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Matteo Messa

Publications and source records attributed to Matteo Messa.

At least 37 records · Page 2Linked to original sources
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.

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arXiv2025

An Investigation of Disk Thickness in M51 from H-alpha, Pa-alpha, and Mid-Infrared Power Spectra

Power spectra (PS) of high-resolution images of M51 (NGC 5194) taken with the Hubble Space Telescope and the James Webb Space Telescope have been examined for evidence of disk thickness in the form of a change in slope between large scales, which map two-dimensional correlated structures, and small scales, which map three-dimensional correlated structures. Such a slope change is observed here in H-alpha, and possibly Pa-alpha, using average PS of azimuthal intensity scans that avoid bright peaks. The physical scale of the slope change occurs at ~120 pc and ~170 pc for these two transitions, respectively. A radial dependence in the shape of the H-alpha PS also suggests that the length scale drops from ~180 pc at 5 kpc, to ~90 pc at 2 kpc, to ~25 pc in the central ~kpc. We interpret these lengths as comparable to the thicknesses of the star-forming disk traced by HII regions. The corresponding emission measure is ~100 times larger than what is expected from the diffuse ionized gas. PS of JWST Mid-IR Instrument (MIRI) images in 8 passbands have more gradual changes in slope, making it difficult to determine a specific value of the thickness for this emission.

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arXiv2025

Power Spectra of JWST Images of Local Galaxies: Searching for Disk Thickness

JWST/MIRI images have been used to study the Fourier transform power spectra (PS) of two spiral galaxies, NGC 628 and NGC 5236, and two dwarfs, NGC 4449 and NGC 5068, at distances ranging from 4 to 10 Mpc. The PS slopes on scales larger than 200 pc range from -0.6 at 21 microns to -1.2 at 5.6 microns. These slopes for one-dimensional PS are consistent with the PS slopes observed elsewhere using HI and dust emission. They are likely related to turbulence, but they may also be viewed as a hierarchical distribution of objects having a size-luminosity relation and size distribution function. There is no evidence for a kink or steepening of the PS at some transition from two-dimensional to three-dimensional turbulence on the scale of the disk thickness. This lack of a kink could be from large positional variations in the PS depending on two opposite effects: local bright sources that make the slope shallower and exponential galaxy profiles that make the slope steeper. The sources could also be confined to a layer of molecular clouds that is thinner than the HI or cool dust layers where PS kinks have been observed before. If the star formation layers observed in the mid-infrared here are too thin, then the PS kink could also be hidden in the broad tail of the JWST point spread function.

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arXiv2025

Anatomy of a z=6 Lyman-α emitter down to parsec scales: extreme UV slopes, metal-poor regions and possibly leaking star clusters

We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally-lensed galaxy ($\rm μ=17-21$) at redshift 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is overall a typical compact and UV-faint ($\rm M_{UV}=-17.8$) Lyman-$α$ emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical $\rm Hα$, $\rm Hβ$ and [OIII]$λ\lambda4959,5007$ lines are spatially resolved with the high spectral resolution grating (G395H, R~2700), with large equivalent widths, EW($\rm Hβ$+[OIII])$\gtrsim1000$ Å, and elevated ionising photon production efficiencies $\rm log(ξ_{ion}/erg^{-1}Hz)=25.2-25.7$. NIRCam deep imaging reveals the presence of compact rest-UV bright regions along with individual star clusters of $\rm R_{eff}=3-8~pc$ in size and $\rm M\sim2\cdot10^5-5\cdot10^{6}~M_\odot$ in mass. These clusters are characterised by steep UV slopes, $\rmβ_{UV}\lesssim-2.5$, which in some cases are associated with a dearth of line emission, indicating possible leaking of the ionizing radiation, as also supported by a Lyman-$\rm α$ emission peaking at $\rm \sim100~km~s^{-1}$ from the systemic redshift. While the entire system is characterised by low-metallicity, $\sim0.1~Z_\odot$, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with $\rm Z\lesssim2\%~Z_\odot$, barely detected in NIRCam imaging; this region is displaced by $\rm >200~pc$ from one of the brightest structures of the system in UV, and would have been too faint to detect if not for the large magnification of the system.

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arXiv2025

Metallicity Scatter Originating from Sub-kiloparsec Starbursting Clumps in the Core of a Protocluster at z=7.88

We present new JWST NIRSpec integral field unit (IFU) G395H/F290LP observations of a merging galaxy system at $z=7.88$, part of A2744-z7p9, the most distant protocluster to date. The IFU cube reveals [OIII] emissions in two previously known galaxies (ZD3 and ZD6) and a newly identified galaxy, ZD12, at $z_{\rm spec}=7.8762$. One of the detected \oiii-emitting regions has a detection of the auroral [OIII]4363, line, allowing us to derive a direct metallicity of $\log$(O/H)$+12=7.4\pm0.2$, while metallicities in other regions are measured using strong line calibration methods. We find large deviations within the measured metallicity ($Δ\log {\rm (O/H)}\sim1$), which suggests a fast chemical enrichment from intense star formation and merger-driven growth, as expected in early galaxies. Our analysis shows that metal-poor regions could easily be outshone by more enriched regions, posing a challenge for spectroscopic analysis based on integrated light (i.e., NIRSpec MSA) against identifying metal-free star formation in the early universe. NIRCam imaging reveals seven UV-bright clumps in ZD12, in the range of stellar mass $\log M_*/M_\odot\sim7.6$--8.9. Four of them are unresolved ($< 100$pc) and intensely star-forming ($>30 M_\odot {\rm yr^{-1} kpc^{-2}}$), likely contributing to the scatter in metallicity by producing an ideal environment for rapid chemical cycles. Lastly, we revisit the nature of the host protocluster by including new member galaxies identified here and in the literature, and obtain local overdensity factor $δ=44_{-31}^{+89}$, total halo mass $M_{\rm h} = 5.8_{-0.3}^{+0.2}\times10^{11}\,M_\odot$, and a formal velocity dispersion of $1100\pm500$ km s$^{-1}$.

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arXiv2024

In-situ formation of star clusters at z > 7 via galactic disk fragmentation; shedding light on ultra-compact clusters and overmassive black holes seen by JWST

We investigate the nature of star formation in gas-rich galaxies at $z > 7$ forming in a markedly overdense region, in the whereabouts of a massive virialized halo already exceeding $10^{12}$ M$_{\odot}$. We find that not only the primary galaxy, but also the lower-mass companion galaxies rapidly develop massive self-gravitating compact gas disks, less than 500~pc in size, which undergo fragmentation by gravitational instability into very massive bound clumps. Star formation proceeds fast in the clumps, which quickly turn into compact star clusters with masses in the range $10^5$-$10^8$ M$_{\odot}$ and typical half-mass radii of a few pc, reaching characteristic densities above $10^5$ M$_{\odot}$ pc$^{-2}$. The properties of the clusters in the lowest-mass galaxy bear a striking resemblance to those recently discovered by the James Webb Space Telescope (JWST) in the lensed Cosmic Gems arc system at $z = 10.2$. We argue that, due to their extremely high stellar densities, intermediate-mass black holes (IMBHs) would form rapidly inside the clusters, which would then swiftly sink and merge on their way to the galactic nucleus, easily growing a $10^7$~M$_{\odot}$ supermassive black hole (SMBH). Due to the high fractional mass contribution of clusters to the stellar mass of the galaxies, in the range $20$-$40\%$, the central SMBH would comprise more than $10\%$ of the mass of its host galaxy, naturally explaining the overmassive SMBHs discovered by JWST at $z > 6$.

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arXiv2024

The Clearing Timescale for Infrared-selected Star Clusters in M83 with HST

We present an analysis of Hubble Space Telescope (HST) data from WFC3/UVIS, WFC3/IR and ACS, investigating the young stellar cluster (YSC) population in the face-on spiral galaxy M83. Within the field of view of the IR pointings, we identify 454 sources with compact F814W continuum and Pa$β$ line emission with a S/N $\geq 3$ as possible YSC candidates embedded in dust. We refine this selection to 97 candidates based on their spectral energy distributions, multi-wavelength morphology, and photometric uncertainties. For sources that are detected in all bands and have mass $> 10^{2.8} M_{\odot}$ (53 sources), we find that by 2 Myr $75\%$ of infrared-selected star clusters have an $A_{V} \leq 1$, and that by 3 Myr the fraction rises to $\sim 82\%$. This evidence of early clearing implies that pre-supernovae feedback from massive stars are responsible for clearing the majority of the natal gas and dust that surround infrared-selected star clusters in M83. Further, this result is consistent with previous estimates based on WFC3 observations, and adds to the growing body of literature suggesting pre-supernova feedback to be crucial for YSC emergence in normal star-forming galaxies. Finally, we find a weak correlation between the YSC concentration index and age over the first 10 Myr, which matches previous studies and indicates little or no change in the size of YSCs in M83 during their early evolution.

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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.

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arXiv2024

Bound star clusters observed in a lensed galaxy 460 Myr after the Big Bang

The Cosmic Gems arc is among the brightest and highly magnified galaxies observed at redshift $z\sim10.2$. However, it is an intrinsically UV faint galaxy, in the range of those now thought to drive the reionization of the Universe. Hitherto the smallest features resolved in a galaxy at a comparable redshift are between a few hundreds and a few tens of parsecs. Here we report JWST observations of the Cosmic Gems. The light of the galaxy is resolved into five star clusters located in a region smaller than 70 parsec. They exhibit minimal dust attenuation and low metallicity, ages younger than 50 Myr and intrinsic masses of $\sim10^6$ M$_{\odot}$. Their lensing-corrected sizes are approximately 1 pc, resulting in stellar surface densities near $10^5$~M$_{\odot}$/pc$^2$, three orders of magnitude higher than typical young star clusters in the local universe. Despite the uncertainties inherent to the lensing model, they are consistent with being gravitationally bound stellar systems, i.e., proto-globular clusters. We conclude that star cluster formation and feedback likely contributed to shape the properties of galaxies during the epoch of reionization. [Abridged]

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arXiv2024

Feedback in Emerging Extragalactic Star Clusters (JWST--FEAST): Calibration of Star Formation Rates in the Mid-Infrared with NGC 628

New JWST near-infrared imaging of the nearby galaxy NGC 628 from the Cycle 1 program JWST-FEAST is combined with archival JWST mid-infrared imaging to calibrate the 21 $μ$m emission as a star formation rate indicator (SFR) at $\sim$120 pc scales. The Pa$α$ ($λ$1.8756 $μ$m) hydrogen recombination emission line targeted by FEAST provides a reference SFR indicator that is relatively insensitive to dust attenuation, as demonstrated by combining this tracer with the HST H$α$ imaging. Our analysis is restricted to regions that appear compact in nebular line emission and are sufficiently bright to mitigate effects of both age and stochastic sampling of the stellar initial mass function. We find that the 21 $μ$m emission closely correlates with the nebular line emission, with a power-law with exponent=1.07$\pm$0.01, in agreement with past results. We calibrate a hybrid SFR indicator using a combination of H$α$ and 24 $μ$m (extrapolated from 21 $μ$m) tracers and derive the proportionality constant between the two tracers $b=0.095\pm0.007$, which is $\sim$ 3-5 times larger than previous derivations using large regions/entire galaxies. We model these discrepancies as an increasing contribution to the dust heating by progressively older stellar populations for increasing spatial scales, in agreement with earlier findings that star formation is hierarchically distributed in galaxies. Thus, use of hybrid SFR indicators requires prior knowledge of the mean age of the stellar populations dominating the dust heating, which makes their application uncertain. Conversely, non-linear calibrations of SFRs from L(24) alone are more robust, with a factor $\lesssim$2.5 variation across the entire range of L(24) luminosities from HII regions to galaxies.

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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$α$).

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arXiv2024

The Timescales of Star Cluster Emergence: The Case of NGC 4449

We survey the young star cluster population in the dwarf galaxy NGC4449 with the goal of investigating how stellar feedback may depend on the clusters' properties. Using Ultraviolet(UV)-optical-NearIR(NIR) photometry obtained from the Hubble Space Telescope, we have recovered 99 compact sources exhibiting emission in the Pa$β$ hydrogen recombination line. Our analysis reveals these sources possess masses $10^{2}<M_{\odot}<10^{5}$, ages 1-20 Myr, and color excess E(B - V) in the range 0-1.4. After selecting clusters with mass above 3,000M$_{\odot}$ to mitigate stochastic sampling of the stellar initial mass function, we find that our IR-selected clusters have a median mass of $\sim$7$\times{10^{3}\text{ M}_{\odot}}$ and remain embedded in their surrounding gas and dust for 5-6 Myr. In contrast, line-emitting sources selected from existing UV/optically catalogs have a median mass of $\sim$3.5$\times{10^{4}\text{ M}_{\odot}}$ and have cleared their surroundings by 4 Myr. We further find that the environment in NGC4449 is too low pressure to drive these differences. We interpret these findings as evidence that the clearing timescale from pre-supernova and supernova feedback is cluster mass-dependent. Even in clusters with mass$\sim$7,000~M$_{\odot}$, stochastic sampling of the upper end of the stellar initial mass function is present, randomly decreasing the number of massive stars available to inject energy and momentum into the surrounding medium. This effect may increase the clearing timescales in these clusters by decreasing the effectiveness of both pre-supernova and supernova feedback; neither models nor observations have so far explored such dependence explicitly. Future studies and observations with, e.g., the JWST, will fill this gap.

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arXiv2024

Properties of the brightest young stellar clumps in extremely lensed galaxies at redshifts 4 to 5

We study the populations of stellar clumps in three high-redshift galaxies, at z=4.92, 4.88 and 4.03, gravitationally lensed by the foreground galaxy clusters MS1358, RCS0224 and MACS0940, respectively. The lensed galaxies consist of multiple counter-images with large magnifications, mostly above $\rm μ>5$ and in some cases reaching $\rm μ>20$. We use rest-frame UV observations from the HST to extract and analyse their clump populations, counting 10, 3 and 11 unique sources, respectively. Most of the clumps have derived effective radii in the range $\rm R_{eff}=10-100$ pc, with the smallest one down to 6 pc, i.e. consistent with the sizes of individual stellar clusters. Their UV magnitudes correspond to $\rm SFR_{UV}$ mostly in the range $\rm 0.1-1\ M_\odot yr^{-1}$; the most extreme ones, reaching $\rm SFR_{UV}=5\ M_\odot yr^{-1}$ are among the UV-brightest compact ($\rm R_{eff}<100$ pc) star-forming regions observed at any redshift. Clump masses span a broad range, from $10^6$ to $\rm 10^9\ M_\odot$; stellar mass surface densities are comparable, and in many cases larger, than the ones of local stellar clusters, while being typically 10 times larger in size. By compiling published properties of clump populations at similar spatial resolution between redshift 0 and 5, we find a tentative evolution of $\rm Σ_{SFR}$ and $\rm Σ_{M_\star}$ with redshift, especially when very compact clumps ($\rm R_{eff}\leqslant20$ pc) are considered. We suggest that these trends with redshift reflect the changes in the host galaxy environments where clumps form. Comparisons with the local universe clumps/star clusters shows that, although rare, conditions for elevated clump $\rm Σ_{SFR}$ and $\rm Σ_{M_\star}$ can be found.

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arXiv2024

CLusters in the Uv as EngineS (CLUES). II. Sub-kpc scale outflows driven by stellar feedback

We analyze the far-ultraviolet spectroscopy of 20 young and massive star clusters (YSCs) in 11 nearby star-forming galaxies. We probe the interstellar gas intervening along the line of sight, detecting several metal absorption lines of a wide range of ionization potentials, from 6.0 eV to 77.5 eV. Multiple-component Voigt fits to the absorption lines are used to study the kinematics of the gas. We find that nearly all targets in the sample feature gas outflowing from 30 up to 190 km per second, often both in the neutral and ionized phase. The outflow velocities correlate with the underlying stellar population properties directly linked to the feedback: the mass of the YSCs, the photon production rate and the instantaneous mechanical luminosity produced by stellar winds and SNe. We detect a neutral inflow in 4 targets, which we interpret as likely not associated with the star cluster but tracing larger scale gas kinematics. A comparison between the outflows energy and that produced by the associated young stellar populations suggests an average coupling efficiency of 10 per cent with a broad scatter. Our results extend the relation found in previous works between galactic outflows and the host galaxy star-formation rate to smaller scales, pointing towards the key role that clustered star formation and feedback play in regulating galaxy growth.

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arXiv2024

ALMA-LEGUS I: The Influence of Galaxy Morphology on Molecular Cloud Properties

We present a comparative study of the molecular gas in two galaxies from the LEGUS sample: barred spiral NGC 1313 and flocculent spiral NGC 7793. These two galaxies have similar masses, metallicities, and star formation rates, but NGC 1313 is forming significantly more massive star clusters than NGC 7793, especially young massive clusters (<10 Myr, >10^4 Msol). Using ALMA CO(2-1) observations of the two galaxies with the same sensitivities and resolutions of 13 pc, we directly compare the molecular gas in these two similar galaxies to determine the physical conditions responsible for their large disparity in cluster formation. By fitting size-linewidth relations for the clouds in each galaxy, we find that NGC 1313 has a higher intercept than NGC 7793, implying that its clouds have higher kinetic energies at a given size scale. NGC 1313 also has more clouds near virial equilibrium than NGC 7793, which may be connected to its higher rate of massive cluster formation. However, these virially bound clouds do not show a stronger correlation with young clusters than that of the general cloud population. We find surprisingly small differences between the distributions of molecular cloud populations in the two galaxies, though the largest of those differences are that NGC 1313 has higher surface densities and lower free-fall times.

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arXiv2024

ALMA-LEGUS II: The Influence of Sub-Galactic Environment on Molecular Cloud Properties

We compare the molecular cloud properties in sub-galactic regions of two galaxies, barred spiral NGC 1313, which is forming many massive clusters, and flocculent spiral NGC 7793, which is forming significantly fewer massive clusters despite having a similar star formation rate to NGC 1313. We find that there are larger variations in cloud properties between different regions within each galaxy than there are between the galaxies on a global scale, especially for NGC 1313. There are higher masses, linewidths, pressures, and virial parameters in the arms of NGC 1313 and center of NGC 7793 than in the interarm and outer regions of the galaxies. The massive cluster formation of NGC 1313 may be driven by its greater variation in environments, allowing more clouds with the necessary conditions to arise, although no one parameter seems primarily responsible for the difference in star formation. Meanwhile NGC 7793 has clouds that are as massive and have as much kinetic energy as clouds in the arms of NGC 1313, but have densities and pressures more similar to the interarm regions and so are less inclined to collapse and form stars. The cloud properties in NGC 1313 and NGC 7793 suggest that spiral arms, bars, interarm regions, and flocculent spirals each represent distinct environments with regard to molecular cloud populations. We see surprisingly little difference in surface densities between the regions, suggesting that the differences in surface densities frequently seen between arm and interarm regions of lower-resolution studies are indicative of the sparsity of molecular clouds, rather than differences in their true surface density.

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arXiv2023

Universal Upper End of the Stellar Initial Mass Function in the Young and Compact LEGUS clusters

We investigate the variation in the upper end of stellar initial mass function (uIMF) in 375 young and compact star clusters in five nearby galaxies within $\sim 5$ Mpc. All the young stellar clusters (YSCs) in the sample have ages $\lesssim 4$ Myr and masses above 500 $M_{\odot}$, according to standard stellar models. The YSC catalogs were produced from Hubble Space Telescope images obtained as part of the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. They are used here to test whether the uIMF is universal or changes as a function of the cluster's stellar mass. We perform this test by measuring the H$α$ luminosity of the star clusters as a proxy for their ionizing photon rate, and charting its trend as a function of cluster mass. Large cluster numbers allow us to mitigate the stochastic sampling of the uIMF. The advantage of our approach relative to previous similar attempts is the use of cluster catalogs that have been selected independently of the presence of H$α$ emission, thus removing a potential sample bias. We find that the uIMF, as traced by the H$α$ emission, shows no dependence on cluster mass, suggesting that the maximum stellar mass that can be produced in star clusters is universal, in agreement with previous findings.

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