Giacomo Bortolini

We present the spatially resolved star formation history (SFH) of the interacting, low-mass, gas-rich galaxy pair NGC 4485–NGC 4490, based on deep JWST/NIRCam imaging obtained as part of the FEAST program. Using resolved stars’ near-infrared color–magnitude diagrams (CMDs) and the SFERA 2.0 CMD fitting code, we reconstruct the SFH of the system, including the tidal bridge connecting the two galaxies. We find that recent star formation is widespread across the system, extending beyond the main bodies of the galaxies into the tidal bridge and other off-center areas. In particular, a significant fraction of the recent star formation in NGC 4485 occurs outside its central region. All major regions of the system exhibit two prominent episodes of enhanced star formation, occurring around 0.5–1 Gyr and 10–30 Myr ago, with the tidal bridge showing the largest increase relative to its average star formation rate. These peaks in star formation activity occurred a few hundred million years after the epochs of the first and second pericenter passages predicted by independent dynamical models, supporting a scenario in which repeated tidal interactions triggered the observed bursts through gas redistribution and compression. Our results place direct observational constraints on the formation history of NGC 4485–NGC 4490 and provide evidence for the important role of galaxy interactions in driving spatially extended, bursty star formation in low-mass galaxies.

We combine imaging data from the HST, JWST, and ground-based millimeter facilities to investigate the correlation between star formation rate (SFR) and molecular gas at the ∼100 pc scale of H II regions in three nearby galaxies: NGC628, NGC5194, and NGC5236. The JWST 21 μ m maps of the three galaxies offer a unique insight into the dust-absorbed SFR at high resolution. We find that the relation between the surface densities of SFR and molecular gas has a slope of ∼1.85 in log–log scale, significantly steeper than previous results for nearby galaxies but closer to the trends found for molecular clouds in the Milky Way. The steep relation also holds on larger, ∼500 pc, scales, and results from the high-resolution imaging that cleanly isolates the star-forming region emission from the underlying galaxy’s diffuse contribution. The diffuse emission at 21 μ m is, in fact, found to correlate with the galaxy’s stellar mass. Comparisons with physical models of star formation are inconclusive; they overlap with the locus of the 100 pc data, but have difficulties in reproducing the data scatter. Possible exceptions are models that add a power-law tail to the gas density probability distribution due to the large range of free parameters allowed. We find that local H II regions, high-redshift star-forming clumps, and low- and high-redshift starburst galaxies form a single sequence of star formation over 3 orders of magnitude in gas surface density.

With its peculiar appearance, I Zw 18 has long been considered a unique example of a young galaxy in the nearby Universe. In this paper, we summarize the observational history of this famous galaxy, discuss the controversies surrounding its evolutionary state, and present new insights gained from JWST/NIRCam observations. These recent findings shed light on one of the most intriguing mysteries in extragalactic astronomy.

With Hubble Space Telescope and James Webb Space Telescope (JWST) observations of NGC 628 spanning 0.3–7.7 μ m, we fit the spectral energy distributions of over 12,000 optically selected star clusters, emerging young star clusters (eYSCs), and MIRI-selected sources with CIGALE to derive their ages, masses, extinctions, and dust properties. We find that near-IR selected eYSC-I (compact Pa α and 3.3 μ m polycyclic aromatic hydrocarbon (PAH) emission) and eYSC-II (compact Pa α and diffuse 3.3 μ m PAH emission) sources peak at ∼3–5 Myr, where ∼12% of the clusters have an E ( B − V ) > 2, demonstrating the presence of dust-embedded populations as clusters emerge. Further, the distributions of the fractional PAH abundance ( q _PAH ) and stellar-to-nebular attenuation ratio ( E ( B − V ) _⋆ / E ( B − V ) _neb ) suggest an evolutionary sequence in which sources evolve from eYSC-I to eYSC-II as clusters clear their surrounding dust and gas. The photodissociation region clearing timescale inferred from the ratio of eYSC-I to optically visible stellar clusters is ∼4 Myr. Additionally, we find that star clusters in the spiral arms of NGC 628 are preferentially more massive and more dust reddened than those in interarm regions. Finally, we find that ∼65% of eYSC-I, ∼27% of eYSC-II, and ∼40% of F335M-selected sources coincide with an F770W peak in our MIRI-selected catalog within 4 pixels, confirming that F770W-bright sources preferentially trace the youngest and dustiest regions. Overall, our results highlight the ability of JWST together with CIGALE model grids to identify and characterize eYSCs during their short-lived embedded phases, and provide constraints on the feedback mechanisms that govern the emergence of stellar clusters.

Quantifying the timescales of star cluster emergence from their natal clouds remains one of the main challenges in understanding the star formation process. These timescales are fundamental measurements of the star formation cycle within galaxies, yet are difficult to constrain due to the complex interplay between stellar feedback and star formation across multiple physical scales. Here we present Hubble Space Telescope and James Webb Space Telescope observations of thousands of young star clusters in four nearby galaxies (M51, M83, NGC 628 and NGC 4449). A substantial fraction of these clusters are still embedded within their natal gas and remain invisible at optical wavelengths. We constrain their emergence process by measuring the timescales required to disperse the surrounding material. We find a strong correlation between dispersal timescale and cluster stellar mass, with massive clusters emerging faster than their lower-mass counterparts. This is a critical constraint on star formation and stellar feedback simulations, which struggle to fully reproduce star clusters formation and emergence. Our results emphasize the central role of massive clusters in driving the escape of ionizing radiation into the galactic medium. Finally, they impose time limitations for planet formation in massive cluster environments where disks get exposed to ultraviolet irradiation and further gas infall is halted.

JWST can pierce through dusty molecular clouds to study the early stages of star formation, where young star clusters are actively driving stellar feedback and still emerging from their natal cloud. We present a first look of the JWST/NIRSpec multiplex spectroscopy observations acquired by the Feedback in Emerging extrAgalactic Star clusTers program for the nearby spiral galaxy NGC 628. We showcase JWST’s ability to resolve the spectral properties of emerging young star clusters (eYSCs) and their immediate interstellar medium by focusing on a bright star-forming complex (0.5 × 0.5 kpc ² ) in the northern spiral arm as a science proof-of-concept. The eYSC spectra are rich in ionized gas (from H II regions), as well as warm H ₂ and polycyclic aromatic hydrocarbon (PAH) emission from photodissociation regions (PDRs), consistent with young star formation. Pa α equivalent widths and H/He ionizing photon fluxes both indicate the presence of hot, young massive stars (O8.5V–O8V), consistent with photometry spectral energy distribution estimates. The ionized gas is highly correlated with H ₂ and PAH emission, suggesting that the PDR morphology evolves as clusters emerge from their natal cloud. We find a photoionization-dominated regime from independent line diagnostics, with little contribution from supernovae-driven shocks, highlighting the importance of presupernovae feedback when massive stars are present. This pilot study showcases how JWST’s multiplex spectroscopy mode can disentangle the mechanisms present in the youngest stages of star formation for the first time outside the Local Group.

We apply the angular two-point correlation function (TPCF) to the spatial distributions of young star clusters (YSCs) in four nearby star forming galaxies (NGC 628, NGC 4449, M51, and M83) in order to investigate their underlying hierarchical structuring. Using newly constructed catalogs of YSCs in the emerging phase (eYSCs), identified in the infrared with JWST, and optical YSCs detected in archival Hubble Space Telescope data, we compute the TPCFs for various cluster samples and age bins across the four galaxies, as part of the Feedback in Emerging extrAgalactic Star ClusTers (FEAST) program. We find clear evidence of hierarchical structuring, especially in eYSCs and YSCs with ages D ₂ values of the YSCs’ underlying distributions are calculated from model fits to the TPCFs. Our values of D ₂ derived from the youngest YSC populations align better with the expected value of D ₂ ∼ 1.3 for a universal star formation process compared to previous findings.

Stellar wind mass loss of massive stars is often assumed to depend on their metallicity Z . Therefore, evolutionary models predict that massive stars in lower- Z environments are able to retain more of their hydrogen-rich layers and evolve into brighter cool supergiants (cool SGs; T _eff < 7 kK). Surprisingly, in galaxies in the metallicity range 0.2 ≲ Z / Z _⊙ ≲ 1.5, previous studies have not found a metallicity dependence on the upper luminosity limit L _max of cool SGs. Here, we add four galaxies to the sample studied for this purpose with data from the Hubble Space Telescope and the James Webb Space Telescope (JWST). Observations of the extremely metal-poor dwarf galaxy I Zw 18 from JWST allow us to extend the studied metallicity range down to Z / Z _⊙ ≈ 1/40. For cool SGs in all studied galaxies, including I Zw 18, we find a constant value of L _max ≈ 10 ^5.6 L _⊙ , similar to literature results for 0.2 ≲ Z / Z _⊙ ≲ 1.5. In I Zw 18 and the other studied galaxies, the presence of Wolf-Rayet stars has been previously inferred. Although we cannot rule out that some of them become intermediate-temperature objects, this paints a picture in which evolved stars with L > 10 ^5.6 L _⊙ burn helium as hot, helium-rich stars down to extremely low metallicity. We argue that metallicity-independent late-phase mass loss would be the most likely mechanism responsible for this. Regardless of the exact stripping mechanism (winds or, for example, binary interaction), for the Early Universe our results imply a limitation on black hole masses and a contribution of stars born with M ≳ 30 M _⊙ to its surprisingly strong nitrogen enrichment. We propose a scenario in which single stars at low metallicity emit sufficiently hard ionizing radiation to produce He II and C IV lines. In this scenario, late-phase metallicity-independent mass loss produces hot, helium-rich stars. Due to the well-understood metallicity dependence on the radiation-driven winds of hot stars, a window of opportunity would open below 0.2 Z _⊙ , where self-stripped helium-rich stars can exist without dense Wolf-Rayet winds that absorb hard ionizing radiation.

We use JWST/NIRCam and MIRI imaging acquired by the Feedback in Emerging extrAgalactic Star clusTers (or FEAST) program along with archival Hubble Space Telescope imaging to map ionized gas (Pa α , Br α , and H α ) and polycyclic aromatic hydrocarbon (PAH) emission (3.3 and 7.7 μ m) across a sample of four nearby galaxies (NGC 5194, 5236, 628, and 4449). These maps are utilized to calibrate the PAH features as star formation rate (SFR) indicators in 40 pc-size regions around massive emerging young star clusters. We find a tight, sublinear (power-law exponent α ∼ 0.8) relation between the PAH luminosities (3.3 and 7.7 μ m) and SFR (extinction-corrected Pa α ) in near-solar-metallicity environments. PAH destruction in more intense ionizing environments and/or variations in the age of our sources may drive the deviation from a linear relation. In the metal-poor environment of NGC 4449 (∼1/3 Z _⊙ ), we see substantial deficits in the PAH feature strengths at fixed SFR and significantly higher scatter in the PAH–SFR relations. We determine that the 3.3/7.7 μ m PAH luminosity ratio increases towards lower-metallicity environments. This is interpreted as a result of a shift in the size distribution towards smaller PAHs at lower metallicities, possibly due to inhibited grain growth. Focusing on the regions in NGC 4449, we observe a decreasing 3.3/7.7 μ m ratio towards higher SFR, which could indicate that small PAHs are preferentially destroyed relative to larger PAHs in significantly subsolar-metallicity conditions. We estimate that ∼two-thirds of the PAH emission in typical local star-forming galaxies is excited by older stars and unrelated to recent (<10 Myr) star formation.

We examine the spatial distribution of star clusters in NGC628 using the statistical tool INDICATE to quantify clustering tendencies. Our sample, based on HST and JWST observations, is the most complete to date, spanning ages from 1 Myr to >100 Myr. We find cluster spatial behavior varies with galactic position, age, and mass. Most emerging young clusters are tightly spatially associated with each other, while fully emerged clusters are in ∼1.5 times looser spatial associations, irrespective of age. Young Massive Clusters (YMCs ≥104 M⊙) tend to associate with lower-mass clusters but not strongly with other YMCs, implying that intense star formation regions produce a few YMCs alongside many lower-mass clusters rather than multiple YMCs together. Young concentrated clusters show a wide radial distribution in the galactic disk, which narrows with age; with concentrated clusters > 100 Myr mostly residing between 2–6 kpc. This pattern may reflect either faster dispersal of isolated tight cluster spatial ‘structure’ in a lower gas density outer disk or gradual inside-out growth, with the formation of this structure shifting outward over time. We also detect distinct spatial behaviors for clusters within 2 kpc, linked to the inner Lindblad resonance (≤1 kpc), nuclear ring (∼0.5–1 kpc), and the start of spiral arms (∼1.25–2 kpc), suggesting these regions exhibit strong radial motions that could hinder clusters from forming and remaining in tight concentrations. Our results highlight how spatially-resolved studies of clusters can reveal the influence of galactic dynamics on star formation and cluster evolution.