What are those spiral patterns we observe in galaxies and what causes them?
Do they represent a stable pattern of differing star/nebula density or are they some sort of wave that travels through the stars in the disk?
This is an image of NGC2403, a galaxy classified as an "intermediate" type. This means that NGC2403 is somewhere between a grand design spiral galaxy and a barred spiral.
If we look closely at a spiral galaxy it appears that there are far more stars within the spiral arms than in the rest of the galactic disk. We also tend to see nebulae with new star formation regions in these same spiral arms and (generally) these are not visible outside the spiral arms. Simplistically this would suggest that the spiral patterns are not waves but real and permanent structural features in the galaxies.
One problem with this is what I'll call the "licorice wheel problem". Astronomers can measure the radial velocity in several galaxies from the core out to the outer reaches. A model can then propagate this forward in time. What do we see? We see the spiral galaxies quickly becoming a tightly wound roll. Like this licorice wheel candy...
So there is something wrong with this model. With time spiral galaxies should be "wound up" and there is no evidence of this. The spiral arms are similar for old and young galaxies and appear to be stable features.
In the 1960s two American Astronomers created the Density Wave Theory, where the spiral structure is a wave propagating among the stars. The stars themselves (and other galactic matter) move through the spiral features.
Consider our sun moving through a spiral arm:
- it will begin a certain distance from the neighboring stars.
- in the center of the spiral arm the neighboring stars will all be closer
- as it moves out the other side it will return to the initial state.
Now consider that a dense cloud of gas (known as a molecular cloud) would do the same. In the spiral arm the cloud of gas will be squeezed (more dense), meaning that star formation (collapse under the effects of gravity) is more likely to occur. Many young stars means many very hot and bright stars which live very short lives and soon burn out or explode. Such stars will be very visible in the arms and practically unseen elsewhere. Similarly with the familiar red nebulae associated with star-forming regions.
A similar phenomenon is seen on a single-lane highway with a slow truck hindering traffic flow. At a good distance in front of or behind the truck the density of vehicles is low. Near the truck there is a clump of vehicles, with a slow trickle of vehicles passing the truck and new ones arriving at the rear. Overall the vehicles are moving at a similar speed. A given car will arrive at the higher density region, propagate through it until it finally passes the truck, and it will then resume its normal travels.
This is an excellent video/simulation of Galaxy dynamics as in the Density Wave theory (Ingo Berg, CC BY-SA 3.0 via Wikimedia Commons). Consider the right hand side only which matches typical galaxy radial velocities. Note how the red nebulae and bright stars are formed in the arms but then die out. Note also how dimmer and longer-lived stars (like our Sun) enter and exit the spiral arms. The spiral patterns persist.
So the answer is that today's dominant scientific theory holds that the spiral arms are some sort of wave propagating through the galaxy. These waves are stable over enormous lengths of time. Like all theories this one cannot ever be "proven", but it is the best we have until a better theory comes along.
The image above of NGC2403 is the result of stacking over six hours of data collected on the night of October 28-29, 2021 at the Lac Teeples Deep Sky Observatory. The RASA-11 telescope and the ZWO ASI 6200 camera were used.
Comments