How astronomers solved the “Zone of Avoidance” puzzle

From their earliest discovery, grand cosmic spirals have posed a tremendous puzzle.

The featured image shows galaxy NGC 7331 along with other members of its galactic group, including the prominent galaxies NGC 7335, 7336, 7337, and 7340. We now know that a large fraction of galaxies beyond the Milky Way are spiral-shaped in nature, and that all of the spiral nebulae we were considering in ~1920 are indeed galaxies beyond our own. Like nearly all galaxies visible to even the aided human eye, these galaxies are located far away from the Milky Way’s central galactic plane.

Most nebulae — dark nebulae, star clusters, planetary nebulae — are found everywhere: omnidirectionally.

Near Orion’s Belt, the reflection nebula known as the Flame Nebula (left) as well as the star-forming emission nebula known as IC 434 (in red) are joined by a series of dark molecular clouds in the foreground that create spectacular silhouettes known as dark nebulae. The Horsehead Nebula (at center) is arguably the most famous dark nebula of them all, with nebulous features found wherever new stars are forming or will form in the future within the Milky Way.

Some prefer the Milky Way’s plane: where stars, gas, and dust are most concentrated.

This complex of some 20 independent star-forming regions is within a 3-degree span and inside the Milky Way’s galactic plane. Taken with the far-infrared Herschel spacecraft, this region, Westerhout 43, is located at the intersection of our central galactic bar and one of our spiral arms. Where multiple infalling streams of cold matter intersect, new star-formation events usually arise.

But not spiral nebulae; they’re found everywhere except in or near the galactic plane.

Spirals, initially recorded as faint, fuzzy objects with no discernible structure through more primitive telescopes, were clearly observed since the mid-1800s to be prevalent in the night sky. But their nature remained a mystery until the 1920s, and a satisfactory explanation for why there were none of them observed within about 20 degrees of the Milky Way’s plane would remain elusive until the birth of infrared astronomy in the 1960s.

We called the Milky Way’s plane the “Zone of Avoidance” for lacking these objects.

This expansive photograph showcases the Milky Way, the two Magellanic Clouds, and several other impressive celestial features at the Cerro Tololo Inter-American Observatory. Although many spiral and elliptical nebulae, now known to be galaxies, can be found all throughout the sky, there are very few found near the galactic plane of the Milky Way itself, leading to that region being known as a the “Zone of Avoidance” for that reason.

Spiral nebula IC 342, found in 1892, was the closest one known.

A similar galaxy to our own Milky Way, galaxy IC 342 has already been imaged in greater detail by ESA’s Euclid than by any other ground-based or space-based observatory. As viewed from the Milky Way, IC 342 is the third largest galaxy by angular size: behind Andromeda and the Triangulum galaxy only. Although it can be seen in visible light, its details are far more apparent when infrared light is included as well.

Elliptical nebulae “avoided” the galactic plane, too, but the reason remained obscure.

A map of star density in the Milky Way and surrounding sky, clearly showing the Milky Way, large and small Magellanic Clouds, and if you look more closely, NGC 104 to the left of the SMC, NGC 6205 slightly above and to the left of the galactic core, and NGC 7078 slightly below. All told, the Milky Way contains some 200-400 billion stars over its disk-like extent. There are a great many galaxies to be discovered, but within about 10 degrees above and below the galactic plane, visible light is a lousy tool for revealing them.

Then, in 1923, we discovered that these spirals and ellipticals were galaxies beyond our own.

Perhaps the most famous photographic plate in all of history, this image from October of 1923 features the great nebula (now galaxy) in Andromeda along with the three novae that Hubble observed within them. When a fourth brightening event happened in the same location as the first, Hubble recognized this was no nova, but a Cepheid variable star. The “VAR!” written in red pen was Hubble having a spectacular realization: this meant Andromeda was an extragalactic object, located far beyond the Milky Way.

Could dust, gas, and foreground matter block the light from these distant, extragalactic objects?

By viewing the Milky Way in infrared wavelengths of light, we can see through large amounts of 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, but the center of the plane of the Milky Way is where the dust is densest. Infrared and visible light views both showcase this, but in vastly different ways.

Certainly, light-blocking material exists.

The dark regions show very dense dust clouds through which light cannot penetrate. The red stars are severely reddened by dust, while the blue stars must be located in front of the dust clouds. These images are part of a survey of the southern galactic plane, which helped map out the dust distribution throughout the galaxy in a distance-dependent way.

Small matter grains, preferentially, would obscure shorter-wavelength photons.

Dust grain size is location independent, allowing infrared telescopes to peer “through” the galactic plane.

The majority of the dust signatures seen in our galaxy arise from our galaxy itself, as this full-sky map from the Planck satellite shows. By mapping out the dust in our galaxy in a variety of wavelengths of light, we can understand its properties: size, density, and distribution. While dust grain density varies greatly with location within the Milky Way, dust grain size remains relatively constant.

Only with the development of infrared astronomy did galaxies finally “appear” within this zone.

The WISE mission from NASA, the Wide-field Infrared Survey Explorer, collected all-sky data in infrared light for the region around the Milky Way. Many nebulae, including extragalactic objects, are found ubiquitously, including in the plane of the Milky Way itself. Although the vast majority of infrared emission comes from the plane of the Milky Way itself, where stars and gas and dust are primarily located, many galaxies can be viewed beyond it.

The first were Maffei 1 and 2, named after infrared astronomy pioneer Paolo Maffei.

Italian astronomer Paolo Maffei’s promising work on infrared astronomy culminated in the discovery of galaxies — like Maffei 1 and 2, shown here — in the plane of the Milky Way itself. Maffei 1, the giant elliptical galaxy at the lower left, is the closest giant elliptical to the Milky Way, yet went undiscovered until 1967. For more than 40 years after the Great Debate, no spirals in the plane of the Milky Way were known, due to light-blocking dust that’s very effective at visible wavelengths.

Over 99.5% of their visible light is blocked.

Maffei 1 (left), a giant elliptical galaxy, and Maffei 2 (right), a spiral galaxy, are named after their discoverer and infrared astronomy pioneer: Paolo Maffei. They were only first discovered in 1967, as more than 99.5% of their light is blocked by the intervening, obscuring material of the Milky Way. Although Hubble can successfully image them, they’re much better viewed in infrared light.

With infrared astronomy, however, galaxies appear just as rich in the “Zone of Avoidance” as anywhere else.

This map of large-scale structure in the nearby Universe color-codes every galaxy beyond our own with redshift/distance, with the plane of the Milky Way shown in infrared light as imaged by the 2-micron all-sky survey (2MASS). Although clusters and walls appear individually, as labeled, the overall structure of the Universe is homogeneous: the same everywhere, including within the so-called “Zone of Avoidance.”

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