions and locations of more than one billion stars in our Milky Way galaxy: the most of all-time. The ability to measure stellar parallax, or how the actual (rather than apparent) position of a star changes over the course of a calendar year, is greatly aided by superior instrumentation, large aperture size, Gaia’s location in space, and the development of photography and computerized identification of the relative shifting of stars.

Even our best spaceborne views leave much ambiguity in our galaxy’s overall structure.

The Whirlpool Galaxy (M51) appears pink along its spiral arms due to a large amount of star formation that’s occurring. In this particular case, a nearby galaxy gravitationally interacting with the Whirlpool galaxy is triggering this star formation, but all spirals rich in gas exhibit some level of new star birth.

We’re not a grand spiral galaxy, as we’re lacking extended outer arms.

This full-scale view of the Andromeda Galaxy, M31, showcases its star-forming regions lining its spiral arms, its dust lanes, and its central, gas-poor region. However, unlike the Milky Way, Andromeda lacks a prominent central bar.

Nor are we similar to Andromeda, our nearest large neighbor, which lacks a central bar.

The enormous bar at the core of galaxy NGC 1300 spans many tens of thousands of light-years, nearly the full width of the galaxy. While many spiral galaxies contain large, prominent bars such as this one, our Milky Way’s central bar is far more modest, extending only about a third of the way out to the Sun’s position. The pink regions found along the spiral arms are evidence of new star formation, triggered by the interaction of internal gas and the density waves of the internal structure.

While one-third of spiral galaxies have bars, ours is smaller than many, like NGC 1300’s.

The galaxy NGC 2775, shown here, displays one of best known examples of flocculent spiral arms, where the arms have wound up many times over in the outskirts of this galaxy. Although there are many visual similarities between a face-on spiral such as this and a protostellar system that’s forming, complete with a surrounding disk rife with imperfections, visual similarities are not sufficient to confirm an object’s nature.

The outer arms are neither irregular nor tightly wound; we’re not “flocculent.”

Many spiral galaxies, such as the Sombrero galaxy (M104), possess both spiral features and also a large central bulge. For comparison, the Milky Way possesses only a small central bulge, but it is still a significant feature in describing our galaxy.

Additionally, the Milky Way possesses a small but significant central bulge.

The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including a multi-armed spiral structure and a central bar, as well as spurs and minor arms, plus a central bulge of stars. The pink regions showcase transitions in hydrogen atoms driven by ultraviolet light: produced by new stars. The Southern Pinwheel galaxy is one of the closest and brightest barred spiral galaxies at a distance of just 15 million light-years, and has a similar diameter (118,000 light-years) to our own Milky Way.

We also exhibit major arms, minor arms, and spurs, with the Orion spur home to our Sun.

The Milky Way has two major arms, called the Perseus Arm and the Scutum-Centaurus Arm. There are also two minor arms, and two smaller ‘spurs’. Earth, its sun, and the rest of our solar system, is embedded within the Orion spur. While the general features of the Milky Way are thought to match this picture, the finer details of the galaxy, particularly once we look a few thousand light-years away from our own location, are largely unknown.

While many galaxies copiously form stars, the Milky Way is relatively quiet.

This ground-based, wide-field image of the Eagle Nebula shows the star-forming region in all its glory, with new stars, reflection and emission nebulae, and dusty features all present. Note how the material around the stars gets ionized and over time becomes transparent to all forms of light. Star-forming regions in the Milky Way are few in number and small in nature, particularly in comparison to the more active galaxies in our Universe.

Only within the arms themselves are new stars primarily formed.

The spiral arms of galaxy NGC 6384 are where new stars primarily form in this galaxy. Under normal circumstances, spiral arms in the disk of a spiral galaxy are where the majority of new stars form. With many features in common with our own Milky Way, NGC 6384 is one of the best candidates for a near-twin of the Milky Way. Its similarities to MCG+01-02-015 are also striking.

It’s as though the Milky Way is a large, barred spiral galaxy with a small, elliptical-like center.

The spiral galaxy NGC 772 has no central bar, but exhibits enormous levels of star formation and a lopsided dust distribution: evidence of large populations of bright stars on the far side of the dusty galaxy. This galaxy, despite numerous superficial properties in common with our Milky Way, cannot be a very good analogue.

Many similar galaxies are known, but nobody knows exactly which one is most like our Milky Way.

The spiral galaxy UGC 12158, with its arms, bar, and spurs, as well as its low, quiet rate of star formation and hint of a central bulge, may be the single most analogous galaxy for our Milky Way yet discovered. It is neither gravitationally interacting nor merging with any nearby neighbor galaxies, and so the star-formation occurring inside is driven primarily by the density waves occurring within the spiral arms in the galactic disk

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.