Selections from 2021: Discovery of the Milky Way’s First Feather

Editor’s note: In these last two weeks of 2021, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

A Kiloparsec-scale Molecular Wave in the Inner Galaxy: Feather of the Milky Way?

Published November 2021

Main takeaway:

New observations from a team led by Veena Vadamattom Shaji (University of Cologne, Germany) have revealed a long, skinny cloud of molecular gas in the Milky Way. This is the first structure discovered in our galaxy analogous to the wispy gas filaments called “feathers” that emerge nearly perpendicularly from the spiral arms of other galaxies.

Why it’s interesting:

diagram of the Milky Way's spiral arms

The location of the Gangotri wave (green) on a model of the Milky Way’s spiral arms. [Veena et al. 2021]

Like trying to map a forest when you’re surrounded by trees, it’s hard to discern the structure of our galaxy when we’re tucked away inside it. Over time, our understanding of the Milky Way’s structure has coalesced into a spiral with four major arms and a central bar, plus several smaller arms and spurs. The newly discovered gas cloud, named the Gangotri wave after the glacier that feeds the Ganges River in India, contains roughly 9 million solar masses of gas, dust, and stars, and it stretches at least 6,500 light-years across. Based on the velocity of the cloud, the Gangotri wave is likely either a subbranch of the Norma arm or a filament connecting two arms.

What’s causing this feathery feature:

Although spiral-arm feathers have been explored in models, there isn’t yet a consensus on how these structures arise. Potential causes include self-gravity, shear from the Milky Way’s rotation, and instabilities like the wiggle instability. Whichever model prevails must account for the Gangotri wave’s location as well as its curious morphology: the gas filament appears to have a sinusoidal-wave-like structure in the direction perpendicular to the galactic plane, with an amplitude of 220–650 light-years. The gas, dust, and stars within the Gangotri wave all seem to follow this sinusoidal pattern, suggesting that gravitational instabilities are the likeliest cause.


V. S. Veena et al 2021 ApJL 921 L42. doi:10.3847/2041-8213/ac341f