REVIEW 2 major objections 4 minor 1 cited by
MIGHTEE: A first look at MIGHTEE quasars
T0 review · 2 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read For the same 104 quasars, the fraction whose radio emission is blamed on the black hole rather than star formation changes from 22-58 per cent to 11-20 per cent depending on which star-formation calibration is used.
desk verdict A genuinely useful first-look MIGHTEE quasar sample; the qualitative calibration-sensitivity result survives, but the headline fractions are computed with over-tight error bars that omit the IRRC intrinsic scatter. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery is the radio-excess method: each quasar's radio luminosity is converted into a star-formation rate (SFR) under the assumption that all the radio emission is from star formation, and this is compared with an independent SFR derived from infrared light. The comparisons use two radio-based calibrations, the redshift-independent relation of Yun et al. (2001) and the redshift-dependent infrared-radio correlation of Delhaize et al. (2017), and two infrared-based estimates, the Kennicutt (1998b) conversion and SED3FIT (Berta et al. 2013). Where the radio-derived SFR exceeds the infrared-derived one by more than $1\sigma$, the paper labels the source 'AGN-dominated'; where it falls short by more than $1\sigma$, the source is a 'possible starburst'. The paper shows that the choice of the radio-based calibration alone changes the AGN-dominated fraction by more than a factor of two.
What would settle it
Take the 45 quasars in the sub-sample with SNR_FIR $> 3$ and 1.3-GHz flux $> 3\sigma$ and observe them with arcsecond-resolution radio imaging (e.g., VLBI) and sub-arcsecond far-infrared imaging (e.g., ALMA). If the sources classed as 'possible starbursts' exhibit compact AGN radio cores and FIR emission concentrated on the quasar rather than extended disk emission, the paper's AGN-heated-dust explanation is supported; if they show resolved star-forming disks and no radio core, the starburst classification is real and the redshift trend is physical.
Extended reading notes
Core claim
For a sample of 104 Type-1 quasars with deep MeerKAT radio data, the paper finds that the fraction of sources whose radio emission is dominated by the active galactic nucleus depends crucially on the star-formation-rate (SFR) estimate derived from the radio luminosity. Considering only quasars detected at $>3\sigma$ in both radio and far-infrared, the AGN-dominated fraction is 22 per cent when the radio-derived SFR is based on the Yun et al. (2001) relation and compared with the Kennicutt (1998b) SFR, and 58 per cent when compared with the SED3FIT SFR; using the redshift-dependent infrared-radio correlation of Delhaize et al. (2017) instead lowers this fraction to 11-20 per cent. The paper also reports that the fraction of 'possible starbursts' rises with redshift, from 31-38 per cent in the lower bins to 63 per cent in the highest-redshift bin, and interprets this trend most plausibly as AGN-heated dust inflating the far-infrared-derived SFRs, so the AGN-dominated fractions should be treated as lower limits.
Load-bearing premise
The load-bearing premise is that the Delhaize et al. (2017) infrared-radio correlation, fitted to normal star-forming galaxies, accurately describes quasar host galaxies (including starbursts) out to $z \approx 3.4$; if that correlation is wrong for quasars, the inferred AGN and starburst fractions—and their redshift trend—change.
Editorial extensions
If this is right
- The radio-excess method produces calibration-dependent fractions: the same sample yields 22-58 per cent AGN-dominated with the Yun et al. (2001) radio-SFR relation versus 11-20 per cent with the Delhaize et al. (2017) redshift-dependent infrared-radio correlation.
- Using the redshift-dependent correlation shifts the population toward 'possible starbursts', whose fraction climbs from roughly a third to 63 per cent in the highest-redshift bin ($z \approx 2.2$-$3.4$).
- The paper concludes that at high redshift, AGN-heated dust likely inflates far-infrared SFRs, so the quoted AGN-dominated fractions are lower limits rather than true values.
- Because the results depend so strongly on calibration, future samples need larger numbers and stellar-mass information (as the paper plans) before the AGN-versus-starburst balance of the quasar population can be pinned down.
Reading between the lines
- A natural but unstated consequence is that AGN fractions from surveys that adopt different radio-to-SFR calibrations are not directly comparable; a community-standard, redshift- and stellar-mass-dependent infrared-radio correlation would be needed for a single set of numbers.
- If the AGN-heated-dust explanation is correct, the apparent rise of starbursts with redshift is mostly a calibration artifact, and the true AGN contribution to radio emission at high redshift is likely larger than the Delhaize-based fractions suggest.
- The same two-calibration comparison could be applied to X-ray-selected AGN or submillimetre galaxies to see whether the calibration sensitivity is generic to the radio-excess method rather than specific to these quasars.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents a spectroscopically complete sample of 104 Type-1 quasars from the COSMOS and XMM-LSS fields of MIGHTEE, with 1.3-GHz radio imaging reaching an rms of about 3 microJy per beam. It extracts radio flux densities, computes radio luminosities, and compares radio-derived SFR estimates (Yun et al. 2001; Delhaize et al. 2017 IRRC) with FIR-derived SFR estimates (Kennicutt 1998b; SED3FIT) to classify sources as AGN-dominated or possible starbursts via a radio-excess method. The central claim is that the AGN-dominated fraction depends strongly on the radio-derived SFR calibration: 22–58 per cent for the Yun relation versus 11–20 per cent for the Delhaize relation among sources detected at 3 sigma in both radio and FIR, and that the possible-starburst fraction rises with redshift.
Significance. If the central quantitative claim holds, the paper is an important cautionary result: the radio-excess technique yields calibration-dependent AGN-versus-star-formation fractions, not intrinsic ones, with direct implications for the interpretation of radio-quiet quasars in deep radio surveys. The paper has clear strengths: transparent measurement and error propagation, retention of negative radio pixel values in statistical analyses, use of multiple independent SFR estimators, spectroscopic completeness to K_s = 21, new SALT redshifts for four objects, and a machine-readable table of radio luminosities and SFRs. The authors also explicitly flag several limitations, including the low resolution of the FIR data and the possible overestimate of FIR-derived SFRs from AGN-heated dust. However, the headline percentages depend on external calibrations and on a classification threshold that, as currently constructed, omits the intrinsic scatter of the infrared-radio correlation; this needs to be addressed before the quantitative claim can be accepted.
major comments (2)
- [§4.3, Eqs. (6)–(8), Table 4] The Delhaize-SFR uncertainties are stated in Section 4.3 to include only radio flux-density and redshift measurement errors, and the 1-sigma_1:1 thresholds used for classification in Figures 8–9 and Tables 4–5 are therefore based only on the reported fit-parameter uncertainties of the IRRC. Delhaize et al. (2017) report an intrinsic scatter in q_IR of about 0.34 dex, which is an order of magnitude larger than the quoted parameter uncertainties of 0.01–0.03 dex. Because each quasar is placed exactly on the relation to derive its Delhaize-SFR, a source that is genuinely star-forming but has a typical q_IR offset would be misclassified as AGN-dominated or starburst at the 1-sigma level. I request that the classification be repeated with the intrinsic scatter added in quadrature to the threshold, and that the resulting AGN-dominated and possible-starburst fractions in Table 4 and the redshift trend in Table 5 be reported. Without this, the headline contrast between the Yun et al. (2001) and Delhaize et al. (2017) fractions, which is the central quantitative statement of the paper, may be an artifact of over-tight thresholds.
- [§4.4.3, Table 5] The rise of the possible-starburst fraction to 63 per cent in the highest redshift bin and 67 per cent in the brightest optical-magnitude bin is presented as a main result (conclusion viii). The paper itself argues in Section 4.4.3 that AGN heating of dust on kpc scales can overestimate FIR-derived SFRs precisely in these regimes (Symeonidis 2022; Symeonidis et al. 2022), which would manufacture exactly this trend. Since the FIR luminosities have already had the AGN component subtracted during SED fitting, the effect must come from extended AGN-heated dust not captured by the templates, making it a systematic rather than random uncertainty. The manuscript should either provide a quantitative estimate of the magnitude of this effect (for example, by re-fitting with a wider AGN-heating model or by comparing to a sample without AGN) or explicitly frame the redshift trend as a possible artifact of the SFR calibration, rather than listing it as a standalone finding.
minor comments (4)
- [Table 4] The whole-sample rows of Table 4 appear corrupted in the manuscript (for example, '0.37σ 1:1 2431 14 / 104 = 2330 13%' and '0 / 104 = 534 0 %'), with overlapping numbers that make the fractions unreadable; these rows should be reformatted.
- [§4.3] The statement that the difference between the 22 per cent and 58 per cent AGN-dominated fractions is 'in large part due to the more-tightly-constrained calibration of the IR-related SFR' should be quantified by explicitly listing the adopted calibration uncertainties for the Kennicutt and SED3FIT SFRs (currently given as '+0.3/−0.5 dex' and '0.1 dex', respectively) and how they translate into the sigma_1:1 values used in the threshold.
- [§2.4] The FIR data over the two fields come from different pipelines (super-deblended for COSMOS, HELP for XMM-LSS) with very different detection limits (for example, 1.44 mJy versus 12.5 mJy at 100 micron). The paper should discuss whether the systematic difference in FIR depth could bias the combined-sample fractions in Table 4, especially for the radio-detected subsample.
- [Appendix E] There are several typographical issues, including 'starbust' instead of 'starburst' in Section 4.3 and Appendix E, and a garbled sentence in the caption of Figure E1 that should be corrected.
Circularity Check
No significant circularity: the AGN/starburst fractions follow from external calibrations and measured fluxes, with self-citations only providing context and selection methodology.
full rationale
The paper's central claim (Abstract; Section 4.4.5, Table 4) is a transparent sensitivity statement: the fraction of quasars classed as AGN-dominated depends on which external radio-to-SFR calibration is adopted (Yun et al. 2001 versus Delhaize et al. 2017). The classifications are obtained by comparing measured 1.3-GHz radio luminosities and FIR-derived SFRs against external relations (Equations 4-8), with no parameter of this paper fitted to define the target result. The Delhaize-SFR is constructed by placing each quasar exactly on the externally determined IRRC, and the Yun-SFR uses another external relation; the resulting contrast in fractions is a direct, openly discussed consequence of the different calibrations, not a prediction forced by an internal fit. Self-citations to White et al. (2015, 2017) appear in the selection method (the gJK_s colour box and K_s < 22.4 cut) and in literature comparison, but they do not carry the derivation of the AGN-dominated fractions; the sample, radio data, and FIR data are externally benchmarked (MIGHTEE DR1, Herschel/HELP, SALT spectroscopy). The possible omission of the IRRC intrinsic scatter from the Delhaize-SFR error budget is a statistical robustness concern, not a circularity: it does not make the output equal to an input by construction. No uniqueness theorem or ansatz is imported from the authors' prior work, and no fitted parameter is renamed as a prediction. Verdict: no significant circularity.
Assumptions & free parameters
free parameters (3)
- Radio spectral index alpha =
-0.7 (assumed)
- Optical spectral index alpha_c =
-0.5 (assumed)
- IMF conversion factor f_IMF =
1/1.72 (Salpeter to Chabrier IMF)
assumptions (5)
- domain assumption Lambda CDM cosmology with H0 = 70 km/s/Mpc, Omega_m = 0.3, Omega_Lambda = 0.7
- domain assumption The gJKs colour-selection box and AGN template fitting identify unobscured quasars reliably
- domain assumption FIR photometry and SED-fitting correctly separate AGN and star-formation contributions to the infrared luminosity
- domain assumption The Yun et al. (2001) and Delhaize et al. (2017) calibrations are applicable to quasar host galaxies
- domain assumption The radio-loudness parameter R uses a non-thermal spectral index alpha = -0.7 and assumes the optical continuum is AGN-dominated
Cite this review
Pith. "Pith review of MIGHTEE: A first look at MIGHTEE quasars." pith.science (2026). https://pith.science/paper/HL7ZOFBW
@misc{pith2026250712046,
author = {Pith},
title = {Pith review of: MIGHTEE: A first look at MIGHTEE quasars},
year = {2026},
howpublished = {\url{https://pith.science/paper/HL7ZOFBW}},
note = {Machine review of arXiv:2507.12046}
}
abstract
In this work we study a robust, $K_s$-band complete, spectroscopically-confirmed sample of 104 unobscured (Type-1) quasars within the COSMOS and XMM-LSS fields of the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Survey, at 0.60 < $z$ < 3.41. The quasars are selected via $gJK_s$ colour-space and, with 1.3-GHz flux-densities reaching rms ~ 3.0$\mu$Jy beam$^{-1}$, we find a radio-loudness fraction of 5 per cent. Thanks to the deep, multiwavelength datasets that are available over these fields, the properties of radio-loud and radio-quiet quasars can be studied in a statistically-robust way, with the emphasis of this work being on the active-galactic-nuclei (AGN)-related and star-formation-related contributions to the total radio emission. We employ multiple star-formation-rate estimates for the analysis so that our results can be compared more-easily with others in the literature, and find that the fraction of sources that have their radio emission dominated by the AGN crucially depends on the SFR estimate that is derived from the radio luminosity. When redshift dependence is not taken into account, a larger fraction of sources is classed as having their radio emission dominated by the AGN. When redshift dependence $is$ considered, a larger fraction of our sample is tentatively classed as 'starbursts'. We also find that the fraction of (possible) starbursts increases with redshift, and provide multiple suggestions for this trend.
Figures
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Reference graph
Works this paper leans on
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via the radio-loudness parameter,𝑅, defined by Kellermann et al. (1989). The ‘Yun-SFR’ and the ‘Delhaize-SFR’ are based upon the radio luminosity,andapplyingtherelationbyYunetal.(2001)(Equation4)andbyDelhaizeetal.(2017)(Equations6–8),respectively.The‘Kennicutt-SFR’isderivedthroughtwo-componentSED-fittingover8–1000μm (Jin et al
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and applying Equation 5 (Kennicutt 1998b), whilst the ‘SED3FIT-SFR’ is that output by the SED-fitting code of Berta et al. (2013). MNRAS000, 1–29 (2015) 28S. V. White et al. R.A. Dec. spec-𝑧 𝐿 1.4 GHz / WHz−1 Yun–SFR /M⊙ yr−1 Kennicutt–SFR SED3FIT–SFR Delhaize-SFR /M ⊙ yr−1 (hms) (dms)±error (+ error,−error) (+ error,−error)±error /M ⊙ yr−1 ±error /M⊙ yr−...
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via the radio-loudness parameter,𝑅, defined by Kellermann et al. (1989). The ‘Yun-SFR’ and the ‘Delhaize-SFR’ are based upon the radio luminosity, and applying the relation by Yun et al. (2001) (Equation
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(2017) (Equations 6–8), respectively
and by Delhaize et al. (2017) (Equations 6–8), respectively. The ‘Kennicutt-SFR’ is derived through two-component SED-fitting over 8–1000μm (Jin et al
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and applying Equation 5 (Kennicutt 1998b), whilst the ‘SED3FIT-SFR’ is that output by the SED-fitting code of Berta et al. (2013). MNRAS000, 1–29 (2015) A first look at MIGHTEE quasars29 101 102 103 SFR [/ M yr 1] via qIR from Delhaize et al. (2017) 100 101 102 103 SED3FIT SFR [/ M yr 1] via SED-fitting SNRFIR > 3, S1.3 GHz > 3 SNRFIR < 3, S1.3 GHz > 3 SN...
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2916.01 (+926.24,−909.08) 539.59±24.86 2094.00±97.81 126.49 (+943.02,−145.33) 09:58:26.67 +02:28:18.1 0.692±0.135 1.81×10 22 (+1.88×1022,−1.09×10
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for six sources, with varying degrees of coverage in the FIR (Appendix C). MNRAS000, 1–29 (2015) 26S. V. White et al. R.A. Dec. spec-𝑧 𝐿 1.4 GHz / WHz−1 Yun–SFR /M⊙ yr−1 Kennicutt–SFR SED3FIT–SFR Delhaize-SFR /M ⊙ yr−1 (hms) (dms)±error (+ error,−error) (+ error,−error)±error /M ⊙ yr−1 ±error /M⊙ yr−1 (+ error,−error) 09:57:58.40 +02:17:29.1 2.420±0.050 1...
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a very secure redshift
and the zCOSMOS-bright𝑧value (with a quality flag that confirms that it is a quasar, has “a very secure redshift”, and that “the spectroscopic MNRAS000, 1–29 (2015) A first look at MIGHTEE quasars21 andphotometricredshiftsareconsistenttowithin0.08(1+𝑧)”;Lilly et al. 2009). Als...
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and applying Equation 5 (Kennicutt 1998b), whilst the ‘SED3FIT-SFR’ is that output by the SED-fitting code of Berta et al. (2013). MNRAS000, 1–29 (2015) A first look at MIGHTEE quasars27 R.A. Dec. spec-𝑧 𝐿 1.4 GHz / WHz−1 Yun–SFR /M⊙ yr−1 Kennicutt–SFR SED3FIT–SFR Delhaize-SFR...
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Reviewed August 6, 2026 · model on record in the stance chip above.
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