When Hot Disks Meet Spinning Halos: How Bars Can Still Form
Bar-shaped structures, simply called “bars”, are incredibly common in spiral galaxies today, including in our own Milky Way. But high-redshift galaxies (those seen when the universe was young) are known to have thicker and more turbulent disks that should, according to classical theory, resist forming bars. Yet recent observations with JWST show many such galaxies do host bars. In this study, Kataria investigates whether the spin of a dark matter halo might enable bar formation even in disks that are otherwise too “hot” and thick to become unstable.
Setting Up the Simulations
Kataria begins by reviewing the classical picture: bar formation is generally expected in kinematically cold disks, where stars move mostly in ordered circular orbits. Hotter disks, those with high random motion, have large Toomre Q values and should remain stable. To test whether halo spin alters this expectation, Kataria constructs two simulated Milky Way–like galaxies using N-body methods. Both galaxies contain a thick, dispersion-dominated stellar disk, but only one sits inside a spinning dark matter halo (with spin parameter Λ = 0.1). Importantly, the disk satisfies several well-known bar stability criteria, including the Toomre Q > 5.35 condition, ensuring that it starts out strongly bar-stable.
A Tale of Two Halos: What the Simulations Reveal
The simulations show a striking difference between the two setups. In the non-spinning halo, the disk remains smooth and axisymmetric for nearly 10 billion years. The face-on density maps display round, undisturbed contours, and the bar-strength measure A₂/A₀ stays below the instability threshold. Only the Jang–Kim stability criterion correctly predicts this behavior; both the Ostriker–Peebles and ELN criteria incorrectly suggest the disk should be unstable. When the same disk is placed in a spinning halo, however, a bar begins to form around 7 Gyr, as elongated, non-axisymmetric features.
The Role of Angular Momentum Transfer
Kataria traces this unexpected bar growth to an eight-fold increase in angular momentum transfer from the disk to the halo. In a spinning halo, resonant interactions more effectively pull angular momentum out of the disk, allowing a bar to grow even when classical stability criteria insist it should not. In short, halo spin changes the dynamical environment enough to flip a bar-stable disk into a bar-forming one. This means that current bar-formation criteria, which mostly ignore halo spin, are incomplete when applied to galaxies with rotating halos.
Implications for High-Redshift Galaxies
In the broader context, these results help explain why JWST finds bars in high-redshift galaxies that appear too thick and turbulent for bar formation. If such galaxies lived inside rapidly spinning halos, bar formation becomes not only possible but expected. Kataria suggests that future bar-formation criteria should incorporate halo spin alongside traditional measures of disk kinematics and mass distribution.
Conclusions: Rethinking Bar Formation Theory
Overall, the paper offers a compelling simulation-based demonstration that dark matter halo spin can act as a crucial driver of bar formation, even in disks that classical theory would deem stable. This insight reshapes our understanding of how some of the earliest galactic structures emerged in the young universe.
Source: Kataria
