The impact of radial migration on disk galaxy star formation histories: II. Role of bar strength, disk thickness, and merger history

Abstract

Reconstructing the star formation history (SFH) of disk galaxies is central to understanding their growth and evolution, yet such estimates can be strongly biased by stellar radial migration over cosmic time. Using 186 Milky Way (MW) and Andromeda (M31) analogs from the TNG50 cosmological simulation, we compare star formation rates (SFRs) inferred from present-day stellar positions with those based on stellar birth radii to quantify the magnitude, spatial structure, and temporal evolution of migration-induced biases. We find that radial migration systematically produces artificial star formation in regions that had not yet formed stars. Notably, ~80% of galaxies exhibit outer-disk stars older than 10 Gyr, which we find to have formed at radii interior to the outer disk and to have reached their present locations via substantial outward migration. Similar effects appear in ~45% of galaxies at intermediate radii during early epochs, and in 30% of quenched inner disks within the past 4 Gyr. Migration also smooths SFHs, washing out localized bursts and suppressions by dispersing stars across neighboring radii. The strength and imprint of these distortions depend sensitively on galactic structure and evolutionary history: strong bars drive mean SFR overestimates of up to 75% in the inner disk and 150% in the outskirts; thinner, dynamically cold disks suffer average outer-disk biases up to 160%; while thick disks exhibit typical inner-disk biases up to 125%. Merger timing further modulates these patterns. Our results demonstrate that failing to account for stellar migration can lead to severe misinterpretations of when and where stars formed, with direct implications for the chemical and evolutionary histories of the MW and external galaxies.

Figures (15)

• Figure 1: Comparison of galaxy stellar density maps across different bar strength bins, using bar strength measurements from Khoperskov2024. In each panel, galaxies are shown face-on. Bar strength increases from left to right and from top to bottom. Galaxies with weak bars (top panels) appear more symmetric and round, while galaxies with stronger bars (bottom panels) clearly show more elongated and asymmetric central structures, characteristic of prominent bars.
• Figure 2: Comparison between the ratios $h_{z,\text{thick}}/h_d$ and $h_{z,\text{thin}}/h_d$. Disk thickness is classified using $h_{z,\text{thick}}/h_d$: thinner ($\leq 0.3$), intermediate (0.3–0.46), and thicker ($\geq 0.46$) disks, with red dashed vertical lines marking the boundaries. The positive correlation and broader spread in thick disk ratios suggest that vertical heating affects both thin and thick disk components, allowing a systematic distinction between disk thickness categories.
• Figure 3: Face-on (top row) and edge-on (bottom row) stellar density maps for three galaxies representing the three disk thickness categories used in this study: a thinner disk (ID: 560751, left), an intermediate thickness disk (ID: 575356, center), and a thicker disk (ID: 419618, right). The vertical extent of each disk becomes progressively larger from left to right in the edge-on views, illustrating the morphological differences captured by the scale-height-to-length ratio $h_{z,thick}/h_d$.
• Figure 4: Example of early merger (galaxy ID 447914), in face-on (upper panels) and edge-on (lower panels) views at three stages in cosmic time: at pre-merger, $t_p=2.84$ Gyr (left), at the time of merger, $t_m=3.90$ Gyr (center), and at final time, $t_f=13.80$ Gyr (right). Initially, the galaxy appears thin and relatively low in mass. Following the merger, the disk becomes more massive and significantly thicker, with stars redistributed to larger vertical distances, suggesting that the merger dynamically heated the disk and contributing to the structural growth of the galaxy.
• Figure 5: Example of late merger galaxy (ID: 372754), shown in face-on (top) and edge-on (bottom) views before merger, $t_p=10.65$ Gyr (left), at the time of the merger, $t_m = 11.14$ Gyr (center), and at the final time, $t_f = 13.80$ Gyr (right). At the time of the merger, the galaxy retains some semblance of a disk-like structure, though it already shows signs of disruption. By the final snapshot, the disk becomes more disturbed and puffed up, especially in the vertical direction. This suggests that the late merger deposited energy into the system too recently for the galaxy to fully relax, leading to a disrupted morphology and significant vertical thickening.