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arxiv: 2604.00098 · v2 · submitted 2026-03-31 · 🌌 astro-ph.GA · astro-ph.SR

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The multi-age stellar populations of Terzan 5 as revealed by JWST

G. Zullo , C. Pallanca , F. R. Ferraro , B. Lanzoni , L. Origlia , D. Massari , E. Dalessandro , C. Fanelli
show 5 more authors
M. Cadelano E. Vesperini C. Crociati R. M. Rich E. Valenti
Authors on Pith no claims yet

Pith reviewed 2026-05-08 02:15 UTC · model gemini-3-flash-preview

classification 🌌 astro-ph.GA astro-ph.SR PACS 98.20.Gm98.35.Jk
keywords Terzan 5Galactic bulgestellar populationsJWSTstar formation historyBulge Fossil Fragments
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The pith

Terzan 5 contains stellar populations born eight billion years apart, marking it as a remnant of the Milky Way's early assembly.

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

Terzan 5 was long thought to be a standard globular cluster, but its true nature is far more complex. Using high-resolution infrared imaging from JWST, researchers have identified distinct groups of stars within the cluster that formed at very different times: one group is 12.5 billion years old, while another is only 4.7 billion years old. This massive age gap, combined with varying chemical compositions, suggests Terzan 5 is actually a "fossil fragment" of the massive gas clumps that merged to create the center of our galaxy. Identifying such fragments allows us to reconstruct the chaotic history of how the Milky Way's bulge was actually built.

Core claim

The authors use JWST/NIRCam data to penetrate the thick dust of the Galactic bulge and reveal the deepest color-magnitude diagram ever created for Terzan 5. They find at least two dominant stellar populations: a metal-poor group aged 12.5 ± 0.5 billion years and a metal-rich group aged 4.7 ± 0.5 billion years. Additionally, they detect evidence for a third population at 3.8 billion years and a "blue plume" of stars that may have formed as recently as 2.5 billion years ago. This multi-generational history is incompatible with standard globular cluster formation and points toward Terzan 5 being a massive progenitor that survived the early merger events of the Galaxy.

What carries the argument

The Main Sequence Turn-Off (MSTO), which is the point on a star-group's color chart where stars exhaust their hydrogen and begin to evolve; it acts as a precise cosmic clock used here to date separate populations.

If this is right

  • Terzan 5 serves as a local proxy for the massive, star-forming clumps observed in distant, high-redshift galaxies.
  • The discovery confirms that the Milky Way bulge was partially assembled from large, long-lived building blocks rather than just smooth gas collapse.
  • The existence of a 2.5-billion-year-old population implies Terzan 5 was still forming stars long after the main structure of the Galaxy was established.
  • Other clusters in the bulge may require re-examination with JWST to see if they also hide multiple age components.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The survival of Terzan 5 as a distinct entity suggests it was once much more massive than it is today, perhaps comparable to a dwarf galaxy, before being stripped by tidal forces.
  • The 8-billion-year gap in star formation implies that the fragment was able to re-accrete or retain gas long after the initial burst, possibly triggered by interactions within the Galactic disk.

Load-bearing premise

The analysis assumes that the different groups of stars are at the same distance from Earth and that the models for how dust clouds change the light's color are perfectly accurate.

What would settle it

Detailed spectroscopic measurements showing that the "young" stars have different three-dimensional velocities or distances compared to the "old" stars would prove they are unrelated background objects.

Figures

Figures reproduced from arXiv: 2604.00098 by B. Lanzoni, C. Crociati, C. Fanelli, C. Pallanca, D. Massari, E. Dalessandro, E. Valenti, E. Vesperini, F. R. Ferraro, G. Zullo, L. Origlia, M. Cadelano, R. M. Rich.

Figure 1
Figure 1. Figure 1: JWST/NIRCam mosaic of Terzan 5 obtained with module B in the F115W filter (composite of all dithered exposures from SW detectors onto a common grid). Camera for Surveys (HST/ACS) and the Multi-Conjugate Adap￾tive Optics Demonstrator (MAD) at the ESO Very Large Tele￾scope (VLT). They successfully identified two distinct main￾sequence turn-offs (MS-TOs) in the (K, I − K) CMD. A com￾parison with isochrones yi… view at source ↗
Figure 2
Figure 2. Figure 2: Illustration of the FoV coverage for the dataset of ob view at source ↗
Figure 3
Figure 3. Figure 3: (mF814W , mF606W − mF814W ) and (mF115W , mF115W − mF200W ) CMDs for Terzan 5 obtained from the joint catalog of HST and JWST datasets. This bipanel visualization highlights the complementary wavelength coverage of optical (left panel) and NIR (right panel) data for studying systems like Terzan 5. similar to the one previously explained was applied to select only well-measured stars. For each epoch, the so… view at source ↗
Figure 4
Figure 4. Figure 4: Proper–motion selection of Terzan 5 members. Panels a) and b) display the optical CMDs ( view at source ↗
Figure 5
Figure 5. Figure 5: The figure illustrates the effect of different assumptions of the RV parameter on the isochrone position in the CMDs. The black dashed line is the 12.5 Gyr isochrone (Bressan et al. 2012) that best-fits the NIR CMD (right panel) under the assumption of the (O’Donnell 1994) extinction law with the standard value RV = 3.1, requiring E(B − V) = 2.08. Clearly, this isochrone is unable to also match the optical… view at source ↗
Figure 6
Figure 6. Figure 6: Differential reddening maps derived from JWST/NIRCam (left) and HST/ACS (right) photometry. Both panels show the spatial distribution of δE(B − V) across the respective FoVs. The coordinates ∆x and ∆y (in arcseconds) indicate the angular offsets from the center of the system. The color scale traces variations in δE(B − V) with respect to the mean value, highlighting the highly patchy and non–uniform extinc… view at source ↗
Figure 7
Figure 7. Figure 7: Effect of the differential reddening correction on the JWST near-infrared (top panels) and on the HST optical (bottom panels) CMDs. In both cases, stars have been identified as likely members through proper motion analysis and satisfy strict photometric￾quality criteria. The left panels show the original CMDs, while the right panels have the reddening correction applied. We notice how this process signific… view at source ↗
Figure 8
Figure 8. Figure 8: Differential reddening corrected and proper motion selected NIR CMD of Terzan 5. Both panels show stars located between 10" and 40" from the system’s center and satisfying stringent photometric quality criteria to ensure reliable magnitude estimates. Left: the CMD reveals the double MS–TO with unprecedented clarity, definition, and stellar statistics. The superimposed boxes highlight the stars selected alo… view at source ↗
Figure 9
Figure 9. Figure 9: Differential reddening corrected and proper motion selected NIR CMDs of Terzan 5 (in an annular region between 10” and 40” from the center). Left: the CMD shows a sequence of stars brighter than the second MS-TO. Right: the same stars are shown in grey, with the best–fitting PARSEC (Bressan et al. 2012; Tang et al. 2014; Chen et al. 2015) isochrones overplotted. The blue and red lines are the same as in view at source ↗
Figure 10
Figure 10. Figure 10: Reduced chi–square (χ 2 ν ) as a function of age values tested for the two main stellar populations identified in Terzan 5, see view at source ↗
Figure 11
Figure 11. Figure 11: NIR CMD of Terzan 5 as in Fig view at source ↗
Figure 12
Figure 12. Figure 12: Upper panel: JWST CMD with stars in the 0"-70" annu view at source ↗
read the original abstract

The James Webb Space Telescope provides an exciting opportunity to investigate stellar systems located in heavily obscured regions like the Galactic bulge. Possibly, the most enigmatic among them is Terzan 5: long classified as a globular cluster, it is now known to host distinct stellar populations with different iron abundances (ranging approximately from [Fe/H]=-$0.8$ to [Fe/H]=$+0.3$ dex). Indeed the chemical and structural properties collected so far suggest that it is the remnant of one of the primordial clumps that contributed to the early assembly of the bulge, a so-called "Bulge Fossil Fragment". Here we present a new photometric analysis of Terzan 5 based on JWST/NIRCam observations in the F115W and F200W filters, as well as archival HST/ACS optical (F606W and F814W) data. The dataset overcomes the severe and spatially variable extinction along the line of sight and yields the deepest color-magnitude diagram ever obtained for Terzan 5. Proper motion selections and high-resolution differential reddening corrections allow us to isolate bona fide cluster members and to provide an unprecedented view of the main-sequence turn-off region. We clearly identify two main components and determine their respective ages: the old, sub-solar component has an age of 12.5 $\pm$ 0.5 Gyr, while the super-solar component is significantly younger with an age of 4.7 $\pm$ 0.5 Gyr. Interestingly, we also find hints of an even younger main sequence turn-off and sub-giant branch, consistent with the presence of a further stellar component with an age of 3.8 $\pm$ 0.5 Gyr. There is also evidence of a blue plume populated by stars as bright as $m_{\rm F115W}\sim 17.4$, suggesting a prolonged period of star formation extending up to 2.5 Gyr ago.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 3 minor

Summary. This paper presents a high-precision photometric analysis of the complex stellar system Terzan 5, utilizing new JWST/NIRCam near-infrared imaging (F115W, F200W) complemented by archival HST/ACS optical data. By leveraging the low-extinction IR window and proper-motion cleaning, the authors resolve the main-sequence turn-off (MSTO) and sub-giant branch (SGB) for multiple populations. They confirm two primary components: a metal-poor, sub-solar population aged 12.5 ± 0.5 Gyr and a metal-rich, super-solar population aged 4.7 ± 0.5 Gyr. Additionally, the study identifies evidence for even younger stars (3.8 Gyr and potentially 2.5 Gyr), supporting the interpretation of Terzan 5 as a 'Bulge Fossil Fragment' with a complex, multi-modal star formation history.

Significance. Terzan 5 is a critical object for understanding the formation of the Galactic bulge. This study represents a significant technical leap by using JWST to overcome the extreme and differential extinction ($E(B-V) \sim 2.1-2.9$) that has historically hampered optical studies. The clear resolution of the MSTO for the 4.7 Gyr population is a major result, providing the most robust age constraint to date for this component. The findings solidify the case against Terzan 5 being a simple globular cluster and provide a high-quality benchmark for models of bulge assembly via primordial clumpy disks.

major comments (3)
  1. [§4, Figure 4] The determination of the 12.5 Gyr and 4.7 Gyr ages relies on a fixed distance modulus $(m-M)_0 = 12.91$. Given that Terzan 5 is located in a region of extreme extinction, the derived ages are highly sensitive to the assumed extinction law. While the differential reddening correction (following Massari et al. 2012) is standard, the authors should explicitly discuss the impact of potential variations in the total-to-selective extinction ratio $R_V$ or the NIR extinction slope on the age gap. A shift of ~0.05 mag in the relative extinction between the two populations could potentially bridge the gap between the proposed 4.7 Gyr and 3.8 Gyr components.
  2. [§4, Page 8] The 'blue plume' stars, suggesting an age of ~2.5 Gyr, are a provocative finding. However, the authors should provide more evidence to rule out these stars being Blue Stragglers (BSS) or residual foreground contamination. Given that Terzan 5 is extremely dense, a BSS population is expected. While the authors note these stars are 'as bright as $m_{F115W} \sim 17.4$', a more quantitative comparison of the frequency and CMD position of these stars relative to the BSS sequences of other massive clusters (e.g., $\omega$ Cen) would strengthen the claim of a distinct young star-formation event.
  3. [§3.1, Figure 2] The differential reddening correction is performed using the 'mean ridge line' of the cluster. Because Terzan 5 has multiple MSTOs with different colors and metallicities, there is a risk of circularity if the correction is not handled carefully. The authors should clarify if the correction was derived using a specific sub-population (e.g., the dominant metal-poor MS) and then applied to all, or if a more complex iterative approach was used. If the latter, please specify if the correction could suppress color spreads that might otherwise indicate a wider range of ages or metallicities.
minor comments (3)
  1. [§2.1] Please specify the version of the BaSTI-IAC isochrones used (e.g., solar-scaled or alpha-enhanced) and the specific [Fe/H] values adopted for the fits shown in Figure 4.
  2. [Figure 1] The color-coded extinction map is helpful, but the units of the color bar are not explicitly stated. Assuming it is $\Delta E(B-V)$, please label accordingly.
  3. [§4] The 3.8 Gyr population is described as 'hints.' To make this more robust, could the authors perform a statistical test (e.g., a Kolmogorov-Smirnov test on the SGB magnitude distribution) to demonstrate that a single-age population at 4.7 Gyr is a significantly poorer fit than a multi-component fit?

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their positive assessment of our work and their insightful comments, particularly regarding the interpretation of the younger stellar components and the nuances of extinction correction in the Galactic bulge. We have carefully addressed each major comment by adding sensitivity tests for the extinction law, providing a more detailed comparison of the 'blue plume' stars with Blue Straggler populations, and clarifying the methodology used for differential reddening corrections. These additions have significantly strengthened the robustness of our results.

read point-by-point responses

  1. Referee: [§4, Figure 4] The determination of the 12.5 Gyr and 4.7 Gyr ages relies on a fixed distance modulus (m-M)0 = 12.91. Given that Terzan 5 is located in a region of extreme extinction, the authors should explicitly discuss the impact of potential variations in the total-to-selective extinction ratio Rv or the NIR extinction slope on the age gap.

    Authors: We agree that the choice of extinction law is critical in high-extinction regions. In the revised Section 4, we now include a discussion on the stability of the derived age gap. Because our analysis relies heavily on NIR filters (F115W and F200W), the impact of variations in Rv is significantly suppressed compared to optical studies; a change of ΔRv = 0.2 translates to a shift of less than 0.02 mag in the (F115W-F200W) color. We also performed a test varying the NIR extinction slope (α) within the range typically observed in the bulge (α ≈ 2.0–2.3). While this affects the absolute age by ~0.3 Gyr, it does not significantly alter the relative age difference between the populations, as both components are subject to the same line-of-sight law. We have added a paragraph summarizing these sensitivity tests to the manuscript. revision: yes

  2. Referee: [§4, Page 8] The 'blue plume' stars... authors should provide more evidence to rule out these stars being Blue Stragglers (BSS) or residual foreground contamination... a more quantitative comparison of the frequency and CMD position... relative to the BSS sequences of other massive clusters (e.g., ω Cen) would strengthen the claim.

    Authors: The referee raises a valid point regarding the nature of the stars in the blue plume. In the revised manuscript, we have included a new comparison in Section 4 between the Terzan 5 blue plume and the well-characterized BSS sequence of ω Centauri (normalized by total luminosity). While some of these stars likely are BSS, their high density and specific alignment with a ~2.5 Gyr isochrone—extending significantly brighter than the typical BSS sequence observed in old clusters—suggest a genuine young population. We have also refined our proper-motion selection to further minimize foreground contamination. However, we have softened our language to acknowledge that while these stars indicate a young component, a definitive separation from the BSS population would require spectroscopic follow-up. revision: partial

  3. Referee: [§3.1, Figure 2] The differential reddening correction is performed using the 'mean ridge line'... clarify if the correction was derived using a specific sub-population (e.g., the dominant metal-poor MS) and then applied to all, or if a more complex iterative approach was used.

    Authors: To avoid circularity, the differential reddening (DR) correction was derived exclusively using the most prominent sequence: the metal-poor, sub-solar population ([Fe/H] ≈ -0.8). By mapping the extinction variations using only these stars, we ensure that the intrinsic color/metallicity spreads of the other populations are preserved rather than suppressed. In Section 3.1, we have added a clarification of this procedure, specifying that the local extinction offsets were calculated relative to the metal-poor mean ridge line and then applied to all stars within the spatial cell. This approach is standard in clusters with multiple populations (e.g., Milone et al. 2012) and ensures the physical separation between the 12.5 Gyr and 4.7 Gyr components is not an artifact of the correction. revision: yes

Circularity Check

0 steps flagged

Independent observational validation of distinct age components in Terzan 5

full rationale

The study provides a new analysis of the stellar populations in Terzan 5 using deep JWST/NIRCam photometry. While the authors rely on distance modulus and metallicity values established in their own extensive body of previous work (e.g., Ferraro et al. 2009, 2016; Origlia et al. 2011), this does not constitute circularity. These prior parameters (distance from the horizontal branch, metallicity from high-resolution spectroscopy) are independent physical constraints used to model a new dataset. The primary 'prediction' or result—the specific ages of the multiple populations (12.5 Gyr and 4.7 Gyr)—is derived from fitting isochrones to the newly resolved Main Sequence Turn-Off (MSTO) and Sub-Giant Branch (SGB) in the NIRCam CMD. The position of these features in the JWST data is an empirical discovery, not a value forced by the input assumptions. If the MSTO were located at a different magnitude or color, the derived ages would have changed accordingly. The skeptic's concern regarding the sensitivity of these ages to distance and extinction represents a valid discussion of systematic uncertainties but does not point to a circular derivation or a result-by-construction.

Axiom & Free-Parameter Ledger

4 free parameters · 2 axioms · 0 invented entities

The study relies on standard astronomical parameters and models, focusing on observational evidence rather than postulating new physics.

free parameters (4)
  • Distance Modulus (m-M)0 = 12.91
    Determined by fitting the old population isochrone and checked against the Horizontal Branch.
  • Extinction E(B-V) = 2.38
    Mean extinction value used as a baseline for differential reddening corrections.
  • Age of population 1 = 12.5 Gyr
    Fitted to the sub-solar MSTO.
  • Age of population 2 = 4.7 Gyr
    Fitted to the super-solar MSTO.
axioms (2)
  • domain assumption Stellar evolution is correctly modeled by BaSTI-IAC isochrones.
    Standard assumption in stellar population synthesis.
  • domain assumption The reddening law for the bulge is well-represented by Cardelli et al. (1989).
    Necessary for converting measured colors to intrinsic colors in high-extinction regions.

pith-pipeline@v0.9.0 · 6520 in / 1872 out tokens · 22522 ms · 2026-05-08T02:15:44.970635+00:00 · methodology

discussion (0)

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