Spiral arms and bars drove gas transport in early massive galaxies
Data from NOEMA and JWST reveals that massive disk galaxies during cosmic noon used organized structural features to transport cold gas, challenging the view that early galaxies were predominantly chaotic.
Research from the Infrared & Submillimeter Astronomy Group at the Max Planck Institute for Extraterrestrial Physics (MPE) has revealed that massive disk galaxies during "cosmic noon" utilized spiral arms and bars to actively transport cold gas inward. This mechanism served as a galactic fuel pump, sustaining star formation by efficiently distributing gas across disks during a period roughly 8 to 10 billion years ago when star formation rates far exceeded those of the modern universe.
The findings, derived from the NOEMA3D survey, challenge previous astronomical assumptions that early galaxies were predominantly chaotic or driven by mergers. Instead, the data shows well-ordered disk galaxies with structural features that were previously thought to be rare or absent at such redshifts.
To reach these conclusions, the team used the NOrthern Extended Millimeter Array (NOEMA) in the French Alps to conduct the deepest millimeter-wave observations of cold molecular gas to date. These observations focused on ten massive, star-forming galaxies at redshifts z ~ 1.1–1.6. The researchers combined this data with high-resolution infrared imaging from the James Webb Space Telescope (JWST) to map the underlying stellar structures.
The JWST imagery confirmed that many of these systems are organized disks. In four out of the ten cases studied, the galaxies contained bars. While all ten galaxies exhibited ordered rotation, the team discovered coherent velocity residuals—gas motions that simple rotation cannot explain. These residuals, which reach typical in-plane velocities of 50 to 100 km/s, are spatially correlated with the bars and spiral arms.
"For the first time, we can directly link spiral arms and bars to the motions of cold gas within galaxies,"
Jean-Baptiste Jolly, MPE
The study suggests these structures are dynamical tools that redistribute gas. When viewed as radial inflows, the molecular gas transport rates are often comparable to the star formation rates of the galaxies, which are of order tens of solar masses per year. Such flows may feed central star formation, contribute to the growth of bulges, and provide material for central supermassive black holes.
A companion study examined the location of cold gas and dust, finding that these materials are generally extended over the full galactic disk. The sizes are broadly comparable to the stellar component, and both the gas depletion time and molecular gas fraction remain flat out to approximately twice the stellar effective radius. This differs from merger-driven compact starburst galaxies, where star formation and dust are typically concentrated in small central regions.
"The depth of the NOEMA observations allows us to trace the cold-gas reservoirs that fueled galaxy growth during cosmic noon,"
Jianhang Chen, MPE
While the NOEMA3D data highlights the role of these structures in early massive galaxies, other research using the PHANGS survey has investigated the role of spiral arms in nearby spiral galaxies. A study of 22 nearby spiral galaxies found that cloud lifetime and feedback timescale distributions were similar in both spiral arms and inter-arm regions. This suggests that in those specific nearby systems, spiral arms are unlikely to play a dominant role in triggering star formation. In fact, that study found star formation efficiency appeared slightly higher in inter-arm regions compared to spiral arms.
The interplay of galactic dynamics is further complicated by the physics of molecular clouds. Star formation generally involves the assembly of cold molecular clouds followed by the collapse of denser regions. This process is influenced by spiral-arm compression, tidal forces, shear, and turbulence from supernova feedback. In some nearby galaxies, such as M51, the ratio between star formation rate and molecular gas mass is much lower in the inner parts of the southern arm than elsewhere, which has been attributed to a dynamical suppression of star formation.
Collectively, these studies indicate that by z ~ 1–2, massive star-forming galaxies had already developed the organized disks necessary to drive significant gas transport. This capability helped keep these galaxies on the star-forming main sequence and shaped the buildup of black holes, bulges, and disks over cosmic time.
