STEP survey: III. STEPping stones between the clouds: the star formation history of the Magellanic Bridge

TL;DR

This study reconstructs the star formation history (SFH) of the Magellanic Bridge using deep STEP optical data and a synthetic color-magnitude diagram approach. By employing two stellar evolution libraries (PARSEC-COLIBRI and BaSTI) across a wide age (10 Myr–13.4 Gyr) and metallicity (-2.0 ≤ [Fe/H] ≤ 0) grid, and incorporating distance/extinction, Milky Way foregrounds, and photometric uncertainties, the authors derive spatially resolved SFHs and metallicity histories for 14 Bridge tiles. They find a pronounced recent star-formation peak around 100 Myr ago, strongest toward the SMC, with young stars having metallicity near [Fe/H] ≈ -0.6 dex, while older populations associated with the LMC are more metal-poor. The global Bridge SFH implies a total stellar mass of about (5.1 ± 0.2) × 10^5 M⊙, and the age–metallicity relation suggests most Bridge stars come from SMC material, with in situ formation largely limited to the youngest component. Additionally, the study tests the Kennicutt–Schmidt relation in this low-density, tidally influenced environment and finds consistency with KMT09 predictions, extending the relation to regimes of low Σ_gas. Overall, these results provide quantitative constraints on the MC interaction history and illuminate the tidal processes that assemble the Magellanic Bridge.

Abstract

The Magellanic Clouds (MCs) offer a unique laboratory for studying galaxy interaction and the evolution of dwarf galaxies. By investigating when and how stars formed, the star formation history (SFH) is a powerful tool to provide constraints for dynamical modeling of the system's past interactions and understand the processes of stripping and triggered star formation in tidally influenced environments. We aim to reconstruct the SFH of the Magellanic Bridge, the gaseous and stellar stream connecting the two Clouds. We used data from the deep optical STEP survey, which covers 54 $\mathrm{deg\, {^{2}}}$ across the Small Magellanic Cloud (SMC) and the Bridge, reaching stars below the oldest main sequence turnoff at the distance of the MCs. We applied the synthetic color-magnitude diagram (CMD) technique to 14 deg$^2$ of STEP data. We constructed two libraries of synthetic stellar populations based on the PARSEC-COLIBRI and BaSTI stellar evolutionary models, with metallicities in the range $-2.0\leq[$Fe/H$]\leq0$ across the whole Hubble time. We find a clear peak of recent star formation $\sim100$ Myr ago in the Magellanic Bridge, which becomes increasingly pronounced toward the SMC. The low metallicity of this population suggests that it formed from gas stripped from the SMC during its most recent close encounter with the LMC. In the eastern part of the Bridge (LMC side), the star formation peaks at earlier times, around 10 Gyr and 2 Gyr ago. We estimate a total stellar mass in the Bridge of $ (5.1 \pm 0.2) \times 10^5 M_\odot$ and a present-day stellar metallicity of $[$Fe/H$]\sim-0.6$ dex, close to SMC value.

Figures (18)

• Figure 1: Footprint of the STEP survey pointings (blue boxes), showing the location of the SMC body and the Bridge. The area covered by this work is highlighted in red. The LMC is in the left direction, outside the map. The black dots mark the position of star clusters and associations from the catalogue of 2020AJ....159...82B.
• Figure 2: Hess diagrams of the completeness for all analyzed tiles. In each cell of the CMD the completeness is computed as the ratio of recovered artificial stars with respect to the number of input stars. The red line indicates the 50% completeness level.
• Figure 3: Difference between the output and input magnitude vs. input magnitude in $g$ (top) and $i$ (bottom) filters derived from AS tests for tile $3\_17$. The red solid line marks the median of the distributions, while the dashed lines indicate the $5^{th}$ and $95^{th}$ percentiles of the distributions.
• Figure 4: CMD of the tile 4_14 from the YMCA survey, used to model the MW contamination in our analysis.
• Figure 5: Stellar density distribution for Galactic longitude $l=289^\circ$ across different latitude bins ($-44$° $\leq b \leq -38$°). Each panel displays the normalized $G_{BP}-G_{RP}$ color distribution for different magnitude intervals sourced from the TRILEGAL Galactic model 2005AA...436..895G2012ASSP...26..165G. The Gaia EDR3 distributions at $b=-42°$ are plotted in gray.