Theories explaining spiral formation and the role of bars in redistributing gas and stars
Galaxies often display impressive spiral arms or central bars – dynamic features that captivate both professional astronomers and amateurs. In spiral galaxies, the arms mark glowing star-forming regions rotating around the center, while in barred spiral galaxies, there is an elongated star concentration crossing the nucleus. These are not just static decorations – these structures reflect ongoing gravity, gas flows, and star formation processes in the disk. In this article, we will examine how spiral patterns form and persist, the importance of bars, and how both factors influence the distribution of gas, stars, and angular momentum in long-term cosmic evolution.
1. Spiral arms: general overview
1.1 Observed properties
Spiral galaxies typically have a disk shape with prominent arms extending from the central nucleus. The arms often appear bluish or bright in optical images, indicating active star formation. Based on observations, we distinguish:
- "Grand-design" spirals: Several bright, continuous arms clearly extending around the entire disk (e.g., M51, NGC 5194).
- "Flocculent" spirals: Many fragmented spiral segments without an obvious global pattern (e.g., NGC 2841).
The arms are rich in H II regions, young star clusters, and molecular clouds, so they play a crucial role in "supporting" a new population of stars.
1.2 The "winding" problem of arms
One obvious difficulty is that due to the different rotation speeds of the disk, any fixed pattern should wind up quite quickly and thus "stretch out" over a few hundred million years. However, observations show that spirals persist much longer, so the arms cannot be considered "material hands" rotating with the stars. Rather, they are density waves or certain patterns moving at a different speed than individual stars and gas [1].
2. Theories of spiral pattern formation
2.1 Density wave theory
Density wave theory, proposed in the 1970s by C. C. Lin and F. H. Shu, states that spiral arms are quasi-stationary waves in the galactic disk. Key points:
- Wave patterns: Arms are higher density regions (like “traffic jams on a highway”), moving slower than the stars' orbital speed.
- Star formation triggering: Gas entering a denser zone compresses and forms stars. These young, bright stellar clusters highlight the arm.
- Longevity: Pattern stability is determined by the wave solution of gravitational instabilities in the rotating disk [2].
2.2 Swing Amplification
“Swing Amplification” – another frequently mentioned mechanism in numerical simulations. When a density excess forms in a rotating disk, cut in the disk shape, gravity under certain conditions (related to the Toomre Q parameter, disk gradient, and thickness) can amplify it. This creates spiral structures that sometimes maintain a “grand-design” nature or break into many arm segments [3].
2.3 Tidal origin spirals
In some galaxy cases, tidal interactions or minor mergers can create prominent spiral features. A passing neighbor disturbs the disk, thus maintaining spiral arms. Systems like M51 (Whirlpool Galaxy) have very pronounced spirals, apparently stimulated by the satellite galaxy's gravitational pull [4].
2.4 “Flocculent” vs. “Grand-Design”
- “Grand-design” spirals often correspond to density wave solutions, which can be enhanced by interactions or bars generating global patterns.
- “Flocculent” spirals may arise from local instabilities and short-lived waves that constantly form and dissipate. Overlapping wavelets give a more chaotic disk appearance.
3. Bars in spiral galaxies
3.1 Observed features
Bar – an elongated or oval shaped stellar concentration crossing the galaxy center and connecting the disk sides. About two-thirds of spiral galaxies have bars (e.g., SB galaxies in the Hubble classification, including our Milky Way). Bars are characterized by:
- Protrusion from the bulge into the disk.
- Rotation approximately like a rigid body wave.
- Ring or nuclear zones, where gas concentrated by the bar causes intense star formation or nuclear activity [5].
3.2 Formation and stability
Dynamic instabilities in a rotating disk can spontaneously create a bar if the disk is sufficiently self-gravitating. Important factors:
- Angular momentum (AM) redistribution: The bar can help exchange AM between different parts of the disk (and halos).
- Interaction with dark matter halos: The halo can absorb or transfer AM, affecting the growth or decay of the bar.
Once formed, bars typically last billions of years, although strong interactions or resonant effects can alter bar strength.
3.3 Gas flow driven by the bar
The essential bar effect — transporting gas to the center:
- Shock fronts in bar dust lanes: Gas clouds experience gravitational torques, lose angular momentum, and migrate toward the galactic nucleus.
- Active star formation: Gas accumulated this way can form ring resonance structures or disk configurations around the bulge, triggering nuclear starburst or an active nucleus (AGN).
Thus the bar effectively regulates bulge and central black hole growth, linking disk dynamics with nuclear activity [6].
4. Spiral arms and bars: connected processes
4.1 Resonances and pattern speeds
Throughout much of the galaxy, bar and spirals coexist. The pattern speed of the bar (when the bar rotates as a wave) can resonantly match the disk orbital frequencies, possibly "anchoring" or aligning spiral arms starting at the bar ends:
- "Manifold" theory: Some simulations show that spiral arms in barred galaxies can arise as manifolds extending from the bar "ends," thus creating a "grand-design" structure linked to the bar's rotation [7].
- Inner and outer resonances: Resonances at the bar edges can form rings or transition zones where bar flows meet spiral wave regions.
4.2 Bar strength and spiral support
A strong bar can enhance spiral patterns or, in some cases, redistribute gas so effectively that the galaxy changes morphological type (e.g., from late-type spiral to early-type with a large bulge). In some galaxies, bar-spiral interactions occur cyclically: bars can weaken or strengthen over cosmic timescales, changing the brightness of spiral arms.
5. Observational data and specific examples
5.1 Milky Way bar and arms
Our Milky Way is a barred spiral, whose central bar extends several kiloparsecs, and several spiral arms are traced by the distribution of molecular clouds, H II regions, and OB stars. Infrared sky maps confirm the bar, behind which lie dust layers, while radio/CO observations show massive gas flows moving along the bar's dust lanes. Detailed models support the idea that the bar continuously drives material inflow into the nuclear region.
5.2 Prominent bars in other galaxies
Galaxies like NGC 1300 or NGC 1365 have prominent bars that transition into clear spirals. Observations show dust lanes, ring star formation, and molecular gas motion, confirming that the bar significantly transfers angular momentum. In some barred galaxies, the bar "end" position smoothly merges with the spiral arm pattern, indicating a resonance junction.
5.3 Tidal spirals and interactions
In systems such as M51 It can be seen that the small satellite can sustain and strengthen two prominent arms. Differences in rotation and periodic gravitational pull create one of the most beautiful “grand-design” patterns in the sky. Studying such “tidally forced” arms confirms that external disturbances can enhance or “lock” spiral patterns [8].
6. Galaxy evolution and secular change processes
6.1 Secular evolution via bars
Over time, bars can drive secular (gradual) evolution: gas accumulates in the central nucleus or pseudobulge region, star formation reshapes the galaxy core, and bar strength may vary. This “slow” morphological change differs from sudden major merger transformations and shows how internal disk dynamics can gradually alter a spiral galaxy from within [9].
6.2 Regulation of star formation
Spiral arms, whether based on density waves or local instabilities, are factories of new stars. Gas crossing the arms experiences compression that initiates star formation. Bars further accelerate this by transporting additional gas to the center. Over billions of years, these processes thicken the stellar disk, enrich the interstellar medium, and feed the central black hole.
6.3 Connections to bulge growth and AGN
Bar-driven flows can concentrate large amounts of gas near the nucleus, sometimes triggering AGN episodes if gas reaches the supermassive black hole. Repeated bar formation or dissolution periods can produce bulge properties, creating pseudobulges (with disk-like kinematics), unlike classical merger-formed bulges.
7. Future observations and simulations
7.1 High-resolution imaging
Future telescopes (e.g., extremely large ground-based, Nancy Grace Roman Space Telescope) will provide more detailed near-IR data on transverse spirals, allowing the study of star formation rings, dust lanes, and gas flows. This information will help refine models of bar-driven evolution over a wider redshift range.
7.2 Integral field spectroscopy (IFU)
IFU projects (e.g., MANGA, SAMI) capture velocity fields and chemical abundances across the galaxy disk, providing two-dimensional kinematic maps of bars and spirals. Such data clarify inflows, resonances, and star formation bursts, emphasizing the synergy of bar and spiral waves that grow the disk.
7.3 Advanced disk simulations
The latest hydrodynamic simulations (e.g., FIRE, IllustrisTNG submodels) aim to realistically create bar and spiral formation, including star formation and black hole feedback. Comparing these simulations with observational data on spiral galaxies allows for more accurate predictions of secular evolution, bar lifetime, and morphological change scenarios [10].
8. Conclusion
Spiral arms and bars – dynamic structures closely linked to the evolution of disk galaxies, embodying patterns of gravitational waves, resonances, and gas flows that regulate star formation and galaxy shape. Whether formed from long-lived density waves, swing amplification, or tidal interactions, spiral arms distribute star formation along graceful arc shapes, while bars act as powerful “angular momentum engines,” funneling gas inward to feed the nucleus and grow the bulge.
Together these features show that galaxies are not static – they constantly move inside and out through cosmic history. By further exploring bar resonances, spiral density waves, and changing stellar populations, we better understand how galaxies like our Milky Way developed into well-known but ever-changing spiral structures.
Links and further reading
- Lin, C. C., & Shu, F. H. (1964). “On the Spiral Structure of Disk Galaxies.” The Astrophysical Journal, 140, 646–655.
- Lin, C. C., & Shu, F. H. (1966). “A Theory of Spiral Structure in Galaxies.” Proceedings of the National Academy of Sciences, 55, 229–234.
- Toomre, A. (1981). “What amplifies the spirals?” Structure and Evolution of Normal Galaxies, Cambridge University Press, 111–136.
- Tully, R. B. (1974). “The kinematics and dynamics of M51.” The Astrophysical Journal Supplement Series, 27, 449–457.
- Athanassoula, E. (1992). “Formation and evolution of bars in galaxies.” Monthly Notices of the Royal Astronomical Society, 259, 345–364.
- Sanders, R. H., & Tubbs, A. D. (1980). “Bar-driven infall of interstellar gas in spiral galaxies.” The Astrophysical Journal, 235, 803–816.
- Romero-Gómez, M., et al. (2006). “The origin of the spiral arms in barred galaxies.” Astronomy & Astrophysics, 453, 39–46.
- Dobbs, C. L., et al. (2010). “Spiral galaxies: Flow of star-forming gas.” Monthly Notices of the Royal Astronomical Society, 403, 625–645.
- Kormendy, J., & Kennicutt, R. C. (2004). “Secular Evolution and the Formation of Pseudobulges in Disk Galaxies.” Annual Review of Astronomy and Astrophysics, 42, 603–683.
- Garmella, M., et al. (2022). “Simulations of Bar Formation and Evolution in FIRE Disks.” The Astrophysical Journal, 924, 120.