Radio-loudness along the quasar main sequence

A. Del Olmo; C.A. Negrete; E. Bon; M. D'Onofrio; N. Bon; P. Marziani; V. Ganci

REVIEW 3 major objections 4 minor 147 references

Radio-loudness along the quasar main sequence

T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read The standard radio-loudness criterion $R_K>10$ picks out jetted quasars only on part of the main sequence; in extreme Population A it can select star formation instead.

desk verdict A systematic, useful MS-resolved radio-loudness study whose Population B jetted conclusion is solid, but whose xA thermal headline rests on external FIR data and is explicitly under-supported in the paper itself. read the letter →

arxiv 1908.07308 v1 pith:5USJCSDM submitted 2019-08-20 astro-ph.GA

V. Ganci , P. Marziani , M. D'Onofrio , A. del Olmo , E. Bon , N. Bon , C.A. Negrete This is my paper · ORCID

classification astro-ph.GA

keywords quasarmainsequenceradio-loudnessKellermanncriterionrelativisticjetsstarformationbroad-lineregionbinaryblackholesFIR-radiocorrelation

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 tests whether the standard criterion for calling a quasar radio-loud—the radio-to-optical flux ratio $R_K>10$—actually identifies quasars whose radio emission comes from a relativistic jet. Using 355 radio-detected SDSS quasars arranged along the quasar main sequence, it finds that the answer depends on where a source falls on that sequence. In the most extreme Population A spectral types (A3 and A4), sources classed as radio-intermediate and even radio-loud reach radio powers compatible with star formation, not with a jet. In Population B, radio-loud sources are characteristically jetted, and their broad H$\beta$ profiles are often consistent with a binary broad-line region. The paper concludes that the Kellermann criterion is not universally sufficient, and that main-sequence position must be considered before classifying a source as jetted.

What carries the argument

The organizing device is the quasar main sequence, the plane defined by FWHM(H$\beta$) versus the FeII strength $R_{\rm FeII}$, subdivided into spectral types from A1 to A5 and B1 to B1+++. Within each spectral type, the analysis compares three radio-loudness classes ($R_K<10$, $10\le R_K<70$, and $R_K\ge70$) and two radio morphologies, compact core-dominated and extended FRII-like. The decisive comparison is between the radio pseudo-star-formation rate derived from the 1.4 GHz power and the star-formation rate derived from 70/100 $\mu$m infrared photometry; sources that fall inside the star-forming, radio-quiet zone of the FIR-radio correlation are interpreted as thermally powered, while sources outside it are read as jetted.

What would settle it

Measure infrared brightness at 70–160 microns for the radio-intermediate and radio-loud extreme-A quasars in the paper's candidate list. If they fall below the star-forming infrared-radio relation and show compact flat-spectrum core-jet structure, the thermal interpretation fails; if they stay on it, the dichotomy holds.

Watch

Extended reading notes

Core claim

The central discovery is that radio-loudness is not one physical condition along the quasar main sequence. The authors build a sample of 355 type-1 quasars from a FIRST-SDSS cross-match, split into compact core-dominated and extended FRII-like radio morphologies, and into radio-detected, radio-intermediate, and radio-loud classes inside each optical spectral type. Extreme Population A (xA: A3 and A4, strong FeII and narrow H$\beta$) contains a notable prevalence of radio-intermediate and radio-loud sources, but their radio powers overlap those expected from star formation and their infrared-radio positions match radio-quiet quasars. Population B radio-loud sources, especially the FRII double-lobed ones, are jetted, and several of their H$\beta$ composites are better described by two displaced broad components than by the usual broad-plus-very-broad model, pointing to a binary broad-line region. The paper therefore argues that $R_K>10$ is sufficient to identify jetted sources in Population B and in spectral bin A1, but not in extreme Population A.

Load-bearing premise

The thermal interpretation of the extreme Population A sources assumes that the infrared-radio correlation from previous samples correctly marks star formation as the dominant radio mechanism and that those samples represent the present selection; only seven objects here have infrared data and none is an extreme-A source.

Editorial extensions

If this is right

In Population B and in spectral bin A1, the condition $R_K>10$ (or $\log R_K>1$) remains a workable identifier of jetted sources, so those bins can be used as a clean jetted sample.
In extreme Population A (A3/A4), radio-intermediate and radio-loud sources should not be counted as jetted without infrared or high-resolution radio follow-up; they are prime targets for testing the thermal interpretation.
FRII double-lobed quasars appear almost exclusively in Population B, with a few A1 objects interpretable as the same jetted population seen at smaller viewing angle, so orientation must be included when using H$\beta$ FWHM to classify sources.
The H$\beta$ profile fits of several Population B bins are consistent with two broad-line regions orbiting each other; if secure, these sources are binary black hole candidates, and radio-loud extreme Population B is the preferred hunting ground for such binaries.

Reading between the lines

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

If the thermal interpretation holds, radio-loudness-selected samples that ignore main-sequence position will systematically overestimate the number of jetted quasars at the highest Eddington ratios, and may inflate binary black hole candidate lists by mixing starbursts into them.
A direct extension would be to obtain high-resolution radio maps of the RL CD A3 candidates: compact symmetric or GPS/CSS morphologies combined with steep spectra would argue for a young jetted source, while diffuse or resolved disk-like radio emission would support a star-formation origin.
The binary-broad-line-region interpretation predicts that the two H$\beta$ components should change radial velocity on decade timescales; single-epoch double-component fits alone cannot confirm a binary, so monitoring campaigns on the B1++ candidates would settle the matter.
If xA radio power is substantially thermal, then feedback at the most extreme accretion rates is probably driven by outflows and star formation rather than by radio jets, which would shift how high-accretion quasars are treated in galaxy evolution models.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Dear colleague,

Quick take: this paper is worth engaging seriously. It is the first systematic spectral-type-resolved analysis of a FIRST-selected radio sample along the quasar main sequence, and the most robust part is the Population B side. The RI/RL Population B sources look genuinely jetted by multiple independent indicators: FRII morphology, radio power distributions, and the radio-power vs. RK' correlation. The suggestion that extreme Population B sources, especially B1++ and B1+, are good targets for binary broad-line region searches is concrete and testable, and the candidate list has practical value.

What is actually new here is the atlas-like breakdown: 355 objects classified by spectral type, radio-loudness, and morphology, with the FRII maps in the appendix. The flat P_radio vs. RK' slope for xA sources compared with the steep slope for Population B is an interesting observational fact that does not depend on the FIR interpretation. The paper is also honest about alternatives, for example outflow vs. binary BLR for the H-beta profiles.

The soft spot is the xA thermal claim. In the primary sample, only seven sources have FIR data and none is xA. The supporting Fig. 9 relies on external samples that were vetted to over-select Population A and xA sources, not on the FIRST-selected sample itself. The paper states this almost explicitly in Sect. 5.3.2: there is no direct evidence that logRK >= 1 fails to select truly jetted sources for xA. So the abstract's "we show" is stronger than the data justify. This is not a fatal flaw because the conclusion is ultimately framed as a suggestion and Table 2 is proposed for follow-up, but a revised version should either label the xA thermal interpretation as an externally motivated hypothesis or back it with new FIR and high-resolution radio data for the Table 2 candidates.

The binary BLR F-test is done on composite spectra and is suggestive rather than decisive. Minor concerns include the fixed FIR K-correction assumptions and the use of FIRST peak fluxes for compact sources, but these are not load-bearing. The citation pattern is fine and the core classification is not circular.

Bottom line: this deserves a serious referee, not a desk reject. The Population B results and the target list stand on their own; the xA thermal claim needs to be scaled down or further tested. I would send it to review and ask for that revision.

Best, [You]

Referee Report

3 major / 4 minor

Summary. The paper cross-matches the FIRST radio survey with the SDSS DR14 quasar catalog to build a sample of 355 radio-detected type-1 AGNs, classifies them along the quasar main sequence (spectral types A1-A5, B1-B3) and by radio morphology (core-dominated vs. FRII-like) and radio-loudness (RD, RI, RL). It analyzes composite optical spectra per spectral type and radio class, measuring H-beta and [O III] profile parameters, estimating black hole masses and Eddington ratios, and comparing radio-derived pseudo-SFRs with FIR-derived SFRs. The central claims are that (i) radio-loud and radio-intermediate sources in extreme Population A (A3/A4) may have thermal, star-formation-dominated radio emission rather than relativistic jets, so that Kellermann's RK>10 criterion may not identify truly jetted sources in this region of the main sequence; (ii) radio-loud Population B sources are characteristically jetted, with some broad H-beta profiles consistent with a binary broad-line region; and (iii) a list of binary black hole AGN candidates is presented.

Significance. If the thermal-origin interpretation for extreme Population A were established, the paper would provide an observationally grounded refinement of the standard radio-loudness classification and would identify a population of quasars where the Kellermann criterion fails. The paper also delivers useful resources: a systematically selected sample, a FIRST atlas of FRII candidates, detailed spectral decompositions, and a candidate list for SMBBH searches. The Population B jetted classification is well supported by the radio morphology and power distributions. However, the headline claim that RK>10 is insufficient for extreme Population A is not established by the primary sample: as the authors explicitly concede in Sect. 5.3.2, there is no direct evidence that thermal xA sources enter the RL range. The external FIR comparison samples are differently selected and their xA members are mostly RD. This weakness is load-bearing because the main-sequence-dependent validity of Kellermann's criterion rests on it. The binary-BLR F-test evidence is suggestive but based on composite spectra and not decisive.

major comments (3)

[§4.4, §5.3.2, Table 5, Fig. 9] The central claim that RI/RL sources in extreme Population A (xA, spectral types A3/A4) are thermal rather than jetted is not supported by the present sample. Table 5 lists only seven sources with FIR data, and none of them is an xA source; their spectral types are A2, A1, B1, B1+, or unclassified. The xA thermal inference therefore rests entirely on the external 73-source sample of Fig. 9, which was 'vetted to preferentially select Pop. A and especially xA' (Sect. 4.4) but is not FIRST-selected, and whose xA members are 'mainly classified as RD' (Sect. 4.4). The paper itself states in Sect. 5.3.2: 'in the samples used for Fig. 9 we have no indication of thermal xA sources entering into the RL radio-loudness range, and thus no direct evidence that the condition logRK >= 1 does not select truly jetted sources.' This is an explicit, load-bearing limitation: if the RL/RI xA sources in Table 2 are actually jetted, the claim that RK>10 is insufficient for xA collapses. The authors should either obtain or identify FIR data for the RL/RI xA sources in their own sample, or explicitly reframe the conclusion as hypothesis-generating rather than established.
[§5.3.1, Fig. 11] The flat trend of logP_nu versus logRK' for xA sources is used as evidence of a different radio emission mechanism, but the xA subsample spans a very narrow range in logRK': in Table 1, xA sources are predominantly RI (15 out of 25) with only 5 RL and 5 RD, and the RI class is limited to 1.0 <= logRK' < 1.8 by definition. A regression slope that is not significantly different from zero over such a compressed dynamic range (slope 0.136 +/- 0.168) provides almost no discriminating power between a thermal and a jetted origin. The contrast with the well-constrained Population B slope is expected even if all sources share the same mechanism but have different luminosity/radio-loudness covariance. This point should be acknowledged and the evidential weight of Fig. 11 reduced accordingly.
[§5.5, Table 6] The F-test comparison between the BC+VBC and BC+BC models is performed on composite (median) spectra, not on individual spectra, so the quoted probabilities (e.g., P = 8.3e-10 for RD CD B1+) quantify the improvement of fit to the composite profile, not the frequency of binary-BLR signatures among the individual sources. The authors themselves note alternative explanations (unresolved outflows, accretion disk instabilities, changing-look variability) and state that it is 'obviously not possible to demonstrate' a sub-pc binary from these data. Given that the candidate list is presented as a main result, the p-values should be interpreted as model-selection diagnostics for the composite, and the candidate list should be presented as a sample for follow-up rather than a statistically significant detection of binaries.

minor comments (4)

[Fig. 11 caption] The word 'exagons' appears to be a typo for 'hexagons'; please correct it.
[Table 4 footnote] The units for MBH are listed as 'M⊙/yr' but should be 'M⊙'; the table footnote should be corrected.
[Abstract and §2.2] The abstract defines RD as RK' < 10 while the text in §2.2 uses log RK < 1.0, which corresponds to RK' < 10; the notation should be made consistent so that the reader does not confuse the threshold with the value of the physical ratio.
[§4.1.2] In the description of Figure 5, the sentence 'This translates in a redshifted line' should read 'This translates into a redshifted line'.

Circularity Check

1 steps flagged · score 4.0 of 10

Partial circularity in the Pν–RK′ evidence and an acknowledged data gap weaken, but do not reduce, the central jetted/thermal classification.

other [Sect. 5.3.1, Eq. (7) and Fig. 11]
"The behavior of the Pν vs. RK′ for the various STs (Fig. 11) adds further evidence in favor of an intrinsic difference between the two populations: Pop. B sources of the three radio loudness classes are significantly correlated, with a unweighted least square fit yielding logPν≈ (0.952±0.041)RK′ + (23.245±0.097)."

By Eq. (7), log P1.4GHz = 20.08 + 2 log dL + log f1.4GHz, while log RK′ is defined (Sect. 2.2) from the same f1.4GHz divided by the g-band flux. Thus log Pν = log RK′ + log f_g + 2 log dL + const: the abscissa and ordinate of Fig. 11 share the same radio flux density by construction. A slope near unity for Pop. B is therefore partly guaranteed whenever f_g and dL have small leverage, and the flat xA slope can arise from distance/luminosity scatter rather than from a different emission mechanism. The paper presents this shared-variable correlation as independent evidence of an intrinsic difference, which is a partial reduction of the argument to its own input.

full rationale

The paper's primary classification (radio-loudness from FIRST/SDSS fluxes) and its FIR-radio comparison use independently measured quantities, and the FRII/Pop. B jetted conclusion does not reduce to a fit. The xA 'thermal' interpretation is the central claim, yet the paper itself concedes (Sect. 5.3.2) that 'in the samples used for Fig. 9 we have no indication of thermal xA sources entering into the RL radio-loudness range, and thus no direct evidence that the condition logRK >= 1 does not select truly jetted sources.' Table 5 contains no xA source with FIR data, so the thermal inference is imported from external samples that were 'vetted to preferentially select Pop. A and especially xA' (Sect. 4.4) and whose xA members are mostly RD or borderline RI. That is an evidence/representativeness gap, not a self-definitional reduction, so it does not by itself raise the circularity score. However, the supporting Fig. 11 regression is partially built from the same 1.4 GHz flux in both axes, as detailed in the step. Self-citations (Negrete et al. 2018, Zamfir et al. 2008, Sulentic/Marziani MS papers) supply the MS/xA framework and the RI/RL thresholds, but the thresholds are empirical and the optical classification is independent of radio power; these citations are not the mechanism forcing the result. Overall, one evidentiary correlation is partially circular by construction, while the central claim retains independent observational content, giving a moderate score.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

The central claim does not introduce new physics, but it depends on the adopted binning of the main sequence, the assumed FIR SED corrections, the radio morphological classification, and the use of literature calibrations for SFR and BH mass. The xA thermal inference additionally depends on external samples that the authors vetted, creating a chain of assumptions that is not independently anchored in the 355-object sample.

free parameters (4)

Radio-loudness thresholds (RK'=10 and RK'=70) = 10, 70 (logRK' 1.0, 1.8)
Classification boundaries adopted from Kellermann et al. (1989) and Zamfir et al. (2008); they determine which sources are called RD/RI/RL and hence shape all prevalence comparisons.
FIR dust temperature T = 60 K
Assumed single modified blackbody temperature for the K-correction at 70/100 micron (Sect. 3.3), affecting SFR(FIR) values in Table 5.
Emissivity index beta = 1.82
Assumed for FIR SED K-correction following Smith et al. (2014), affecting SFR(FIR).
Spectral type bin boundaries = Delta RFeII = 0.5; Delta FWHM = 4000 km/s
Hand-chosen bins defining the main sequence spectral types; the xA designation is defined as A3+A4 (RFeII>1).

assumptions (5)

domain assumption FWHM(Hbeta) traces virial motion in a flattened BLR
Used to convert FWHM to MBH (Eq. 11) and to interpret Pop A/B differences; Sect. 3.4 and 5.1.
domain assumption SDSS DR14 spectra allow reliable RFeII and FWHM(Hbeta) measurement after screening
Objects with strong host contamination or poor spectra were excluded, but the reliability of the remaining measurements is assumed; Sect. 2.3.
domain assumption FIR-radio correlation of Bonzini et al. (2015) applies to these quasars
Basis for comparing SFR(FIR) and pSFR(radio) to classify thermal vs jet emission (Fig. 9).
domain assumption The FIRST 1.4 GHz flux is complete for unresolved CD sources after accounting for ~30% underestimation
The RK' values are not corrected for the FIRST resolution bias, which the paper notes in Sect. 2.2.
domain assumption Radio morphology classification (CD vs FRII) is reliable using the de Vries et al. (2006) opening-angle criterion
FRII candidates are selected by a statistical criterion with a posteriori NED optical-counterpart rejection; Sect. 2.1.

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Cite this review

Pith. "Pith review of Radio-loudness along the quasar main sequence." pith.science (2026). https://pith.science/paper/5USJCSDM

@misc{pith2026190807308,
  author       = {Pith},
  title        = {Pith review of: Radio-loudness along the quasar main sequence},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/5USJCSDM}},
  note         = {Machine review of arXiv:1908.07308}
}

read the original abstract

Following the established view of the AGNs inner workings, an AGN is radio-loud (RL) if associated with relativistic ejections emitting a radio synchrotron spectrum (i.e., a jetted AGN). If large samples of optically-selected quasars are considered, AGNs are identified as RL if their Kellermann's radio loudness ratio RK > 10. Our aims are to characterize the optical properties of different classes based on radio-loudness within the quasar main sequence (MS) and to test whether the condition RK > 10 is sufficient for the identification of RL AGNs. A sample of 355 quasars was selected by cross-correlating the FIRST survey with the SDSS DR14 quasar catalog. We classified the optical spectra according to their spectral types along the quasars MS. For each spectral type, we distinguished compact and extended morphology, and three classes of radio-loudness: detected (specific flux ratio in the g band and at 1.4GHz, RK' < 10, RD), intermediate (10 < RK' < 70, RI), and radio loud (RK' > 70). The analysis revealed systematic differences between RD, RI, and RL in each spectral type along the MS. We show that spectral bins that contain the extreme Population A sources have radio power compatible with emission by mechanisms ultimately due to star formation processes. RL sources of Population B are characteristically jetted. Their broad H-beta profiles can be interpreted as due to a binary broad-line region. We suggest that RL Population B sources should be preferential targets for the search of black hole binaries, and present a sample of binary black hole AGN candidates. The validity of the Kellermann's criterion may be dependent on the source location along the quasar MS. The consideration of the MS trends allowed to distinguish between sources whose radio emission mechanisms is jetted from the ones where the mechanism is likely to be fundamentally different.

Figures

Figures reproduced from arXiv: 1908.07308 by the authors.

**Figure 1.** Sketch illustrating the definition of the spectral types along the MS, as a function of RFeII and FWHM(Hβ). The numbers in square brackets yield from top to bottom, the prevalences of each spectral bin (nST) in an optically selected sample (Marziani et al. 2013), of the RI and RL sources (nRI+RL) and of RD (nRD) in each spectral bin. The total number of sources is N=680. The gray and pale green ares trace the sour… view at source ↗

**Figure 2.** Redshift (top) and luminosity at 5100 Å (bottom) distributions of the sample. The left plots refer to CD sources, the right ones to FRII ones. Blue refers to the RD class, green to the RI one and red to the RL one. 3.2. Radio power and pseudo SF R From the flux density at 1.4 GHz we calculated a formal estimate of the star formation rate (hereafter pseudo-SF R, pSF R) of each object following Yun et al. (2001): pSF … view at source ↗

**Figure 3.** Composite of spectral types along the MS, ordered from B1++ to B2. Each panel shows the original composite, along with the modeled continuum (cyan line). The lower part of the panels shows the result of the specfit analysis on the emission lines. HβBC: black line, HβVBC: red line; Feii: green line; yellow lines: Hβ narrow components and [Oiii]λλ4959,5007. RFeII, along with a final classification code that includes r… view at source ↗

Figures from the paper (11 more)

**Figure 4.** Figure 4: Composite of spectral types along the MS, ordered from A1 to A4. Color scheme same as in [PITH_FULL_IMAGE:figures/full_fig_p008_4.png]

**Figure 5.** Figure 5: CD Hβ centroid shifts (km/s) for different E1 classes and for different line height. From top to bottom: x = 1/4, x = 1/2, x = 9/10. Bottom: equivalent width (Å) distribution. Blue refers to the RD class, green to the RI one, red to the RL one and black to the RL FRII …

**Figure 6.** Figure 6: shows that in the CD class the values of c(1/4) become increasingly negative from Pop. B to A, reflecting an increasing blueshift toward the line base. The blueshifting of the [OIII] wing component, represented by c(1/4), is very common in AGNs but not every AGNs show…

**Figure 7.** Figure 7: Equivalent width of [Oiii]λ5007 as a function of spectral types for CD RD, RI, RL sources. The upper panel refers to the base, semi-broad component, the bottom one to the narrower core of the [Oiii]λ5007 line. reflected by the increase in blueshift amplitude toward the…

**Figure 9.** Figure 9: FIR luminosity vs. radio power. The shaded areas trace the occupation zone of star-forming galaxies (gray) and RQ quasars (pink) following Bonzini et al. (2015). Color codes are: red (RL), green (RI), blue (RD). Left: sources identified on the basis of spectral types; …

**Figure 11.** Figure 11: Behavior of radio power Pν in units of W as a function of the Kellermann’s parameter RK0 . Symbol coding corresponds to different radio morphologies and to different STs. Triangles: RL FRII; squares: RL CD, exagons: CD RI, circles: CD RQ. Red: union of B1++ and B1+, o…

**Figure 10.** Figure 10: Distribution of radio power Pν in units of W for RI (top) and RL (bottom), for different spectral types along the main sequence. The top panels show the distributions for the union of B1++ and B1+ (red), B1 (orange) spectral types, the bottom ones for A1 (gray), pale …

**Figure 12.** Figure 12: Behavior of log RK0 Top: Pop. A and B FRII (red: union of B1+++, B1++ and B1+, orange: B1; gray: A1): Middle: Pop. B CD with the same color coding. Bottom: Pop. A (gray: A1, pale blue: A2, xA): blue. The dot-dashed lines indicate the median values for the samples show…

**Figure 13.** Figure 13: Top: Core-to-lobe radio flux density ratio vs. FWHM(Hβ), adapted from Wills & Browne (1986). Colored dots represent sources of our sample: red dots represent A1 RL FRII sources, yellow dots B1 RL FRII, green B1+ RL FRII and blue ones B1++ RL FRII. Black dots are the o…

**Figure 14.** Figure 14: In the case of CD RD B1+, the BC+BC model was tested against a BC+VBC model fit in which the peak of the BC was set to 0 [PITH_FULL_IMAGE:figures/full_fig_p019_14.png]

**Figure 14.** Figure 14: Comparison between broad + very broad and double broad Hβ fitting models. From left to right, top to bottom: CD-RDB1+, CD-RI-B1+, CD-RL-B1+ and FRII-RL-B1++. For each pair the left plot represents the broad + very broad fitting BC+VBC model and the right plot represe…

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Pith tools

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

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

ENTRY address archiveprefix author booktitle chapter edition editor howpublished institution eprint journal key month note number organization pages publisher school series title type volume year label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts #0 'before.all := #1 ...

[2] [2]

write newline

" write newline "" before.all 'output.state := FUNCTION n.dashify 't := "" t empty not t #1 #1 substring "-" = t #1 #2 substring "--" = not "--" * t #2 global.max substring 't := t #1 #1 substring "-" = "-" * t #2 global.max substring 't := while if t #1 #1 substring * t #2 global.max substring 't := if while FUNCTION word.in bbl.in " " * FUNCTION format....

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