Research on disk galaxies sheds light on movement of stars

Research on disk galaxies sheds light on movement of stars
Credit: University of Arkansas

University of Arkansas astrophysicists have taken an important step toward solving the mystery of how disk galaxies maintain the shape of their spiral arms. Their findings support the theory that these arms are created by a wave of denser matter that creates the spiral pattern as it travels across the galaxy.

"The structure of spiral arms in disk galaxies is a mystery," said 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."

Disk galaxies, including the Milky Way, comprise 70 percent of known galaxies. They are characterized by their spiral-shaped arms, but astronomers are not sure how these form and maintain themselves.

The mystery begins with a simple paradox: stars in a disk galaxy orbit a central mass called a "galactic bulge," and the stars closer to the center orbit faster than the stars toward the edge. But, if the spiral arms were composed of a fixed group of stars, the ones on the edges of the pattern would have to cover more distance than the stars in the middle in order to maintain the spiral pattern. Like runners in the outer lane of a circular track, they would need to move faster to keep their position in the group.

In the 1960s, astronomers proposed the " wave theory" to explain this paradox. The theory holds that the arms of disk galaxies are not formed from static bundles of stars. Instead, these arms are waves of denser areas that move through the stars. The stars move in accordance to the laws of physics, and as they orbit the center of the galaxy, they encounter these denser areas.

Many astronomers have compared the wave of denser matter to a traffic jam in which the speed of stars traveling in a circle around the center of a galaxy is affected by the denser matter in the same way that motor vehicles are affected by a congested part of a highway. They slow as they meet the congestion and then move more easily after getting past the traffic jam.

The denser areas also affect clouds of gas that pass through these regions.They become compressed, collapsing into new stars.

Miller worked with associate professors Julia and Daniel Kennefick, postdoctoral scholar Rafael Eurfrasio, doctoral graduate student? Douglas Shields, and graduate students Mahamed Shameer Abdeen and Erik Monson, as well as Benjamin Davis of Swinburne University of Technology in Australia, also a graduate of the U of A. They have published their results in the Astrophysical Journal.

Miller and his colleagues provided support for the density wave theory by looking at stars of different ages and comparing their locations to that of the center of the density wave.

According to the theory, there would be a point on each arm of the galaxy where the rotation speed of the density wave and the speed of the stars is the same. This is called the co-rotation radius. Stars inside the co-rotation radius should be moving faster than the density wave because they are closer to the center. Therefore, the older a star gets, the further ahead it should travel from its birthplace near the wave. On the outer side of the co-rotation radius, where stars are traveling more slowly than the density wave, the older stars should fall further behind the wave.

The researchers examined images of in the NASA/IPAC Extragalactic Database, which is operated by the NASA Jet Propulsion Laboratory at the California Institute of Technology. For each galaxy, they examined images of different wavelengths of light, representing stars of different ages. They found that each group of stars formed an arm with a slightly different "pitch angle," which is the angle of the arm in relation to the center of the galaxy. By comparing these different angles to the angle formed by the center of the density wave, they showed that the location of these groups of matches the prediction of the density wave theory.

Although the research provides evidence for why the spiral arms maintain their shape, questions remain. It's easy to understand why a traffic jam occurs when you get to a car accident that reduced three lanes to one, but determining what creates the denser waves is still an open question.


Explore further

Astrophysicists discover mechanism for spiral-arm formation in disk galaxies

More information: Ryan Miller et al. Investigating the Origins of Spiral Structure in Disk Galaxies through a Multiwavelength Study, The Astrophysical Journal (2019). DOI: 10.3847/1538-4357/ab0d26
Journal information: Astrophysical Journal

Citation: Research on disk galaxies sheds light on movement of stars (2019, April 23) retrieved 23 May 2019 from https://phys.org/news/2019-04-disk-galaxies-movement-stars.html
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Apr 23, 2019
Water flowing over flat concrete produces waves that remain in-place, even as the water flows through them. Have physicists explained that?

Apr 23, 2019
This happens every time I flush my toilet, or let the water out of my bath. Maybe space is contracting?

Apr 23, 2019
Which direction is this galaxy rotating?
as
it appears to be rotating in the direction of the spiral
as in this graphic would be anticlockwise
as this is the start of the puzzle
for
when this arm 13billion years ago was forming
it looks as though these stars were playing a gravitational follow my leader

Apr 23, 2019
There are some very complicated issues of galaxy formation. Unfortunately, here is the same problem as with the stars. The origin of galaxies remains unclear, in spite of huge activity in the field. What the "formation" means? It means that we have the material that is assembling into galaxies.
https://www.acade...ome_From

Apr 23, 2019
I was watching a video of the waves of a tsunami going through and around some obstacles on their way to a beach in Thailand. The waves kept their shape all the way up through an inlet to a river while still keeping their shape washing through it. Fascinating how Nature does these tricks and it never gets old seeing it happen.
I/we don't see any 'paradox' that is described in the article. A spiral galaxy could be likened to a Wagon Wheel where the outer rim turns more slowly than the hub of the wheel which is twirling at a good speed.

Apr 24, 2019
This whole theory is based on a very shaky foundation, namely that stars can 'form' all by themselves from clouds of gas. Given the physical impossibility of that event ever occurring, anyone basing their explanation for the existence of spiral arms on it is chasing their own tail.
This is precisely what the researchers have done here. They are following the purely naturalistic paradigm of self-creation [ because an intelligent and all-powerful creator is not allowed ] and will inevitably get stuck and forced to go back to the drawing board. This happens time and time again.
We'll see how long the density wave theory holds up. Time will tell.

Apr 24, 2019
This whole theory is based on a very shaky foundation, namely that stars can 'form' all by themselves from clouds of gas. Given the physical impossibility of that event ever occurring.....


It's called gravity. The same phenomenon that keeps your dopey ass from floating off the planet. PS, it's been observed; so much for your impossibility.

Apr 24, 2019
FWIW, our solar system is 'near' the co-rotation circle.
Only a 'stub' but plenty more reports if you search...
https://en.wikipe...n_circle

Apr 24, 2019
Water flowing over flat concrete produces waves that remain in-place, even as the water flows through them. Have physicists explained that?
Yep. They're called standing waves. Might wanna look that up.

Apr 24, 2019
This whole theory is based on a very shaky foundation, namely that stars can 'form' all by themselves from clouds of gas. Given the physical impossibility of that event ever occurring.....


It's called gravity. The same phenomenon that keeps your dopey ass from floating off the planet. PS, it's been observed; so much for your impossibility.

Although gravity does keep dopey asses such as yourself from floating off the planet, it has never been observed that gravitational accretion forms anything more than pebbles.

Apr 24, 2019
& then what do the pebbles do?

Since you have already acknowledged the ungodly influence of Gravity?
Those pebbles are Mass.
Gravity is a Constant of Mass.

Unless you are claiming these are magical, voodoo hoodoo pebbles?
The normal gravitational attraction of every single pebble in the accretion disk is pulling at every other pebble.

Did you mean to say "planetesmal disk?
Not that it matters.
The gravitational influence remains a Constant.

Apr 24, 2019
we have the material that is assembling into galaxies.
https://www.acade...ome_From


Val: Don't give us links that require signing into something. :/

Apr 24, 2019
It's perfectly reasonable to assume when a solar system forms the entire thing is under massive E&M effects from the intense explosions. That wouldn't require gravity accretion.

But the E&M is just an EFFECT of the actual physics causing the mess.
Cantdrive85 insists the E&M magically out of the blue creates everything.

Apr 24, 2019
It's perfectly reasonable to assume when a solar system forms the entire thing is under massive E&M effects from the intense explosions. That wouldn't require gravity accretion.

But the E&M is just an EFFECT of the actual physics causing the mess.
Cantdrive85 insists the E&M magically out of the blue creates everything.


One of the measurements made by the Rosetta lander, Philae, was of how magnetised the comet was. Given that they are primordial, this was expected to give us a reasonable idea of the conditions in the very early solar system. The comet was 'remarkably unmagnetic'. As was an asteroid whose name escapes me now. It somewhat surprised the scientists, as they had expected more EM in the early system to help explain formation scenarios.

Apr 24, 2019
More EM effects? Yeah? Nahh...

The only time you get all these sparkly, electromuggering, plasmspazma effects?

Are when Gravity lines up some matter or energy & gleefully smashes it all together.
Gravity, playing demolition derby with the Universe.

EM/plasma, WN & SN, are just pretentious sadsacks.
Temporary/Local phenomena.

Apr 24, 2019
It's perfectly reasonable to assume when a solar system forms the entire thing is under massive E&M effects from the intense explosions. That wouldn't require gravity accretion.

But the E&M is just an EFFECT of the actual physics causing the mess.
Cantdrive85 insists the E&M magically out of the blue creates everything.


One of the measurements made by the Rosetta lander, Philae, was of how magnetised the comet was. Given that they are primordial, this was expected to give us a reasonable idea of the conditions in the very early solar system. The comet was 'remarkably unmagnetic'. As was an asteroid whose name escapes me now. It somewhat surprised the scientists, as they had expected more EM in the early system to help explain formation scenarios.

More circular reasoning, however as observations shows comets are not "primordial" or left overs from the speculative formation scenario.

Apr 24, 2019
Why did something go that way?....it was the path of least resistance.

Apr 24, 2019
It's perfectly reasonable to assume when a solar system forms the entire thing is under massive E&M effects from the intense explosions. That wouldn't require gravity accretion.

But the E&M is just an EFFECT of the actual physics causing the mess.
Cantdrive85 insists the E&M magically out of the blue creates everything.

"What we call mass would seem to be nothing but an appearance, and all inertia to be of electromagnetic origin." Henri Poincaré, Science and Method

Apr 25, 2019
It is not stated, but since dark matter cannot be concentrated in the disk, it - and its density waves - is primarily a star/gas phenomena.

FWIW, our solar system is 'near' the co-rotation circle.


I did not know this, thanks! That means we would be located in a denser region of the disk, where star populations of all ages would keep up with the density waves [ c.f. the figure here: https://www.skyan...8201623/ ].

You know, I am selectively biased :-) right now, but the anthropic/survival bias universe that I am looking into for feasibility do seem a decent hypothesis in these cases. A calm, low star formation rate history but relatively large galaxy, with at least one inhabited planet situated in its densest calm (disk) region ... against a uniform prior.

I can't see an immediate way to test that though, density of inhabited planets would tend to mirror star density.

Apr 25, 2019
torbjorn. you seem to be conflicted between what you have observed & what you wish our existence to have as a meaning.

The basic problem of the anthropic, philanthropic desire to explain Us.
Our reason for existing?

Observation of the Cosmos leaves us with two choices
The creator deities are bungling incompetents.
Or, at this early, infantile stage of the Universe? We had the bad luck to be sooners.

When the Universe matures? Maybe by then the percentage of successful, thriving, Living Worlds will increase exponentially?

Right now? We are stuck with being surrounded by dead, failed worlds.

If this sector of the Galactic Arm is considered optimal for the possibility of Living Worlds?
That conjecture needs proof.

Apr 25, 2019
Galactic standing waves

Oscillatory galaxies
fourinfinities> Water flowing over flat concrete produces waves that remain in-place, even as the water flows through them. Have physicists explained that?
Da Schneib> Yep. They're called standing waves. Might wanna look that up.

These spirals are wave forms
just as waves form in traffic jams that continue long after the road is clear
part of the existence of these waves is in traffic jams
of
building up of dust billions of years ago
in time
These galactic wave forms will subside as the arms fade away

Apr 25, 2019
The spiral arms are the same spiral arms which are created by two interacting Birkeland currents;
https://www.plasm...rmation/
The same results were demonstrated by the Plasmakristall-3 Plus experiments on the ISS. No density wave explanation is needed as these are electromagnetic structures.

Apr 25, 2019
Helicity.
Helical expression of gravity over time...

Apr 26, 2019
The spiral arms are the same spiral arms which are created by two interacting Birkeland currents;
https://www.plasm...rmation/
The same results were demonstrated by the Plasmakristall-3 Plus experiments on the ISS. No density wave explanation is needed as these are electromagnetic structures.


Lol. No they aren't. You are lying again.

Apr 26, 2019
Helicity.
Helical expression of gravity over time...

Helicity is the signature of electric currents twisting about themselves, there is nothing about gravity which creates helical motion.

Apr 26, 2019
Helicity.
Helical expression of gravity over time...

Helicity is the signature of electric currents twisting about themselves, there is nothing about gravity which creates helical motion.


Yet more pseudoscientific crap.

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