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Tracing the earliest stages of star and cluster formation in 19 nearby galaxies with PHANGS-JWST and HST: compact 3.3 $\mu$m PAH emitters and their relation to the optical census of star clusters

T0 review · 3 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash

Pith's one-line read Compact 3.3 µm PAH sources in 19 nearby galaxies are dust-embedded young star clusters, and their PAH-bright phase fades within about 3 Myr, so the optically visible cluster census is missing a substantial embedded population.

desk verdict Solid multi-galaxy census of embedded cluster candidates, but the headline ~3 Myr fade timescale rests on optically selected ages and should be softened. read the letter →

arxiv 2412.07862 v1 pith:GIXEWCRU submitted 2024-12-10 astro-ph.GA

classification astro-ph.GA
keywords starformationclustersdust-embedded3.3micronPAHemissionJWSTNIRCamnearbygalaxiesclustertimescalesHSTcatalogs
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper argues that compact sources glowing at 3.3 µm, the signal of polycyclic aromatic hydrocarbons heated by very young massive stars, are star clusters still wrapped in their natal dust and gas, too obscured for HST's optical bands. Using NIRCam photometry in 19 nearby galaxies, it selects 1,816 such sources by an F300M–F335M color excess above 5σ. When these sources are placed against the ages of optically selected clusters from the same galaxies, PAH emission is common only among clusters younger than about 3 Myr, and the PAH emitters have Hα equivalent widths 1–2.8 times higher than the youngest optical clusters. The bright end of the luminosity function therefore implies that including these dusty clusters would raise the number of known ≤3 Myr clusters by a factor of roughly 1.8–8.5 per galaxy.

What carries the argument

The central object is the compact 3.3 µm PAH emitter, selected as a 5σ excess of F335M over the F300M continuum in NIRCam color-magnitude diagrams. The 3.3 µm feature is a C–H stretching mode of small PAH molecules, excited by single UV photons, so its presence marks photodissociation regions and the hardest, most intense radiation fields around very young massive stars; the F300M–F335M excess therefore acts as a short-lived clock for the embedded phase. Two auxiliary diagnostics carry the argument: the F200W concentration index, an aperture-difference measure used to separate extended cluster-like objects from point-like stars, and the Hα equivalent width, which is higher for younger populations and does not rely on UV-optical SED ages.

What would settle it

Measure independent, extinction-robust ages for a sample of the 3.3 µm PAH emitters, for example from resolved stellar populations in the nearest galaxies or from radio free-free emission probing embedded ionizing stars; if a substantial fraction turn out older than about 5 Myr, the central claim that PAH emission marks a ≤3 Myr embedded phase would be refuted.

Watch

Extended reading notes

Core claim

The central claim is that the compact 3.3 µm PAH emitter is a distinct, earlier phase of cluster evolution: a dusty, optically invisible or faint object that becomes a normal young HST cluster after it clears its surroundings. The paper establishes a selection recipe with a median 5σ color threshold of F300M−F335M = 0.67 mag at F335M = 20, removes likely evolved stars with a F2100W/F1000W > 3 cut, and keeps 1,816 sources. About 87% of these have F200W concentration indices in the cluster regime, and they sit preferentially in dust lanes, spiral arms, rings, bar ends, and galaxy centers. On the HST cluster age scale, roughly 30% of clusters younger than 3 Myr show the PAH color excess, while essentially none of the 4–10 Myr clusters do; this leads the authors to conclude that PAH emission at cluster scale fades within about 3 Myr. The Hα equivalent widths and the bright-end luminosity function comparison support the idea that the PAH emitters are younger than the youngest optically detected clusters, increasing the census of ≤3 Myr clusters by factors of 1.8–8.5 (median 3.3) when restricted to bright, complete samples.

Load-bearing premise

The argument that the PAH-bright phase lasts only about 3 Myr rests on trusting the HST-derived SED ages to separate clusters younger than 3 Myr from 4–10 Myr clusters, even though the paper notes that UV-optical colors of very young clusters are highly degenerate and that optically selected clusters are biased toward objects that have already cleared their dust.

Editorial extensions

If this is right

  • The dust-embedded phase of cluster formation, as traced by compact 3.3 µm PAH emission, lasts no more than about 3 Myr.
  • At the bright, complete end of the luminosity function, PAH-selected embedded clusters increase the count of ≤3 Myr clusters by a factor of 1.8–8.5 per galaxy, with a median of 3.3.
  • Around 87% of the 1,816 PAH emitters have concentration indices consistent with star clusters rather than individual stars.
  • PAH emitters have Hα equivalent widths 1–2.8 times higher than the youngest optically visible clusters, placing them in an earlier evolutionary phase.
  • The number of compact PAH emitters tracks galaxy star formation rate and CO luminosity, tying the hidden embedded population to the gas supply of the galaxy.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Inference: if the ~3 Myr fading timescale is correct, optical cluster catalogs underestimate the current cluster formation rate by the same factor, and the correction may be largest in high-star-formation, gas-rich galaxies such as NGC 1385 and NGC 1365.
  • Inference: the steeper bright-end luminosity function of PAH emitters compared with HST clusters hints that the embedded population is weighted toward lower-mass clusters; a completeness-corrected mass function would test whether the embedded-to-visible ratio depends on mass.
  • Inference: cross-matching PAH emitters with ALMA molecular gas maps would test the paper's implied emergence sequence, predicting that PAH emitters sit near CO intensity peaks while HST clusters lie toward the cloud edges.
  • Inference: follow-up mid-infrared spectroscopy could decide whether the apparent 10 µm dip in the median SED is silicate absorption or simply continuum between the 7.7 and 11.3 µm PAH features, which would refine the selection diagnostics.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. This paper presents a census of compact 3.3 micron PAH emitters in 19 PHANGS-JWST galaxies, identifying 1816 sources via F300M-F335M color excess, per-galaxy 5-sigma thresholds, and mid-IR color cuts to remove evolved stars. The authors characterize the sources with concentration indices, spatial distributions, SEDs, H-alpha equivalent widths, and luminosity functions, and compare them with PHANGS-HST cluster catalogs. The headline results are that PAH emission fades within about 3 Myr and that dusty embedded clusters may increase the number of optically visible <=3 Myr clusters in PHANGS-HST by a factor of 1.8-8.5.

Significance. If the timescale claim holds, this is a valuable systematic measurement of the duration of the compact PAH-emitting phase and the first large, multi-galaxy census of embedded cluster candidates selected at 3.3 microns. The paper's strengths include the per-galaxy treatment of detection limits and color dispersions, contamination cuts validated against an external LMC sample, a transparent presentation of the selection criteria, and an explicit discussion of the bright-end LF caveats. The catalog itself and the characterization of 1816 sources are likely to be useful reference products for the PHANGS community and for future JWST studies of embedded star formation.

major comments (3)
  1. [Sec. 6.1 and Sec. 7.1 (Figs. 12-14, Table 3)] The central claim that "PAH emission fades within ~3 Myr" is not established for the general embedded population, because the age clock is the optically selected PHANGS-HST cluster sample. As the paper itself concedes in Sec. 6.1, clusters detected in blue or UV filters are not heavily extinguished, so the HST sample is biased by construction toward clusters that have already cleared their dust. The data show that optically visible clusters lose their F300M-F335M excess by ~3 Myr, but they do not constrain how long clusters that remain embedded continue to emit PAHs. This is load-bearing for the abstract and Section 7.1; the conclusion should be reframed as a constraint on the PAH-emitting phase of optically detected clusters, or supported by an independent age or embedded-phase indicator such as Pa-alpha or mid-IR spectroscopy.
  2. [Sec. 6.1 (age bins used in Table 3 and Figs. 12-14)] The age comparison relies on the reliability of SED-derived ages in exactly the range where the paper says ages are most uncertain: the text notes that UV-optical colors of <=3 Myr clusters are highly degenerate and that 3-5 Myr clusters with A_V >~ 1 can be confused with 5-10 Myr clusters. Because the evolutionary statement depends on separating <=3 Myr from 4-10 Myr, the paper should quantify the effect of these age uncertainties on the reported fractions, for example by propagating SED-fitting age posteriors or showing the sensitivity of the Table 3 fractions to alternative age cuts.
  3. [Sec. 7.2 (LF-based increase factor)] The factor of 1.8-8.5 increase in the number of <=3 Myr clusters is presented as a headline result, but it depends on treating bright PAH emitters as clusters and on the relative completeness of the PAH and HST cluster samples. The paper correctly notes that only the bright end is used and that fainter sources are incomplete, but Fig. 15 shows the PAH LF turning over near the completeness limit and having a steeper bright-end slope than the HST LF. A quantitative completeness and contamination treatment for the overlap region, or an explicit upper/lower bound derived from the CI distribution and the LMC contamination benchmark, is needed before this factor can be taken as a robust measurement rather than an illustrative estimate.
minor comments (5)
  1. [Abstract] The phrase "identify of 1816 sources" should read "identify 1816 sources."
  2. [Sec. 6.1] The word "Neverthelsss" is misspelled; it should be "Nevertheless."
  3. [Sec. 6.3] The text "age < 107 Myr" appears to have a typesetting error; it should presumably be "age < 10^7 yr" or "age < 10 Myr," and the caption for that figure should state the intended meaning.
  4. [Fig. 17 and Fig. 18] The gray region denotes the absence of age estimates for PAH emitters, but the emitters are placed to the left of the HST clusters to suggest an evolutionary sequence. The caption should state explicitly that this horizontal placement is an assumption, not an age measurement.
  5. [References] The two references listed as Whitmore et al. 2023a and 2023b appear to be identical; please check and correct the bibliographic entries.

Circularity Check

0 steps flagged · score 2.0 of 10

No construction-level circularity; the main weakness is an optically-selected age-clock bias that the paper itself concedes.

full rationale

The paper's selection of compact 3.3 µm PAH emitters uses F300M−F335M > 5σ color excess, but the headline fade-timescale is not obtained by re-fitting that same excess. Ages come from the external PHANGS-HST SED fits (Thilker et al. 2024, based on NUV-U-B-V-I photometry), and the comparison in §6.1 measures how the fraction of HST clusters with F300M−F335M excess declines with SED age. This is a correlation between an external clock and the selection variable, not a definitional identity. The Hα EW comparison (§7.1, Fig. 17) is an independent observable not used to set the PAH-emitter threshold, and the CI analysis (§5.1) calibrates cluster-like concentration against HST clusters and red evolved stars, again external to the F300M−F335M cut. The 1.8–8.5× increase factor (§7.2) is a bookkeeping ratio of matched/unmatched counts, not a prediction. The main caveat, acknowledged in §6.1, is that the HST cluster sample is optically selected: 'clusters detected in blue or even UV filters are not heavily extinguished', so the ≤3 Myr fade timescale constrains optically visible clusters, not necessarily the full embedded phase; likewise 'UV-optical colors of clusters ≤ 3 Myr are also highly degenerate'. These are validity/selection-effect concerns, not circular reductions. Self-citations to Rodríguez et al. (2023) and Thilker et al. (2024) supply the tracer premise and the age catalog, but the age catalog is independent of the PAH excess and the tracer premise is re-supported in this paper by Hα, CI, and SED evidence. Minor self-citation presence gives score 2; no construction-level circularity.

Assumptions & free parameters 6 free parameters · 6 assumptions · 0 invented entities

The central claims rest on a chain of hand-chosen and fitted thresholds (5-sigma color curve, MIRI ratio cut, F360M/F335M cut, mass-to-light fit, per-galaxy CI limits) plus external calibration assumptions (HST SED ages, LMC contamination benchmark, TRGB distances). No new physical entities are postulated; 'compact 3.3 micron PAH emitter' is an observational source class, not a new object type. The heaviest burden is the identification of the F300M-F335M excess with 3.3 micron PAH emission from young embedded clusters, which is supported by several indirect checks but not by spectroscopy.

free parameters (6)
  • Per-galaxy 5-sigma color dispersion curve (a, b, c) = Median threshold F300M-F335M = 0.67 at F335M = 20; per-galaxy range 0.35-0.84
    The selection threshold sigma(F300M-F335M) = a*exp(b*F335M) + c is fitted to the negative-color side of each galaxy's CMD (Section 4.1); it sets sample membership for all 1,816 sources.
  • Fnu(F2100W)/Fnu(F1000W) > 3 contamination cut = 3.0 (F1000W-F2100W = 1.2 AB mag)
    Hand-chosen cut to remove evolved stars, validated against the LMC sample of Jones et al. (2017) (Section 4.2 and Appendix A). It is conservative and affects which sources enter the final sample.
  • Fnu(F360M)/Fnu(F335M) < 1 cut = 1.0
    Hand-chosen cut to exclude sources whose F300M-F335M excess reflects a rising near-IR continuum rather than the 3.3 micron PAH feature (Section 4.2).
  • F200W mass-to-light relation = M_F200W = -2.55 (+/-0.03) log(M/Msun) + 2.48 (+/-0.15)
    Line fitted to HST clusters with ages <=3 Myr (Figure 11, Section 5.4); used to convert PAH-emitter F200W magnitudes into masses and to label the LF mass axis, building on Thilker et al. (2024) SED masses.
  • Per-galaxy concentration index limit = CI_lim per galaxy, mean of the 84% quantile of red-star candidates and the 16% quantile of HST clusters
    Each galaxy's star-versus-cluster CI threshold is computed from that galaxy's own populations (Figure 7, Section 5.1); the headline that about 87% of PAH emitters are cluster-like depends on these per-galaxy thresholds.
  • MIRI aperture correction factor = 2.0
    Flux correction applied to F1000W and F2100W point-source photometry measured in an aperture capturing 50% of encircled energy (Table 2); it feeds the contamination-cut color.
assumptions (6)
  • domain assumption The F300M-F335M color excess in compact sources selected in F335M is dominated by 3.3 micron PAH emission from young dust-embedded star-forming regions rather than reddened stellar continuum or other emission processes.
    Introduced in Sections 4.1-4.2; supported only indirectly by the F360M/F335M < 1 cut, five example SEDs (Figure 6), and comparison with LMC source types. The paper acknowledges that follow-up mid-IR spectroscopy is required (Section 4.3).
  • domain assumption The negative-color side of the F300M-F335M distribution is symmetric with the positive side for continuum sources, so the fitted sigma curve is a valid noise model for the color selection.
    Assumed in Section 4.1 to derive the 3-sigma and 5-sigma selection curves; standard in narrowband surveys but not independently verified here.
  • domain assumption PHANGS-HST SED-fitting ages (Thilker et al. 2024) are accurate enough to separate <=3 Myr, 4-5 Myr, and 6-10 Myr clusters for the fade-timescale comparison.
    The fade-timescale conclusion (Sections 6.1 and 7.1) rests on these ages; the text acknowledges UV-optical age degeneracies at <=3 Myr and at 5-10 Myr.
  • domain assumption The Jones et al. (2017) LMC sample of point sources is representative of evolved-star contamination at 10-21 microns in the PHANGS galaxies, validating the F2100W/F1000W > 3 cut.
    Used in Section 4.2 and Appendix A to justify the contamination cut; the LMC differs in metallicity and angular resolution from the PHANGS galaxies.
  • domain assumption The bright end of the F200W luminosity function follows a single power law for both PAH emitters and HST clusters, so fitted slopes can be compared directly.
    Invoked in Section 6.2 (Figure 15); the steeper PAH slope (alpha = 2.4 versus 2.0) is interpreted as a real difference, but color dilution of bright continuum-dominated clusters could bias it.
  • domain assumption TRGB distances from Anand et al. (2020) and Lee et al. (2023) are accurate enough to compute luminosities, physical scales, and the 85 pc background box sizes used throughout.
    External calibration adopted from prior literature; standard practice for this field and not independently checked in this paper.

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Pith. "Pith review of Tracing the earliest stages of star and cluster formation in 19 nearby galaxies with PHANGS-JWST and HST: compact 3.3 $\mu$m PAH emitters and their relation to the optical census of star clusters." pith.science (2026). https://pith.science/paper/GIXEWCRU

@misc{pith2026241207862,
  author       = {Pith},
  title        = {Pith review of: Tracing the earliest stages of star and cluster formation in 19 nearby galaxies with PHANGS-JWST and HST: compact 3.3 $\mu$m PAH emitters and their relation to the optical census of star clusters},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/GIXEWCRU}},
  note         = {Machine review of arXiv:2412.07862}
}
abstract

The earliest stages of star and cluster formation are hidden within dense cocoons of gas and dust, limiting their detection at optical wavelengths. With the unprecedented infrared capabilities of JWST, we can now observe dust-enshrouded star formation with $\sim$10 pc resolution out to $\sim$20 Mpc. Early findings from PHANGS-JWST suggest that 3.3 $\mu$m polycyclic aromatic hydrocarbon (PAH) emission can identify star clusters in their dust-embedded phases. Here, we extend this analysis to 19 galaxies from the PHANGS-JWST Cycle 1 Treasury Survey, providing the first characterization of compact sources exhibiting 3.3$\mu$m PAH emission across a diverse sample of nearby star-forming galaxies. We establish selection criteria, a median color threshold of F300M-F335M=0.67 at F335M=20, and identify of 1816 sources. These sources are predominantly located in dust lanes, spiral arms, rings, and galaxy centers, with $\sim$87% showing concentration indices similar to optically detected star clusters. Comparison with the PHANGS-HST catalogs suggests that PAH emission fades within $\sim$3 Myr. The H$\alpha$ equivalent width of PAH emitters is 1-2.8 times higher than that of young PHANGS-HST clusters, providing evidence that PAH emitters are on average younger. Analysis of the bright portions of luminosity functions (which should not suffer from incompleteness) shows that young dusty clusters may increase the number of optically visible $\leq$ 3 Myr-old clusters in PHANGS-HST by a factor between $\sim$1.8x-8.5x.

Figures

Figures reproduced from arXiv: 2412.07862 by the authors.

Figure 1
Figure 1. F335M image of a crowded region in the galaxy NGC 7496. The left panel displays the sources detected using both the find peaks algorithm of Photutils and the SExtractor Mexicanhat filter. It is noticeable that the SExtractor detects three sources (red circles) missed by find peaks (green circles) which can be attributed to its better performance in deblending sources. In the right panel, we present the final catalog… view at source ↗
Figure 2
Figure 2. Color-magnitude diagrams (CMDs) F335M vs. F300M − F335M for the 19 galaxies included in the PHANGS–JWST cycle 1 Treasury Program. The plots are organized by increasing galaxy distance from the upper left corner to the bottom right. The contours represent the 5th, 10th, 30th, 50th, 70th, and 90th density percentiles. The small grey dots indicate regions where the density is lower than the 5th percentile. The red and … view at source ↗
Figure 3
Figure 3. F1000W vs. F1000W − F2100W CMDs showing the compact 3.3 µm PAH emitters (green and blue points) from this analysis and a comparison sample from Jones et al. (2017) which includes point-like sources in the LMC: PN (purple points), YSOS and HII regions (orange contours) and different kind of stars (black contours representing 98th and 99.9th percentiles). Objects that were removed from our 3.3 µm PAH emitters sample a… view at source ↗
Figures from the paper (17 more)
Figure 4
Figure 4. Figure 4: Number of PAH emitters detected above the 5σ color dispersion (see Sect 4.1 and 4.2) against the total galaxy SFR, (left) and the integrated CO (2-1) luminosity (LCO, right). Values for SFR and LCO are from Lee et al. (2023, [PITH_FULL_IMAGE:figures/full_fig_p012_4.png]
Figure 5
Figure 5. Figure 5: Example of compact 3.3µm PAH emitters (green) detected in a northern region of the galaxy NGC3627, located at a distance of 11.32Mpc. The left panel shows an HST F555W image of the northern part of the galaxy, with the red box indicating the region displayed in the zoo…
Figure 6
Figure 6. Figure 6: Observed SED for the five objects shown in [PITH_FULL_IMAGE:figures/full_fig_p013_6.png]
Figure 7
Figure 7. Figure 7: F200W concentration index (CI) – difference between F200W photometry measured in circular apertures with radii of 1 and 4 pixels– distribution for PHANGS-HST Class 1 and 2 star clusters (blue), candidate red evolved stars (red) and > 5σ PAH emitters (green). The red da…
Figure 8
Figure 8. Figure 8: Spatial distribution of the < 5σ PAH emitters overlaid in a JWST F335M image of the galaxies. The plots are organized by increasing galactic distance from the upper left corner to the bottom right. The red footprints shows the HST H-α observations [PITH_FULL_IMAGE:fig…
Figure 9
Figure 9. Figure 9 [PITH_FULL_IMAGE:figures/full_fig_p017_9.png]
Figure 10
Figure 10. Figure 10: Spatial distribution of PAH emitters in NGC3627 and NGC4321 overlaid on the HST F555W image of the galaxies. We observe that the PAH emitters are mainly located in and around the dust lanes. Here we show both 5σ (green) and 3-5σ PAH (blue) samples. not compact; we hav…
Figure 11
Figure 11. Figure 11: Mass-to-light ratio in the F200W band for the HST-cluster catalogs Class 1 and 2 with ages≤3 Myr. The relation derived here was used to estimate the masses of the PAH emitters. detected clusters, then the computed masses will under￾estimate the true stellar mass; neve…
Figure 12
Figure 12. Figure 12: F335M vs. F300M-F335M CMDs. The contours represent the 5th, 10th, 30th, 50th, 70th, and 90th density percentiles of all the sources detected over the F335M images (see Sect 3.1). The small grey dots indicate regions where the density is lower than the 5th percentile. …
Figure 13
Figure 13. Figure 13: F335M vs. F300M-F335M CMDs. The contours represent the 5th, 10th, 30th, 50th, 70th, and 90th density percentiles of all the sources detected over the F335M images (see Sect 3.1). The small grey dots indicate regions where the density is lower than the 5th percentile. …
Figure 14
Figure 14. Figure 14: Age histograms for the PHANGS-HST clusters from (Maschmann et al. 2024) with ages determined via SED fitting from (Thilker et al. 2024) with F300M −F335M > 3σ are detected using our method, even though they are not selected as PAH emitters. This rules out the pos￾sibi…
Figure 16
Figure 16. Figure 16: Median values of the SED normalized to the flux density in F200W for compact 3.3 µm PAH emitters in the ‘not detected in HST’, ‘only detected in Hα’ and ‘detected in HST’ categories (see S7.1 for definition of these categories). These SEDs reveal three distinct charac…
Figure 17
Figure 17. Figure 17: Evolution of Hα EW with Age. The plot presents the EW derived for HST Clusters and PAH Emitters. The grey region denotes the absence of estimated ages for the PAH emitters; however, they are positioned to the left of the HST clusters to suggest a potential evolutionar…
Figure 18
Figure 18. Figure 18: F300M − F335M vs. Age. The grey region denotes the absence of estimated ages for the PAH emitters; however, they are positioned to the left of the HST clusters to suggest a potential evolutionary sequence (refer to text for details). Median values are depicted with la…
Figure 19
Figure 19. Figure 19: An example F555W-F814W vs. F814W color-magnitude diagram for the galaxy NGC 628. The black points are sources from the PHANGS-HST DOLPHOT catalog (Thilker et al. 2022), while the red points show a candidate sample of red evolved stars, used in this paper to help deter…
Figure 20
Figure 20. Figure 20: F1000W-F2100W vs. F300M-F335M color-color diagram for >5σ (green) and 3-5σ (blue) PAH emitters. The HST candidate samples of red evolved stars are plotted if their F1000W and F2100W photometry are above their respective 5σ point source limits (Lee et al. 2023, [PITH_…
Figure 21
Figure 21. Figure 21: F300M − F335M photometric errors. The contours represent the 5th, 10th, 30th, 50th, 70th, and 90th density percentiles of all sources detected in the F335M images. Small gray dots mark regions with densities below the 5th percentile. The blue dashed line indicates the…

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Forward citations

Cited by 3 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

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  3. Probing Obscured Star Formation in Galaxy Clusters Using JWST Medium Band Images: 3.3$\mu\rm m$ PAH Emitter Sample in Abell 2744

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Pith tools

Reviewed August 11, 2026 · model on record in the stance chip above.