Kelsey E. Johnson

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.

We use JWST NIRCam and MIRI imaging at 2, 4, 10, and 21 μ m to study young, dusty compact sources in four nearby galaxies at distances of ∼1–5 Mpc (M33, NGC 300, NGC 7793, and NGC 5068). This work bridges well-characterized massive young stellar objects (MYSOs) in the Magellanic Clouds from the Spitzer Space Telescope SAGE survey to new studies of embedded clusters in more distant galaxies with JWST. Guided by the SAGE-LMC catalog, we define JWST color–magnitude selection criteria (F1000W versus F1000W − F2100W) and test them using resolution-degradation experiments. We identify 216, 32, 80, and 139 dusty young objects in the four galaxies, respectively. The selected population spans sources from systems dominated by a single MYSO to compact marginally resolved sources hosting multiple MYSOs. The color selection remains stable across 1–5 Mpc, and the 10 μ m luminosity function retains a slope of α ∼ −2. However, blending and surface-brightness dilution remove fainter sources, leading to incompleteness of up to ∼50% at 5.2 Mpc and biasing the sample toward brighter objects (F1000W < 19 mag). The sample spans approximate stellar masses of ∼10–2 × 10 ⁵ M _⊙ . Spatial resolution affects the interpretation of mid-IR emission: clustering increases the fraction of emission attributed to compact sources in active regions, while blending into diffuse emission dominates in quiescent environments. Comparisons with polycyclic aromatic hydrocarbon (PAH)-selected young clusters in the PHANGS galaxy NGC 5068 show that our selection recovers ∼80% of the PAH-selected sources. We show that the practical limit for studying individual MYSOs with JWST is ∼3 Mpc. The resulting catalog provides a foundation for future resolved studies of star formation rates and early cluster evolution.

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.

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.

We report on the H I content of an isolated, compact group of six dwarf galaxies at a distance of 145 Mpc. The distribution and kinematics of the H ​​​​​​ I , including multiple gaseous bridges, indicate the group is a gravitationally bound system. The H I maps further reveal two newly discovered dwarf satellites easily identified by their gas but only barely visible in optical images. The four dwarf group members previously identified in the Sloan Digital Sky Survey have 9.06 < log( M _* / M _⊙ ) < 9.43 and 9.42 < log( M _{H I} / M _⊙ ) < 9.73. The two newly discovered dwarf satellites have log( M _* / M _⊙ ) = 6.10 with log(M _{H I} / M _⊙ ) = 8.71 and log( M _* / M _⊙ ) = 7.07 with log( M _{H I} / M _⊙ ) = 9.18. New Gemini optical spectra link the H I detections and their optical counterparts. The group’s 3D velocity dispersion (188 km s ⁻¹ ), mass-to-light ratio ( M / L _B ∼ 44), dynamical-to-baryonic mass ratio ( M _dyn / M _bar ∼ 21), size (69 kpc), and gas fraction (0.56) are all consistent with the compact dwarf groups in the TNG50 simulation. The group has a top-heavy satellite mass function that is inconsistent with predictions for LMC-sized hosts and may instead be two or more groups coming together. A Voronoi tessellation reveals the group resides in a tendril outside the intersection of two filaments. These intermediate-density environments within the large-scale structure provide the conditions needed for groups of star-forming, gas-rich dwarf galaxies to form and eventually merge. Our results further show that it is possible to uncover fainter dwarf satellites around dwarf galaxy hosts via H I maps.

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 report on the HI content of an isolated, compact group of 6 dwarf galaxies at a distance of 145 Mpc. The distribution and kinematics of the HI, including multiple gaseous bridges, indicate the group is a gravitationally bound system. The HI maps further reveal two newly discovered dwarf satellites easily identified by their gas but only barely visible in optical images. The four dwarf group members previously identified in SDSS have 9.06 < log(Mstar/Msun) < 9.43 and 9.42 < log(MHI/Msun) < 9.73. The two newly discovered dwarf satellites have log(Mstar/Msun) = 6.10 with log(MHI/Msun) = 8.71 and log(Mstar/Msun) = 7.07 with log(MHI/Msun) = 9.18. New Gemini optical spectra link the HI detections and their optical counterparts. The group's 3D velocity dispersion (188 km/s), mass-to-light ratio (M_L/B ~44), dynamical-to-baryonic mass ratio (Mdyn/Mbary ~ 21), size (69 kpc), and gas fraction (0.56) are all consistent with the compact dwarf groups in the TNG50 simulation. The group has a top-heavy satellite mass function that is inconsistent with predictions for LMC-sized hosts and may instead be two or more groups coming together. A Voronoi tessellation reveals the group resides in a tendril outside the intersection of two filaments. These intermediate density environments within large scale structure provide the conditions needed for groups of star forming, gas-rich dwarf galaxies to form and eventually merge. Our results further show that it is possible to uncover fainter dwarf satellites around dwarf galaxy hosts via HI maps.

Stephan’s Quintet (SQ) is a local compact galaxy group system that exhibits significant star formation activity. A history of tidal interactions between its four member galaxies and a recent collision between an intruder galaxy and the original group are associated with active star formation, particularly in many shocked regions in the intragroup medium. Using an existing star cluster candidate (SCC) catalog constructed from Hubble Space Telescope (HST) UV/optical images, we integrate flux measurements from five near-infrared filters (F090W, F150W, F200W, F277W, and F356W) obtained from JWST NIRCam observations in 2022. Leveraging the extended photometric baseline from HST and JWST, spanning ∼300 to ∼3500 nm, we perform spectral energy distribution (SED) fitting using the C igale code to derive reliable estimates of age, mass, and extinction for the 1588 high-confidence SCCs. We confirm earlier results that very young SCCs (∼a few Myr) are predominantly located along previously identified shock regions near the merging galaxies, while older (>100 Myr) and globular clusters are more widely distributed. Our analysis shows that near-IR photometry helps break the age–extinction degeneracy, reclassifying many SCCs from older to younger, moderately dust-extincted clusters when added to HST-based SED fits. We also observe a strong spatial correlation between young clusters and CO-traced molecular gas, although active star formation is present in several regions with no detectable CO. We find that the two prominent epochs of star formation, around 5 and 200 Myr, correspond to the two major interaction events in SQ that gave rise to the observed extended tidal features.