Bars, Spins, and Neighbors: What Shapes the Formation of Galactic Bars?
Galactic bars are elongated structures of stars that cross the centers of many spiral galaxies, and understanding how they form helps astronomers learn how galaxies evolve over time. In this paper, Aquino-Ortíz and collaborators study why some disc galaxies host bars while others do not, focusing on both internal properties (like mass and motion of stars) and external influences (like nearby galaxies). Using observations from the MaNGA survey, which provides detailed, spatially resolved measurements of thousands of nearby galaxies, the authors compare barred and unbarred disc galaxies to identify the conditions that favor bar formation.
Background: Internal Instabilities and Environmental Effects
The paper begins by outlining the two main ideas for how bars form. One idea emphasizes internal dynamical instabilities: if a galaxy’s stellar disc is massive and dominates over its dark matter halo in the central regions, the disc can become unstable and naturally form a bar. Another idea highlights environmental effects, where gravitational interactions with neighboring galaxies can trigger or modify bar formation. The authors also stress the importance of angular momentum, which describes how strongly stars are supported by rotation. Discs with lower angular momentum are generally more prone to forming bars. These concepts motivate the main goal of the study: to disentangle how internal structure and environment work together to shape bar formation.
Data and Sample Selection
To tackle this question, the authors describe the MaNGA data and how they select their sample. They focus on nearly face-on disc galaxies that are not undergoing major mergers and have reliable bar classifications from the Galaxy Zoo 2 project. By excluding edge-on systems and weakly classified bars, they ensure that their final sample, about 1,900 galaxies, is clean and well suited for kinematic analysis. This careful selection allows for fair comparisons between barred and unbarred galaxies.
Measuring Mass and Motion in Galaxies
Next, the paper explains how key physical quantities are measured. The authors estimate the dynamical mass (the total mass inferred from galaxy motions) within one and two effective radii, and compare it to the stellar mass to see whether stars or dark matter dominate the inner regions. They also calculate the stellar angular momentum using the parameter λ_R, which summarizes how ordered the stellar motions are within a galaxy. Importantly, these measurements come from spatially resolved velocity maps, making them sensitive to the internal structure of galaxies, including the presence of bars.
Main Results: Internal Differences Between Barred and Unbarred Galaxies
The main results show clear differences between barred and unbarred galaxies. Bars are more common in galaxies with higher stellar mass and, crucially, in systems where stars dominate over dark matter in the central regions. Barred galaxies also tend to have lower stellar angular momentum in their inner parts. At fixed total stellar mass, barred galaxies are more centrally concentrated and less rotationally supported than unbarred ones. This strongly supports the idea that internal conditions, especially stellar mass dominance and low angular momentum, play a primary role in bar formation.
The Role of Environment and Galaxy Interactions
The authors then examine the role of environment using a parameter that measures the tidal influence of the nearest neighboring galaxy. Interestingly, the bar fraction shows a bimodal trend: bars are common in very isolated galaxies, where they likely form through internal secular processes, and they become common again in galaxies experiencing strong tidal interactions. In contrast, galaxies with intermediate interaction strengths show fewer bars. This suggests that while bars do not require interactions to form, external tides can trigger or enhance bar formation in galaxies that are already close to instability.
Conclusions: A Combined Picture of Bar Formation
In the final section, the authors combine all these factors, stellar mass dominance, angular momentum, and environment, to paint a unified picture. No single parameter fully determines whether a galaxy will host a bar. Instead, bars emerge most easily in galaxies that are internally primed for instability, with environmental interactions acting as a secondary influence. Overall, the study provides strong observational evidence that internal gravitational instabilities are the main driver of bar formation, while a galaxy’s surroundings can either help or hinder this process depending on the circumstances.
Source: Aquino-Ortíz
