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REVIEW 5 major objections 4 minor 1 references

O CAESAR: The Optical CAtalogue of Extragalactic SupernovA Remnants

T0 review · 5 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read A new homogeneous optical survey aims to compile the largest catalogue of extragalactic supernova remnant candidates, with the first galaxy yielding 129 candidates.

desk verdict A useful survey announcement and method description, but the scientific payload (including the NGC 3344 counts) is deferred to companion papers, and the confirmation step is not fully independent of the selection cut. read the letter →

arxiv 1909.00766 v1 pith:JTZZ6EJA submitted 2019-09-02 astro-ph.GA

I. Moumen , C. Robert , D. Devost , L. Rousseau-Nepton , D. Patnaude , S. Safi-Harb , R. P. Martin , L. Drissen
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T. Martin
This is my paper · ORCID
classification astro-ph.GA
keywords supernovaremnantsextragalacticopticalspectroscopyintegralfieldemission-linediagnosticsNGC3344SITELLEgalaxysurveys
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

The paper introduces O CAESAR, a catalogue that will collect supernova remnant candidates from an imaging Fourier transform spectrograph on a 3.6-meter telescope, covering a volume-limited sample of roughly forty galaxies within 10 megaparsecs. The authors argue that, because all data come from the same instrument and a uniform analysis pipeline, the catalogue will be the largest homogeneous optical sample of extragalactic supernova remnants. To demonstrate the method, they apply four automatic selection criteria to the galaxy NGC 3344 and find 129 emission regions that satisfy them. A follow-up spectroscopic classification, using emission-line ratio diagrams, then splits these into 42 confirmed, 45 probable, and 42 less likely remnants. If the approach works at scale, it gives astronomers a consistent census of supernova remnants across galactic environments.

What carries the argument

The load-bearing object is the automatic identification pipeline built on SITELLE's spatially resolved spectra. For every emission region with [S II]/H-alpha at least 0.4, the pipeline applies three further filters: signal-to-noise at least 5 for the key lines, projected size at most 120 parsecs, and a correlation coefficient of at least 0.5 for the H-alpha flux profile. The selected regions are then checked spectroscopically against the shock-heating diagnostic diagrams of Sabbadin and of Baldwin, Phillips and Terlevich, which separate shock-heated gas from photoionized gas. This combination of a fixed automatic selection with a physical classification is what the catalogue generalizes to about 40 galaxies.

What would settle it

Observe a well-studied spiral galaxy with existing radio and X-ray supernova remnant catalogues, run the pipeline on its SITELLE cubes, and compare the optical candidates to the non-thermal detections; if either the recovery fraction of known remnants is far below 100% or the fraction of candidates with no non-thermal counterpart is large, the adopted thresholds are not selectively finding remnants. A sharper test is to measure [S II]/H-alpha for a sample of confirmed remnants and check whether a substantial number lie below 0.4.

Watch

Extended reading notes

Core claim

The central claim is that an automated, uniform analysis of SITELLE integral-field spectra can identify extragalactic supernova remnant candidates across an entire galaxy disk, and that the same pipeline will be applied to about 40 nearby galaxies to build the largest homogeneous optical catalogue to date. Applied to NGC 3344, the four criteria — [S II]/H-alpha at least 0.4, H-alpha and [S II] signal-to-noise at least 5, region size no more than 120 parsecs, and a correlation coefficient of at least 0.5 for the H-alpha profile — select 129 candidates. A self-consistent spectroscopic analysis using Sabbadin plots and BPT diagrams then classifies them as 42 confirmed, 45 probable, and 42 less likely supernova remnants.

Load-bearing premise

The entire identification rests on the premise that a fixed line-ratio cut ([S II]/H-alpha at least 0.4) and a fixed size cut (at most 120 parsecs) cleanly separate supernova remnants from H II regions and diffuse gas in every target galaxy, regardless of that galaxy's metallicity or ionization conditions.

Editorial extensions

If this is right

  • If O CAESAR's method is correct, the result is a uniform set of supernova remnant candidates across about 40 galaxies of different types, enabling statistical comparisons of remnant populations as a function of galactic environment.
  • The NGC 3344 list itself becomes the largest homogeneous optical sample of extragalactic supernova remnant candidates for that galaxy, available for follow-up at radio and X-ray wavelengths.
  • The automatic criteria mean that the same thresholds are applied without human intervention, so the catalogue is reproducible and can be updated as new SITELLE observations are reduced.
  • The spectroscopy attached to each candidate provides physical parameters such as electron density, shock velocity, and metallicity diagnostics, turning a candidate list into a physical database.

Reading between the lines

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

  • The paper's uniform threshold [S II]/H-alpha at least 0.4 is inherited from Galactic samples; if metallicity or ionization conditions vary strongly across the target galaxies, the catalogue may systematically miss remnants in low-metallicity environments or include bright H II regions.
  • The 120 parsec size cut, while appropriate for the young remnants seen in NGC 3344, could exclude older, larger remnants, so the catalogue is probably a census of young to middle-aged remnants rather than the full remnant population.
  • A testable extension is to cross-match the NGC 3344 candidates with radio and X-ray detections: a high confirmation rate would validate the optical criteria, while a low rate would tell where the optical selection fails.
  • One could also check whether the confirmed fraction correlates with galactocentric radius, which would hint that background subtraction or the correlation criterion behaves differently in crowded central regions.
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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

5 major / 4 minor

Summary. This paper presents O CAESAR, an ongoing catalogue of extragalactic supernova remnant (SNR) candidates detected with the SITELLE imaging Fourier transform spectrograph on the Canada-France-Hawaii Telescope. The survey is volume-limited (D <= 10 Mpc), draws mostly on the SIGNALS Large Program, and uses three filters to measure strong emission lines including H-alpha, [N II], [S II], [O II], H-beta, and [O III]. Candidate identification is based on four automatic criteria: [S II]/H-alpha >= 0.4, S/N >= 5 in H-alpha and [S II], an emission-region size <= 120 pc, and an H-alpha flux-profile correlation >= 0.5. Applying this method to NGC 3344 yields 129 SNR candidates, classified as 42 confirmed, 45 probable, and 42 less likely using Sabbadin plots and BPT diagrams. The paper describes the method and first results, but it does not include the candidate list, measured line fluxes, or a quantitative definition of the classification scheme.

Significance. If fully realized, O CAESAR would be a valuable homogeneous resource: the sample is volume-limited, uses a single instrument with uniform spectral and spatial resolution, and the selection criteria are explicitly stated and automatic. The use of SITELLE's wide-field integral field spectroscopy for extragalactic SNR surveys is a notable methodological step, and the promised synergy with radio, X-ray, and near-IR follow-up is scientifically attractive. However, in its current form the paper is a methods announcement rather than a self-contained catalogue paper: the central NGC 3344 counts are not reproducible from the text, the confirmation step partly reuses the primary selection ratio, and no uncertainties or validation of the detection pipeline are presented. The strengths are real, but the load-bearing numerical claims need additional support before the results can be evaluated.

major comments (5)
  1. [Abstract; Section 3] The central reported result, the 129 SNR candidates in NGC 3344 split as 42 confirmed, 45 probable, and 42 less likely, is not reproducible from this manuscript. No candidate list, coordinates, line fluxes, sizes, or line-ratio values are given in the text, tables, or figures. A catalogue paper must provide at least a machine-readable table of detected regions with their measured properties, or explicitly state that the counts are provisional results from the companion paper. As submitted, the reader cannot verify the 129 count or the three-class breakdown.
  2. [Section 3] The confirmation stage is not independent of the primary selection. Criterion (i) selects regions with [S II]/H-alpha >= 0.4, and the Sabbadin and BPT diagnostics used in confirmation include [S II]/H-alpha as a principal axis. A region admitted by criterion (i) is therefore already displaced toward the shock-heated locus in these diagrams, and the 42 'confirmed' candidates do not represent an independent validation of their SNR nature. The paper should quantify the expected contamination from H II regions and diffuse ionized gas under the same selection, or confirm candidates with a diagnostic that does not reuse this ratio, such as [O I]/H-alpha, X-ray, or radio observations, and report the resulting revised counts.
  3. [Section 3, Fig. 1e] The classification scheme that assigns 129 candidates to 42 confirmed, 45 probable, and 42 less likely is referenced only through a figure panel. No quantitative definitions of these classes are given, such as threshold distances from model loci in the Sabbadin or BPT planes or line-ratio ranges. Without an explicit classification rule, the reported split is not reproducible and the 'confirmed/probable/less likely' labels cannot be assessed.
  4. [Section 3] No uncertainties are propagated into the line ratios or into the classification. The S/N >= 5 threshold applies to individual lines, but the errors on [S II]/H-alpha and on the BPT coordinates are not presented. Because candidates near the 0.4 boundary will scatter across the selection threshold, the paper should include error bars on the reported ratios or an estimate of how many candidates lie within 1 sigma of the selection threshold; otherwise the completeness and contamination of the 129-count sample cannot be evaluated.
  5. [Sections 2-3] The fixed thresholds [S II]/H-alpha >= 0.4 and size <= 120 pc are applied without discussing their behaviour across the volume-limited sample. At the 10 Mpc distance limit with 0.32'' seeing, the physical resolution is roughly 15 pc, so the 120 pc size cut may not separate individual remnants from complexes of H II regions or diffuse ionized gas in all galaxies. The paper should either justify the universality of these thresholds, for example through metallicity- and resolution-dependent tests, or restrict the homogeneous-survey claim to regimes where the thresholds have been verified.
minor comments (4)
  1. [Title; Abstract; Section 2] The acronym is spelled inconsistently as 'O CAESAR', 'OCAESAR', and 'O CEASAR'. Please standardize the spelling throughout.
  2. [Figure 1] The composite figure is difficult to read because panels (c)-(g) are very small in the printed layout. Since the spectra, line-ratio maps, and classification scheme are central to the claims, split the figure or enlarge the individual panels.
  3. [Section 4] There are several typos: 'Finaly' should be 'Finally', 'makeitpossible' should be 'make it possible', and 'Exemples' in the Figure 1 caption should be 'Examples'.
  4. [Section 2] The sentence 'Only ~3 hours are needed to reach the spectral resolution requested for each filter for a total of 10 hours of integration time' is ambiguous. Please clarify whether the 3 hours is per filter and how the 10-hour total is composed.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the selection and confirmation stages share [S II]/H-alpha but are not equivalent by construction, and the counts are imported from a peer-reviewed companion paper rather than derived from fitted inputs.

full rationale

The paper's quantitative chain is a selection pipeline, not a reduction. It fixes four criteria (line ratio [S II]/H-alpha >= 0.4; S/N >= 5; size <= 120 pc; correlation coefficient >= 0.5), applies them to emission regions in NGC 3344, and then uses external Sabbadin (1977) and BPT (1981) diagnostics to classify candidates. There is no fitted parameter renamed as a prediction, and no equation defines an output as an input. The skeptical concern that criterion (i) and the confirmation step share [S II]/H-alpha as a variable does not amount to circularity: [S II]/H-alpha >= 0.4 is a necessary condition for candidacy, but the Sabbadin/BPT placement also uses [O III]/H-beta, [N II]/H-alpha, and other independent axes, and the pipeline still splits candidates into 42 confirmed, 45 probable, and 42 less likely, which would not happen if the confirmation were forced by construction. The NGC 3344 counts are imported from the companion paper Moumen et al. (2019), a peer-reviewed MNRAS article by partially overlapping authors; this is a normal citation of an externally checkable dataset, not a self-citation chain used to foreclose alternatives. Any concern that the 0.4 threshold may be poorly suited to low-metallicity host galaxies is a scientific robustness issue, not circularity, and the paper itself discloses that it adopts a 'self-consistent' spectroscopic analysis rather than claiming independent confirmation. The paper self-cites its own instrument and software (SITELLE, ORCS, ORBS), but those are concrete tools used to acquire and fit spectra, not circular load-bearing arguments. Consequently the derivation chain is self-contained in the relevant sense: the output counts follow the stated selection and diagnostic rules.

Assumptions & free parameters 4 free parameters · 4 assumptions · 0 invented entities

The central claim rests on four hand-chosen threshold parameters and on diagnostic assumptions imported from prior SITELLE work and the broader nebular astrophysics literature. No new free parameters are fitted in this paper, and no new physical entities are introduced.

free parameters (4)
  • [S II]/H-alpha threshold = 0.4
    Chosen by hand as criterion (i) in Section 3; it defines SNR candidacy, so all catalogue counts depend on this value.
  • Minimum S/N for H-alpha and [S II] = 5
    Criterion (ii) in Section 3; hand-selected to suppress noise, directly affecting which regions enter the candidate list.
  • Maximum emission-region size = 120 pc
    Criterion (iii) in Section 3; hand-selected physical size cut intended to reject larger H II regions and diffuse gas.
  • Minimum H-alpha flux-profile correlation = 0.5
    Criterion (iv) in Section 3; hand-selected to reject blended or irregular regions.
assumptions (4)
  • domain assumption The [S II]/H-alpha >= 0.4 ratio is a reliable optical diagnostic of shock-heated gas and identifies supernova remnants.
    Used as the primary selection criterion (Section 3, criterion i); it is standard in the field but is an external calibration assumption inherited from Galactic SNR studies.
  • domain assumption BPT (Baldwin et al. 1981) and Sabbadin (Sabbadin et al. 1977) diagnostic diagrams correctly separate ionization mechanisms for the metallicity and density conditions of the target galaxies.
    Adopted in Section 3 for confirmation of shock-heated nature; the diagrams are calibrated on samples that may not cover all galaxy environments.
  • domain assumption Emission-line fluxes fitted by ORCS/ORBS are reliable and unbiased after the SITELLE data reduction.
    The analysis assumes the spectral fitting software outputs accurate line fluxes; no validation is presented in this paper.
  • domain assumption All SNR candidates in NGC 3344 share the same adopted distance, allowing a fixed 120 pc size cut.
    The size criterion (iii) converts angular to physical size using the galaxy distance, whose uncertainty is not propagated.

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Cite this review

Pith. "Pith review of O CAESAR: The Optical CAtalogue of Extragalactic SupernovA Remnants." pith.science (2026). https://pith.science/paper/JTZZ6EJA

@misc{pith2026190900766,
  author       = {Pith},
  title        = {Pith review of: O CAESAR: The Optical CAtalogue of Extragalactic SupernovA Remnants},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/JTZZ6EJA}},
  note         = {Machine review of arXiv:1909.00766}
}
abstract

We present O CAESAR, the Optical CAtalogue of Extragalactic SupernovA Remnants. O CAESAR will provide the largest homogeneous optical survey of extragalactic SNR candidates taken by the same telescope, the same instrument, and under similar observational conditions. Our sample is volume-limited (D $\leq$ 10 Mpc) and includes mostly galaxies ($\sim$40) from the Large Program SIGNALS. The Observations are carried out using SITELLE, the imaging Fourier transform spectrograph of the Canada-France-Hawaii Telescope. Using three filters, we are able to measure the strong emission lines [O II]$\lambda$3727, H$\beta$, [O III]$\lambda\lambda$4959,5007, H$\alpha$, [N II]$\lambda\lambda$6548,6583, and [S II]$\lambda\lambda$6716,6731. Identification of the SNR candidates will be done automatically and will be based on four criteria for regions where the emission lines flux ratio [S II]/H$\alpha$ $\ge$ 0.4. To confirm the shock-heated nature of the ionization mechanism in the candidates derived from our sample, we adopted a self-consistent spectroscopic analysis using the whole set of emission lines available with our SITELLE data and considering Sabbadin plots and BPT diagrams. We here present our method and first results for the spiral galaxy NGC 3344.

Figures

Figures reproduced from arXiv: 1909.00766 by the authors.

Figure 1
Figure 1. (a) SIGNALS’ galaxies (Rousseau-Nepton et al. 2019); (b) The SITELLE deep image of NGC 3344 obtained by summing the interferograms of the SN1 (blue) and SN3 (red) filters (Moumen et al. 2019); (c) Hα and [S II]λλ6716,6731 flux maps near of the candidate SNR-C159; (d) SITELLE spectra for one pixel within SNR-C159 and SNR-C1050 (in black, the observed spectrum and in red, the fit obtained with ORCS); (e) Scheme of our… view at source ↗

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Works this paper leans on

1 extracted references · 1 linked inside Pith

  1. [1]

    et al., 2018, MNRAS, 477, 4152 || Rousseau-Nepton L

    Baldwin J.A., Phillips M.M., Terlevich R., 1981, PASP, 93, 5 || Drissen L., et al., 2019, MNRAS, 485, 3930 Martin T.B., Prunet S., Drissen L., 2016, MNRAS, 463, 4223 || Moumen, I., et al., 2019, MNRAS, 488, 803 Rousseau-Nepton L. et al., 2018, MNRAS, 477, 4152 || Rousseau-Nepton L. et al., 2019, MNRAS, accepted, arXiv:1908.09017 || Sabbadin F., Minello S....

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Reviewed August 14, 2026 · model on record in the stance chip above.