arXiv subjects

Giacomo Bortolini

Publications and source records attributed to Giacomo Bortolini.

18 recordsLinked to original sources

arXiv2026

The HII Regions' Molecular Law of Star Formation

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 HII regions in three nearby galaxies: NGC628, NGC5194 and NGC5236. The JWST 21 micron 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 micron 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 HII regions, high redshift star-forming clumps, and low and high redshift starburst galaxies form a single sequence of star formation over three orders of magnitude in gas surface density.

astro-ph.GA↗

arXiv2026

Feedback in Extragalactic Star Clusters (FEAST): Spectral Energy Distributions and the Physical Properties of Star Clusters in NGC 628 with CIGALE

With Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) observations of NGC~628 spanning 0.3--7.7\,$μ$m, we fit the spectral energy distributions (SEDs) of over 12,000 optically-selected star clusters, emerging young star clusters (eYSCs), and MIRI-selected sources with \textsc{cigale} to derive their ages, masses, extinctions, and dust properties. We find that near-infrared selected eYSC-I (compact Pa$α$ and 3.3,$μ$m PAH emission) and eYSC-II (compact Pa$α$ and diffuse 3.3,$μ$m PAH emssion) sources peak at $\sim$3--5~Myr, where $\sim 12\%$ 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 polycyclic aromatic hydrocarbon (PAH) abundance ($q_{\mathrm PAH}$) and stellar-to-nebular attenuation ratio ($E(B{-}V)_{\rm \star}/E(B{-}V)_{\rm neb}$) suggest an evolutionary sequence in which sources evolve from eYSC-I to eYSC-II as clusters clear their surrounding dust and gas. The photo-dissociation region (PDR) clearing timescale inferred from the ratio of eYSC-I to optically visible stellar clusters is $\sim$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 inter-arm regions.~Finally, we find that $\sim$65\% of eYSC-I, $\sim$27\% of eYSC-II, and $\sim$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 \textsc{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.

astro-ph.GA↗

arXiv2026

The emerging timescale of young star clusters regulated by cluster stellar mass

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.

astro-ph.GA↗

arXiv2026

A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18

Stellar wind mass loss is often assumed to depend on their metallicity $Z$. Therefore, evolutionary models of massive stars at lower $Z$ are able to retain more of their H-rich layers and evolve into brighter cool supergiants (cool SGs; $T_\mathrm{eff} < 7$ kK). Surprisingly, in galaxies in the range $0.2 \lesssim Z / Z_\odot \lesssim 1.5$ previous studies did not find a $Z$-dependence of the upper luminosity limit $L_\mathrm{max}$ of cool SGs. Here, we study four extra galaxies with HST and JWST. Observations of the extremely low-$Z$ dwarf galaxy I Zw 18 from JWST allow us to go down to $Z / Z_\odot \approx 1/40$. For cool SGs in all studied galaxies including I Zw 18, we find a constant $L_\mathrm{max} \approx 10^{5.6}$L$_\odot$, similar to literature results for $0.2 \lesssim Z / Z_\odot \lesssim 1.5$. In I Zw 18 and the other studied galaxies, the presence of Wolf-Rayet stars has been claimed. 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$_\odot$ burn He as hot He-rich stars down to extremely low $Z$. We argue that $Z$-independent late-phase mass loss would be the most likely mechanism responsible. Regardless of the exact stripping mechanism, for the Early Universe our results imply a limitation on black hole masses and a contribution of stars born with $M\gtrsim 30$M$_\odot$ to its surprisingly strong nitrogen enrichment. We propose a scenario in which single stars at low $Z$ emit sufficiently hard ionizing radiation to produce lines of He II and C IV. In this scenario, late-phase $Z$-independent mass loss produces hot He-rich stars. Due to the well-understood $Z$-dependence of radiation-driven winds of hot stars, a window of opportunity would open below 0.2Z$_\odot$, where self-stripped He-rich stars can exist without dense WR winds that absorb hard ionizing radiation.

astro-ph.SR↗

arXiv2026

FEAST: a NIRSpec/MOS survey of emerging young star clusters in NGC 628

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 (FEAST) program for the nearby spiral galaxy NGC628. We showcase JWST's ability to resolve the spectral properties of emerging young star clusters (eYSCs) and their immediate interstellar medium (ISM) by focusing on a bright star-forming complex ($0.5\times0.5~\mathrm{kpc}^2$) in the northern spiral arm as a science proof-of-concept. The eYSC spectra are rich in ionized gas (from HII regions), as well as warm H$_2$ and polycyclic aromatic hydrocarbon (PAH) emission from photodissociation regions (PDRs), consistent with young star formation. $\mathrm{Pa}α$ equivalent widths and H/He ionizing photon fluxes both indicate the presence of hot, young massive stars (O8.5V-O8V), consistent with photometry SED 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 pre-Supernovae 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.

astro-ph.GA↗

arXiv2026

FEAST: Probing Hierarchical Star Formation with the Spatial Distributions of Young Star Clusters

We apply the angular two-point correlation function (TPCF) to the spatial distribution 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 HST data, we compute TPCFs for various cluster samples and age bins across the four galaxies as part of the FEAST (Feedback in Emerging extrAgalactic Star ClusTers) program. We find clear evidence of hierarchical structuring, especially in eYSCs and YSCs with ages < 10 Myr (referred to as oYSCs), which show similar TPCFs within each galaxy. NGC 628 exhibits a clear distinction between the TPCFs of eYSCs and oYSCs, implying a shorter randomization timescale. In contrast, clusters aged 10 to 300 Myr exhibit progressively more random spatial distributions, becoming effectively random after $\sim$ 100 Myr, consistent with earlier studies. The two-dimensional fractal index $D_2$ of the YSCs underlying distribution is calculated from model fits to TPCFs. Our values of $D_2$ derived from the youngest YSC populations align better with the expected value of $D_2 \sim $1.3 for a universal star formation process compared to previous findings.

astro-ph.GA↗

arXiv2025

The Spatial Evolution of Star Clusters in NGC 628 with JWST

We examine the spatial distribution of star clusters in NGC 628 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 behaviour varies with galactic position, age, and mass. Most emerging young clusters are tightly spatially associated with each other, while fully emerged clusters are in \sim1.5 times looser spatial associations, irrespective of age. Young Massive Clusters (YMCs \ge 10^4 M_{\odot}) 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 behaviours for clusters within 2 kpc, linked to the inner Lindblad resonance (\le1 kpc), nuclear ring (\sim0.5-1 kpc), and the start of spiral arms (\sim1.25-2 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.

astro-ph.GA↗

arXiv2025

The Calibration of Short Wavelength Polycyclic Aromatic Hydrocarbon Emission as Star Formation Rate Indicators with JWST

We use JWST/NIRCam and MIRI imaging acquired by the Feedback in Emerging extrAgalactic Star clusTers (FEAST) program along with archival HST 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 (eYSCs). We find a tight, sub-linear (power-law exponent, $α{\,}{\sim}{\,}0.8$) 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 (${\sim}$1/3 Z$_{\odot}$), 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 sub-solar metallicity conditions. We estimate that ${\sim}$2/3 of the PAH emission in typical local star-forming galaxies is excited by older stars and unrelated to recent ($<$10 Myr) star formation.

astro-ph.GA↗

arXiv2025

The near infrared SED of young star clusters in the FEAST galaxies: Missing ingredients at 1-5 $μ$m

We present a combined HST and JWST 0.2 - to - 5 $μ$m analysis of the spectral energy distributions (SEDs) of emerging young star clusters (eYSCs) in four nearby galaxies from the Feedback in Emerging extrAgalactic Star clusTers (FEAST) survey: M51, M83, NGC 628, and NGC 4449. These clusters, selected for their bright Pa$α$ and 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) emission, are still associated to their natal gas cloud and have been largely missed in previous HST optical campaigns. We modeled their SEDs using the CIGALE fitting code and identified: i) a systematic flux excess at 1.5 - 2.5 $μ$m that is not accounted for by current stellar population models; ii) the preference for a set of dust model parameters that is not aligned with expectations from self-consistent analyses of star-forming regions, suggesting model shortcomings also in the 3 - 5 $μ$m. The near-infrared (NIR) excess is most prominent in low-mass ($\leq 3000$ M$_\odot$) and young ($\leq 6$ Myr) clusters. Additionally, we see that the SED fitting analysis wrongly assigns ages $\geq 6$ Myr to a fraction of strong Pa$α$ emitters with equivalent widths suggestive of significantly younger ages. A parallel analysis with the slug code suggests that stochastic initial mass function (IMF) sampling of pre-main-sequence stars combined with extinction might partially reduce the gap. We conclude that the inclusion of young stellar object SEDs, along with more realistic sampling of the cluster IMF, might be needed to fully account for the stellar population and dust properties of eYSCs.

astro-ph.GA↗

arXiv2025

FEAST: JWST/NIRCam view of the Resolved Stellar Populations of the Interacting Dwarf Galaxies NGC~4485/NGC~4490

We present new JWST/NIRCam observations of the interacting dwarf galaxy system NGC 4485/NGC 4490 (a.k.a. Arp 269), obtained as part of the Cycle 1 Feedback in Emerging extrAgalactic Star clusTers (FEAST) program. NGC 4485 and NGC 4490 form the closest known pair of interacting late-type dwarf galaxies (at $\sim 7.4$ Mpc), excluding the Magellanic Clouds. Near-infrared color-magnitude diagrams (CMDs) reveal a wide range of stellar populations in both galaxies, including young ($\lesssim 200$ Myr) upper main sequence stars, core helium-burning stars, and oxygen-rich asymptotic giant branch (AGB) stars. We also identify intermediate-age ($\sim 200$ Myr -- $1$ Gyr) carbon-rich AGB stars and a well-populated old ($\gtrsim 1$ Gyr) red giant branch (RGB). The CMDs show two distinct bursts of star formation beginning $\sim 30$ Myr and $\sim 200$ Myr ago, the latter consistent with the most recent pericenter passage predicted by N-body simulations. The spatial distribution of stars reveals a tidal bridge extending from NGC 4485 and connecting to the disk of NGC 4490. Compact star-forming regions are seen along NGC 4490's spiral arms, possibly originating from its infrared nucleus. A significant metallicity gradient is observed in the young stellar populations forming the bridge. These findings suggest that during the last pericenter passage, gas was stripped from NGC 4485 via tidal forces or ram pressure, accreted by NGC 4490, and mixed with in-situ material, fueling ongoing star formation. This system provides a unique nearby laboratory for studying how tidal interactions shape the star formation and chemical enrichment history of dwarf galaxies.

astro-ph.GA↗

arXiv2025

Quantification of The Age Dependence of Mid-Infrared Star Formation Rate Indicators

We combine James Webb Space Telescope images of the nearby galaxy NGC 5194 in the hydrogen recombination line Pa-alpha (lambda=1.8756 micron) from the Cycle 1 program JWST-FEAST with 21 micron dust continuum images from the Cycle 2 Treasury program JWGT to quantify the difference in the calibration of mid-infrared star formation rates (SFR) between HII regions and galaxies. We use the archival HST H-alpha image to correct the Pa-alpha emission for the effects of dust attenuation. Our data confirm previous results that the dust-corrected Pa-alpha flux is tightly correlated with the 21 micron emission at the scales of HII regions. When combined with published JWST data for the HII regions of the galaxy NGC 628 and Spitzer 24 micron data for whole galaxies and for kpc-size galaxy regions, we show that the L(24)-L(Pa-alpha) correlation has exponent >1 across six decades in luminosity. In addition, the hybrid 24 micron+H-alpha SFR indicator has a scaling constant about 4.4 times higher for HII regions than for whole galaxies, also in agreement with previous results. Models of stellar populations with a range of star formation histories reveal that the observed trends can be entirely ascribed to and quantified with the contribution to the IR emission by stellar populations older than ~5-6 Myr. Based on the models' results, we provide: (1) a calibration for the infrared SFR across six orders of magnitude in L(24), from HII regions to luminous galaxies, and (2) a prescription for the scaling constant of the hybrid infrared SFR indicators as a function of the star formation timescale.

astro-ph.GA↗

arXiv2025

FEAST: probing the stellar population of the starburst dwarf galaxy NGC4449 with JWST/NIRCam

We present new JWST/NIRCam observations of the starburst irregular galaxy NGC 4449, obtained in Cycle 1 as part of the Feedback in Emerging extrAgalactic Star clusTers (FEAST) program, which we use to investigate its resolved stellar populations and their spatial distributions. NGC4449 NIR color-magnitude diagrams reveal a broad range of stellar populations, spanning different evolutionary phases, from young main sequence stars, to old red giant branch stars and asymptotic giant branch (AGB) stars. The analysis of their spatial distributions shows that younger (< 10 Myr) populations form an S-shaped distribution aligned with the galaxy's north-south axis, while stars aged 10 - 60 Myr show shifting concentrations from the north to the south, consistent with the possibility that external interactions or tidal effects may have triggered star formation in spatially distinct bursts. Clusters of comparable ages generally follow these distributions, suggesting that cluster and field stars form at the same pace in each galaxy region. Thanks to the unprecedented high-spatial resolution and sensitivity of the JWST data we recover a clear gap between Oxygen-rich and the carbon star branch of the AGB population and the presence of a massive AGB star "finger". The analysis of these stars can provide constraints on AGB evolution models and dust production in this galaxy. These results confirms NGC 4449 status as a compelling example of a local dwarf starburst galaxy undergoing complex and possibly external driven star formation and underscore the power of JWST in probing the full lifecycle of stars in nearby starburst systems.

astro-ph.GA↗

arXiv2025

FEAST: JWST uncovers the emerging timescales of young star clusters in M83

We present JWST NIRCam observations of the emerging young star clusters (eYSCs) detected in the nearby spiral galaxy M83. The NIRcam mosaic encompasses the nuclear starburst, the bar, and the inner spiral arms. The eYSCs, detected in Pa$α$ and Br$α$ maps, have been largely missed in previous optical campaigns of young star clusters (YSCs). We distinguish between eYSCI, if they also have compact 3.3~$μ$m PAH emission associated to them, and eYSCII, if they only appear as compact Pa$α$ emitters. We find that the variations in the 3.3~$μ$m PAH feature are consistent with an evolutionary sequence where eYSCI evolve into eYSCII and then optical YSCs. This sequence is clear in the F300M-F335M (tracing the excess in the \PAHlambda\ feature) and the F115W-F187N (tracing the excess in Pa$α$) colors which become increasingly bluer as clusters emerge. The central starburst stands out as the region where the most massive eYSCs are currently forming in the galaxy. We estimate that only about 20~\% of the eYSCs will remain detectable as compact YSCs. Combining eYSCs and YSCs ($\leq$10 Myr) we recover an average clearing timescale of 6~Myr in which clusters transition from embedded to fully exposed. We see evidence of shorter emergence timescales ($\sim$5~Myr) for more massive ($>5\times10^3$ \msun) clusters, while star clusters of $\sim 10^3$ \msun\ about 7~Myr. We estimate that eYSCs remain associated to the \PAHlambda\ emission 3--4~Myr. Larger samples of eYSC and YSC populations will provide stronger statistics to further test environmental and cluster mass dependencies on the emergence timescale.

astro-ph.GA↗

arXiv2025

Unveiling IZw18 age's mystery: Resolved Stellar Populations and Star Formation History Study with JWST/NIRCam

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.

astro-ph.GA↗

arXiv2024

Feedback in Emerging extragAlactic Star clusTers, FEAST: JWST spots PAH destruction in NGC 628 during the emerging phase of star formation

We investigate the emergence phase of young star clusters in the nearby spiral galaxy NGC 628. We use JWST NIRCam and MIRI observations to create spatially resolved maps of the Pa$α$-1.87 $μ$m and Br$α$-4.05 $μ$m hydrogen recombination lines, as well as the 3.3 $μ$m and 7.7 $μ$m emission from polycyclic aromatic hydrocarbons (PAHs). We extract 953 compact HII regions and analyze the PAH emission and morphology at $\sim$10 pc scales in the associated photo-dissociation regions (PDRs). While HII regions remain compact, radial profiles help us to define three PAH morphological classes: compact ($\sim$ 42%), extended ($\sim$ 34%) and open ($\sim$ 24%). The majority of compact and extended PAH morphologies are associated with very young star clusters ($<$5 Myr), while open PAH morphologies are mainly associated with star clusters older than 3 Myr. We observe a general decrease in the 3.3 $μ$m and 7.7 $μ$m PAH band emission as a function of cluster age, while their ratio remains constant with age out to 10 Myr and morphological class. The recovered PAH$_{3.3 μ{\rm m}}$/PAH$_{7.7 μ{\rm m}}$ ratio is lower than values reported in the literature for reference models that consider neutral and ionized PAH populations and analyses conducted at galactic scales. The 3.3 $μ$m and 7.7 $μ$m bands are typically associated to neutral and ionised PAHs, respectively. While we expected neutral PAHs to be suppressed in proximity of the ionizing source, the constant PAH$_{3.3 μ{\rm m}}$/PAH$_{7.7 μ{\rm m}}$ ratio would indicate that both families of molecules disrupt at similar rates in proximity of the HII regions.

astro-ph.GA↗

arXiv2024

Imaging of I Zw 18 by JWST. I. Strategy and First Results of Dusty Stellar Populations

We present a JWST imaging survey of I Zw 18, the archetypal extremely metal-poor, star-forming (SF), blue compact dwarf galaxy. With an oxygen abundance of only $\sim$3% $Z_{\odot}$, it is among the lowest-metallicity systems known in the local Universe, and is, therefore, an excellent accessible analog for the galactic building blocks which existed at early epochs of ionization and star formation. These JWST data provide a comprehensive infrared (IR) view of I Zw 18 with eight filters utilizing both Near Infrared Camera (F115W, F200W, F356W, and F444W) and Mid-Infrared Instrument (F770W, F1000W, F1500W, and F1800W) photometry, which we have used to identify key stellar populations that are bright in the near- and mid-IR. These data allow for a better understanding of the origins of dust and dust-production mechanisms in metal-poor environments by characterizing the population of massive, evolved stars in the red supergiant (RSG) and asymptotic giant branch (AGB) phases. In addition, it enables the identification of the brightest dust-enshrouded young stellar objects (YSOs), which provide insight into the formation of massive stars at extremely low metallicities typical of the very early Universe. This paper provides an overview of the observational strategy and data processing, and presents first science results, including identifications of dusty AGB, RSG, and bright YSO candidates. These first results assess the scientific quality of JWST data and provide a guide for obtaining and interpreting future observations of the dusty and evolved stars inhabiting compact dwarf SF galaxies in the local Universe.

astro-ph.GA↗

arXiv2024

Imaging of I Zw 18 by JWST: II. Spatially resolved star formation history

The blue compact dwarf galaxy I Zw 18 is one of the most metal-poor ($Z \sim 3% Z_{\sun}$) star-forming galaxies in the local Universe. Its evolutionary status has sparked debate within the astronomical community. We aim to investigate the stellar populations of I Zw 18 in the near-IR using JWST/NIRCam's high spatial resolution and sensitivity. Additionally, we aim to derive the galaxy's spatially resolved star formation history (SFH) over the last 1 Gyr and provide constraints for older epochs. We used DOLPHOT to measure positions and fluxes of point sources in the F115W and F200W filters' images of I Zw 18. To derive I Zw 18's SFH, we applied the color-magnitude diagram (CMD) fitting technique SFERA 2.0, using two independent sets of stellar models. Our analysis reveals three main stellar populations: one younger than $\sim30$ Myr, mainly in the northwest star-forming (SF) region; an intermediate-age population ($\sim 100 - 800$ Myr) in the southeast SF region; and a red and faint population linked to the underlying halo, older than 1 Gyr and possibly as old as 13.8 Gyr. The main body of the galaxy shows a very low star formation rate (SFR) of $\sim 10^{-4} M_{\odot} \text{yr}^{-1}$ between 1 and 13.8 Gyr ago. In the last billion years, I Zw 18 shows increasing SF, with strong bursts around $\sim10$ and $\sim100$ Myr ago. Component C mirrors the main body's evolution but with lower SFRs. Our findings confirm that I Zw 18 contains stars of all ages, indicating it is not a young galaxy but has an old stellar halo, similar to other BCDs. The low SF activity over the past billion years supports the "slow cooking" dwarf scenario, explaining its low metal content. Currently, the galaxy is undergoing its strongest SF episode ($\sim 0.6 M_{\odot} \text{yr}^{-1}$) mainly in the northwest region, likely due to a recent gravitational interaction with Component C.

astro-ph.GA↗

arXiv2024

Feedback in emerging extragalactic star clusters, FEAST: The relation between 3.3 $μ$m PAH emission and Star Formation Rate traced by ionized gas in NGC 628

We present maps of ionized gas (traced by Pa$α$ and Br$α$) and 3.3 $μ$m Polycyclic Aromatic Hydrocarbon (PAH) emission in the nearby spiral galaxy NGC 628, derived from new JWST/NIRCam data from the FEAST survey. With this data, we investigate and calibrate the relation between 3.3 $μ$m PAH emission and star formation rate (SFR) in and around emerging young star clusters (eYSCs) on a scale of ${\sim}40$ pc. We find a tight (correlation coefficient $ρ$${\sim}$0.9) sub-linear (power-law exponent $α$${\sim}$0.75) relation between the 3.3 $μ$m PAH luminosity surface density and SFR traced by Br$α$ for compact, cospatial (within 0.16$''$ or ${\sim}$7 pc) peaks in Pa$α$, Br$α$, and 3.3 $μ$m (eYSC-I). The scatter in the relationship does not correlate well with variations in local interstellar medium (ISM) metallicity due to a radial metallicity gradient, but rather is likely due to stochastic sampling of the stellar initial mass function (IMF) and variations in the PAH heating and age of our sources. The deviation from a linear relation may be explained by PAH destruction in more intense ionizing environments, variations in age, and IMF stochasticity at intermediate to low luminosities. We test our results with various continuum subtraction techniques using combinations of NIRCam bands and find that they remain robust with only minor differences in the derived slope and intercept. An unexpected discrepancy is identified between the relations of hydrogen recombination lines (Pa$α$ versus Br$α$; H$α$ versus Br$α$).

astro-ph.GA↗