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Redshifts of Objects near 3C 220.3

T0 review · 1 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read This paper builds a spectroscopic catalog of 511 objects in the field of the lensing radio galaxy 3C 220.3, establishing 146 reliable galaxy redshifts, 126 stellar identifications, and a large-scale overdensity at the system's redshift.

desk verdict Useful, honest redshift catalog; the body hedges the flashy claims appropriately, though the abstract slightly overstates the QSO. read the letter →

arxiv 2505.21613 v1 pith:IYDWK4H2 submitted 2025-05-27 astro-ph.GA

Sóley Ó. Hyman , S.P. Willner , Belinda J. Wilkes This is my paper
classification astro-ph.GA
keywords galaxyredshiftscatalogsredshiftsurveysactivegalacticnucleiquasarsgravitationallensing3C220.3MilkyWay–LMCanalog
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 reports a spectroscopic census of the field around 3C 220.3, the radio galaxy that lenses a submillimeter galaxy into an Einstein ring. Using MMT/Binospec slit masks taken while studying the lens system, the authors obtained spectra of 511 additional objects and establish that 146 of them are galaxies with good-quality redshifts and 126 are Galactic stars, publishing the redshifts as a catalog. The redshift histogram shows a narrow peak at the system's redshift, which places 3C 220.3 in a large-scale overdensity, but the paper finds no evidence for or against a galaxy group within 2 Mpc of the radio host itself. The same data reveal 12 active-galactic-nucleus candidates, including a likely z≈4.64 broad-line QSO, and HST imaging gives morphological types for 14 galaxies. One pair of galaxies at z≈0.10, separated by 53 kpc with a SED-fitted stellar mass ratio of about 0.6, is proposed as a potential analog of the Milky Way–Large Magellanic Cloud system.

What carries the argument

The argument is carried by multi-object spectroscopy: Binospec on the MMT with the 270 lpm grating (R≈1340, coverage about 3900–9240 Å), reduced by the Binospec data pipeline and interpreted three ways—xcsao cross-correlation, SpecPro template fitting, and direct visual examination of the 2D spectra. The redshift quality scheme is the load-bearing classification: a definitive Q=4 requires unambiguous multiple features, while a single resolved emission line whose width matches the [OII] doublet—which Binospec barely resolves at z≳0.3—is assigned Q=2. For the potential Milky Way–LMC pair, the stellar masses come from SED fitting with Bagpipes using HST F606W/F814W/F160W and WISE W1–W3 photometry, a delayed-exponential star formation history, a Kroupa IMF, and a Calzetti dust law.

What would settle it

Re-observe the slit 2021.093 target at higher signal-to-noise and with enough resolution to see whether the 7276 Å feature splits into the [OII] doublet; if it does, that object's redshift is 0.9576 rather than the adopted 0.6760. A broader test would compare all Q=2 single-line galaxies against multi-line redshifts to measure how often the single-line rule misidentifies [OII].

Watch

Extended reading notes

Core claim

The central deliverable is a public redshift catalog built from Binospec spectra of 511 position-selected targets in the two 8′×15′ fields around 3C 220.3. Redshifts were assigned by cross-correlation, template fitting, and visual inspection of two-dimensional spectra, with a quality rating Q from 0 (no redshift) to 4 (definitive); 146 galaxies have Q≥2. The redshift distribution peaks at z≈0.685, so 3C 220.3 sits in a large-scale overdensity, but only four candidate companions lie within a projected 2 Mpc and 1000 km/s, and the paper states that the search neither confirms nor refutes a galaxy group around the radio host. Five galaxies at ⟨z⟩=0.6749 with a rest-frame velocity dispersion of 236 km/s form a separate group about 720 kpc away with a −1800 km/s offset, too far for 3C 220.3 to belong to it. Beyond the catalog, the spectra identify 12 AGN candidates and HST imaging morphologically classifies 14 galaxies; SED fitting of the z≈0.10 pair yields stellar masses of 6.3×$10^{10}$ and 3.7×$10^{10}$ M⊙, giving the ~0.6 mass ratio that motivates the Milky Way–LMC analog suggestion.

Load-bearing premise

The load-bearing assumption is that a single resolved emission line with the width of the [OII] doublet is really [OII] when no other spectral features confirm it; the paper's own ambiguous case, slit 2021.093, shows that such a line can instead be [OII] at a different redshift, changing the object's z from 0.6760 to 0.9576.

Editorial extensions

If this is right

  • Future projects in the 3C 220.3 field can use the published redshifts to avoid re-observing these 146 galaxies and 126 stars.
  • The histogram places 3C 220.3 inside a large-scale overdensity at z≈0.685, so its environment is part of that structure even though a bound group is unproven.
  • The four candidate companions within 2 Mpc and 1000 km/s are the specific targets that follow-up spectroscopy should check to settle whether a group exists.
  • The five-galaxy structure at ⟨z⟩=0.6749, 720 kpc away and offset by −1800 km/s, is a separate group or small cluster along the same line of sight.
  • If the z≈4.64 QSO candidate is confirmed by follow-up spectroscopy, it provides a bright AGN at that redshift within this field.

Reading between the lines

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

  • The Q=2 single-line rule is the most probable source of systematic redshift error in the catalog; comparing single-line objects against later multi-line measurements would quantify its failure rate.
  • Because targets were chosen by position rather than by redshift, the inconclusive group result may be an artifact of sparse sampling; a redshift-selected survey around the radio host would answer the open question directly.
  • The Milky Way–LMC analog rests on separation and stellar mass ratio alone; resolved kinematics or a search for tidal features would test whether the pair is genuinely interacting.
  • The overdensity plus the nearby group at z≈0.675 hints that 3C 220.3 lies in a filamentary large-scale structure, a claim the paper does not make but that the data permit.
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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

1 major / 6 minor

Summary. The paper reports MMT/Binospec spectroscopy of 511 objects in the field of the 3C 220.3 lens system. It presents a catalog of 146 galaxy redshifts with a four-tier quality rating (Q=0-4), 126 identified Galactic stars, 12 AGN candidates (including a tentative z~4.64 broad-line QSO), and morphological classifications of 14 galaxies from HST imaging. The redshift histogram shows a large-scale overdensity near the 3C 220.3 redshift, but the authors find no conclusive evidence for a bound galaxy group within 2 Mpc of the radio galaxy. A secondary result is a potential Milky Way-LMC analog at z~0.10 with a stellar mass ratio ~0.6 from SED fitting.

Significance. The catalog is a useful community resource: it provides redshifts in a field selected for a lensing study, and the authors make an explicit effort to define redshift-quality criteria and to flag ambiguous cases. The strengths of the paper are its clear quality-rating scheme, the acknowledgment of the single-line [OII] degeneracy that affects Q=2 entries, the honest statement of the inconclusive group-membership test, and the public availability of the HST data. The headline result—146 good-quality galaxy redshifts—is, on inspection, consistent with the Q>=3 subset, and the group-search conclusion is appropriately cautious. The paper is a solid, incremental contribution to extragalactic spectroscopy in a targeted field, suitable for a journal like PASP.

major comments (1)
  1. [Abstract and Section 2 (Q definition)] The abstract and Section 4 state that the survey provides '146 good-quality galaxy redshifts,' but 'good-quality' is never explicitly defined in terms of the Q scale introduced in Section 2. Since Q=2 is defined as 'likely but not certain' and is assigned to a single resolved emission line assumed to be [OII], the headline number should either state the Q cutoff (e.g., Q>=3) or report separately how many of the 146 have Q=2. This distinction is load-bearing for the catalog's reliability, because a user cannot otherwise determine how many of the published redshifts rest on the single-line [OII] assumption. I recommend adding a sentence to the abstract or Section 2 that maps 'good-quality' to Q>=3 and, if possible, gives the Q=2 subset count.
minor comments (6)
  1. [Section 3.3] The text refers to the 'Large Magellanic/Milky Cloud (LMC)'—change 'Milky Cloud' to 'Magellanic Cloud' or simply 'LMC.'
  2. [Section 2] The paper does not state where the reduced Binospec spectra are made available; authors should provide a link to the data products or state that they are available upon request/through the observatory archive, in line with the closing remark that results should be made public.
  3. [Figure 5 and Section 3.2] The histogram is described as including Q>=2 galaxies, and the text refers to 'the 30 galaxies in the peak' near z=0.685, but the redshift window used to define this peak is not specified; adding the bin range (or the exact criterion) would make the statement reproducible.
  4. [Table 3 and Section 3.2] The text states that the search for potential companions considered galaxies with Q>=2, but all entries in Table 3 have Q=3 or Q=4; either the search actually required Q>=3, or the table should include any Q=2 candidates (or the text should note that none were found).
  5. [Table 6a and Section 2] The template names 'hemtemp,' 'habtemp,' 'eltemp,' 'eatemp,' and 'sptemp' are used without definition; a brief footnote explaining these abbreviations would improve the table's usability.
  6. [Section 3.3 and Table 4] The Bagpipes SED fitting uses only six photometric points (three HST bands plus W1/W2/W3) with no explicit statement of priors on metallicity or dust; the reported stellar-mass uncertainties are likely underestimated, so the 'potential analog' claim should be toned down or the fitting assumptions stated more fully.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: the catalog redshifts are observational measurements compared with independent templates, and no predicted quantity is derived from fitted inputs.

full rationale

The paper's central claims are an observed catalog: 146 good-quality galaxy redshifts and 126 stellar identifications from Binospec spectra, plus a redshift histogram and a group-search statement. Redshifts are derived by cross-correlation with spectral templates (xcsao, SpecPro) and by visual inspection of 2D spectra against identified emission and absorption features; no redshift is computed from a fitted parameter that is itself derived from the claimed result. The Q=2 rule that a resolved emission line with no other features is assumed to be [OII] is an identification convention with an explicit quality caveat; it is not a 'prediction' forced by a fit, and the paper itself flags the ambiguous case of slit 2021.093. The group search uses observed redshifts and projected separations with stated cuts, and the paper explicitly concludes the 2 Mpc group question is inconclusive, so no unsupported central claim is being protected. The SED fitting with Bagpipes uses the spectroscopically measured redshifts as fixed inputs to estimate stellar masses of two galaxies; the resulting mass ratio is a derived quantity, not an input to the redshift or group-membership criteria. Self-citations to Hyman et al. (2024) provide lensing-system context and prior spectra of the 3C 220.3 system, but they do not supply the load-bearing evidence for the new catalog or for any uniqueness claim. No fitted-input-called-prediction, self-definitional, imported-uniqueness, or ansatz-smuggled-via-citation pattern is present. The acknowledged Q=2 single-line [OII] ambiguity is a genuine observational limitation, but it is a completeness/contamination concern, not circularity.

Assumptions & free parameters 2 free parameters · 4 assumptions · 0 invented entities

The central catalog rests on standard astronomical assumptions: template matching, the single-line [OII] rule, and SED model choices. The only fitted quantities are the stellar masses in the analog claim; the mass ratio is their quotient. No new physical entities are introduced.

free parameters (2)
  • Stellar mass of primary galaxy (2020.193/2021.196) = 6.3e10 M_sun (+1.6/-1.3)
    Derived from Bagpipes SED fit using HST F606W/F814W/F160W and WISE W1-W3 photometry; used for the MW-LMC analog mass ratio.
  • Stellar mass of secondary galaxy (2020.196/2021.193) = 3.7e10 M_sun (+1.0/-0.9)
    Same SED fit setup as the primary; the quotient of the two masses gives the quoted ~0.6 ratio.
assumptions (4)
  • domain assumption A single resolved emission line with no other features at z≳0.3 is the [OII] doublet
    Used to assign Q=2 redshifts (Section 3, definition of Q=2); the paper notes the ambiguity risk, e.g., slit 2021.093.
  • domain assumption Spectral templates in SpecPro and xcsao are appropriate for galaxy and star classification at R≈1340
    Central to all redshift assignments; no independent external validation set is presented.
  • domain assumption Bagpipes SED models (delayed exponential SFH, Kroupa IMF, Calzetti dust) give unbiased stellar masses
    Underlies the mass ratio ~0.6 and the MW-LMC analog interpretation (Section 3.3).
  • standard math The cosmology from Hyman et al. (2024) gives an angular to physical scale of 7.312 kpc/arcsec
    Used to compute projected separations in Table 3 and Sections 3.2 and 3.3.

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Cite this review

Pith. "Pith review of Redshifts of Objects near 3C 220.3." pith.science (2026). https://pith.science/paper/IYDWK4H2

@misc{pith2026250521613,
  author       = {Pith},
  title        = {Pith review of: Redshifts of Objects near 3C 220.3},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/IYDWK4H2}},
  note         = {Machine review of arXiv:2505.21613}
}
abstract

In the course of studying the 3C 220.3 lensing system, spectra were obtained with the Binospec instrument on the MMT for 511 additional objects in 3C 220.3's vicinity. These gave 146 good-quality galaxy redshifts and identified 126 Galactic stars. The galaxy redshift histogram shows a peak near 3C 220.3's redshift, but there is no evidence for or against a galaxy group within 2 Mpc of 3C 220.3 itself. The spectra revealed 12 AGN candidates including a likely $z\approx4.64$ broad-line QSO. Visible and near-infrared imaging with HST allowed morphological classifications of 14 galaxies. One system is a potential analog of the Milky Way-LMC system with stellar mass ratio $\sim$0.6.

Figures

Figures reproduced from arXiv: 2505.21613 by the authors.

Figure 1
Figure 1. Binospec field layout. The background nega￾tive image of the field is the coadded g+r+z MzLS+BASS data from the DECaLS Data Release 10 (credit to Legacy Surveys/D. Lang, Perimeter Institute). The magenta square shows the HST footprint, which is centered on 3C 220.3. The blue rectangles indicate the Binospec 8′×15′ fields of view. The spatial dimension of the slit is parallel to the long di￾mension of each FoV, which… view at source ↗
Figure 2
Figure 2. 2014 HST visible light data cutouts (combined F606W/F814W, log scale) of sources in [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. 2014 HST IR data cutouts (log scale) of sources in [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: 2D spectrum closeups of the [O II] doublet and Hβ line in a galaxy at z = 0.3308 (slit 2021.210). Observed wavelength increases left to right as shown by the green la￾bels, and the y (spatial) coordinate is vertical. Magenta ticks mark the rest wavelengths of the [O II…
Figure 5
Figure 5. Figure 5: Histogram of galaxy redshifts with redshift qual￾ity Q ≥ 2. The 3C 220.3 z = 0.6850 (Hyman et al. 2024) is marked. sive, and spectroscopy of more galaxies near the system is needed. Modern multi-object spectrographs observe a plethora of targets with each exposure. It …

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Works this paper leans on

32 extracted references · 7 canonical work pages

  1. [1]

    , " * write output.state after.block = add.period write newline

    ENTRY address archivePrefix author booktitle chapter doi edition editor eprint howpublished institution journal key month number organization pages publisher school series title misctitle type volume year version url label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts ...

  2. [2]

    write newline

    " write newline "" before.all 'output.state := FUNCTION format.url url empty "" new.block "" url * "" * if FUNCTION format.eprint eprint empty "" archivePrefix empty "" archivePrefix "arXiv" = new.block " " eprint * " " * new.block " " eprint * " " * if if if FUNCTION format.doi doi empty "" " " doi * " " * if FUNCTION format.pid doi empty eprint empty ur...

  3. [3]

    :rB^Ѣ\[=lX[2 Z u RV` 8 +jB u 㴳C`P(txM+yMz?m|R#Q jiӦͭ -[ m۶NRpң& = 1I4TtTn vӋ Rvc-x (oj

    thebibliography [1] 20pt to REFERENCES 6pt =0pt -12pt 10pt plus 3pt =0pt =0pt =1pt plus 1pt =0pt =0pt -12pt =13pt plus 1pt =20pt =13pt plus 1pt \@M =10000 =-1.0em =0pt =0pt 0pt =0pt =1.0em @enumiv\@empty 10000 10000 `\.\@m \@noitemerr \@latex@warning Empty `thebibliography' environment \@ifnextchar \@reference \@latexerr Missing key on reference command E...

    2021

  4. [4]

    P., Tollerud , E

    Astropy Collaboration , Robitaille , T. P., Tollerud , E. J., et al. 2013, , 558, A33, 10.1051/0004-6361/201322068

    doi:10.1051/0004-6361/201322068 2013

  5. [5]

    M., Sip o cz , B

    Astropy Collaboration , Price-Whelan , A. M., Sip o cz , B. M., et al. 2018, , 156, 123, 10.3847/1538-3881/aabc4f

    doi:10.3847/1538-3881/aabc4f 2018

  6. [6]

    M., Lim , P

    Astropy Collaboration , Price-Whelan , A. M., Lim , P. L., et al. 2022, , 935, 167, 10.3847/1538-4357/ac7c74

    doi:10.3847/1538-4357/ac7c74 2022

  7. [7]

    2022, astropy/regions: v0.7, v0.7, Zenodo, 10.5281/zenodo.7259631

    Bradley, L., Deil, C., Ginsburg, A., et al. 2022, astropy/regions: v0.7, v0.7, Zenodo, 10.5281/zenodo.7259631

    doi:10.5281/zenodo.7259631 2022

  8. [8]

    2023, astropy/photutils: 1.8.0, 1.8.0, Zenodo, 10.5281/zenodo.7946442

    Bradley, L., Sip o cz, B., Robitaille, T., et al. 2023, astropy/photutils: 1.8.0, 1.8.0, Zenodo, 10.5281/zenodo.7946442

    doi:10.5281/zenodo.7946442 2023

Show all 32 references
  1. [9]

    C., et al

    Calzetti , D., Armus , L., Bohlin , R. C., et al. 2000, , 533, 682, 10.1086/308692

    2000 doi

  2. [10]

    C., McLure , R

    Carnall , A. C., McLure , R. J., Dunlop , J. S., & Dav \'e , R. 2018, , 480, 4379, 10.1093/mnras/sty2169

    2018 doi

  3. [11]

    2001, , 328, 1039, 10.1046/j.1365-8711.2001.04902.x

    Colless , M., Dalton , G., Maddox , S., et al. 2001, , 328, 1039, 10.1046/j.1365-8711.2001.04902.x

    2001

  4. [12]

    M., Wright , E

    Cutri , R. M., Wright , E. L., Conrow , T., et al. 2013, Explanatory Supplement to the AllWISE Data Release Products , Explanatory Supplement to the AllWISE Data Release Products, by R. M. Cutri et al

    2013

  5. [13]

    2019, , 131, 075004, 10.1088/1538-3873/ab1d78

    Fabricant , D., Fata , R., Epps , H., et al. 2019, , 131, 075004, 10.1088/1538-3873/ab1d78

    2019 doi

  6. [14]

    2002, , 576, 135, 10.1086/341730

    Gavazzi , G., Bonfanti , C., Sanvito , G., Boselli , A., & Scodeggio , M. 2002, , 576, 135, 10.1086/341730

    2002 doi

  7. [15]

    2014, , 790, 46, 10.1088/0004-637X/790/1/46

    Haas , M., Leipski , C., Barthel , P., et al. 2014, , 790, 46, 10.1088/0004-637X/790/1/46

    2014 doi

  8. [16]

    R., Millman, K

    Harris, C. R., Millman, K. J., van der Walt, S. J., et al. 2020, Nature, 585, 357, 10.1038/s41586-020-2649-2

    2020 doi

  9. [17]

    P., Verheijen , M

    Healy , J., Willner , S. P., Verheijen , M. A. W., & Blyth , S. L. 2021, , 162, 193, 10.3847/1538-3881/ac0bc6

    2021 doi

  10. [18]

    Hunter, J. D. 2007, Computing in Science & Engineering, 9, 90, 10.1109/MCSE.2007.55

    2007 doi

  11. [19]

    \'O ., Wilkes , B

    Hyman , S. \'O ., Wilkes , B. J., Willner , S. P., et al. 2024, , 974, 171, 10.3847/1538-4357/ad68f7

    2024 doi

  12. [21]

    A., & Mandel , E

    Joye , W. A., & Mandel , E. 2003, in Astronomical Society of the Pacific Conference Series, Vol. 295, Astronomical Data Analysis Software and Systems XII, ed. H. E. Payne , R. I. Jedrzejewski , & R. N. Hook , 489

    2003

  13. [22]

    2019, , 131, 075005, 10.1088/1538-3873/ab1ceb

    Kansky , J., Chilingarian , I., Fabricant , D., et al. 2019, , 131, 075005, 10.1088/1538-3873/ab1ceb

    2019 doi

  14. [23]

    E., Erkal , D., Li , T

    Koposov , S. E., Erkal , D., Li , T. S., et al. 2023, , 521, 4936, 10.1093/mnras/stad551

    2023 doi

  15. [24]

    Kroupa , P., & Boily , C. M. 2002, , 336, 1188, 10.1046/j.1365-8711.2002.05848.x

    2002

  16. [25]

    J., & Mink , D

    Kurtz , M. J., & Mink , D. J. 1998, , 110, 934, 10.1086/316207

    1998 doi

  17. [26]

    K., Croom , S

    Mahony , E. K., Croom , S. M., Boyle , B. J., et al. 2010, , 401, 1151, 10.1111/j.1365-2966.2009.15705.x

    2010

  18. [27]

    2011, , 123, 638, 10.1086/660023

    Masters , D., & Capak , P. 2011, , 123, 638, 10.1086/660023

    2011 doi

  19. [28]

    1999, IRAF: Image Reduction and Analysis Facility , Astrophysics Source Code Library, record ascl:9911.002

    National Optical Astronomy Observatories . 1999, IRAF: Image Reduction and Analysis Facility , Astrophysics Source Code Library, record ascl:9911.002

    1999

  20. [29]

    A., Cooper , M

    Newman , J. A., Cooper , M. C., Davis , M., et al. 2013, , 208, 5, 10.1088/0067-0049/208/1/5

    2013 doi

  21. [30]

    1986, , 161, 89

    Sandage , A. 1986, , 161, 89

    1986

  22. [31]

    2000, SAOImage DS9: A utility for displaying astronomical images in the X11 window environment , Astrophysics Source Code Library, record ascl:0003.002

    Smithsonian Astrophysical Observatory . 2000, SAOImage DS9: A utility for displaying astronomical images in the X11 window environment , Astrophysics Source Code Library, record ascl:0003.002

    2000

  23. [32]

    1986, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol

    Tody , D. 1986, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 627, Instrumentation in astronomy VI, ed. D. L. Crawford , 733, 10.1117/12.968154

    1986 doi

  24. [33]

    1993, in Astronomical Society of the Pacific Conference Series, Vol

    Tody , D. 1993, in Astronomical Society of the Pacific Conference Series, Vol. 52, Astronomical Data Analysis Software and Systems II, ed. R. J. Hanisch , R. J. V. Brissenden , & J. Barnes , 173

    1993

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