JWST probes emerging young star clusters in nearby spiral galaxy NGC 628

Using the James Webb Space Telescope (JWST), an international team of astronomers has observed a nearby spiral galaxy known as NGC 628. Results of the observational campaign, published March 10 on the arXiv pre-print server, shed more light on the population of emerging young star clusters in this galaxy.

Star clusters in their youth

Young star clusters (YSCs) are dense, gravitationally bound groups of newly formed stars that form the fundamental building blocks of galaxies. They are generally less than 100 million years old and often form in the gas-rich environments of spiral or starburst galaxies. Studying YSCs is important to advance our knowledge about star formation, stellar evolution, and galactic dynamics.

However, understanding of YSCs cannot be complete without the investigation of a very young stage of their life—the emerging phase. In this phase, YSCs are still embedded in their dusty natal cloud and stellar feedback from massive stars dramatically impacts the surroundings, shaping a multiphase interstellar medium (ISM). Due to this, emerging young star clusters (eYSCs) are for a certain time invisible at optical wavelengths and therefore largely missed by optical surveys.

Nearby galaxy hosting eYSCs

Recently, a group of astronomers led by Helena Faustino Vieira of Stockholm University in Sweden decided to take a closer look at eYSCs in NGC 628, also known as Messier 74, located some 30 million light years away. NGC 628 is a large spiral galaxy containing two clearly defined spiral arms.

The galaxy, which is estimated to be 10–13 billion years old, is known for its vibrant, ongoing star formation (with a global star-formation rate at a level of 1.7 solar masses per year), and hosts many YSCs in its spiral arms.

Vieira’s team employed JWST’s Near Infrared Spectrograph (NIRSpec) to explore NGC 628’s population of YSCs, as it is capable of piercing through the dusty molecular clouds and reveal the emerging clusters. The observations were conducted as part of the Feedback in Emerging extrAgalactic Star clusTers (FEAST) program.

NIRSpec allowed the team to characterize the spectral properties of an initial sample of 14 eYSCs in NGC 628, as well as the photodissociation regions, and the diffuse ISM. The astronomers found star clusters at early evolutionary stages, still partly embedded in the natal cloud and actively driving stellar feedback.

What does JWST tell us about eYSCs in NGC 628?

In particular, the observations detected numerous helium and hydrogen recombination lines tracing regions of ionized atomic hydrogen (H II regions) powered by eYSCs. Moreover, multiple molecular hydrogen transitions and bright 3.3 µm polycyclic aromatic hydrocarbons (PAH) emission were identified, originating from the photodissociation regions (PDRs) still associated with these young clusters.

The collected data indicate that the investigated YSCs are at an early and energetic evolutionary phase, with hot, young massive stars with high ionizing outputs. The measured ionizing photon fluxes suggest that stars of spectral types O8.5V–O8V dominate the emission from the eYSCs.

The study confirmed that the explored eYSCs in NGC 628 are indeed young, and are consistent with the ages derived from photometric spectral energy distribution (SED) fits (median age of 3 million years). The eYSCs that have spectral signatures consistent with more evolved stars (like red supergiants) have age estimates of more than 9 million years.

Results of the study also suggest that as the star clusters age and begin to emerge from their natal cloud, both the molecular hydrogen and PAH emission decrease. This, according to the authors of the paper, indicates a tight connection between eYSCs and their PDR, with PDR morphology evolving as the clusters emerge from their natal cloud.

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