First Known "Feather" Connecting Two Of The Milky Way’s Spiral Arms Discovered

A wavy filament has been found that appears to be a 'feather' linking two of the Milky Way's spiral arms, but may just be a subbranch of the Norma arm. Image credit: Alex Mi/Shutterstok.com 

The structure of our galaxy, the Milky Way, is very hard to determine from the inside. However, telescopes operating at under-explored wavelengths are starting to overcome this with a slew of new discoveries. The latest is a wavy filament of cold, dense gas stretching at least 6,000 light-years close to the galactic center, which may be the galaxy's first known "feather", a structure seen in other spiral galaxies. 

Described in The Astrophysical Journal, astronomers posit the unusual structure could be a subbranch of the galaxy's Norma arm, or it could be a "feather" connecting between the Norma arm and the 3 kpc arm, the innermost arm yet discovered.

We can see many other spiral galaxies much more clearly than our own, and from this astronomers have created a taxonomy of the structures that protrude from spiral arms, referring to them as branches, splinters, or feathers depending on their angle or shape. Based on what the authors of the new paper have found, it seems the Milky Way, like many dinosaurs, probably has feathers (at least this one).

Rather than being shaped like a bird's feather, however, the discovery has a pattern like a sinusoidal wave, which inspired them to name it Gangotri Wave, after the glacier that feeds the Ganges river.

This reconstruction of the galaxy's spiral arms is now slightly out of date after the new discovery of a filament that either comes from the Norma arm, or bridges the gap between the Norma and 3kpc arm. Image Credit: Milky Way Galaxy (Robert Hurt,SSC/JPL/Caltech) CC BY-NC-SA 2

Gangotri is between 6,000 and 13,000 light-years long, and lies within 17,000 light-years of the Galactic Center. Its total mass is thought to be at least 9 million times that of the Sun.

The path of Gangotri is determined by tracing carbon monoxide across the sky as traced by several sky survey projects. Only a tiny proportion of Gangotri's mass would actually be CO, and even less with the carbon-13 molecule whose velocity is tracked, but the gas acts as a tracer for more abundant, but harder to detect, hydrogen and helium.

Suspected location of Gangotri. Image Credit: Veena et al/Astrophysical Journal

Although Gangotri will need to be mapped further for its true status to be confirmed, the paper notes “The filament being a bone/spine can be ruled out, as such features, per definition, are closely associated with the spiral arms.” Gangotri, on the other hand, extends far out from Norma, and probably reaches to the 3 kpc arm, forming a bridge between the two. Such interarm spurs have been seen in other nearby spiral galaxies, particularly barred spirals like our own. Despite their frequency, however, there is little agreement on their cause.

Our increased capacity to trace the movements of stars and the locations of clouds of gas has revealed something of the smaller galaxies that were amalgamated into the Milky Way to make it what it is today. More recently, astronomers revealed an extra arm within our galaxy, and a “splinter” or “spur” that comes off the Sagittarius Arm and includes astronomical monuments such as the Eagle Nebula, home to the breathtaking Pillars of Creation.

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