Detection of an excess of young stars in the Galactic Centre Sagittarius B1 region

The study investigates where and how recent star formation has occurred in the Galactic Centre (GC), focusing on the Sagittarius B1 (Sgr B1) region. The GC, within ~8 kpc, exhibits extreme conditions (high stellar densities, strong tidal and magnetic fields, turbulence, and high ISM temperatures) yet has hosted substantial recent star formation (~0.1 M⊙ yr−1 over the past 10–100 Myr). Despite this, only two young massive clusters (Arches and Quintuplet) are known, accounting for <10% of the expected young stellar mass, leading to the “missing clusters” problem. Possible rapid dissolution of clusters in the GC tidal field and observational challenges (crowding, patchy extinction) may hide distributed young stars. Sgr B1 is a well-known H II region with evidence of widely spaced hot stars and several massive young stars. The research question is whether Sgr B1 harbors a significant, previously unrecognized young stellar population and what its star formation history (SFH) reveals about GC star formation and the assembly of the nuclear stellar disk.

Prior work has shown the GC contributes >10% of the Galaxy’s Lyman continuum flux while occupying <1% of its volume, with multiple indicators of recent elevated star formation (massive young stars, Cepheids, inferred SFHs). Only the Arches and Quintuplet clusters are well established, suggesting either rapid cluster dissolution or observational biases as explanations for the missing clusters. Sgr B1 has been identified as a patchwork of H II regions with far-infrared evidence for widely spaced hot stars and contains known O-type and WN7–9ha stars. Studies have proposed that exciting sources in Sgr B1 may not have formed in situ. There is also evidence for metallicities around twice solar in the GC and for potential age gradients in nuclear stellar disks of external galaxies, supporting inside-out formation scenarios.

Data and fields: Used H and Ks-band observations from the GALACTICNUCLEUS JHKs survey (~0.2″ resolution) covering part of Sgr B1 (~160 pc^2) and a control field of similar size in the inner nuclear stellar disk; both observed under excellent seeing (~0.4″). Additional comparisons used a central nuclear disk field (~1,600 pc^2) from prior work. Bright-star saturation in Ks was addressed by replacing saturated photometry with SIRIUS/IRSF data and cross-checking with 2MASS. Foreground removal and extinction correction: Applied a color cut in H−Ks (~1.3 mag) to remove foreground disk/bulge stars. Constructed high-resolution extinction maps using red clump and red giant stars with intrinsic (H−Ks) ≈ 0.10 ± 0.01 mag. Adopted extinction law AH/AKs = 1.84 ± 0.03, pixel size ~2″, using the five nearest reference stars (within 7.5″) with inverse-distance weighting; typical statistical uncertainty ~3%, systematic ~5%. Dereddened photometry and culled over-dereddened outliers. K-band luminosity functions (KLFs): Built dereddened KLFs (including stars lacking H detections) after foreground removal. Bin widths were chosen via Freedman–Diaconis and Sturges criteria. Corrected for completeness using artificial star tests (20 modified images per field, inserting ~5% of stars per 0.5 mag bin starting at Ks ~12 mag). Set a completeness floor at 75%; propagated completeness and Poisson uncertainties. Restricted analysis to Ks > 8.5 mag to avoid saturation biases; verified with 2MASS that brighter bins are incomplete. SFH inference via model fitting: Fitted observed KLFs with linear combinations of synthetic single-age populations using two independent stellar evolution model sets to assess systematics: PARSEC and MIST, both at ~2× solar metallicity. For PARSEC, sampled ages at 14, 11, 8, 6, 3, 1.5, 0.6, 0.4, 0.2, 0.1, 0.04, 0.02, 0.01, 0.005 Gyr; for MIST, a similar coverage. Assumed a Kroupa IMF (PARSEC; corrected for unresolved binaries) and a Salpeter IMF (MIST). Included a distance modulus parameter (~14.6) within uncertainties and a Gaussian smoothing to represent distance/differential extinction spread. Created 1,000 Monte Carlo realizations of each KLF by perturbing star counts by their uncertainties, fit each realization by χ^2 minimization, and combined ages into five bins: >7 Gyr, 2–7 Gyr, 0.5–2 Gyr, 0.06–0.5 Gyr, and 0–0.06 Gyr. Final SFHs are averages of PARSEC and MIST results with quadratically combined uncertainties. Targeted sub-region: Performed a focused analysis of a ~40 pc^2 Sgr B1 sub-region with intense 4.5 μm hot dust emission, building and fitting a dedicated KLF to assess the youngest populations (5, 10, 20, 40 Myr components). Mass estimates and robustness checks: Estimated total originally formed stellar mass by scaling KLF fits (PARSEC) and averaging over Monte Carlo draws. Extensively tested systematics: metallicity variations (solar, 1.5× solar), KLF bin widths (half/double), cumulative KLF fitting (to remove binning effects), faint/bright-end limits, completeness alternatives, IMF variations (top-heavy), and unresolved multiplicity using SPISEA with MIST. Also validated the method with artificial SFHs and by applying it to the Quintuplet cluster field (detected expected young mass).

• Sgr B1 exhibits a distinct SFH compared to a control inner nuclear disk field and the central nuclear disk region. The Sgr B1 population is on average younger, with a substantial intermediate-age (2–7 Gyr) component and a significant contribution from young stars.
• Young stars: The youngest age bin (<60 Myr) contributes >5% of the total stellar mass in Sgr B1, about six times higher than in the control field and roughly two times higher than in the central nuclear disk (where known young clusters reside).
• Intermediate-age population: Sgr B1 shows an enhanced 2–7 Gyr component (~40% of the total stellar mass), which appears rare or absent in the innermost nuclear disk, suggesting an age gradient consistent with inside-out formation.
• Quantitative SFH for Sgr B1 (from Fig. 4a; average of PARSEC and MIST): approximate mass fractions per bin are consistent with ~42% (>7 Gyr), ~42% (2–7 Gyr), ~7.7% (0.5–2 Gyr), ~2.2% (0.06–0.5 Gyr), and ~5.7% (0–0.06 Gyr).
• Total mass and young mass: The Sgr B1 field contains an originally formed stellar mass of ~7.6 ± 0.7 × 10^6 M⊙. The mass in young stars is at the level of several 10^5 M⊙ formed ~10 Myr ago, exceeding the combined mass of the Arches and Quintuplet clusters and addressing part of the missing clusters problem.
• Hot dust sub-region: In a ~40 pc^2 sub-region with intense 4.5 μm emission, >7% of the stellar mass is <60 Myr old. The 5–10 Myr components contribute 6 ± 1% of the total stellar mass there and dominate the young mass (contributing to ~99% of Monte Carlo realizations), indicating an excess of ~5–10 Myr stars relative to the surroundings. Across the whole Sgr B1 field, 5–10 Myr stars account for 2 ± 2% of the total mass.
• Formation site and dynamics: At a galactocentric radius of ~80 pc, the stellar rotation period is ~5 Myr. The ~5–10 Myr age implies the young population likely did not form in situ and has orbited at least once, consistent with prior suggestions that Sgr B1’s ionizing sources did not originate there.
• Cluster dissolution and association: The inferred young stellar mass (~10^5 M⊙ scale) and theoretical upper limits (~10^4 M⊙) for bound clusters in the central molecular zone argue against formation as a single bound cluster; instead, the stars likely formed as a coeval association (possibly including multiple short-lived bound clusters) that dispersed rapidly due to tidal shocking by giant molecular clouds (~6 Myr disruption timescale).
• Method validation: Application of the KLF-fitting method to the Quintuplet field recovered a young mass of order 10^4–10^5 M⊙, consistent with expectations, demonstrating sensitivity to young populations despite completeness limits.

The findings reveal that Sgr B1 hosts a substantial young stellar component, sufficient to account for a significant fraction of the expected recent star formation in the GC outside of the two known clusters. This distributed young population supports a scenario in which stars form in massive associations (which may temporarily include bound clusters) that subsequently disperse within a few Myr under GC tidal conditions, explaining the scarcity of long-lived, easily identifiable clusters and addressing the missing clusters problem. The elevated young fraction relative to the control and even the central nuclear disk indicates localized recent star formation or the recent passage of a young association through Sgr B1 that now ionizes the surrounding gas and dust. The detection of a strong intermediate-age (2–7 Gyr) component in Sgr B1, contrasting with the inner nuclear disk, implies an age gradient across the nuclear stellar disk and supports inside-out growth, consistent with trends observed in external galactic nuclei. The kinematic timescales (rotation period ~5 Myr at ~80 pc) and age estimates (~5–10 Myr) suggest the young stars likely formed elsewhere in the nuclear disk and have dispersed throughout Sgr B1. The presence of nearby X-ray-emitting O-type and WN7–9ha stars and a candidate supernova remnant is consistent with this young stellar population. Overall, the results integrate with models of GC star formation cycles that peak around ~10 Myr ago and with theoretical expectations for rapid cluster disruption in the central molecular zone.

This study provides evidence for an excess of young stars in the Sgr B1 region of the Galactic Centre, identifying several 10^5 M⊙ of ~5–10 Myr-old stars and a total young (<60 Myr) mass fraction exceeding 5%. The results demonstrate that recent star formation in the GC is not confined to the known massive clusters but includes widely distributed young populations, consistent with rapid dissolution of clusters and the formation of massive associations. The pronounced intermediate-age component in Sgr B1, compared to inner nuclear regions, points to an inside-out formation of the nuclear stellar disk. These insights advance our understanding of GC star formation under extreme conditions, the fate of young clusters, and possible IMF variations. Future work should focus on spectroscopic follow-up to refine age and metallicity estimates of the young populations, deeper photometry to improve KLF constraints at the bright and faint ends, kinematic measurements to reconstruct orbits and formation sites, and broader surveys to complete the census of distributed young stars across the nuclear disk.

• Distance and line-of-sight uncertainties hinder precise orbit reconstruction and exact formation site identification for the young population.
• Photometric saturation and completeness limits constrain the bright and faint ends of the KLFs (analysis restricted to Ks > 8.5 mag; completeness >75%).
• Dereddening relies on extinction maps and an assumed extinction law; residual differential extinction and distance spread are modeled but remain sources of uncertainty.
• SFH inferences depend on stellar evolution models (PARSEC, MIST), assumed metallicity (~2× solar), and IMF choices; some degeneracies between nearby ages persist despite binning.
• Possible systematic effects from unresolved multiplicity, binning choices, and completeness corrections were tested and found not to alter the young-star conclusion, but residual systematics may affect detailed mass fractions, especially for older bins.
• The control and central comparison fields have differing data quality and completeness, which can inflate uncertainties in comparative SFHs.