A significant advancement in understanding how disk galaxies preserve the shape of their spiral arms

Scientists Might Understand How Spiral Arms Form and Maintain Shape in Disk Galaxies

New study published by University of Arkansas has revealed that the arms of disk galaxies are sustained through waves of density, the problem that scientists have puzzled over for a long time. These waves propagated through stars in a galaxy yield the spiral pattern as is seen normally as a traffic jam on a freeway. Density wave theory was given a nod by astrophysicists when they were able to track the motions and the age of stars and decreed that inside a co-rotation radius stars of old move differently from those outside of it. Remarkably, though the study gives evidence for waves of increase, it raises questions on where these waves come from. There is no doubt this article contains a good array of information pertaining to the formation of galaxies.

After reading the article, David, a Reddit user, gained more than 192 upvotes with this comment: “So it’s as if the stars reached a ‘traffic jam’ in space where this ‘wave of denser matter’ is theorised, yet they don’t understand why these waves exist in this manner. Very interesting and leaves a lot to think about.”

Scientists at the University of Arkansas in the field of astrophysics have come up with part of the answer to the question of how the spiral arms of the disks of galaxies are sustained. They embarked that these arms are formed as waves of denser matter and chasten the spiral pattern as they broadcast themselves across the galaxy.

“Spiral arm structure in disk galaxies is still a mystery,” said a visiting assistant professor of physics, Ryan Miller. “The nature of the spirals and the question of the number of arms nobody knows, and our work has given us a definite answer to one part of it.

A majority of about 70% of all galaxies are spiral galaxies as can be seen observed with the Milky Way as an example. These galaxies are spirals and this is characterized by their long, winding arms and astronomers are a bit cloudy as to how these structures are maintained and developed.

The mystery starts with a paradox: Many stars in a disk galaxy revolve around a central mass referred to as a ‘galactic bulge ‘ The speed of such stars is determined by the proximity to the bulge. If the spiral arms were composed of a fixed number of stars, the outer stars for example would have to rotate at a much faster rate as do runners in the outermost lane of a running track.

To explain this paradox astronomers came up with what is known as the “density wave theory” in the 1960s. It postulates that disk of galaxies are not structures composed out of definite clusters of stars. But they are waves of denser areas passing through the stars. As stars move in circles around the galactic centre they come across such regions of higher density.

Some astronomers have likened this wave of denser matter to a traffic jam where stars move around the galaxy at a certain pace, they slow down when they are near the denser matter, and then speed up once they are past the ‘traffic’ jam.

These areas also influence the clouds of the gas, which is in motion, squeezing it and hence developing new stars.

Co-authors include Julia and Daniel Kennefick, both associate professors at SWOSU; Rafael Eurfrasio, a postdoctoral scholar; Douglas Shields, a doctoral graduate student; Mahamed Shameer Abdeen, and Erik Monson, both of SWOSU; and Benjamin Davis, a Swinburne University of Technology, Australia alumnus and SWOSU graduate. The research was intended and subsequently published in the Astrophysical Journal.

Miller and his team supplied density wave theory through observing such stars of different ages and relating their position to the density wave centre.

Took, from the density wave theory, it was anticipated that there is a point on each arm of the galaxy where the rotation speed of the density wave and the stars’ speed are equal, what is referred to as the co-rotation radius. Stars within this radius should move faster than the density wave because they are nearer the centre. Hence, as a star ages, it should progress beyond what it was, at least nearer the wave of birth. Beyond the co-rotation radius where stars orbit at slower pace than the density wave, the older stars are expected to lag even further behind the wave.

The authors analyzed the photographs of galaxies in the NASA/IPAC Extragalactic Database which is run by the Jet Propulsion Laboratory under the aegis of Caltech. They analyse pictures with various wavelengths illustrating the stars of various age. That is, each group of stars formed an arm with different the so called pitch angle which is the angle between the arm and the center of the galaxy. They proved that the location of these star groups correspond to the density wave theory by comparing these angles with the angle given by the centre of the density wave.

Still, there are some unanswered questions regarding the phenomenon itself However, the research offers data as to why the spiral arms manage to hold their form. It is rather obvious as to why traffic congestion happens when three lanes are condensed to one because of an accident, but what results in the occurrence of denser waves is still an enigma.

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