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Mapping the Filamentary Nebula of NGC 1275 with Multiwavelength SITELLE Observations

T0 review · 4 major / 5 minor · reviewed 2026-08-08 · deepseek-v4-flash

Pith's one-line read The AGN does not power the extended filaments of NGC 1275; a distributed hard-ionizing source, likely the cooling intracluster medium, does.

desk verdict The new SITELLE maps are a genuine dataset, but the paper's headline claim that they rule out AGN photoionization of the extended filaments overreaches what the diagnostics can actually discriminate. read the letter →

arxiv 2502.05406 v2 pith:QCBIRVGT submitted 2025-02-08 astro-ph.GA astro-ph.IM

classification astro-ph.GAastro-ph.IM
keywords filamentarynebulaNGC1275PerseusclusterAGNphotoionizationWHANdiagramBPTcool-coreSITELLE
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 uses new multiwavelength optical observations of the entire filamentary nebula around NGC 1275 to determine what ionizes the cool gas that extends dozens of kiloparsecs from the cluster's central galaxy. It produces flux maps for [O II], [O III], Hβ, [N II], and Hα and applies BPT and WHAN emission-line diagnostics to each region. The central result is that the AGN is not responsible for powering the extended filaments; instead, a spatially uniform hard-ionizing phenomenon, most plausibly the cooling intracluster medium acting through collisional excitation or mixing, maintains the optical emission, with magnetic fields stabilizing the filaments. A compact central region does show signs of AGN photoionization, and star formation appears only in a few clumps such as the blue-loop.

What carries the argument

The key machinery is the combination of the WHAN diagram (equivalent width of Hα versus log([N II]/Hα)) and the BPT diagram ([O III]/Hβ versus [N II]/Hα), with diagnostic cuts adopted from Stasińska et al. (2006) at log([N II]/Hα) = -0.4 and Kewley et al. (2006) at W_Hα = 6 Å. Because [O III] is undetected in the extended filaments, the WHAN diagram is the primary tool there; it classifies each spaxel as star-forming, high-equivalent-width (HEW), or low-equivalent-width (LEW), and the spatial maps of these classifications reveal the distribution of ionizing mechanisms across the nebula.

What would settle it

A spatially resolved spectrum of the large northern filament that detects [O III] λ5007 at greater than 3σ significance, or that shows W_Hα increasing toward the galaxy center, would falsify the claim that a uniform distributed source rather than the AGN ionizes the extended filaments.

Watch

Extended reading notes

Core claim

The paper establishes that [O III] λ5007 is detected only in the central core of NGC 1275 and is absent from the extended filaments, placing an upper limit on its flux in the small northern filament. In the BPT diagram, all central-region spaxels lie above the Kauffmann et al. (2003) line, indicating that neither pure AGN nor star-formation photoionization alone explains the observed line ratios, while the WHAN diagram places nearly the entire nebula in the high-equivalent-width region that requires a highly energetic ionizing source. Critically, the northern filament shows no radial trend in either [N II]/Hα or W_Hα with distance from the AGN, which the authors interpret as evidence that the ionizing source is uniformly distributed along the filament rather than centrally concentrated. The conclusion is that AGN photoionization does not power the filaments; a distributed hard-ionizing mechanism, consistent with cooling ICM gas via collisional excitation and/or mixing, is responsible, and magnetic fields play a key role in the filaments' formation and persistence.

Load-bearing premise

The diagnostic boundaries calibrated on galaxy-integrated spectra apply to the low-density, $10^{4}$ K cluster filaments, and the non-detection of [O III] in the extended filaments can be read as the absence of hard AGN photoionization there.

Editorial extensions

If this is right

  • The extended optical filaments of NGC 1275 are not lit by the central AGN, so models of cool-core clusters must invoke distributed energy input, such as thermal conduction, mixing, or particle heating from the ICM.
  • Magnetic fields must play a structural role: thin 10^4 K filaments survive without being shredded, supporting the idea that field lines guide and confine the gas.
  • The central high-dispersion region is kinematically and ionizingly distinct, consistent with a separate mechanism such as an AGN-driven outflow or jet interaction.
  • Most of the nebula contains no [O III], so future ionization studies of such filaments cannot rely on classic BPT diagrams alone; WHAN-style diagnostics are needed.
  • Star formation is confined to a few clumps, so the nebula is not a star-forming system; its optical emission traces cooling or mixing of hot ICM gas.

Reading between the lines

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

  • If the distributed hard-ionizing source is the cooling ICM, the same diagnostic pattern should appear in other cool-core clusters with resolved filaments; its absence in another cluster would separate the Perseus case from them.
  • The transferability of the S06/K06 diagnostic cuts could be tested by comparing resolved filament spectra with photoionization-plus-precipitation models; a mismatch would shift classifications without necessarily changing the [O III] upper-limit argument.
  • The non-detection of [O III] in the extended filaments can be read as a constraint on the local ionizing spectrum: fewer than roughly one hard photon per recombining atom, which would rule out unshielded AGN radiation even if the AGN's luminosity varies over time.
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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

4 major / 5 minor

Summary. This paper presents new SITELLE observations of NGC 1275 covering the SN1, SN2, and SN3 filters, from which the authors produce flux maps of [O II] λ3726/3729, [O III] λ5007, Hβ, [N II] λ6548/6583, and Hα across the full filamentary nebula. Using LUCI for spectral fitting and applying BPT and WHAN diagnostics, the authors find that [O III] is detected only in the central core, that the extended filaments are consistent with a hard-ionizing source that is uniformly distributed, and that the AGN does not play a critical role in ionizing the extended filaments. They interpret the results as supporting cooling ICM and/or mixing as the ionization mechanism, with magnetic fields important for filament support.

Significance. The observational product is valuable: this is the first complete, spatially resolved multi-filter map of the NGC 1275 filamentary nebula in [O II], [O III], Hβ, [N II], and Hα at a common spectral resolution, and the paper gives a detailed description of the LUCI fitting procedure plus the exact fit commands in Appendix B, which aids reproducibility. The authors also repeatedly and appropriately caution that BPT/WHAN classification boundaries should not be overinterpreted. If the central conclusion—that a distributed hard-ionizing source rather than the AGN powers the filaments—withstood scrutiny, it would strengthen the case for cooling ICM, collisional excitation/mixing, and magnetic field support. However, the inference rests on the transfer of galaxy-calibrated diagnostic cuts to low-density cluster filaments and on interpreting non-detections as absences, and the key radial-gradient argument in §4.2.4 is not discriminating between central and distributed ionizing sources.

major comments (4)
  1. [§4.2.4] The conclusion that AGN photoionization is not responsible for the filaments, based on the absence of a radial trend in the WHAN indicators, is not supported by the data as presented. The two WHAN quantities, W_Hα and log([N II]/Hα), are nearly flat functions of ionization parameter U in the low-U regime that applies to these filaments, whereas [O III]/Hβ is the diagnostic that actually tracks U. The paper's own BPT analysis (§4.1, Fig. 4) shows [O III]/Hβ decreasing with distance in the core, which directly demonstrates a decreasing U; extrapolating this trend would place [O III] below the SITELLE detection limit in the extended filaments. Thus a central AGN illuminating the gas at low U would naturally produce the observed pattern: undetectable [O III] outside the core, high and roughly constant [N II]/Hα, and a flat W_Hα because both Hα and the continuum fall approximately as r^-2 for a point source. The no-radial-trend argument therefore does not exclude AGN photoionization. To support the distributed-source claim, the authors need a quantitative test, for example a CLOUDY or MAPPINGS photoionization grid covering low U values to show that AGN-like spectra fail to reproduce the observed line ratios and upper limits, or an energy-budget argument showing the AGN is too faint. Absent that, the data support only the more modest statement that the WHAN diagnostics are consistent with a hard-ionizing source without requiring radial variation.
  2. [Abstract and §3.2.1/§4.1] The abstract states that the paper 'confirms the absence of [O III] λ5007 in the extended filaments,' but the data only provide a non-detection, with a 3σ upper limit computed from a single pixel: the brightest pixel in the small northern filament (cyan arrow in Fig. 4). A non-detection is not confirmation of absence, especially when the detection limit is defined from one pixel rather than from stacked or co-added spectra across the extended filaments. Since the reading of the [O III] upper limit is load-bearing for the conclusion that AGN photoionization is unimportant, the authors should either place (and present) spatially resolved upper limits along the filaments, generate a stacked limit, or soften the claim to 'do not detect [O III] in the extended filaments' and describe the corresponding upper limits explicitly.
  3. [§3.2.2] The WHAN classification used throughout §4.2 is partly definitional. Spaxels are classified as HEW (i.e., as requiring a 'highly energetic ionizing source') when they lie above W_Hα = 6 Å and to the right of log([N II]/Hα) = -0.4, and the subsequent interpretation treats this classification as evidence for a hard ionizer. But the diagnostic lines themselves are calibrated on galaxy-integrated spectra of star-forming galaxies and AGN (K03, K06, S06), and the paper acknowledges in §3.2 that these classification systems 'may fall short of diagnostic' for emission-line regions with more physical processes in play. The transfer of these cuts to low-density, 10^4 K cluster filaments—where collisional ionization, mixing, magnetic reconnection, and low ionization parameters dominate—is not established. The inference that a hard ionizing source is present is therefore vulnerable to circularity: the HEW category is defined by the same equivalent-width threshold that is then interpreted as indicating a hard ionizer. Independent checks, such as comparing observed line ratios against photoionization and collisional-ionization models (e.g., CLOUDY grids with varying U, density, and input spectra), are needed to demonstrate that the HEW classification corresponds to a genuinely hard ionizing mechanism in this environment.
  4. [§3.2.1 and §4.1] The statement that the [O III] upper bound in the filaments indicates 'that the ionization mechanism at play in the outer filaments is distinct from ionized gas regions in our galaxy or nearby AGN' goes beyond what the data show. The comparison relies on the position of a single upper-limit point relative to galaxy-integrated BPT loci, which are not appropriate benchmarks for spatially resolved, low-density, low-U gas. Moreover, the earlier claim in §4.1 that the radial decrease of [O III]/Hβ implies that 'the strength of the ionizing source decreases further from the center' conflates ionization parameter with the intrinsic hardness of the source; a constant SED with decreasing U produces the same observed trend. These interpretations should be reframed as possibilities rather than conclusions, unless accompanied by a model that distinguishes SED hardness from ionization parameter.
minor comments (5)
  1. [§4.2.4] The reference to the northern filament figure appears as 'Figure ??' in the text; the correct figure number must be inserted.
  2. [§4.2.1] The '2-dimensional Kolmogrov-Smirnoff test' should be 'Kolmogorov-Smirnov', and the authors should report the test statistic, p-value, and the number of spaxels in each region so that the statistical significance of the difference between the shock bar and the small northern filament can be evaluated.
  3. [Throughout] There are numerous typographical errors that should be corrected in a revision: 'Minkowki 1959' (→ Minkowski), 'refered' (→ referred), 'occuring' (→ occurring), 'theshocked' (→ the shocked), 'ioinzing' (→ ionizing) in the summary, 'pre-calibarated' (→ pre-calibrated), 'classification classification' (duplicated word in §3.2), and 'W e' (→ We) in §3.2.2.
  4. [Fig. 4 caption] The caption reads 'the cyan arrow represents the upper bound the brightest pixel in the small northern filament'; this should be 'the upper bound for the brightest pixel'.
  5. [§3.2.1] The sentence 'All points lie above the Kauffmann et al. 2003 line' is stated before the caveat that the literal BPT classifications are not definitive for this object; given the paper's own emphasis on not overinterpreting diagnostic lines, this phrasing should be softened to 'all points lie in the composite/AGN region of the diagram according to the K03 and K06 demarcations.'

Circularity Check

1 steps flagged · score 2.0 of 10

Observational analysis with externally calibrated diagnostics; the only mildly circular step is the WHAN/HEW classification itself, and it is not load-bearing for the independent [O III] and line-ratio evidence.

  1. self definitional [Section 3.2.2 (WHAN Diagram); conclusion applied in Section 4.2.4]
    "Any spaxel above this line is generally classified as a HEW and any spaxel below this line is classified as a LEW. The distinction here comes from the fact that the equivalent width measures the ratio of the contributions of the highly energetic ionizer and the non-ionizing stellar component; therefore, the sHIS classification refers to a strong hard-ionizing agent while a wHIS refers to a weaker hard-ionizing agent."

    The HEW label is defined by construction as indicating a strong hard-ionizing source, so reporting that most spaxels fall in the Seyfert/HEW region and then concluding that a hard-ionizing source is required is partly a restatement of the classification rather than an independent detection. The subsequent Section 4.2.4 claim that 'a uniformly distributed physical phenomenon that is a hard-ionizing source appears to be responsible' inherits the 'hard-ionizing source' component from that same definition. The circularity is mild because the paper repeatedly cautions against overinterpreting the cuts, and the independent evidence (non-detection of [O III] in extended filaments and the constant [N II]/Halpha ratio) does not reduce to the WHAN definitions.

full rationale

This is an observational analysis rather than a derivation from first principles, so most of the paper is naturally self-contained against external diagnostics. The BPT and WHAN cuts are imported from published galaxy-integrated calibrations (K03, K06, S06, Cid Fernandes et al.), not fitted to the NGC 1275 data, and the line fluxes are produced by a public fitting code, LUCI, with stated assumptions. There is no equation whose output is equivalent to its input and no fitted parameter that is later relabeled as a prediction. Self-citations to the authors' prior SITELLE kinematics and LUCI software are used for context and reduction, but they are not the load-bearing support for the central ionization claim. The one mildly self-definitional element is the WHAN/HEW classification itself: the category 'HEW' is defined as requiring a strong hard-ionizing agent, so the inference that such an agent exists partly restates the diagnostic definition. However, the paper's independent evidence, particularly the absence of [O III] in the extended filaments and the roughly constant [N II]/Halpha ratio along the northern filament, gives the central conclusion content beyond the tautological part. The skeptic concern that no radial WHAN gradient is also expected for low-ionization-parameter AGN photoionization is a legitimate scientific underdetermination, but it is not a circularity of the kind defined by the review criteria. Overall score 2 reflects one minor definitional component while the central observational result has independent support.

Assumptions & free parameters 6 free parameters · 5 assumptions · 0 invented entities

The central claim rests on a chain of measurement and classification assumptions: line-profile model, velocity linkage, HVS separation, adoption of galaxy-calibrated BPT/WHAN cuts for cluster filaments, and masking choices that emphasize Hα-bright gas. None of these are fitted to a target result, and no new physical entity is introduced; the paper selects among mechanisms already in the literature.

free parameters (6)
  • Spatial binning of 3x3 SITELLE pixels = 3x3 pixels (~340 pc; text incorrectly says 350 kpc)
    Chosen to maximize detection of faint lines in SN1 and SN2; sets effective resolution and mixes gas along the line of sight (Section 2.1).
  • Flux masking threshold = 1×10^-17 erg s^-1 cm^-2 Å^-1
    Applied to every emission-line map; spaxels below this cut are excluded, shaping which regions enter the diagnostic diagrams (Section 2.1).
  • SNR masking cut = 3
    Spaxels with SNR below 3 after sky subtraction are masked; this may suppress faint [O III] and Hβ in outer filaments, directly affecting the central claim (Section 2.1).
  • Hβ mask based on Hα contours = null
    Hβ is retained only where Hα is above threshold, after a visual check of contours; this can imprint Hα morphology on Hβ and bias BPT [O III]/Hβ ratios (Section 3.1).
  • Manual ds9 removal of noisy pixels = null
    Unquantified human selection step; may remove real faint emission or leave artifacts, and is not reproducible from the text (Section 2.1).
  • Single-pixel [O III] 3-sigma upper limit = null
    Used to infer the absence of [O III] in extended filaments; it is computed only for the brightest pixel of the small northern filament, not as a map-wide sensitivity map (Section 3.2.1, Figure 4).
assumptions (5)
  • domain assumption Line profile is a sinc function convolved with a Gaussian with instrument-defined widths (Equation 1).
    Forward model used for all flux measurements in LUCI (Section 2.1); if the profile is wrong, fluxes and ratios shift.
  • domain assumption Emission lines in the same filter are emitted by the same gas, so their velocities and dispersions are linked.
    Explicit assumption in Section 2.1; if wrong, BPT ratios mix different gas phases and the inferred ionization mechanism is distorted.
  • domain assumption The High-Velocity System is cleanly separated in velocity, and its only contamination is [O III] 4959 overlapping [O III] 5007 of the LVS.
    Used to mask HVS spaxels in the [O III] map (Section 2.1); residual contamination would alter the central [O III] morphology.
  • domain assumption BPT and WHAN diagnostic boundaries calibrated on galaxy samples (K03, K06, S06) are applicable to low-density cluster filaments as indicators of energy per ionization.
    Load-bearing for the WHAN interpretation in Sections 3.2.2 and 4.2; the authors themselves caution against overinterpretation, so this is a fragile premise.
  • domain assumption Hβ emission follows Hα morphology, justifying masking Hβ with the Hα map.
    Stated in Section 3.1 after visual verification; if false, the Hβ map and hence the BPT diagram are biased toward Hα-bright regions.

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Cite this review

Pith. "Pith review of Mapping the Filamentary Nebula of NGC 1275 with Multiwavelength SITELLE Observations." pith.science (2026). https://pith.science/paper/QCBIRVGT

@misc{pith2026250205406,
  author       = {Pith},
  title        = {Pith review of: Mapping the Filamentary Nebula of NGC 1275 with Multiwavelength SITELLE Observations},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/QCBIRVGT}},
  note         = {Machine review of arXiv:2502.05406}
}
abstract

The filamentary nebula encompassing the central galaxy of the Perseus Cluster, NGC 1275, is a complex structure extending dozens of kiloparsecs from NGC 1275. Decades of previous works have focused on establishing the primary formation and ionization mechanisms in different filaments. These studies have pointed to a lack of star formation in the majority of the filaments, the importance of magnetic fields and turbulence in several regions, and the role of interactions between the intercluster medium (ICM) and the cool gas in the filaments, as well as the role of interaction between the central radio source, 3C84, and the filaments. In this paper, we present multi-filter observations of the entire filamentary system that cover the optical bandpass, using the SITELLE instrument at the Canada-France-Hawai'i Telescope. Here, we use the data analysis software, \href{https://crhea93.github.io/LUCI/index.html}{\texttt{LUCI}}, to produce flux maps of the prominent emission lines present in the filters: \oii{}$\lambda$3726/3729, \oiii{}$\lambda$5007, H$\beta$, \nii{}$\lambda$6548, \nii{}$\lambda$6583, and H$\alpha$. We use these maps to produce BPT and WHAN diagrams to study the ionization mechanisms at play in each distinct region of the filamentary nebula. First, we confirm the absence of \oiii{}$\lambda$5007 in the extended filaments, although we detect this line in the central core, revealing a compact region where photoionization by the AGN might affect local conditions. Our findings corroborate previous claims that the ionization in the extended filaments could be caused by the cooling ICM via collisional excitation and/or mixing. Moreover, they support the conclusion that magnetic fields play an important role in the formation and continued existence of the filaments.

Figures

Figures reproduced from arXiv: 2502.05406 by the authors.

Figure 1
Figure 1. Composite image of the SN1 (365-385 nm; blue), SN2 (480-520 nm; green), and SN3 (651-685; red) filters emission in NGC 1275 obtained with SITELLE on the CFHT. We constrain the image only to cover approximately a quarter of the SITELLE field of view in order to highlight the optical emission nebula. omi Collaboration et al. 2018; Wright 2006). Assuming the Hubble constant of H0=69.6, the energy density of matter, ΩM=… view at source ↗
Figure 2
Figure 2. Illustrative SN1 (a), SN2 (b), and SN3 (c) spectra (blue) and corresponding fit (purple). The spectrum is integrated from a square region containing 9 pixels (3x3 bin) centered at (3:19:49.7, +41:30:45.4 ) corresponding to the left-hand side of the central region. The x-axis has been shifted by the redshift of NGC 1275. The bottom row shows the SN1 (d), SN2 (e), and SN3 (f) sky emission spectra (orange). Here, the s… view at source ↗
Figure 3
Figure 3. Masked flux maps for each emission line in SN3, SN2, and SN1: Hα, [N ii]λ6548, [N ii]λ6583, [O iii]λ5007, Hβ, [O ii]λ3726+[O ii]λ3729. The calculated flux maps are masked below a SNR of 3 and a flux cut at 1×10−17 ergs s−1 cm−2 ˚A −1 . Remaining noisy pixels were removed manually using ds9. In the background, we display the exposure-corrected, background￾subtracted, merged Chandra image. The images are aligned to ap… view at source ↗
Figures from the paper (13 more)
Figure 4
Figure 4. Figure 4: shows the [N ii]λ6583/Hα versus [O iii]λ5007/Hβ BPT diagram. Since [O iii]λ5007 is only present in the central region of NGC 1275, the figure is only for the central region with the exception of the blue arrow. We measure the 3-σ detection limit of [O iii]λ5007 for the…
Figure 5
Figure 5. Figure 5: Emission line diagram showing the relationship between log([N ii]/Hα) and log(Hα) as a function of the dis￾tance from the AGN (designated here at the center) in kpc. The central 0.5kpc have been excised to remove any issues pertaining to multiple emission lines. least …
Figure 6
Figure 6. Figure 6: WHAN diagram and color-coded map for all pixels in the nebula surrounding NGC 1275 with SNR over 3 and flux over 1e−17 ergs s−1 cm−2 ˚A −1 . The pixels are divided into 3 categories: star-forming (orange), HEW (teal), and LEW (purple). Measurements below the dotted lin…
Figure 7
Figure 7. Figure 7: In the left-hand diagram, we plot the log([N ii]/Hα) values for spaxels labeled as HEW. Similarly, in the right-hand panel, we plot the equivalent width values for spaxels labeled as HEW in units of ˚A [PITH_FULL_IMAGE:figures/full_fig_p012_7.png]
Figure 8
Figure 8. Figure 8: WHAN plot divided by structures. We show the ensemble of points color-coded to each structures studied in detail in §4.2. For each structure, the mean value with a 1-σ error is superimposed on the data points. shock front as determined by Fabian & Sanders (2006) in blu…
Figure 9
Figure 9. Figure 9: The left panel is a diagram of the shock region in NGC 1275. The background image is the Chandra soft X-ray emission between 0.5-2.0 keV described in §2.2; the contours highlight the Hα emission. We highlight the location of the shock front (blue) as detected in Fabian…
Figure 10
Figure 10. Figure 10: The left panel is a diagram of the central region of the filamentary nebula in NGC 1275. The background image is the Chandra soft X-ray emission, while the white contours highlight the Hα emission. We partition the central region into the high dispersion region as ini…
Figure 11
Figure 11. Figure 11: Diagram and WHAN plot of the horseshoe region of the NGC 1275 filamentary nebula. The background image is the Chandra soft X-ray emission, while the white contours highlight the Hα emission. The horseshoe is segmented into four color-coded sections, capturing the diff…
Figure 12
Figure 12. Figure 12: The left-hand plot is a diagram of the larger northern filament. The background image shows the soft X-ray emission captured by Chandra, with white contours following the Hα emission. The WHAN plot on the right is color-coded by the distance to the bottom of the filam…
Figure 13
Figure 13. Figure 13: In the left panel, we have a diagram of the Blue Loop region. The background image shows the soft X-ray emission captured by Chandra, with white contours following the Hα emission. The loop is segmented into a left and right loop. The WHAN plot is color-coded by this …
Figure 14
Figure 14. Figure 14: Left: SN3 velocity map; Right: SN3 broadening map. Although these findings were initially reported in Gendron￾Marsolais et al. (2018), we include the LUCI-based fits here for posterity. We note that globally the values are similar to those calculated previously. APPEN…
Figure 15
Figure 15. Figure 15: Left: SN2 velocity map; Right: SN2 broadening map. The values reported for the SN2 data follows the SN3 data closely. (a) Velocity (b) Velocity dispersion [PITH_FULL_IMAGE:figures/full_fig_p022_15.png]
Figure 16
Figure 16. Figure 16: Left: SN1 velocity map; Right: SN1 broadening map. While the velocity values of the blended [O ii] doublet follow the values reported for SN2 and SN3 closely, the broadening values differ. Instead, they are uniformly between 100 and 120 km/s. This is likely due to the…

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Pith tools

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