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The WISSH quasars project VI. Fraction and properties of BAL quasars in the hyper-luminosity regime

T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Hyperluminous quasars launch C IV broad absorption winds almost twice as often as ordinary quasars, and the winds are faster, indicating radiation-pressure acceleration.

desk verdict A credible first BAL census at Lbol > 1e47 erg/s: the 24% fraction versus ~13% for G09 is likely real in direction, but the comparison is imperfect and the exact size of the excess is not yet pinned down. read the letter →

arxiv 1908.09673 v1 pith:IAJV73F5 submitted 2019-08-26 astro-ph.GA

G. Bruni , E. Piconcelli , T. Misawa , L. Zappacosta , F. G. Saturni , G. Vietri , C. Vignali , A. Bongiorno
show 4 more authors
F. Duras C. Feruglio F. Tombesi F. Fiore
This is my paper · ORCID
classification astro-ph.GA
keywords broadabsorptionlinesBALquasarshyperluminousCIVoutflowsquasarwindsradiationpressuredrivingactivegalacticnucleifeedbackSDSSDR12spectra
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper sets out to establish that BAL winds are more common and more powerful in the hyperluminous regime ($L_{\rm bol}>10^{47}$ erg/s) than in the ordinary quasar population. Using SDSS DR12 spectra of the 86 WISSH quasars, it finds 21 with C IV $BI>0$: an observed fraction of $24\pm5\%$, against the 10\--15\% found in earlier catalogues, with SNR contributing only about 2\%. The paper also reports higher average $BI$ (about 4,000 km/s) and maximum velocities (about 17,000 km/s) than previous samples, plus two objects whose C IV absorption implies outflows at about 0.15c. If right, the result means quasar luminosity itself drives faster nuclear winds, most plausibly through radiation pressure, and that the fastest winds can deliver enough kinetic power to affect the host galaxy.

What carries the argument

The load-bearing objects are the wind-detection indices applied to SDSS spectra: the Balnicity Index ($BI$), the Absorption Index ($AI$), and the intermediate $AI_{1000}$, all integrals of the normalized residual flux $(1 - f(v)/0.9)$ over $0$\,--\,$25,000$ km s$^{-1}$ from the C IV peak, with increasingly strict requirements on contiguous trough widths (2,000 km/s for $BI$). The $BI>0$ criterion is the standard that makes the WISSH fraction comparable to earlier catalogues; the $AI$ variants catch weaker and narrower troughs, and the ultra-fast BAL (UBO) classification—C IV absorption blueward of the Si IV peak, implying $v_{\rm max}>0.1c$—identifies the relativistic tail.

What would settle it

Measure the C IV BAL fraction in a sample covering $L_{\rm bol}$ from $10^{46}$ to $10^{48}$ erg/s at $z\approx2$\,--\,4, selected without a luminosity cut and analysed with a blind automated BAL finder at uniform SNR; if the fraction stays near 15\% across the whole range once selection effects are controlled, the claimed luminosity-driven increase to 24\% would be refuted. A cheaper check is to degrade the 86 WISSH spectra to SNR of about 9 and re-run the classification: if most of the 21 $BI>0$ objects no longer qualify, the excess fraction would not be robust.

Watch

Extended reading notes

Core claim

The paper reports that among 86 WISSH quasars with $L_{\rm bol}>10^{47}$ erg/s, 21 satisfy the standard C IV Balnicity Index criterion ($BI>0$), an observed fraction of $24\pm5\%$—almost twice the 10\--15\% typical of previous optically selected samples—and that signal-to-noise differences contribute only about 2 percentage points of this excess. It further finds that WISSH BAL quasars have stronger absorption (mean $BI\sim4,000$ km/s) and a maximum-velocity distribution distinct from that of the comparison sample ($p<0.015$), with two objects showing C IV absorption at about 0.15c blueward of the Si IV peak. Finally, it estimates that the fastest, densest of these winds carry kinetic power above 0.1\% of $L_{\rm bol}$, enough for meaningful feedback onto the host galaxy.

Load-bearing premise

The 24\% fraction is intrinsic to the hyperluminous regime and not a byproduct of the WISSH sample selection or of inspecting higher signal-to-noise spectra more sensitively than earlier catalogues.

Editorial extensions

If this is right

  • BAL wind incidence is luminosity-dependent: at $L_{\rm bol}>10^{47}$ erg/s about 24\% of quasars show C IV broad absorption, versus roughly 10\--15\% in lower-luminosity samples.
  • Hyperluminous BAL winds are faster and stronger, with a maximum-velocity distribution that differs from the comparison sample at $p<0.015$ and two objects reaching about 0.15c.
  • The $L_{\rm bol}$\,--\,$v_{\rm max}$ relation places BAL winds between galaxy-scale molecular/ionized winds and X-ray ultrafast outflows, supporting radiation-pressure launching.
  • For the highest column densities, BAL kinetic power exceeds 0.1\% of $L_{\rm bol}$, enough to affect the host galaxy if coupling is efficient.
  • Radio properties of hyperluminous BAL quasars match lower-luminosity ones, so the stronger winds do not come with a different jet production rate.

Reading between the lines

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

  • A luminosity-unbiased BAL survey from $10^{46}$ to $10^{48}$ erg/s at $z\approx2$\,--\,4 could test whether the fraction rises smoothly with $L_{\rm bol}$ or switches on above about $10^{47}$ erg/s.
  • If radiation pressure is the driver, the same data imply a dependence on Eddington ratio rather than luminosity alone; splitting WISSH quasars by Eddington ratio would separate these two drivers.
  • The two 0.15c C IV outflows are natural candidates for the ultraviolet counterparts of X-ray ultrafast outflows; coordinated X-ray spectroscopy of these two objects would test whether one wind is seen in both bands.
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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

3 major / 5 minor

Summary. The paper searches for broad absorption line (BAL) quasars in the WISSH sample of 86 hyperluminous quasars (L_bol > 10^47 erg/s, 1.8 < z < 4.6) using SDSS DR12 spectra. The authors visually identify BAL candidates, fit continua with IRAF splines, and compute the AI, AI1000, and BI indices. They report 21/86 (24±5%) objects with C IV BI>0, which is roughly twice the literature fraction (10-15%), as well as larger mean BI and maximum velocities, and two candidate ultra-fast BAL outflows at ~0.15c. They interpret the excess as evidence that hyperluminous quasars accelerate BAL winds more efficiently, likely through radiation pressure, and estimate kinetic powers that could be relevant for feedback.

Significance. If the reported excess BAL fraction is intrinsic to the hyperluminous regime, it would be an important observational constraint on wind-driving mechanisms and on the luminosity dependence of quasar outflows. The paper uses standard BAL definitions and public SDSS spectra, and the appendix provides spectra of all 38 BAL candidates, which is a useful resource. The two UBO candidates are intriguing. However, the central comparison with lower-luminosity samples is not fully controlled: the analysis relies on a coarse SNR correction, a visual selection method that is not calibrated against the automated algorithms used for the comparison sample, and a parent sample whose selection could bias the BAL fraction. The claimed intrinsic luminosity dependence therefore requires additional verification.

major comments (3)
  1. [Sec. 3.2, Fig. 1] The SNR correction is applied to the mean SNR of the BAL-detected subsamples (9 vs 18 from the Shen et al. 2011 catalogue) rather than to the SNR distributions of the parent quasar samples from which the BAL fractions are measured. G09's SNR-BAL fraction relation is a property of the parent sample; to estimate the bias in the WISSH fraction, one must compare the full WISSH parent sample (86 objects) with the full G09 parent sample in matched SNR bins. As written, the 2% correction is not a valid estimate of the detection-efficiency bias, and the conclusion that SNR can account for only a small part of the excess (Sec. 5, second bullet) is not supported. A matched SNR-binned analysis, ideally applying the same automated BAL search to both samples, is needed.
  2. [Sec. 2 and Sec. 3.1] The WISSH BAL selection relies on visual inspection and IRAF spline continuum fits, whereas the G09 comparison sample is classified by an automated algorithm. These methods are not shown to be equivalent; the fact that two WISSH objects (0414+06 and 1210+17) were not found in previous automated catalogues demonstrates that the visual method is more permissive. Without an inter-comparison (e.g., running the visual method on a subsample of G09, or an automated code on WISSH), the observed fraction may be systematically inflated relative to the literature values, undermining the central claim of an intrinsic excess.
  3. [Sec. 3.1 and Sec. 2] The WISSH parent sample is selected using WISE and SDSS photometry, which may preferentially include reddened quasars, and reddened quasars are known to have a higher BAL fraction (as the paper itself notes from Dai et al. 2008). The paper does not test whether the WISSH color distribution or SEDs indicate such a bias. If present, the elevated BAL fraction could be a selection effect rather than a direct consequence of L_bol. The authors should compare the reddening/colors of WISSH to the G09 sample and, if necessary, restrict the comparison to matching color or extinction ranges.
minor comments (5)
  1. [Table 1] Individual BI, AI, vmin, and vmax values are reported without uncertainties; adding error bars would strengthen the statistical comparisons in Secs. 3.2 and 3.4.
  2. [Sec. 3.2] The KS test between the WISSH and G09 BI distributions is based on samples of very different sizes (21 vs 3874) and without accounting for measurement uncertainties; a two-sample test that incorporates errors or a non-parametric approach with proper weighting would be more appropriate.
  3. [Sec. 3.4] There is a typo in the text: 'beetwen' should be 'between'.
  4. [Fig. 5 and Sec. 3.4] The text and figure caption refer to the WISSH BAL region as 'purple' in one place and 'magenta' in another; the colors should be used consistently.
  5. [Sec. 3.3] The identification of the two UBOs as C IV absorption at ~0.15c is based on the absence of strong absorption between Si IV and C IV and on ionization arguments; supporting constraints from other transitions or photoionization modeling would make this more robust, though the authors are appropriately cautious in their wording.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the BAL fraction is an independent measurement using standard external indices and comparison catalogues.

full rationale

This paper is an observational census, not a derivation chain. The headline quantity, the 24±5% observed fraction of C IV BI>0 BAL quasars in WISSH, is computed directly from SDSS DR12 spectra using the Weymann et al. (1991) BI and Hall et al. (2002) AI definitions, and the comparison fractions are taken from external catalogues (Hewett & Foltz 2003; Reichard et al. 2003; Trump et al. 2006; Knigge et al. 2008; Gibson et al. 2009; Allen et al. 2011). The only index originating in the authors' own prior work, AI1000 (Bruni et al. 2012), is a transparent variant used for classification and is not the basis of the headline BI>0 fraction. The WISSH sample and its bolometric luminosities are inputs adopted from earlier collaboration papers, not outputs claimed to be derived here, and the BAL classification itself is performed on public spectra. The SNR correction (about 2%) is adopted from Gibson et al. (2009)'s external relation rather than fitted to the target quantity, and the UBO and kinetic-power estimates are explicitly presented as tentative and assumption-dependent. No equation in the paper reduces the claimed fraction, BI distribution, or velocity distribution to an input by construction, so there is no circularity to flag.

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

The paper contributes measurement rather than derivation. The central claim of a higher BAL fraction depends on the reliability of the standard indices, the representativeness of the WISSH sample, and the UBO identification; the feedback estimate depends on adopted column density, covering factor, and distance assumptions. No new physical entities are introduced.

assumptions (4)
  • domain assumption The standard BAL indices (BI, AI, AI1000) defined by Weymann et al. (1991) and Hall et al. (2002) correctly classify BAL quasars, with the same thresholds applied to WISSH and comparison samples.
    The central fraction comparison rests on the equivalence of these indices across samples; any systematic difference in spectral coverage or continuum fitting could bias the comparison (Sec. 2, Eqs. 1-2).
  • domain assumption For the two UBO candidates, absorption blueward of Si IV is C IV absorption at ~0.15c, based on the ionization argument that C IV dominates over Si IV optical depth for Log(U)>-2 (Dunn et al. 2012).
    This identification underpins the 0.15c velocity claim; no other species or blending is considered, and the authors acknowledge the statistics are very limited (Sec. 3.3).
  • domain assumption The kinetic power estimate assumes a spherical shell geometry, a covering factor Q=0.28, a column density NH=2.2e21 cm^-2 (log mean of literature values), and distances between 1 pc and 1 kpc for the BAL gas.
    These values are adopted from the literature, not measured in this paper; the feedback conclusion depends on them (Sec. 3.4, Eq. 3).
  • domain assumption The WISSH sample of 86 WISE/SDSS-selected quasars is representative of the hyperluminous regime and free of selection effects that would inflate the BAL detection rate beyond the ~2% SNR effect estimated from Gibson et al. (2009).
    The higher BAL fraction is interpreted as intrinsic; the authors test the SNR dependence but not other potential selection biases from the parent sample construction (Sec. 3.1-3.2).

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Pith. "Pith review of The WISSH quasars project VI. Fraction and properties of BAL quasars in the hyper-luminosity regime." pith.science (2026). https://pith.science/paper/IAJV73F5

@misc{pith2026190809673,
  author       = {Pith},
  title        = {Pith review of: The WISSH quasars project VI. Fraction and properties of BAL quasars in the hyper-luminosity regime},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/IAJV73F5}},
  note         = {Machine review of arXiv:1908.09673}
}
abstract

The WISSH quasars project aims at studying the nuclear and host galaxy properties of the most luminous quasars ($L_{bol}>10^{47}$ erg/s, $1.8<z<4.6$). Nuclear winds are manifested as UV broad ($\geq$ 2,000 km/s) absorption lines (BAL) in $\sim$ 15\% of quasars. We aim at studying the incidence and properties of such winds in the WISSH sample, to investigate possible differences with respect to lower luminosity AGN regimes. We collected optical spectra from the SDSS data release 12, and identified those showing absorption troughs in the region between \siiv and \civ emission lines. We find a higher observed fraction of \civ BAL quasars in the WISSH sample (24\%) with respect to previous catalogues (10-15\%). These WISSH BAL quasars are also characterized by a larger average BI ($\sim$4,000 km/s) and maximum velocity ($\sim$17,000 km/s). Moreover, for two objects we discovered BAL features bluewards of the \siiv peak, which can be associated to \civ absorption with velocity of 0.15c. Finally, we updated previous studies on the dependence of maximum outflow velocity upon bolometric luminosity, showing that BAL winds have intermediate properties compared to molecular/ionized winds and ultra fast outflows (UFOs). Finally, the radio properties of the WISSH BAL quasars as a whole are in line with those of samples at lower luminosities from previous studies. Our results suggest that the higher $L_{\rm bol}$ of the WISSH quasars likely favours the acceleration of BAL outflows and that their most likely driving mechanism is radiation pressure. Furthermore we estimate that the kinetic power associated to these winds in hyperluminous quasars is sufficient, for the highest column density and fastest winds, to provide efficient feedback onto the host galaxy.

Figures

Figures reproduced from arXiv: 1908.09673 by the authors.

Figure 1
Figure 1. Top panel: bolometric luminosity normalized distribu [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Zoom on the C iv and Si iv regions for 0947+14 (SDSS DR12 spectrum), one of the two objects showing ultra-fast (>0.1c) BAL signatures associated to Civ absorption, blue￾wards of the Si iv emission line. Top panel: spectrum (blue line) and fit performed in IRAF; dashed lines indicate the position of the Si iv and Civ peaks as calculated from redshift. Bottom panel: residuals between the Civ peak and −50,000 km/s, wit… view at source ↗
Figure 4
Figure 4. Bolometric luminosities versus maximum wind velocity [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figures from the paper (3 more)
Figure 3
Figure 3. Figure 3: Comparison between BI>0 BAL QSOs vmin and vmax in WISSH and G09 BAL QSOs catalogue. conclusions on this trend and additional investigations based on larger samples of QSOs with Lbol > 1047 erg/s are necessary. 3.4. Velocity distribution and potential for feedback of BA…
Figure 5
Figure 5. Figure 5: Left panel: physical distance (dph) of the high velocity outflow component as a function of vmax for the same sources reported in [PITH_FULL_IMAGE:figures/full_fig_p007_5.png]
Figure 6
Figure 6. Figure 6: Normalized distributions of L1.4 GHz for all WISSH ob￾jects with FIRST radio counterpart (blue) and for BAL QSOs in WISSH with FIRST radio counterpart (orange). gas components), indicating that they are likely radiatively driven. – The maximum velocities of the Civ BAL…

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