Understanding the Origin and Dynamical Evolution of the Unique Open Star Cluster Berkeley 20 using FIRE Simulations

TL;DR

Berkeley 20-like open clusters encode a dynamic history of the Milky Way's disk. The authors analyze a FIRE-2 MW-mass zoom-in simulation (m12f) to identify a Be20 analog, track its multi-Gyr orbital evolution with high time resolution, and quantify local environmental perturbations. They find a two-phase history: an internally driven outward migration via spiral-arm–induced cold torquing that increases the guiding radius with minimal vertical heating, followed by a satellite-driven perturbation that significantly raises the maximum vertical excursion and reverses radial migration, yet preserves bound status. This work demonstrates that Be20-like clusters can retain a dynamical memory of both internal disk structure and external satellite interactions, reinforcing the necessity to include migration and satellite heating when interpreting OC orbits in Gaia-era data.

Abstract

Open clusters (OCs) act as key probes that can be leveraged to constrain the formation and evolution of the Milky Way (MW)'s disk, as each has a unique chemical fingerprint and well-constrained age. Significant Galactic dynamic interactions can leave imprints on the orbital properties of OCs, allowing us to use the present day properties of long-lived OCs to reconstruct the MW's dynamic history. To explore these changes, we identify OC analogs in FIRE-2 simulations of MW-mass galaxies. For this work, we focus on one particular FIRE-2 OC, which we identify as an analog to the old, subsolar, distant, and high Galactic latitude MW OC, Berkeley 20. Our simulated OC resides ~6 kpc from the galactic center and ultimately reaches a height $|Z_{\mathrm{max}}|>2$ kpc from the galactic disk, similar to Berkeley 20. We trace the simulated cluster's orbital and environmental history, identifying key perturbative episodes, including: (1) an interaction with a gas overdensity in a spiral arm that prompts an outward migration event and (2) a substantial interaction with a Sagittarius Dwarf Spheroidal Galaxy-mass satellite that causes significant orbital modification. Our simulated OC shows significant resilience to disruption during both its outward migration and the satellite-driven heating event that causes subsequent inward migration. Ultimately, we find these two key processes -- migration and satellite heating -- are essential to include when assessing OC orbital dynamics in the era of Gaia.

Figures (4)

• Figure 1: BOB undergoes major orbital changes. Panel a shows the ratio of the local to annular stellar (blue) and median gas (dark orange) densities over time. Blue/dark orange vertical dotted lines indicate when BOB encounters a significant stellar/gaseous overdensity. We identify stellar density peaks after the disk settles at $\sim$9.25 Gyr Yu2021 and before the merger event. The black, right-most dash-dotted line at $\sim$12.4 Gyr indicates the merger event with a $\sim$Sgr dSph-mass satellite, after which BOB's vertical amplitude increases dramatically. Panel b tracks BOB's height above/below the midplane, while panel c shows its instantaneous and guiding radius over time. Panel d shows BOB's virial ratio, which stays $\lesssim1$ after 10 Gyr, indicating the cluster remains bound. Panel e shows the cluster's angular momentum, $L_z$, with blue/red points indicating BOB's location on the leading/trailing edge of a dominant spiral arm, respectively, otherwise BOB is in an inter-arm region.
• Figure 2: BOB experiences an increase in $L_z$ while on the trailing edge of a dominant spiral arm. The stellar mass distribution is shown with solid/dashed contours quinn2025 indicating the 25%, 15%, 2.5% over/under densities. BOB is indicated by the green star. The bottom right panel shows the ratio of the local to annular stellar and median gas density (top) and $L_z$ (bottom), with red/blue points marking BOB's position on the trailing/leading edge of a dominant spiral arm. The shaded region highlights the time window shown in the five top-down panels (10.32–10.42 Gyr). BOB increases in $L_z$ while it interacts with the trailing edge of the spiral arm, consistent with expectations from cold torquing danielwyse2018 and corresponding to a quick rise in $R_{guiding}$ without a large change in vertical amplitude. An animation of BOB’s dynamic evolution from 8.68 to 13.78 Gyr is available in the online Journal. The animation includes two panels. Left: Top-down view of BOB (green star) overlaid on the stellar mass distribution in the m12f simulation with solid/dashed contours showing the stellar over/under densities. Right: time-series of major orbital changes in BOB (as in Figure \ref{['figure:4panel']}).
• Figure 3: BOB's vertical height changes significantly in response to the satellite's passage. Top-down and side-on visualization of insitu and exsitu stars in m12f during and after the satellite interaction. BOB is marked in green in each panel, with a 4 kpc$^2$ zoom centered on BOB. In-situ stars are shown in the background, and ex-situ stars are highlighted in blue. Arrows indicate the satellite’s center of velocity vectors, originating from its center of mass, computed via an iterative zoom-in method. Left: Satellite appears on the lower right edge-on. The region surrounding BOB is well populated with satellite stars, and BOB is near the mid-plane of the disk. Middle: Satellite passes through the disk, and the bulk of its stars pass near BOB. Right: Satellite is at the outskirts of the disk, above the mid-plane. There are few exsitu stars near BOB, but the impact of the satellite interaction can be seen by the drastic change in $Z$.
• Figure 4: The satellite's passage induces significant asymmetry in the stellar mass distribution, $\mathbf{\rho}$(Z), about the midplane. We measure the asymmetry within a cylindrical volume centered on BOB with a radius of 1.4 kpc and height of $\pm3$ kpc. The stellar disk near BOB is twice as asymmetric as anti-BOB, which corresponds to the same cylindrical region but on the opposite side of the disk azimuthally immediately following the satellite passage (marked by the dashed gray line). This spike is evidence of a strong, localized vertical mass redistribution in the disk surrounding BOB, a direct response of the satellite encounter.