Linda J. Smith

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.

We investigate the presence of high-ionization fine-structure emission lines across a range of 22 OB-type stars observed with JWST-MIRI as part of calibration programs and the Webb Investigation of Silicates, Carbons, and Ices and the Measuring Extinction and Abundances of Dust programs. Mid-infrared (MIR) wind emission is detected in four late O dwarfs (O8 V–O9 V), one early O dwarf (O5 V), and there are tentative detections in an additional three stars (O8.5 II, O8.5 IV, and O9 I). We measure the wind speeds and make estimates of lower limits on the mass-loss rates of five O-type stars from broad, flat-topped emission in the fine-structure line of [Ne v] 14.3 μ m. We find terminal wind speeds that are generally in agreement with empirical trends, but note that in some cases the wind speeds are surprisingly low. We highlight two main takeaways from this sample, which combine to establish an exciting new window into the winds of massive stars. First, a new diagnostic ability is gained from lines formed at much higher ionization, larger spatial extent, and longer wavelengths than typical wind diagnostics. Second, there is a frequent incidence of MIR emission in O-type stars, even in the “weak-wind” regime where wind emission is often not detected in the UV and optical.

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.

We present the spectroscopic analysis of far-ultraviolet (FUV) observations taken with the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) targeting young massive clusters in the nearby, metal-poor, blue compact dwarf galaxy NGC 5253. We characterize the stellar populations observed across seven COS pointings and report on their inferred physical parameters: age, metallicity, mass, and reddening values. Comparisons between our spectroscopic ages and those inferred using photometric methods show that the former are preferentially younger. We also investigate the impact of these young massive clusters on their surrounding interstellar medium. Using Very Large Telescope/MUSE optical observations and matching the size of the COS aperture, we measure outflow velocities of the ionized gas along the line of sight of the COS pointing with values ranging from ∼125 to 300 km s ⁻¹ . We report on strong statistically significant correlations between the outflow velocities and stellar ages, masses, and total mechanical luminosity primarily driven by supernovae (SNe) as derived from our full-spectrum fitting analysis. Although theoretical models predict a delayed injection of mechanical energy and momentum in low-metallicity environments (5 Myr ages, with no apparent delay. One possible explanation is that the decrease and suppression of mechanical feedback due to SN explosions might be dominant at even lower metallicities than those observed in NGC 5253.

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.

The Galaxy UV Legacy Project (GULP) is a Cycle 28 Treasury program with the Hubble Space Telescope designed to characterize resolved massive stars, OB associations, and young star clusters in 26 nearby star-forming galaxies. Utilizing the Advanced Camera for Surveys/Solar Blind Channel F150LP and WFC3/UVIS F218W filters, combined with extensive archival observations, GULP provides an unprecedented panchromatic eight-band view from the far-UV to the I band. The target galaxies were carefully selected to span a broad range of metallicities, masses, morphological types, and star formation rates, thereby enabling detailed studies of star formation processes across different galactic environments. This paper introduces the GULP survey, detailing its observational strategy, data processing, and initial scientific results for the irregular barred starburst dwarf galaxy NGC 4449, used as a test case. We derived the physical parameters and ages for thousands of stars using the Binary Populations And Spectral Synthesis models, and found that the younger stars and clusters are predominantly concentrated along the galaxy’s central bar, and that over the past λ 2175 correlates with the intensity of the UV radiation. The “UV bump” is detected in many areas of the galaxy, but is absent in the regions of most intense and recent star formation. This strongly supports the scenario where UV radiation from young, massive stars effectively destroys the small dust grains responsible for the “UV bump.”

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.

Context. There is considerable debate about the formation of massive stars, including whether a high-mass star must always form with a population of low-mass stars, or if it can also form in isolation. Massive stars found in the field are often considered to be runaways from star clusters or OB associations. However, there is evidence in the Milky Way and the Small Magellanic Cloud of high-mass stars that appear to be isolated in the field and they cannot be related to any known star cluster or OB association. Studies of more distant galaxies have been lacking so far. Aims. We identified massive star candidates that appear isolated in the field of the nearby spiral galaxy NGC 4242 (at a distance of 5.3 Mpc) to explore how many candidates for isolated star formation we find in a galaxy outside the Local Group. Methods. We identified 234 massive ( M _ini ≥ 15 M _⊙ ) and young (≤10 Myr) field stars in NGC 4242 using the Hubble Space Telescope Solar Blind Channel of the Advanced Camera for Surveys, the UVIS channel of the Wide Field Camera 3 from the Galaxy UV Legacy Project (GULP), and optical data from the Legacy ExtraGalactic UV Survey (LEGUS). We investigated the surroundings of our targets within the range of projected distances expected for runaway stars, 74 pc and 204 pc. Results. Within the threshold radii, 9.8% and 34.6% of our targets have no young star clusters, OB associations, or massive stars. This causes them to appear isolated. This fraction reduces to 3.2%−11.5% for the total number of massive stars expected from the observed UV star formation rate. Conclusions. Our results show that there is a small population of young and massive potentially isolated field stars in NGC 4242.

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.