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REVIEW 5 major objections 6 minor 107 references

AGN Feedback Efficiency of NAL Quasars

T0 review · 5 major / 6 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read Narrow absorption-line outflows in quasars can carry as much kinetic energy as broad absorption-line winds, and eight of the 11 systems studied exceed the 0.5% Eddington feedback threshold, with two exceeding it by more than an order of…

desk verdict First NAL outflow efficiency estimates, but the feedback claim rests on unverified intrinsicness and extreme outputs that strain physical plausibility. read the letter →

arxiv 2507.23328 v1 pith:42T72XVB submitted 2025-07-31 astro-ph.GA

Toru Misawa , Jane C. Charlton , Michael Eracleous This is my paper · ORCID
classification astro-ph.GA
keywords quasarabsorptionlinesnarrowAGNfeedbackoutflowspartialcoveragekineticluminosityexcited-stateEddingtonratio
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 asks whether narrow absorption lines (NALs, gas signatures narrower than 500 km/s) in quasar spectra, the most common absorption-line outflow class, carry enough kinetic energy to count as AGN feedback. Using 11 NAL systems in eight optically luminous quasars selected for "partial coverage," a signature thought to mark gas tied to the quasar, the authors place upper limits on electron density from the absence of excited-state C II and Si II lines and convert those into lower limits on distance, mass outflow rate, kinetic luminosity, and feedback efficiency. Eight of the 11 systems exceed the 0.5% Eddington-luminosity feedback threshold, and two reach lower limits of $\varepsilon_k > 7.8$ and $> 22$. The authors' own conclusion is conditional: if the NALs are truly intrinsic, their feedback efficiency is comparable to or larger than that of broad absorption line outflows, but the paper also states that the connection of these distant absorbers to the quasar central engine remains open.

What carries the argument

The load-bearing object is the excited-to-ground-state column density ratio of C II and Si II, which acts as an electron-density probe: because no excited-state lines are detected, each system yields only an upper limit on $n_e$ through the collisional-excitation relation of the paper's equation (3). That upper limit enters the ionization-parameter definition $U = Q(H)/(4\pi R^2 n_H c)$ with an assumed $\log U = -2.6$, turning the density limit into a lower limit on the radial distance $R$ (about 100 kpc to 4 Mpc). Distance, total hydrogen column density, and measured ejection velocity then feed the standard outflow formulas of equations (4) and (5), $\dot M = 4\pi R f_c \mu m_p N_H v_{\rm ej}$ and $\dot E_k = \frac{1}{2}\dot M v_{\rm ej}^2$, with a global covering fraction $f_c = 0.5$, and the result is divided by the Eddington luminosity to get $\varepsilon_k$. All derived quantities are lower limits, because the distances are lower limits and the outflow may be instantaneous rather than continuous.

What would settle it

Detect the excited-state lines C II* 1336 or Si II* 1265/1533 at the absorption redshift of any of these systems; a detection would replace an electron-density upper limit with a measured density, shorten the derived distance $R$, and lower the quoted feedback efficiencies. Conversely, showing that any selected system is an intervening (sub-)DLA cloud, for instance by detecting a foreground galaxy along the line of sight at $z_{\rm abs}$ with no association to the quasar, would remove that system's support for the feedback claim.

Watch

Extended reading notes

Core claim

The central claim is that intrinsic narrow absorption line outflows can be energetically dominant feedback agents, not a minor byproduct of quasar winds. From single-epoch spectra of 11 systems, the paper derives lower limits on the kinetic luminosity, $\log(\dot E_{\rm k}/{\rm erg~s}^{-1}) > 42.9$ to $49.8$, and feedback efficiency $\varepsilon_k = \dot E_{\rm k}/L_{\rm Edd}$ that are comparable to or larger than those measured for BAL, high-ionization S IV, and EUV500 outflows. The physical picture that emerges is of low-density gas ($n_e < 0.2$ to $18~{\rm cm}^{-3}$) located hundreds of kiloparsecs from the nucleus, meaning the energy is deposited in the circumgalactic medium rather than in the immediate vicinity of the black hole. The authors state plainly that this conclusion holds only if the selected NALs are genuinely intrinsic, and that the large distances raise an open question about how these absorbers connect to the quasar-driven outflow.

Load-bearing premise

The argument assumes both that the 11 selected absorption systems are truly gas ejected by the quasar rather than small dense clouds in unrelated foreground galaxies, and that one adopted ionization parameter (log U = -2.6) describes all of them; if either premise fails, the derived distances, outflow rates, and efficiencies no longer describe quasar-driven winds.

Editorial extensions

If this is right

  • NAL outflows, which are present in roughly half of quasars, must be included in AGN feedback budgets; ignoring them would undercount the kinetic energy available to heat or expel gas.
  • Eight of the 11 systems have lower limits on $\varepsilon_k$ above 0.5% of $L_{\rm Edd}$, so if the selection is right these outflows alone meet the threshold thought to affect host-galaxy star formation.
  • The implied distances (hundreds of kpc to roughly 4 Mpc) mean most of the outflow energy is delivered to the circumgalactic medium, not to the inner kiloparsecs, which changes where and when feedback acts.
  • Because the method needs only a single epoch of spectra, it can measure feedback efficiency for stable NAL systems that time-variability studies cannot probe.
  • Two systems have formal lower limits $\varepsilon_k > 7.8$ and $> 22$, implying kinetic luminosities above the Eddington luminosity when the lower limits are treated as actual values.

Reading between the lines

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

  • If partial-coverage NALs really sit at hundreds of kiloparsecs, a direct test of the feedback interpretation is to search for the transverse proximity effect: gas near these quasars' sightlines should be over-ionized compared with the general intergalactic medium at the same redshift.
  • The extreme $\varepsilon_k > 7.8$ and $\varepsilon_k > 22$ limits cannot be steady-state efficiencies; taken literally they demand that either the assumed global covering fraction $f_c = 0.5$ is too large, the single ionization parameter is wrong for those systems, or the Eddington luminosities are underestimated.
  • A detection of C II* or Si II* excited lines in any of these systems, for example with higher signal-to-noise ultraviolet spectra, would convert an upper limit into a density measurement, shrink $R$, and lower the feedback efficiencies, providing a concrete way to sharpen or overturn the claim.
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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

5 major / 6 minor

Summary. The paper selects 11 narrow absorption line (NAL) systems in 8 luminous quasars from Misawa et al. (2007a), based on partial coverage, the presence of low-ionization C II or Si II lines, and coverage of Lyα. Using upper limits on n_e from undetected excited-state lines of C II and Si II, the authors derive lower limits on the absorber distance R, the total hydrogen column N_H (via a Cloudy model at log U = -2.6), and then, following Borguet et al. (2012), lower limits on the mass outflow rate, kinetic luminosity, and feedback efficiency ε_k = Ė_k/L_Edd. Eight of the 11 systems have ε_k > 0.005, with two extreme systems reaching >7.8 and >22, leading the authors to conclude that NAL outflows may be a significant AGN feedback channel. The paper is explicit that the results depend critically on the unverified identification of the absorbers as intrinsic.

Significance. If the central claim holds, the paper would establish that NAL outflows, the most common absorption-line outflow class, can have feedback efficiencies rivaling or exceeding BALs, with important implications for AGN feedback. The work is methodologically useful: it applies the excited-state/resonance-line diagnostic to NALs for the first time, the arithmetic of Eqs. (4)-(6) reproduces the tabulated values given the stated assumptions, and the authors are commendably transparent about the selection caveats. However, the conclusion is conditional on two fragile premises: the intrinsic nature of the absorbers and the single adopted ionization parameter. The small sample and the non-independence of multiple systems from the same quasars further limit the statistical weight of the '8 of 11' headline. The paper is better framed as a method demonstration with conditional limits than as an established measurement of NAL feedback efficiency.

major comments (5)
  1. [§3; Tables 2–3] The headline result that eight of eleven systems exceed ε_k > 0.005 depends entirely on the eleven absorbers being intrinsic, but the only selection test is partial coverage, and §3 explicitly concedes (caveats 1 and 2) that compact low-ionization clouds in (sub-)DLA systems can show partial coverage without any quasar connection and that no independent corroborating test is available. Because Eq. (2) uses the target quasar's Q(H) and the geometric R from that equation enters Eqs. (4)–(6), any intervening contaminant makes Ṁ, Ė_k, and ε_k unphysical; a lower limit on Ė_k for foreground gas is not a conservative bound on AGN feedback. The authors should estimate the expected contamination fraction from the literature or at least show how many of the eleven systems would need to be intervening to eliminate the super-0.5% conclusion, and they should carry that contingency through the abstract and §6 conclusions.
  2. [§4, after Eq. (2)] The ionization parameter log U = -2.6 is not independently constrained: it is chosen because it reproduces the sample-averaged N(CII)/N(CIV) ≈ 0.3, and it is then used both to convert N_HI to N_H with Cloudy and to compute R from Eq. (2). Since Ė_k and ε_k scale directly with N_H and R, the derived efficiencies are contingent on this single-phase, single-U assumption rather than measured. The authors should present a sensitivity test over a plausible range of log U (e.g., -3.5 to -1.5) and discuss the single-phase assumption, quantifying how many of the eight super-threshold systems survive.
  3. [§5, Table 3] Two of the headline systems (Q1548+0917 at z_abs = 2.6082 and HS1700+6416 at z_abs = 2.4330) have ε_k lower limits of 7.8 and 22, implying Ė_k of 4.6 and 17.7 times L_Edd respectively (and even larger multiples of L_bol). This is energetically challenging for radiatively driven quasar outflows and suggests that the assumed N_H (from log U = -2.6), f_c = 0.5, or R lower limits may be mutually inconsistent. The paper should address this tension explicitly and identify which assumption drives the extreme values before using these systems to argue that NALs rival BALs.
  4. [§5, Figure 4] The comparison with BAL, SIV, and EUV500 samples mixes lower limits computed with the assumptions of this paper (f_c = 0.5, log U = -2.6, single-phase) with upper limits and measurements from other work that use different SEDs, covering fractions, and fitting methods. The statement that NALs have 'exceptionally large efficiency compared to the other outflow classes' could be an artifact of these methodological differences; a common-metric comparison or a matched-treatment reanalysis is needed to support it.
  5. [§5, below Eq. (5)] The global covering fraction is set to f_c = 0.5 because intrinsic NALs are found in at least 50% of quasars, but the detection rate of NAL absorbers is not the same as the solid-angle covering fraction of a single outflow; since Ṁ, Ė_k, and ε_k are all proportional to f_c, the choice directly sets the normalization of the feedback efficiencies. The authors should vary f_c over a plausible range (e.g., 0.1–1.0) and report which of the eight systems remain above ε_k = 0.005.
minor comments (6)
  1. [Abstract and §6] The notation 'log(Ṁ/M⊙ s⁻¹) > 79–(3.1×10⁵)' (and the similar expression in §6) is malformed and should be replaced with the actual range of log values (approximately 1.9–5.5 in M⊙ yr⁻¹) or a properly formatted range.
  2. [§3, caveat paragraph] The sentence 'Nonetheless, we do not that, (a) photoionization models ...' contains a typo and should read 'we do note that'.
  3. [§6, summary bullet] The bullet 'ε_k ≳ 0–22' is imprecise; it should read something like 'ε_k > 0.005 to >22' to match Table 3.
  4. [§4 and Figure 2] The text says equation (3) uses critical densities from Tayal (2008a,b), while Figure 2 is described as calculated with the CHIANTI 10.0 database; please clarify which calculation is used for the reported density limits.
  5. [Tables 2–3] Multiple systems from the same quasar (three from Q1548+0917 and two from HS1946+7658) are treated as independent in the '8 of 11' counting; the paper should either report quasar-level statistics or explicitly discuss the clustering.
  6. [Affiliations] The affiliation 'Institute for Gavitation and the Cosmos' should be 'Institute for Gravitation and the Cosmos'.

Circularity Check

0 steps flagged · score 0.0 of 10

No circular derivation: the feedback efficiencies follow from a standard photoionization plus density-diagnostic chain, with the sample-calibrated ionization parameter as an intermediate model input rather than a renamed output, and the acknowledged caveats are correctness risks rather than circular steps.

full rationale

The derivation chain is linear and self-contained: the measured N(CII)/N(CIV) ratio of approximately 0.3 is used to adopt log U = -2.6, with an independent Cloudy check reported in footnote 5, and then Cloudy at that U converts the measured N_HI to N_H, while Eq. (2) combines U with the n_e upper limits (from non-detection of CII* and SiII*) to give R. Equations (4) and (5) then combine N_H, R, and the directly measured v_ej to yield Mdot and E_k, and Eq. (6) normalizes by L_Edd. The adopted U is not the quantity being predicted; it is an intermediate physical parameter. The large epsilon_k values are driven chiefly by the low n_e upper limits and the high v_ej and N_HI measurements, not by the U calibration, and the same chain produces sub-threshold efficiencies for three systems, showing that the output is not forced. The paper's own caveats, namely that the intrinsic classification rests only on partial coverage and cannot be corroborated by an independent test (Section 3) and that f_c = 0.5 is uncertain, are limitations that affect the physical interpretation but are not circular because the equations do not assume the conclusion. The comparison with BALs and other outflow classes uses independent literature data. No equation in the paper reduces an output to an input by construction, and no load-bearing claim rests solely on an unverified self-citation.

Assumptions & free parameters 4 free parameters · 6 assumptions · 0 invented entities

The headline efficiencies rest on four hand-adopted parameters and six domain assumptions. The two that move the result most are log U = -2.6 (it sets both N_H via Cloudy and R via equation (2), and both enter Ė_k linearly) and f_c = 0.5 (it scales all three headline quantities linearly). The least secure assumption is the intrinsic association of the absorbers with the quasar, which the authors themselves flag as uncorroborated (§3). No new entities are invented, but the shell-outflow geometry is a strong modeling assumption for gas observed as small, partially covering clouds.

free parameters (4)
  • Ionization parameter log U = -2.6
    Adopted in §4 because it reproduces the sample's average N(CII)/N(CIV) ~ 0.3. Used to convert N_HI to N_H with Cloudy and to derive R from equation (2); both enter Ė_k and ε_k linearly. Not measured per system.
  • Global covering fraction f_c = 0.5
    Set in §5 from the ~50% intrinsic NAL detection rate of Misawa et al. (2007a). Ṁ, Ė_k, and ε_k scale linearly with f_c; the paper acknowledges this value is uncertain.
  • Gas temperature T = 10^4 K
    Assumed for the CHIANTI-based n_e limits in Figure 2 and equation (3); typical for NAL systems but not measured. Affects the n_e upper limits and hence the R lower limits.
  • Electron-to-hydrogen density ratio n_e/n_H = 1.2
    Assumed relation for highly ionized plasma (Osterbrock and Ferland 2006), used in equation (2) to convert the n_e limits into the hydrogen densities that set R.
assumptions (6)
  • domain assumption The 11 NAL systems selected via partial coverage are intrinsic (quasar-origin) absorbers, so the quasar's ionizing flux illuminates them.
    Load-bearing premise. §3 caveats 1 to 3 state that partial coverage cannot be independently corroborated and could be mimicked by compact clouds in intervening systems.
  • domain assumption A single-phase absorber with log U = -2.6 describes all systems, with C II and C IV tracing the same gas.
    §4 footnote 4; used for the Cloudy N_HI-to-N_H conversion and for equation (2).
  • domain assumption The Borguet et al. (2012) shell-outflow geometry applies: spherically symmetric, continuous outflow with global covering fraction f_c, mass rate Ṁ = 4πR f_c μ m_p N_H v_ej.
    Equations (4) and (5). NALs are observed as small, partially covering clouds; applying a smooth-shell formula over hundreds of kpc is what produces the large Ṁ and Ė_k values.
  • domain assumption The Narayanan et al. (2004) segmented power-law SED converts L_bol to Q(H) for these quasars.
    §4, used in equation (2) to compute ionizing photon rates.
  • domain assumption Cloudy photoionization equilibrium at log U = -2.6 correctly maps neutral hydrogen column density to total hydrogen column density.
    §4, converts N_HI (Table 2) to N_H (Table 3); the conversion amplifies column density by 0.7 to 2.6 dex and directly inflates Ė_k.
  • domain assumption The feedback thresholds of Hopkins and Elvis (2010) and Scannapieco and Oh (2004) apply to energy deposited at hundreds of kpc from the nucleus.
    §1 and §5; the standard thresholds were derived for nuclear-scale winds, and whether CGM-scale deposition couples to the host galaxy is the open question the paper itself flags.

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Cite this review

Pith. "Pith review of AGN Feedback Efficiency of NAL Quasars." pith.science (2026). https://pith.science/paper/42T72XVB

@misc{pith2026250723328,
  author       = {Pith},
  title        = {Pith review of: AGN Feedback Efficiency of NAL Quasars},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/42T72XVB}},
  note         = {Machine review of arXiv:2507.23328}
}
abstract

We consider if outflowing winds that are detected via narrow absorption lines (NALs) with FWHM of $<$ 500 km/s (i.e., NAL outflows) in quasar spectra contribute to feedback. As our sample, we choose 11 NAL systems in eight optically luminous quasars from the NAL survey of Misawa et al. (2007a), based on the following selection criteria: i) they exhibit ``partial coverage'' suggesting quasar origin (i.e., intrinsic NALs), ii) they have at least one low-ionization absorption line (C II and/or Si II), and iii) the Ly$\alpha$ absorption line is covered by available spectra. The results depend critically on this selection method, which has caveats and uncertainties associated with it, as we discuss in a dedicated section of the paper. Using the column density ratio of the excited and ground states of C II and Si II, we place upper limits on the electron density as $n_{\rm e}$ $<$ 0.2 - 18 cm$^{-3}$ and lower limits on their radial distance from the flux source $R$ as greater than several hundreds of kpc. We also calculate lower limits on the mass outflow rate and kinetic luminosity of $\log(\dot{M}/{\rm M_{\odot}~s}^{-1}) > 79$ - (3.1$\times 10^{5})$ and $\log(\dot{E_{\rm k}}/{\rm erg~s}^{-1}) > 42.9$ - 49.8, respectively. Taking the NAL selection and these results at face value, the inferred feedback efficiency can be comparable to or even larger than those of broad absorption line and other outflow classes, and large enough to generate significant AGN feedback. However, the question of the connection of quasar-driven outflows to NAL absorbers at large distances from the central engine remains open and should be addressed by future theoretical work.

Figures

Figures reproduced from arXiv: 2507.23328 by the authors.

Figure 1
Figure 1. Estimated column densities of the ground and excited states (Nl and Nu, respectively) of the C II line at zabs = 2.6517 in the spectrum of Q0805+0441 (top panel) and their corresponding electron density (bottom panel) as an example demonstrating the lack of sensitivity of ne to Cf. The C IV line of the system has covering factor of Cf = 0.43 (marked with a vertical dotted line), but the C II line could have differen… view at source ↗
Figure 2
Figure 2. Column density ratios between excited and ground states of C II and Si II (red and blue curves) as a function of electron density, which is calculated with the CHIANTI 10.0 Database (Dere et al. 1997; Del Zanna et al. 2021) assuming a gas temperature of 10,000 K. Arrows denote upper limits on the column density ratio of C II∗ to C II and Si II∗ to Si II and the corresponding upper limits on electron density [PITH_F… view at source ↗
Figure 3
Figure 3. Normalized flux spectrum (solid histogram) and error spectrum (dotted histogram) around the absorption lines of C II 1335 and C II∗ 1336a,b at zabs = 2.6517 in Q0805+0441. The former is detected at a 3σ level, while the latter is not detected. The blue solid curve is a fitted model to C II 1335. The red dashed curve is a 1-σ upper limit on C II∗ 1336a,b. The strong absorption profile in the gray shaded area is an un… view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Kinetic luminosity (E˙ k) as a function of the Eddington luminosity (LEdd) for the 11 NAL systems of this study (red), along with nine EUV500 (blue) and seven S IV (green) systems from Miller et al. (2020). Filled black squares denote BALs from the literature (Fiore et…

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