Telescópio Espacial James Webb (JWST) has revealed unprecedented details about emerging young star clusters in the spiral galaxy NGC 628, located about 30 million light-years from Terra. An international team of astronomers used the observatory’s NIRSpec instrument to penetrate molecular dust clouds and characterize 14 of these clusters, identified as eYSCs (emerging young star clusters). The observations, carried out under the FEAST program, detected intense emissions of ionized hydrogen, molecular hydrogen and polycyclic aromatic hydrocarbons (PAHs), indicating energetic early evolutionary phases driven by hot massive stars.
The data show that the investigated clusters have median ages of around 3 million years, with ionizing photon fluxes compatible with stars of spectral types O8.5V to O8V. The associated photodissociation regions (PDRs) exhibit bright emissions from PAHs at 3.3 µm and multiple molecular hydrogen transitions. As clusters age and emerge from the natal cloud, both molecular and PAH emission decrease, revealing a direct connection between cluster evolution and PDR morphology.
The observations combine NIRCam images in specific filters, such as F335M for PAHs, F405N for Brα and F444W for stellar continuum, with NIRSpec spectroscopy in MOS mode. The galaxy NGC 628, also known as Messier 74, has well-defined spiral arms and a global star formation rate of 1.7 solar masses per year.
Details of spectral observations
The hydrogen and helium recombination lines track H II regions driven by eYSCs. Emissões of PAHs and H₂ arise predominantly from PDRs close to the clusters.
Younger clusters dominate the ionizing emission, while signatures of evolved stars, such as red supergiants, appear in objects older than 9 million years.
Spectral characterization encompassed PDRs and the diffuse interstellar medium (ISM), providing insights into early stellar feedback.
Evolution of clusters and PDRs
The decrease in PAHs and molecular hydrogen emissions as clusters age indicates that PDRs change morphologically as clusters emerge from natal dust.
Hot massive stars in eYSCs generate intense ionizing radiation, shaping the surroundings and facilitating the transition to more exposed phases.
The process highlights how feedback from young stars impacts the multiphase ISM in spiral galaxies.
Characteristics of the galaxy NGC 628
NGC 628 is estimated to be between 10 and 13 billion years old and exhibits vibrant star formation activity in its spiral arms.
The relative proximity allows for detailed observations of eYSCs, which serve as fundamental building blocks for studying galactic evolution.
The JWST data overcome limitations of optical wavelengths, where dust obscures these young objects.
FEAST program and NIRSpec instrument
The FEAST program focuses on feedback in emerging extragalactic star clusters, utilizing NIRSpec’s capability for multi-object spectroscopy.
The Micro-Shutter Assembly (MSA) settings allowed observing multiple positions with high spatial resolution.
NIRSpec’s slit footprints reveal precise spatial distributions of detected emissions.
Implications for star formation
eYSCs represent critical phases where stellar feedback begins to disperse natal gas and dust.
Observations confirm that dense, gravitationally bound clusters form fundamental structures in gas-rich environments.
Detection of specific lines reinforces models of early evolution of massive clusters.
Published study results
The work, released in preprint on arXiv on March 10, 2026, presents the first detailed spectral characterization of an initial sample of eYSCs in NGC 628.
Ages derived from spectral energy distribution (SED) fits match those inferred spectroscopically.
The authors highlight the connection between molecular/PAH emission and the emergence of clusters.
Additional observational perspectives
NIRSpec allowed us to penetrate dense clouds, revealing objects invisible in optics.
Composite images highlight positions of eYSCs and magnified regions of interest.
Future analyzes could expand the sample to better understand the star formation cycle in the galaxy.