Why to Spiral Galaxies have Spiral Arms?  What type of structure makes up the arms? And how do they maintain their shape?

Why do Galaxies have Arms?

Researchers from the University of Arkansas attempt to answer the questions taking an important step towards solving the mystery of how galaxies maintain the shape of their spiral arms.

An artists illustration of the true shape of the Milky Way, with an S-like warp in the outer reaches of the disk. Image Credit: CHEN Xiaodian.

An artists illustration of the true shape of the Milky Way, with an S-like warp in the outer reaches of the disk. Image Credit: CHEN Xiaodian.

“The structure of spiral arms in disk galaxies is a mystery,” explained Ryan Miller, visiting assistant professor of physics.

“No one knows what determines the shape of these spirals, or why they have certain numbers of arms. Our research provides a clear answer to part of that mystery.”

This is kind of a big deal since astrophysicists argue that disk galaxies, just like the Milky Way, make up around 70 percent of all known galaxies.

Characterized by massive spiral-shaped arms, astronomers have not been able to understand how these galaxies are able to maintain their shape.

The results of the new study have been published in the Astrophysical Journal.

Starting Point: The Density Wave Theory

To unravel the mystery behind the curious structure that holds together the shape of disk galaxies, astronomers looked back to a theory first proposed in the 1960s called the ‘Density Wave Theory‘.

It maintains that the arms of galaxies are not formed from stating a bundle of wars. Instead, these galactic arms are waves of much denser areas that move through the stars of a galaxy.

These stars orbit the center of their galaxy and find denser areas. These denser, galactic areas also play a huge role in the gas that passes through these areas, comprising it and collapsing into new stars.

Researchers offer supporting evidence for the wave theory.  They observed stares of different ages and compared their locations to that of the center of the density wave. The theory holds that there should be a certain point on each galactic arm where the rotation speed of the density wave and the speed of the stars as they orbit the galactic center are the same.

This is referred to as the co-rotation radius, and stars inside it should move at a much faster rate than the density because they are closer to the galactic center.

The outer parts of the co-rotation radius are inhabited by older stats which should fall further behind the wave.

The researchers analyzed a plethora of images of different galaxies from the NASA/IPAC Extragalactic Database, which is operated by the NASA Jet Propulsion Laboratory at the California Institute of Technology.

They examined images of different wavelengths of light for each galaxy, representing stars of different ages.

They discovered that each star group from an arm with a slightly different pitch angle–the angle of the arm in relation to the center of the galaxy.

They compared the different angles to the angle that forms by the center of the density wave, showing that the location of the star groups matches what is predicted in the density wave theory.

While the new study offers new evidence as to why spiral arms have the shapes they have, there are still cosmic engimas flying around, like; what creates the denser waves?

Hopefully, new observations and new studies will help reveal more details about the different galaxies that exist in our universe.