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arxiv: 2605.28948 · v1 · pith:C4SNBP4Qnew · submitted 2026-05-27 · 🌌 astro-ph.GA

Closing the UV Gap: Rest-frame EUV science from high-redshift QSOs as a legacy-defining capability

Rongmon Bordoloi , J. Michael Shull This is my paper

Pith reviewed 2026-06-29 10:39 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords quasarsextreme ultravioletultraviolet spectroscopycircumgalactic mediumintergalactic mediumspectral energy distributionHubble Space Telescope
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The pith

Expanding HST ultraviolet orbit allocations for z=1-2 quasars would capture their rest-frame extreme-ultraviolet spectra before the window closes.

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

The paper makes the case that Hubble is the only high-resolution ultraviolet observatory available until the mid-2040s. Observing quasars at redshifts 1 to 2 shifts their rest-frame extreme-ultraviolet emission into the telescope's far-ultraviolet range, allowing absorption and continuum measurements of both the quasars and surrounding gas. This dataset would advance studies of warm-hot material in the circumgalactic and intergalactic media while creating the first broad empirical census of quasar extreme-ultraviolet output. The opportunity is described as time-limited because detector performance is falling as new bright targets become available. Additional orbits in the 2030s are presented as the way to secure these measurements as a lasting reference for later missions.

Core claim

Rest-frame extreme-ultraviolet absorption and continuum spectroscopy of intermediate-redshift quasars at z=1-2, achieved by shifting those photons into the HST/COS far-ultraviolet bandpass, would deliver a step-change in warm-hot CGM/IGM science and produce the first systematic empirical EUV SED census of QSOs, serving as the foundational low-redshift anchor for future ultraviolet facilities.

What carries the argument

The redshift shift that moves rest-frame EUV photons (1-4 Ryd) from z=1-2 quasars into the HST far-UV bandpass for absorption and continuum spectroscopy.

If this is right

  • Improved empirical constraints on warm-hot gas in the circumgalactic and intergalactic media through rest-frame EUV absorption lines.
  • The first systematic census of quasar extreme-ultraviolet spectral energy distributions.
  • A low-redshift reference dataset that anchors science planning for future ultraviolet observatories.
  • Preservation of scientific return from HST before further sensitivity loss occurs.

Where Pith is reading between the lines

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

  • The resulting spectra could be cross-checked against simulations of quasar-driven gas heating to test feedback prescriptions.
  • An EUV census might identify systematic differences in quasar output that affect ionization balance calculations in the intergalactic medium.
  • These observations could help rank targets for later facilities by providing current-epoch brightness and spectral shape measurements.

Load-bearing premise

Declining COS detector sensitivity combined with new quasar discoveries creates a narrowing perishable window that requires immediate expansion of HST allocations.

What would settle it

A demonstration that COS sensitivity remains stable through the 2030s or that no additional UV-bright z>1 quasars are found would remove the stated urgency for expanded orbits.

Figures

Figures reproduced from arXiv: 2605.28948 by J. Michael Shull, Rongmon Bordoloi.

Figure 1
Figure 1. Figure 1: UV/optical/infrared facility timeline from 1990–2050. Colors denote primary spectroscopic [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Rest-frame EUV diagnostics in the COS bandpass. [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
read the original abstract

The Hubble Space Telescope is the only high-resolution ultraviolet spectroscopic facility that will exist until the Habitable Worlds Observatory (HWO) achieves first light in the mid-2040s. We describe a coherent class of science, coupling rest-frame extreme-ultraviolet (EUV; 1--4 Ryd, 228--912 {\AA}) absorption and continuum spectroscopy of intermediate-redshift quasars at $z = 1-2$, shifting the rest-frame EUV photons into the HST/COS far-UV bandpass. This science on quasars and gas in the IGM and CGM is doubly perishable. The COS detector sensitivity is declining, just as new quasars are found (Milliquas, UVQS, and soon Rubin, Roman, and Euclid). Thus, the window to reach UV-bright quasars at $z>1$ QSOs narrows with every deferred orbit. Expanding HST UV orbit allocations in the 2030s would deliver a step-change in warm-hot CGM/IGM science and produce the first systematic, empirical EUV SED census of QSOs. These datasets will serve as the foundational low-redshift anchor for HWO science. This recommendation makes the scientific and strategic case for an expansion of the HST/COS spectroscopic data base on intermediate redshift AGN in their rest-frame EUV.

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 / 0 minor

Summary. The manuscript argues that HST/COS is the sole high-resolution UV facility until HWO in the mid-2040s, and that rest-frame EUV (1-4 Ryd) absorption and continuum spectroscopy of z=1-2 quasars (shifted into the COS FUV bandpass) represents a doubly perishable science opportunity. Declining COS detector sensitivity coincides with new UV-bright QSO discoveries from Milliquas, UVQS, Rubin, Roman, and Euclid, narrowing the window for high-S/N observations of warm-hot CGM/IGM gas and the first systematic empirical EUV SED census of QSOs; these data would anchor low-redshift science for HWO. The paper therefore recommends expanded HST UV orbit allocations in the 2030s.

Significance. If the quantitative premises on sensitivity decline and target availability are substantiated, the resulting legacy dataset would provide a foundational empirical anchor for EUV science with HWO and enable new constraints on warm-hot baryons in the IGM/CGM at intermediate redshifts, addressing a recognized gap between current HST capabilities and future missions.

major comments (2)
  1. [Abstract / Introduction] Abstract and introduction: the central claim that the observational window is narrowing due to COS sensitivity decline is load-bearing for the urgency recommendation, yet no numerical values are supplied for the rate of effective-area loss (e.g., percent per year), no references to COS monitoring reports or sensitivity curves are cited, and no projected impact on S/N for z>1 targets is quantified.
  2. [Abstract / Science case] Abstract and science-case sections: the argument that new QSO discoveries from Milliquas/UVQS/Rubin/Roman/Euclid will create a perishable opportunity requires estimates of annual yield of UV-bright (FUV flux sufficient for high-S/N EUV spectroscopy) z=1-2 targets and of how many such targets would be lost under a 5- or 10-year delay; none of these quantities or supporting calculations appear in the manuscript.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive feedback, which highlights opportunities to strengthen the quantitative foundation of our arguments. We will revise the manuscript to incorporate the requested details on sensitivity decline and target yields.

read point-by-point responses

  1. Referee: [Abstract / Introduction] Abstract and introduction: the central claim that the observational window is narrowing due to COS sensitivity decline is load-bearing for the urgency recommendation, yet no numerical values are supplied for the rate of effective-area loss (e.g., percent per year), no references to COS monitoring reports or sensitivity curves are cited, and no projected impact on S/N for z>1 targets is quantified.

    Authors: We agree that explicit quantification would strengthen the case. In revision we will cite COS team monitoring reports and sensitivity curves, supply the effective-area loss rate (percent per year), and add projected S/N calculations for representative z=1-2 targets under current and future sensitivity. revision: yes

  2. Referee: [Abstract / Science case] Abstract and science-case sections: the argument that new QSO discoveries from Milliquas/UVQS/Rubin/Roman/Euclid will create a perishable opportunity requires estimates of annual yield of UV-bright (FUV flux sufficient for high-S/N EUV spectroscopy) z=1-2 targets and of how many such targets would be lost under a 5- or 10-year delay; none of these quantities or supporting calculations appear in the manuscript.

    Authors: We concur that numerical estimates of annual UV-bright target yields and the effect of delays would make the perishable opportunity more rigorous. The revised manuscript will include these estimates, derived from the cited catalogs and survey projections, together with calculations of targets rendered inaccessible by a 5- or 10-year postponement. revision: yes

Circularity Check

0 steps flagged

No circularity: forward-looking recommendation without derivations or self-referential logic

full rationale

The manuscript is a policy-oriented recommendation for HST orbit allocations. It contains no equations, no fitted parameters, no predictions derived from data, and no derivation chain. All load-bearing claims rest on external references to instrument performance, survey catalogs (Milliquas, UVQS, Rubin, etc.), and future observatory timelines rather than reducing to self-citation or input-by-construction. The absence of any mathematical or statistical structure precludes the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No mathematical model, physical derivations, or new assumptions are introduced; the text relies on general knowledge of telescope capabilities and survey timelines.

pith-pipeline@v0.9.1-grok · 5782 in / 1148 out tokens · 49850 ms · 2026-06-29T10:39:42.817876+00:00 · methodology

discussion (0)

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