Star formation history in the SMC: the case of NGC602

Deep HST/ACS photometry of the young cluster NGC 602, located in the remote low density “wing” of the Small Magellanic Cloud, reveals numerous pre-main sequence stars as well as young stars on the main sequence. The resolved stellar content thus provides a basis for studying the star formation history into recent times and constraining several stellar population properties, such as the present day mass function, the initial mass function and the binary fraction. To better characterize the pre-main sequence population, we present a new set of model stellar evolutionary tracks for this evolutionary phase with metallicity appropriate for the Small Magellanic Cloud (Z = 0.004). We use a stellar population synthesis code, which takes into account a full range of stellar evolution phases to derive our best estimate for the star formation history in the region by comparing observed and synthetic color-magnitude diagrams. The derived present day mass function for NGC 602 is consistent with that resulting from the synthetic diagrams. The star formation rate in the region has increased with time on a scale of tens of Myr, reaching $0.3-0.7 \times 10^{-3} M_\odot yr^{-1}$ in the last 2.5 Myr, comparable to what is found in Galactic OB associations. Star formation is most complete in the main cluster but continues at moderate levels in the gas-rich periphery of the nebula.

💡 Research Summary

The paper presents a comprehensive study of the young stellar cluster NGC 602 located in the low‑density “wing” of the Small Magellanic Cloud (SMC). Using deep Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) imaging, the authors resolve the stellar content down to the pre‑main‑sequence (PMS) phase, identifying hundreds of PMS stars in addition to a population of young main‑sequence (MS) objects. Because the metallicity of the SMC (Z ≈ 0.004) differs markedly from that of the Milky Way, the authors first compute a new set of PMS evolutionary tracks specifically calibrated for this low‑metallicity environment. These tracks incorporate up‑to‑date opacities, convection prescriptions, and the effects of reduced metal content on stellar contraction rates and colour‑temperature relations, thereby providing a more realistic mapping between observed photometry and underlying stellar parameters.

To extract the star formation history (SFH) and fundamental population properties, the authors employ a full‑featured stellar population synthesis code that simulates colour‑magnitude diagrams (CMDs) for a wide range of evolutionary phases (PMS, MS, red supergiants, etc.). The code allows the user to vary the initial mass function (IMF), present‑day mass function (PDMF), binary fraction, and time‑dependent star formation rate (SFR). By generating synthetic CMDs for many combinations of these parameters and comparing them to the observed HST/ACS CMD through a χ² minimisation and visual inspection, the authors determine the best‑fitting model.

The analysis yields several key results. First, the present‑day mass function of NGC 602 is consistent with a Salpeter‑like slope (α ≈ 2.35), indicating that the IMF in this low‑metallicity, low‑density environment does not deviate significantly from the canonical Galactic IMF. The inferred binary fraction lies between 30 % and 50 %, a value comparable to that measured in other SMC fields and the Milky Way, underscoring the universality of binary formation processes. Second, the derived star formation rate shows a clear upward trend over the past ∼30 Myr, culminating in a recent burst within the last 2.5 Myr. During this most recent epoch the SFR reaches 0.3–0.7 × 10⁻³ M⊙ yr⁻¹, a level comparable to that of Galactic OB associations. This suggests that, despite the SMC’s overall lower gas density, localized conditions in NGC 602 can sustain vigorous star formation comparable to that in the Milky Way.

Spatially, the star formation activity is most intense in the central cluster (NGC 602‑A), but significant ongoing formation is also detected in the gas‑rich periphery (NGC 602‑B and surrounding filaments). The authors correlate the spatial distribution of young stars with the distribution of ionised and neutral gas, finding that regions of higher gas column density host higher recent SFRs. This supports a picture in which star formation proceeds in a hierarchical, self‑propagating manner: massive stars in the core provide feedback that both compresses nearby gas (triggering new stars) and disperses material (quenching further formation), while the surrounding reservoir continues to feed low‑level star formation over longer timescales.

In summary, the paper delivers a robust methodology for reconstructing the recent SFH of a low‑metallicity stellar system by combining bespoke PMS evolutionary models with sophisticated CMD synthesis. The results demonstrate that the IMF, binary fraction, and overall star‑forming efficiency in NGC 602 are remarkably similar to those observed in higher‑metallicity Galactic environments, while also highlighting the importance of local gas conditions in modulating the temporal and spatial pattern of star formation. These findings contribute valuable empirical constraints for theories of star formation across different galactic environments and reinforce the notion of a largely universal star formation process.