Bar formation and evolution in the cosmological context: Inputs from the Auriga simulations (2406.09453v3)

Abstract: Galactic bars drive the internal evolution of spiral galaxies, while their formation is tightly coupled to the properties of their host galaxy and dark matter halo. To explore what drives bar formation in the cosmological context and how these structures evolve throughout cosmic history, we use the Auriga suite of magneto-hydrodynamical cosmological zoom-in simulations. We find that bars are robust and long-lived structures, and we recover a decreasing bar fraction with increasing redshift which plateaus around $\sim20\%$ at $z\sim3$. We find that bars which form at low and intermediate redshifts grow longer with time, while bars that form at high redshifts are born `saturated' in length, likely due to their merger-induced formation pathway. This leads to a larger bar-to-disc size ratio at high redshifts as compared to the local Universe. We subsequently examine the multi-dimensional parameter space thought to drive bar formation. We find that barred galaxies tend to have lower Toomre $Q$ values at the time of their formation, while we do not find a difference in the gas fraction of barred and unbarred populations when controlling for stellar mass. Barred galaxies tend to be more baryon-dominated at all redshifts and assemble their stellar mass earlier, while galaxies that are baryon-dominated but that do not host a bar, have a higher ex-situ bulge fraction. We explore the implications of the baryon-dominance of barred galaxies on the Tully-Fisher relation, finding an offset from the unbarred relation; confirming this in observations would serve as additional evidence for dark matter, as this behaviour is not readily explained in modified gravity scenarios.

• The paper leverages Auriga cosmological simulations to study galactic bar formation, characteristics, and evolution across cosmic time.
• It finds the fraction of barred galaxies decreases with increasing redshift, matching JWST observations, and that high-redshift bars form fully extended unlike lower-redshift bars.
• Barred galaxies are consistently more baryon-dominated and feature dynamically cooler disks with lower Toomre Q values throughout cosmic history, linking internal structure to bar formation potential.

Overview of "Bar formation and evolution in the cosmological context: Inputs from the Auriga simulations"

The paper "Bar formation and evolution in the cosmological context: Inputs from the Auriga simulations" by Francesca Fragkoudi et al. leverages the Auriga suite of magneto-hydrodynamical cosmological zoom-in simulations to investigate the formation, characteristics, and evolution of galactic bars, which are elongated structures of stars and gas within galaxies. The research examines the conditions under which bars form, their longevity, and their impact on the host galaxy's evolution over cosmic time.

Key Findings

1. Bar Fraction Trends:
   • The research observes that the fraction of barred galaxies decreases as redshift increases, stabilizing around 20% at redshift $z \approx 3$. This matches observational findings from tools like JWST, confirming that bars were less common in earlier epochs of the Universe.
2. Bar Formation and Evolution:
   • Bars formed at lower to intermediate redshifts tend to grow longer over time, whereas bars formed at high redshifts are born fully extended, likely due to merger-induced processes. This is indicative of different formation mechanisms depending on the redshift.
3. Baryon-Dominance:
   • Barred galaxies are found to be more baryon-dominated compared to their unbarred counterparts throughout cosmic history. This baryon dominance is linked to a deeper assembly of stellar mass early in the galaxy's evolution, facilitating bar formation.
4. Factors Influencing Bar Formation:
   • Lower Toomre $Q$ values, which signify dynamically cooler and more unstable disks, are more prevalent in galaxies that develop bars.
   • The paper found no significant difference in gas fraction between barred and unbarred galaxies when controlling for stellar mass, indicating that gas fraction does not directly influence bar formation.
5. Numerical and Observational Implications:
   • The alignment of barred galaxies with offsets from the abundance matching relation in the stellar mass-halo mass plane suggests that these galaxies do not conform to typical historical star formation and assembly patterns expected in a $\Lambda$CDM Universe.

Implications

The paper provides robust insights into the role of cosmic environment and internal galaxy dynamics in shaping the bar fraction and properties across cosmic time. The implication that bars at higher redshifts tend to be longer relative to their discs suggests observable properties by which future surveys could identify bars in high-redshift galaxies.

Future Directions

The results open new avenues for testing galaxy formation models under varied initial conditions and evolutionary tracks. Future work could explore:

• The role of detailed feedback processes in the onset of bar instability.
• The potential of bars as indicators for the dynamical age and merger history of galaxies.
• Extending the analysis to a broader range of galactic environments, including lower-mass systems.

In summary, the paper comprehensively models the connection between cosmic structure formation and internal dynamical processes of galaxies. This work enhances our understanding of how barred structures manifest and transform, driven by environmental and intrinsic triggers across the vast timeline of the Universe.