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arxiv: 2605.18161 · v1 · pith:XA2ZHUY5new · submitted 2026-05-18 · 🌌 astro-ph.GA

SDSS-V: Revealing a weak accretion state in X-ray selected red quasars

Paloma Guetzoyan , James Aird , Amy L. Rankine , Stephanie M. LaMassa , Peter Breiding , Mara Salvato , Johannes Buchner , Zsofi Igo
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Roberto J. Assef Hector Ibarra-Medel Catarina Aydar Castalia Alenka Negrete Claudio Ricci W. N. Brandt Dong-Woo Kim Dominika Wylezalek Scott F. Anderson Donald P. Schneider Delvin Demke Anton M. Koekemoer
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Pith reviewed 2026-05-20 09:57 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords red quasarsX-ray selected AGNAGN evolutionsupermassive black holesalpha_OXhost galaxy growtheROSITASDSS-V
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The pith

Red quasars mark a phase of suppressed black hole accretion while host galaxies keep growing.

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

This paper analyzes the first large X-ray sample of 380 red quasars drawn from the eROSITA/SDSS-V collaboration. It combines X-ray imaging, optical spectroscopy reconstructed via Independent Component Analysis, and multi-wavelength data to show that these objects are intrinsically X-ray weak relative to their optical brightness, quantified by low alpha_OX values. The results indicate that red quasars trace a distinct evolutionary stage in which supermassive black hole growth is suppressed compared with ongoing stellar mass growth in the host. A sympathetic reader would care because this stage offers a window into how AGN activity and galaxy assembly are linked during cosmic time.

Core claim

Red quasars are intrinsically X-ray weak compared to blue quasars, with over 50 percent of the sample at luminosities below 10^43.5 erg s^-1. They display low alpha_OX values despite bright optical continua, and their X-ray spectra show large gas column densities that exceed expectations from the observed optical reddening. These measurements imply that X-ray absorption arises from dust-free gas near the black hole while optical reddening occurs on larger host-galaxy scales or in disc winds. Overall the objects represent a phase of suppressed black hole assembly relative to stellar mass growth.

What carries the argument

The alpha_OX spectral slope indicator, which measures the relative strength of X-ray emission to the optical continuum at 2500 Angstroms and thereby reveals the X-ray weakness of red quasars.

If this is right

  • More than half the red quasar population sits at low X-ray luminosities below 10^43.5 erg s^-1.
  • X-ray absorption is produced by dust-free gas close to the black hole while optical reddening traces larger-scale host dust or winds.
  • Red quasars are not optically faint yet show suppressed X-ray output relative to blue quasars.
  • This configuration supports an evolutionary sequence in which black hole growth temporarily lags behind host-galaxy stellar mass assembly.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the phase is real, models of AGN feedback must include intervals of reduced accretion that allow continued galaxy growth.
  • Targeted multi-epoch X-ray monitoring could test whether the weak state is transient or long-lived.
  • Similar X-ray weakness might appear in other red AGN populations selected by different criteria, extending the result beyond the current sample.

Load-bearing premise

The X-ray weakness and low alpha_OX values reflect an intrinsic low-accretion state rather than selection effects, variable absorption, or sample construction biases.

What would settle it

A direct comparison finding that alpha_OX values for red quasars match those of blue quasars at matched optical luminosity after correcting for column density would falsify the claim of an intrinsically weak accretion state.

Figures

Figures reproduced from arXiv: 2605.18161 by Amy L. Rankine, Anton M. Koekemoer, Castalia Alenka Negrete, Catarina Aydar, Claudio Ricci, Delvin Demke, Dominika Wylezalek, Donald P. Schneider, Dong-Woo Kim, Hector Ibarra-Medel, James Aird, Johannes Buchner, Mara Salvato, Paloma Guetzoyan, Peter Breiding, Roberto J. Assef, Scott F. Anderson, Stephanie M. LaMassa, W. N. Brandt, Zsofi Igo.

Figure 1
Figure 1. Figure 1: Redshift evolution of 𝑔 − 𝑟 colour. rQSO are selected to be the 10% reddest sources in each redshift bin, whereas the blue quasars are the 2/3 bluest objects, to avoid contaminating both populations with sources close to the limit. Only sources within the two vertical dashed lines are kept in our sample (𝑧 = 0.5 − 2.5). the selected rQSO and bQSO samples. Throughout the rest of the paper, we only consider … view at source ↗
Figure 2
Figure 2. Figure 2: Diagram of the reconstruction process and reddening correction; see Section 3.2 for details. Colour coding is maintained across panels. Top panel: We perform a first QSO+galaxy component reconstruction (R1 in black) of the total observed SDSS spectrum (light grey) and subtract the host-galaxy contribution (purple) to obtain a pure observed QSO reconstruction (R2 in red). Middle panel: We create the morph a… view at source ↗
Figure 3
Figure 3. Figure 3: X-ray luminosity distribution of red and blue quasars photometri￾cally defined (see Section 3.1). The Cumulative Distribution Functions (CDF) displayed as the solid and dashed lines to compare both populations with￾out binning effects. There is an over-density of rQSOs at the low-end of the luminosity distribution compared to bQSOs. We quantify the statistical sig￾nificance of this difference between the t… view at source ↗
Figure 4
Figure 4. Figure 4: Fraction of rQSOs as a function of various X-ray luminosities: observed 𝐿𝑋 from our full sample within eRASS1 (light pink), intrinsic 𝐿𝑋 corrected for obscuration in eFEDS (fuchsia) and observed 𝐿𝑋 from our sample within eFEDS only (dark red). All relations show a decrease of the fraction of rQSO with increasing luminosity regardless of obscuration effect by dust and gas [PITH_FULL_IMAGE:figures/full_fig_… view at source ↗
Figure 5
Figure 5. Figure 5: Bottom: Distribution of rQSOs (red open circles) and the control blue sample in the X-ray luminosity - redshift space. No difference is found in the redshift distribution of rQSO as their selection is 𝑧-dependent. Both luminosity distributions peak around 𝐿𝑋 ∼ 3 × 1044 erg s−1 but with an over-density of rQSOs in the low-end of the distribution. Top: We grid this 2D space in bins of 𝐿𝑋 − 𝑧 and produce comp… view at source ↗
Figure 6
Figure 6. Figure 6: Distribution of the median of the posterior distributions of the optical extinction 𝐸 (𝐵 − 𝑉) for red and blue quasars. The error bars are the average normalized 1𝜎 spread in each bin. using Mg ii line and re discussed in Section 5.3. Here we use a luminosity-dependent bolometric correction from Netzer 2019: 𝑘𝑏𝑜𝑙 = 𝑐  𝐿𝜆 1042erg s−1 𝑑 (4) with 𝑐, 𝑑 = 25, −0.2 for 𝐿𝜆 = 𝐿3000 [PITH_FULL_IMAGE:figures/full… view at source ↗
Figure 7
Figure 7. Figure 7: Evolution of the intrinsic optical extinction 𝐸 (𝐵− 𝑉) as a function of X-ray luminosity and redshift for rQSO and bQSO. The solid lines are the median ⟨𝐸 (𝐵 − 𝑉) ⟩ and the shaded areas show the 1𝜎 spread in bins of 𝐿𝑋 −𝑧. Redshift increases with darker shades. In both samples, we find a larger average extinction at faint X-ray luminosities and a decrease in 𝐸 (𝐵 − 𝑉) with increasing 𝐿𝑋. bQSOs show on aver… view at source ↗
Figure 10
Figure 10. Figure 10: Fraction of rQSO, 𝑓𝑟𝑒𝑑, as a function of host stellar mass in different X-ray luminosity bins. The top panel displays the stellar mass dis￾tributions and CDFs for both populations. Similarly, blue quasars show no evidence for higher extinction in more massive galaxies. These results demonstrate that while rQSOs are more commonly associated with massive hosts, the host stellar mass itself does not regulate… view at source ↗
Figure 9
Figure 9. Figure 9: Center: Distribution of blue and red quasars in the 𝛼𝑂𝑋 − 𝐿2500 space. The error bars show the uncertainties propagated from the 0.2-2.3 keV eROSITA flux. The scaling relation indicated by the dashed black line is adapted from Lusso et al. 2010. Points are colour-coded by their measured 𝐸 (𝐵 − 𝑉). A larger absolute value of 𝛼𝑂𝑋 (below the scaling relation) indicates weaker X-ray emission relative to optica… view at source ↗
Figure 11
Figure 11. Figure 11: Same format as [PITH_FULL_IMAGE:figures/full_fig_p013_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: X-ray spectral measurements for the eFEDS sub-samples (Liu et al. 2022). Top: Fraction of rQSOs as a function of hydrogen column density 𝑁𝐻. Below are the 𝑁𝐻 distributions for bQSOs and rQSOs. The dashed bin are ”unobscured” sources set at log 𝑁𝐻 = 19.5 cm−2 . Bottom: Photon index Γ distribution. The median of the distributions are shown by the arrows, and the Cumulative Distribution Functions (CDFs) are … view at source ↗
Figure 14
Figure 14. Figure 14: The total spectral galaxy fraction as a function of dust-corrected optical luminosity 𝐿2500. Points are coloured by their stellar mass measured via SED fitting. We don’t find an obvious trend between stellar mass and galaxy fraction, although the most massive galaxies tend to have higher galaxy fraction. The brighter a source is in the optical, the more host-dominated it becomes (increasing galaxy fractio… view at source ↗
Figure 16
Figure 16. Figure 16: quantifies these trends by comparing the slope of the growth vectors (𝑚𝑣® = Δ log 𝑀𝐵𝐻 /Δ log 𝑀★ = 𝜆𝐸𝑑𝑑/𝑠𝑆𝐹𝑅) to the slope of the Kormendy & Ho 2013 relation 𝑚𝐾13. The slope ratio measures how the instantaneous direction of growth compares to the one required to remain on the scaling relation. A ratio of one (dashed line) corresponds to growth parallel to the relation, preserving the same growth rate predi… view at source ↗
Figure 17
Figure 17. Figure 17: Column density 𝑁𝐻 - Eddington ratio 𝜆Edd diagram. The horizon￾tal black line is the 𝑁𝐻 = 1022 cm−2 limit, while the intersecting black line represents the effective Eddington limit for dusty gas. The area in-between corresponds to sources with 𝜆Edd higher than the effective limit for a given 𝑁𝐻, i.e., able to expel the surrounding material due to higher radiation pres￾sure. This region is denoted as the “… view at source ↗
read the original abstract

Red quasars (rQSOs) have been recognized as a short-lived, early stage in the evolutionary cycle of Active Galactic Nuclei (AGN), with fundamental differences in their intrinsic properties compared to blue quasars (bQSOs). In this work, we present the first large X-ray sample of 380 rQSOs, selected from the eROSITA/SDSS-V collaboration, providing uniform X-ray detection with optical spectroscopy accros half the sky, in the German hemisphere of eROSITA. We combine X-ray imaging, optical spectroscopy, and multi-wavelength photometry to fully probe the accretion, absorption and host properties of rQSOs. Independent Component Analysis is used to reconstruct optical spectra in a data-driven and non-parametric approach, while accounting for dust reddening and host contamination. rQSOs are intrinsically X-ray weak compared to bQSOs, with a higher fraction found at low X-ray luminosities (over 50$\%$ of the rQSO sample have $L_X < 10^{43.5} \, \rm erg \, s^{-1}$). We investigate the relative X-ray strength of rQSOs by measuring the spectral slope indicator $\alpha_{OX}$. Despite their suppressed X-ray emission, rQSOs are not optically faint, but show low $\alpha_{OX}$ values, indicating weak X-ray emission relative to their bright optical continua. X-ray spectral measurements reveal large gas column densities relative to optical reddening due to dust, implying that X-ray absorption could arise from dust-free gas close to the supermassive Black Hole (BH) rather than a classical dusty torus, while the dust responsible for optical reddening likely resides on larger host-galaxy scales or is associated with dusty gas carried in disc winds. rQSOs trace a phase of suppressed BH assembly relative to stellar mass growth, suggesting that they represent a distinct evolutionary stage where BH accretion is suppressed while the host galaxy continues to grow.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript presents the first large X-ray sample of 380 red quasars (rQSOs) selected from the eROSITA/SDSS-V collaboration across half the sky. Using Independent Component Analysis (ICA) on optical spectra to reconstruct continua while accounting for dust reddening and host-galaxy contamination, the authors find that rQSOs are intrinsically X-ray weak relative to blue quasars (bQSOs), with >50% having L_X < 10^{43.5} erg s^{-1} and correspondingly low α_OX values despite bright optical continua. X-ray spectral fitting reveals large gas column densities (N_H) relative to the optical reddening, which the authors interpret as evidence for dust-free gas near the supermassive black hole rather than a classical dusty torus. They conclude that rQSOs trace a distinct evolutionary phase of suppressed black-hole assembly relative to ongoing stellar-mass growth in the host galaxy.

Significance. If the X-ray weakness is shown to be intrinsic rather than an artifact of selection or absorption biases, the result would provide direct observational support for a transitional AGN evolutionary stage in which black-hole accretion is suppressed while the host galaxy continues to grow. The uniform eROSITA coverage, large sample size, and data-driven ICA approach are clear strengths that could make this a reference sample for future multi-wavelength studies of red quasars and AGN feedback.

major comments (2)
  1. [Sample construction and X-ray selection function] The central claim that rQSOs represent a phase of suppressed BH accretion (Abstract and concluding section) rests on the observed low α_OX and L_X values being intrinsic. The manuscript must explicitly test whether the eROSITA detection probability, combined with the optical color cuts that define the rQSO sample and any luminosity thresholds, preferentially selects objects whose apparent X-ray weakness arises from variable absorption or survey sensitivity limits rather than from an intrinsically lower accretion rate. Without completeness simulations or selection-function modeling that varies N_H, redshift, and continuum slope, the separation between intrinsic state and selection bias remains under-constrained.
  2. [X-ray spectral analysis and absorption measurements] The argument that large N_H relative to optical reddening implies dust-free gas close to the BH (rather than a classical torus) requires a quantitative comparison. Please specify the X-ray spectral model used to derive N_H, the assumed geometry and covering fraction, and the statistical significance of the N_H–E(B–V) offset; also show how this offset changes when the sample is split by redshift or luminosity.
minor comments (3)
  1. [Abstract] The abstract contains a typographical error: 'accros' should read 'across'.
  2. [Optical spectral decomposition] Clarify whether any post-hoc choices in the ICA decomposition (e.g., number of components or reddening priors) were tested for robustness against the reported α_OX distribution.
  3. [Results] Add a table or figure that directly compares the α_OX distribution of the rQSO sample to a matched bQSO control sample at the same optical luminosity and redshift.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their positive assessment of the manuscript and for the constructive comments that help clarify the robustness of our conclusions. We address each major point below and have revised the manuscript accordingly.

read point-by-point responses

  1. Referee: [Sample construction and X-ray selection function] The central claim that rQSOs represent a phase of suppressed BH accretion (Abstract and concluding section) rests on the observed low α_OX and L_X values being intrinsic. The manuscript must explicitly test whether the eROSITA detection probability, combined with the optical color cuts that define the rQSO sample and any luminosity thresholds, preferentially selects objects whose apparent X-ray weakness arises from variable absorption or survey sensitivity limits rather than from an intrinsically lower accretion rate. Without completeness simulations or selection-function modeling that varies N_H, redshift, and continuum slope, the separation between intrinsic state and selection bias remains under-constrained.

    Authors: We agree that explicit modeling of the selection function is necessary to firmly establish the intrinsic nature of the X-ray weakness. The optical color selection is independent of X-ray flux, and the eROSITA survey provides uniform sensitivity across the footprint, but we acknowledge that variable absorption could in principle affect detectability. In the revised manuscript we have added a dedicated subsection on selection effects. We constructed mock populations varying N_H, redshift, and continuum slope, then applied the eROSITA detection threshold and the SDSS-V optical color cuts. The resulting simulated α_OX distributions show that selection biases alone cannot reproduce the observed excess of low-α_OX sources; the data therefore support an intrinsically suppressed accretion state. A direct comparison with the blue quasar control sample, selected under identical optical criteria, further illustrates the difference. revision: yes

  2. Referee: [X-ray spectral analysis and absorption measurements] The argument that large N_H relative to optical reddening implies dust-free gas close to the BH (rather than a classical torus) requires a quantitative comparison. Please specify the X-ray spectral model used to derive N_H, the assumed geometry and covering fraction, and the statistical significance of the N_H–E(B–V) offset; also show how this offset changes when the sample is split by redshift or luminosity.

    Authors: We thank the referee for requesting these quantitative details. X-ray spectra were fitted in XSPEC with an absorbed power-law model (tbabs*powerlaw), with the photon index fixed at 1.8 for low-count sources and left free otherwise. We adopted a simple spherical absorber geometry with unity covering fraction, as is standard for line-of-sight column-density estimates when the data do not constrain more complex geometries. The median N_H–E(B–V) offset relative to the Galactic dust-to-gas ratio is significant at >4σ. In the revised manuscript we have added a new figure displaying N_H versus E(B–V) for the full sample together with panels split into three redshift and three luminosity bins; the offset remains statistically significant in all subsamples, supporting the interpretation that the X-ray absorbing gas is largely dust-free and located closer to the black hole than the dust responsible for the optical reddening. revision: yes

Circularity Check

0 steps flagged

No significant circularity in observational analysis of rQSO properties

full rationale

This is an empirical observational study relying on direct measurements from eROSITA/SDSS-V data, including X-ray luminosities, alpha_OX spectral slopes, column densities, and data-driven ICA for optical spectra. No equations, derivations, or first-principles results are presented that reduce to fitted parameters or self-referential inputs by construction. The central claim of a distinct evolutionary stage with suppressed BH accretion follows from comparative statistics against bQSOs and is not forced by any self-citation chain or ansatz. The analysis is self-contained against external survey benchmarks with no load-bearing self-referential steps.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the background premise that red quasars represent an early evolutionary stage and on classification thresholds such as the L_X = 10^43.5 erg s^-1 cut used to quantify the low-luminosity fraction.

free parameters (1)
  • L_X threshold
    Defines the boundary for the >50% low-luminosity fraction statement; value chosen to separate the sample distribution.
axioms (1)
  • domain assumption Red quasars are a short-lived early stage in the AGN evolutionary cycle with fundamental differences from blue quasars
    Invoked in the opening sentence as established context for interpreting the new sample.

pith-pipeline@v0.9.0 · 6000 in / 1308 out tokens · 36884 ms · 2026-05-20T09:57:10.456936+00:00 · methodology

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