Spiral galaxies, characterized by their stunning pinwheel shapes, have captivated astronomers and stargazers alike. Our own Milky Way is a prime example, showcasing the beauty and complexity of these cosmic structures. However, the question of how these galaxies acquire and maintain their spiral arms has puzzled scientists for decades. This article explores the theories surrounding the formation of spiral arms, the role of dark matter and dark energy, and the evidence supporting each perspective.
Theories of Spiral Arm Formation
1. Density Wave Theory
This widely accepted theory suggests that spiral arms are not fixed structures but rather density waves that travel through the galaxy’s disk. As stars and gas move through these waves, they experience gravitational interactions that cause them to clump together, creating the appearance of spiral arms.
Evidence: Observations of magnetic fields in galaxies, such as M77, show alignment with spiral arms, indicating that gravity compresses material into these patterns. Additionally, simulations have demonstrated that once spiral arms form, they can persist independently due to gravitational influences, even when the initial disturbances are removed.
2. Transient Spiral Arm Theory
This theory proposes that spiral arms are temporary features that form and dissipate over time due to gravitational disturbances. According to this view, the arms arise from interactions with nearby galaxies or internal instabilities within the galaxy.
Evidence: Some studies suggest that spiral patterns can change significantly over time, indicating that they may not be permanent structures. Long-term observations of galaxies are necessary to confirm the transient nature of these arms.
3. Galactic Mergers and Interactions
This theory posits that mergers between galaxies can lead to the formation of spiral structures. The gravitational interactions during these events can induce the spiral shape, as seen in many galaxies with complex histories.
Evidence: Data from the Hubble Space Telescope and other observatories indicate that many spiral galaxies have evolved from irregular shapes through gas-rich mergers. For instance, the interactions between smaller galaxies and the Milky Way may have contributed to the formation of its spiral arms.
The Role of Dark Matter and Dark Energy
Dark matter and dark energy play crucial roles in the dynamics and evolution of spiral galaxies. Dark matter, an invisible substance that does not emit light but exerts gravitational influence, is believed to be distributed in massive halos surrounding galaxies. This dark matter affects the rotation curves of galaxies, which reveal that stars in the outer regions rotate at unexpectedly high speeds, suggesting the presence of additional unseen mass.
Impact of Dark Matter: The existence of dark matter helps explain the flat rotation curves observed in spiral galaxies. According to the laws of gravity, stars further from the galactic center should rotate more slowly, akin to planets in our solar system. However, observations show that the rotational velocities remain constant, indicating that dark matter contributes significantly to the overall mass of the galaxy, particularly in its outer regions. Studies have shown that the mass of dark matter halos can be substantial, influencing the galaxy’s structure and star formation rates[1][5].
Impact of Dark Energy: While dark energy primarily affects the universe’s expansion on cosmological scales, its implications for galaxy formation and evolution are profound. Dark energy may influence the rate of star formation in galaxies by affecting the gravitational dynamics of the universe. As galaxies interact and merge, the presence of dark energy could alter the gravitational landscape, potentially impacting the stability and longevity of spiral arms.
Recent Discoveries and Insights
Recent research has provided new insights into the nature of spiral arms. A study on the galaxy M106 revealed “anomalous arms” made primarily of gas, which were heated by shock waves from jets ejected by a supermassive black hole. This finding supports the idea that spiral structures can be influenced by dynamic processes within the galaxy.
Furthermore, data from the Gaia mission has suggested that spiral arms in the Milky Way may not be fixed but could form temporarily due to the rotation of the galactic disk. This ongoing research aims to clarify the origins and longevity of spiral arms in various galaxies.
Conclusion
The mystery of how spiral arms form and persist in galaxies remains an active area of research in astrophysics. While several theories exist, including density wave theory, transient spiral arm theory, and the role of galactic mergers, the influence of dark matter and dark energy adds another layer of complexity to our understanding. As new observational data and simulations continue to emerge, our grasp of spiral galaxies and the fundamental forces shaping them will undoubtedly deepen, revealing the intricate processes that govern our universe.
Read More
[1] https://iris.sissa.it/retrieve/dd8a4bf7-0447-20a0-e053-d805fe0a8cb0/1963_5271_PhD_Yegorova_Irina.pdf
[2] https://www.aanda.org/articles/aa/full_html/2013/09/aa21390-13/aa21390-13.html
[3] https://www.thoughtco.com/spiral-galaxies-3072049
[4] https://www.mso.anu.edu.au/~jerjen/researchprojects/darkmatter/darkmatter.html
[5] https://phys.org/news/2019-10-dark-massive-spiral-galaxies-breakneck.html
[6] https://www.iac.es/en/outreach/news/new-findings-spiral-galaxies-challenge-dark-matter-hypothesis
[7] https://www.esa.int/Science_Exploration/Space_Science/Where_did_today_s_spiral_galaxies_come_from
[8] https://www.universetoday.com/101205/wrapping-around-the-mystery-of-spiral-galaxy-arms/
1 comment
If you start with a static gas cloud that gradually collapses under gravity, (Hydrogen being the basic building block of all matter, I would postulate), the center of gravity will become more dense and as it does it will have an asymmetrical gravitational effect on the surrounding particles.
This asymmetry or imbalance will cause a galactically unique, directional turning moment or spin to occur at ever greater speed as more matter adds to it, so the initial creation of the arms is a result of the influence of the rotating center on the outer regions of the relatively static gas or dust cloud.
Even if the cloud is fairly evenly distributed, the faster spinning center will attract areas of higher and lower density of matter as it’s major gravitational waves pass through and this will cause a pulsing or non-regular pull on the cloud, ultimately inducing areas of higher and lower density particles or mass objects, i.e. ‘Arms’.
They may be of varying width and length but as time passes, the forces will tend to even out and become regular, as all matter seeks the path of least resistance.
For example, the faster you spin a pizza base, the larger and thinner it gets but in this case, it’s not caused by centrifugal force at this stage of galactic formation. Maybe the particles were first sprayed out of a black hole as gamma rays that collapsed back into a disc of hydrogen atoms, which then collapsed into gas and dust?
The rotating galactic center is pulling the entire galaxy down the plug hole of it’s black hole, (or eventual black hole), as it collapses under it’s own gravity, if it gets dense enough from swallowing up enough matter.
The spiral arms then, are the end result of a balancing act between the forces pulling them in to the centre and the drag of inertia on the slower moving dust or gas at the outer edge or galactic circumference, which will ultimately be pulled right to the center itself.
The Spiral Arm backward curve is simply a result of the rotational speed differential between the faster spinning center and the slower spinning outer portion, so it’s winding itself up like a clock spring.
The Sun’s equator does this every 11 years apparently and releases it’s tension in solar flares and magnetic storms.