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The Spectral Energy Distributions of Active Galactic Nuclei

T0 review · 1 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read New empirical SEDs for 41 active galactic nuclei, built from archival spectra and matched-aperture photometry, match the colors of AGNs seen by wide-field surveys well enough to deliver photometric redshifts as good as or better than…

desk verdict Genuinely useful SED library that passes a real external photo-z test; the construction is openly hand-stitched and lacks published error estimates, but the central practical claim holds. read the letter →

arxiv 1908.03720 v1 pith:NNTBQFQC submitted 2019-08-10 astro-ph.GA

M. J. I. Brown , K. J. Duncan , H. Landt , M. Kirk , C. Ricci , N. Kamraj , M. Salvato , T. Ananna This is my paper · ORCID
classification astro-ph.GA
keywords galaxies:activequasars:generalSeyfertemissionlinesdistancesandredshiftsspectralenergydistributionsphotometricAGNtemplates
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 assembles spectral energy distributions (SEDs) for 41 active galactic nuclei (AGNs) by stitching together archival spectra from the X-ray to the radio, anchoring them to matched-aperture photometry, and filling gaps with greybody and power-law models. It then mixes four Seyfert nuclei with galaxy SEDs to produce 72 additional Seyfert templates. The central claim is that these empirical templates are functional models for AGNs detected by current wide-field surveys: when used to compute photometric redshifts for 2058 AGNs in the Bootes field, they yield a scatter of $\sigma_{\mathrm{NMAD}} = 0.096 \times (1+z)$, median reduced $\chi^2 \simeq 1$, and median flux residuals of about 9% over restframe 0.1\,--\,3µm. This matters because broad-band AGN colors are shaped by real spectral features\,—\,the big blue bump, the 1µm inflection, silicate emission, strong emission lines\,—\,that earlier composite templates often smoothed away, and photo-z codes need templates that reproduce those features.

What carries the argument

The central object is the SED template itself, built by a specific stitching procedure. For each AGN, archival spectra from many instruments are multiplicatively scaled—by factors usually between 0.5 and 2.0, but occasionally outside 0.33 to 3.0—until they join into one continuous SED, with overlapping wavelength ranges used to fix the relative scalings and matched-aperture photometry used to anchor the absolute level. Gaps are filled by fitting polynomials to the log flux–log wavelength relation, with greybody models for far-infrared thermal dust emission and power-laws or polynomials for radio flux densities. This construction carries the argument because the photo-z test is sensitive to spectral shape: if the rescaling distorts the shape, the template error function and residual statistics would degrade. The test itself uses a template-fitting photometric redshift code that slides each SED in redshift, and the authors report that the templates produce the redshift posterior, scatter, and residual statistics quoted above.

What would settle it

Compare each rescaled template SED to a contemporaneous, well-sampled SED of the same object (or to the object's measured flux densities at a single epoch across many bands); if the rescaled template deviates from the simultaneous measurement by more than the quoted 9% residual level in ways that correlate with the amplitude of the multiplicative scalings, then the stitching assumption is not safe.

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Extended reading notes

Core claim

The discovery the paper argues for is that a continuous, spectrophotometric SED of a single AGN can be built from archival data taken years apart, and that the resulting library outperforms or matches existing SED libraries when used for photometric redshift estimation. In the authors' presentation, AGN photometric redshifts determined with the new AGN SEDs plus galaxy SEDs have typical scatter $\sigma_{\mathrm{NMAD}} = 0.096 \times (1+z)$, median reduced $\chi^2$ values of about 1, and typical flux density residuals of 9%, with the exact value depending on wavelength. Compared to fitting with a galaxy-only library, the new templates reduce model colour biases by a factor of roughly 2 to 5 and cut the median absolute residual from 18% to 9%; compared to a recent AGN-optimised library they are comparable or better, particularly at $z>2.5$. The paper's framing is that previous AGN templates traded spectral resolution for coverage, washing out features like silicate emission that matter for broadband colours, while these SEDs preserve those features at the cost of being tied to individual, variable objects.

Load-bearing premise

The whole library stands or falls on the assumption that multiplicatively scaling spectra taken at different times and through different apertures yields one faithful SED of the AGN, even though variability and aperture bias act differently at different wavelengths.

Editorial extensions

If this is right

  • For X-ray selected AGNs at $z<2.5$, photometric redshifts from the new templates are comparable to those from a recent AGN-optimised library, with improved scatter and outlier fractions at $z>2.5$.
  • For optically and mid-infrared selected AGNs, the new library performs comparably to the X-ray case, indicating it covers the colour space of AGNs selected by different survey techniques.
  • Median absolute flux residuals drop from 18% (galaxy-only templates) to 9% (new AGN+galaxy library), and the template error function is reduced most strongly at restframe wavelengths below 2000 Å and near 1µm.
  • Combining the templates with their own error function and a magnitude prior reduces robust scatter from $\sigma_{\mathrm{NMAD}}=0.12$ to $0.095$ and the outlier fraction from 37% to 31%, showing the quoted performance is not yet optimised.

Reading between the lines

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

  • A testable extension the paper leaves implicit: because several templates include far-infrared and radio extensions, they could predict the sub-mm and radio colours of radio-selected AGNs in new surveys, an application the paper only mentions in passing.
  • The striking $z\sim4$ feature where Hα enters a broad 3.4µm infrared band turns the templates into a possible redshift discriminator for mid-infrared selected samples; the paper notes the colour effect but does not exploit it as a stand-alone diagnostic.
  • The Seyfert composites are built from $z\sim0$ galaxy SEDs, so building analogous composites from higher-redshift galaxy SEDs would naturally extend the library to host-dominated AGNs at $z>1$; the authors explicitly leave this to future work.
  • The scaling factors themselves encode information about AGN variability amplitude as a function of wavelength; restricting photo-z fits to low-scaling templates would test whether the 9% residual figure depends on the most heavily rescaled spectra.
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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 constructs spectral energy distributions (SEDs) for 41 AGNs by combining archival spectrophotometry and photometry over 0.09–30 μm, with extensions into X-ray, far-infrared, and radio for some objects. Individual spectra are multiplicatively scaled to form continuous SEDs; gaps are filled with simple models, and 72 Seyfert composites are made by mixing AGN and galaxy SEDs. The authors validate the library by fitting photometric redshifts for 2058 Boötes AGNs and comparing with existing libraries, reporting comparable or better performance (σNMAD = 0.096, median reduced χ² ≃ 1, ~9% typical flux residuals).

Significance. If the SEDs are reliable, they are a valuable public resource with higher resolution and broader coverage than earlier libraries, useful for photo-z, AGN selection, and SED modeling. The photo-z test is a genuine external validation on an independent sample, and the comparison with Ananna et al. (2017) is a strong benchmark. The templates and synthetic photometry are made available via a DOI. The main caveat is the dependence of the SED construction on subjective multiplicative scaling of archival spectra, so the shapes of individual SEDs, particularly those with large epoch-to-epoch flux differences, remain less firmly established.

major comments (1)
  1. [§3.1, Figure 2] The construction of the SEDs relies on multiplicative scaling of archival spectra taken at different epochs and with different apertures, and the paper explicitly cautions that this is a crude approximation because variability and aperture bias have wavelength dependence. For 7 of the 41 AGNs the scalings exceed a factor of 3. The external photo-z validation in §5 supports the utility of the templates, but the fitting includes 10% flux uncertainties and a template error function that can absorb smooth shape distortions, and the residual metric is averaged over the sample. Since the central conclusion that these are functional models is broader than photo-z utility alone, the paper should provide a quantitative test that the scaling does not introduce wavelength-dependent biases in the SED shapes. I request either (i) a presentation of the distribution of flux ratios in the overlap regions of the input spectra before scaling, or (ii) a demonstration that the photo-z metrics and fixed-redshift residuals are robust to the removal of the 7 AGNs with the most extreme scalings. Without such a test, the shape fidelity of the SEDs, especially for the extreme-scaling objects, is not fully established.
minor comments (6)
  1. [§3.1] The word 'polynominals' is a typo and should read 'polynomials'.
  2. [Figure 2] The seven AGNs with scalings outside 0.33–3.0 are named in the text but not identified in the figure; labelling them would aid the reader.
  3. [§5, paragraph beginning 'When fitting in EAzY'] The extinction ranges are asymmetric (E(B−V) ≤ 0.5 for Ananna et al. (2017) and Brown et al. (2014b), but ≤ 0.2 for the new AGN templates). The text should briefly justify this choice, even though it is conservative.
  4. [§6] The notation 'σNMAD = 0.096 × (1 + z)' is ambiguous because σNMAD is already defined as a normalized scatter; consider writing 'σNMAD = 0.096 (i.e., a redshift uncertainty of 0.096(1+z))'.
  5. [Table 1] The 'SED λ range' column uses exponential notation (e.g., '9.0×10−2−6.4×101'); decimal notation (0.09–64) would be clearer.
  6. [§2.3] The greybody model is said to have four free parameters, but the parameters are not listed; please specify them.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: the new SEDs are validated against an independent Bootes AGN sample, and the Brown et al. (2014b) self-citation is a published galaxy library, not the load-bearing comparison.

full rationale

The paper's construction chain is: archival spectra plus matched-aperture photometry are multiplicatively scaled to form 41 AGN SEDs and 72 Seyfert composites (Section 3), and these templates are then tested by fitting photometry of 2058 Bootes AGNs with spectroscopic redshifts (Section 5). The test sample is external to template construction: no Bootes photometry enters the scaling, interpolation, or model fitting that defines the SEDs, so the reported sigma_NMAD = 0.096(1+z), reduced chi-squared near 1, and 9% flux residuals are not fitted inputs renamed as predictions. The Brown et al. (2014b) galaxy SEDs used for composites and in the combined photo-z library are a first-author citation, but they are an independent published data product, are not fitted to the Bootes targets, and the central comparison is against the external Ananna et al. (2017) library under identical EAzY settings. The authors' explicit caution that multiplicative scaling of individual input spectra is a crude approximation (Section 3.1) is a limitation affecting accuracy, not a circular step; the photo-z test is precisely the external check on that assumption. The template error function in Figure 14 is computed from residuals of the same fits and then applied in the 'semi-optimised' estimates, so the small improvement from sigma_NMAD = 0.12 to 0.095 is partly in-sample calibration, but the paper labels it semi-optimised and the comparative conclusions versus Ananna et al. are unaffected. No load-bearing derivation reduces to its own input.

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

The SED construction uses a number of fitted or hand-chosen parameters: per-spectrum multiplicative scalings, greybody and polynomial coefficients, toy-model UV values, and AGN-to-host ratios. These are all data-processing parameters rather than new physical constants, and the external photo-z test provides some check on the resulting template shapes. However, the absence of per-wavelength error bars on the released SEDs means users cannot propagate template uncertainty directly.

free parameters (5)
  • Multiplicative scalings of input spectra = Per-object, range about 0.33 to 3, with 7 objects outside this range
    Applied to each archival spectrum to produce continuous SEDs; values are fit by comparing overlapping flux densities and via visual inspection (Section 3.1).
  • Greybody model parameters (far-infrared) = Not tabulated; 4 free parameters per object
    Fit to up to six Herschel, WMAP, and Planck photometric points; fits are visually inspected (Section 2.3).
  • Polynomial coefficients for gap interpolation and radio SEDs = Not tabulated
    Fitted to log flux density versus log wavelength to fill gaps and to approximate radio spectra (Sections 2.3 and 3.1).
  • Toy-model UV parameters for obscured quasars = Not tabulated
    Constant f_lambda values and a Lyman-alpha step function are chosen by hand to extrapolate the UV where no data exist (Section 3.1).
  • AGN-to-host light ratios for Seyfert composites = 2:1 through 1:64
    Hand-picked ratios used to sum AGN and galaxy SEDs, producing 72 templates (Section 3.2).
assumptions (4)
  • domain assumption AGN SEDs can be represented as a sum of disk, torus, and host galaxy components over 0.09 to 30 micron.
    Underlies the combination of AGN and host SEDs and the interpretation of features like the 1 micron inflection and mid-IR silicate features (Introduction and Section 3.2).
  • domain assumption Milky Way foreground extinction follows the Fitzpatrick (1999) model with E(B-V) from Planck dust maps.
    Used to correct all photometry and spectra before combination (Section 2.4).
  • domain assumption EAzY single-template fitting with 10% flux uncertainty and zero-point offsets yields reliable photo-z posteriors.
    The validation of the SED library depends entirely on this fitting procedure (Section 5).
  • domain assumption The Bootes AGN sample selected by the Duncan et al. (2018a) criteria is representative of AGNs detected in wide-field surveys.
    The photo-z test's external validity rests on this representativeness (Section 5).

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Cite this review

Pith. "Pith review of The Spectral Energy Distributions of Active Galactic Nuclei." pith.science (2026). https://pith.science/paper/NNTBQFQC

@misc{pith2026190803720,
  author       = {Pith},
  title        = {Pith review of: The Spectral Energy Distributions of Active Galactic Nuclei},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/NNTBQFQC}},
  note         = {Machine review of arXiv:1908.03720}
}
read the original abstract

We present spectral energy distributions (SEDs) of 41 active galactic nuclei, derived from multiwavelength photometry and archival spectroscopy. All of the SEDs span at least 0.09 to 30 micron, but in some instances wavelength coverage extends into the X-ray, far-infrared and radio. For some AGNs we have fitted the measured far-infrared photometry with greybody models, while radio flux density measurements have been approximated by power-laws or polynomials. We have been able to fill some of the gaps in the spectral coverage using interpolation or extrapolation of simple models. In addition to the 41 individual AGN SEDs, we have produced 72 Seyfert SEDs by mixing SEDs of the central regions of Seyferts with galaxy SEDs. Relative to the literature, our templates have broader wavelength coverage and/or higher spectral resolution. We have tested the utility of our SEDs by using them to generate photometric redshifts for 0 < z < 6.12 AGNs in the Bootes field (selected with X-ray, IR and optical criteria) and, relative to SEDs from the literature, they produce comparable or better photometric redshifts with reduced flux density residuals.

Figures

Figures reproduced from arXiv: 1908.03720 by the authors.

Figure 1
Figure 1. Illustrative examples of quasar spectral energy distributions from the past two decades (Francis et al. 1991; Vanden Berk et al. 2001; Richards et al. 2006; Polletta et al. 2007; Assef et al. 2010; Lyu & Rieke 2017), along with our SED for PG 0052+251. These templates were created using different methods and with different purposes, so some caution is required directly comparing them. However, the trend towards impr… view at source ↗
Figure 2
Figure 2. The multiplicative scalings applied to the individual spectra used to produce the AGN SEDs. Variability and aperture bias results in the range of scalings increasing as one moves to shorter wavelengths. For half the sample the scalings never exceed the 0.5 to 2.0 range, but 7 AGNs have scalings that fall outside the 0.33 to 3.0 range (3C 390.3, Ark 120, Mrk 110, Mrk 509, NGC 5548, NGC 5728 and PKS 1345+12). rescalin… view at source ↗
Figure 3
Figure 3. A comparison of measured aperture photometry and photometry synthesised from the SEDs. The photometry includes more host galaxy light than the SEDs, resulting in systematic offsets at 1 µm, while variability results in increased scatter with decreasing wavelength. quasar SEDs show significant diversity at these wavelengths (e.g., Netzer et al. 2007; Lyu et al. 2017; Lyu & Rieke 2017), a point that we shall return to… view at source ↗
Figures from the paper (9 more)
Figure 4
Figure 4. Figure 4: The SEDs of the 41 individual AGNs sorted by restframe u − g colour, with the bluest templates at the top-left of the plot. UV and optical emission lines are immediately evident. The 9.7 µm silicate absorption is evident for some red AGN templates while 9.7 µm silicate…
Figure 5
Figure 5. Figure 5: The SEDs of AGNs that have radio coverage, normalised at 1.25 µm. For comparison we also plot the median radio-loud and radio-quiet SEDs of Elvis et al. (1994). Several features that are evident, including the change of spectral index for 3C 120, 3C 273 and PKS 1345+12…
Figure 6
Figure 6. Figure 6: The restframe optical colours of the AGN SED tem￾plates (including AGN and host combinations) along with the colours of galaxy SED templates from Brown et al. (2014b). Blue continuum and strong Hα emission offset templates down and to the right (respectively) from othe…
Figure 7
Figure 7. Figure 7: The restframe WISE mid-infrared colours of the AGN SED templates (including AGN and host combinations) along with the colours of galaxy SED templates from Brown et al. (2014b). The AGN templates are clustered in colour-space, and this is exploited by mid-infrared AGN s…
Figure 9
Figure 9. Figure 9: Observed WISE W1 − W2 colour as a function of red￾shift for the Ananna et al. (2017) AGN SED templates, the Brown et al. (2014b) galaxy templates and the AGN templates from this work. One clear difference between the mid-infrared colours of our templates and some of th…
Figure 10
Figure 10. Figure 10: Spectroscopic vs photometric redshift comparison for the Bo¨otes X-ray selected AGN sample at 0 < z < 4.5 using three different template sets. Unsurprisingly, Brown et al. (2014b) has the worst performance for AGN photometric redshifts as it (by con￾struction) exclude…
Figure 12
Figure 12. Figure 12: Photometric redshifts, determined with our SEDs, for AGNs that meet optical selection criteria, Spitzer infrared selec￾tion criteria, and when separated into blue and red apparent op￾tical colours. As in [PITH_FULL_IMAGE:figures/full_fig_p014_12.png]
Figure 14
Figure 14. Figure 14: The template error function for each set of template SEDs (solid coloured lines), calculated by subtracting in quadra￾ture the scaled average fractional error (black dotted line) from the median absolute rest-frame residuals (coloured symbols). ing simple models to in…
Figure 13
Figure 13. Figure 13: Rest-frame residuals for the template fits (with red￾shifts are fixed to the known spectroscopic redshift) using each of the three SED libraries employed in this work. Background grey points correspond to individual datapoints (i.e. one for each fitted filter per sour…

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    write newline

    " write newline "" before.all 'output.state := FUNCTION fin.entry write newline FUNCTION new.block output.state before.all = 'skip after.block 'output.state := if FUNCTION new.sentence output.state after.block = 'skip output.state before.all = 'skip after.sentence 'output.stat...

Pith tools

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