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arxiv: 2605.29949 · v1 · pith:VAY4THSVnew · submitted 2026-05-28 · 🌌 astro-ph.GA

High-S/N Quasar Observations with HST/COS: Deep Fields for Spectroscopy

Andrew J. Fox , Jerry Kriss , Philipp Richter , J. Michael Shull , Frances Cashman , Sapna Mishra , Annelia Anderson , Nahum Arav
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Ramona Augustin Kathleen Barger Michelle Berg Rongmon Bordoloi Sanchayeeta Borthakur Joseph Burchett Jane Charlton Hsiao-Wen Chen Christopher Churchill Ryan Cooke Annalisa de Cia Gisella de Rosa Romeel Dav\'e Yakov Faerman Travis Fischer David French Farhan Hasan Svea Hernandez Cameron Hummels Sean Johnson Glenn Kacprzak Vikram Khaire Doyeon Avery Kim Brad Koplitz Varsha Kulkarni Nicolas Lehner Matilde Mingozzi Talawanda Monroe Sowgat Muzahid Benjamin Oppenheimer Molly Peeples C\'eline P\'eroux Patrick Petitjean Andreea Petric Max Pettini Zhijie Qu Kate Rowlands Ravi Sankrit Debopam Som Raghunathan Srianand Nicolas Tejos Jason Tumlinson Bart Wakker Jessica Werk
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Pith reviewed 2026-06-29 06:36 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords quasarsUV spectroscopyHST/COSCGMIGMabsorption lineslegacy surveycircumgalactic medium
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The pith

A deep HST/COS survey of 20 quasars would reach H I column densities of log N=12.6 in the CGM and IGM.

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 for obtaining high signal-to-noise ultraviolet spectra of about 20 quasars at redshifts 0.5 to 1.5 using the COS instrument on Hubble. These spectra, taken at 20 km/s resolution with S/N above 30, would fill a gap in existing archives and create a reusable legacy resource modeled on the success of deep imaging fields. The resulting data would permit measurements of gas properties at column densities and metallicities far below current limits. A reader would care because this level of sensitivity directly affects our ability to quantify the distribution of baryons and metals in the space around and between galaxies.

Core claim

The central claim is that a high-S/N UV spectroscopic survey with HST/COS targeting approximately 20 QSOs would produce a legacy dataset enabling frontier science programs in CGM and IGM studies at unparalleled sensitivity, covering a wide range of UV metal lines and reaching very low H I column densities of log N=12.6 and low metallicities near [Z/H]=-2, along with studies of Milky Way diffuse gas and AGN outflows in the EUV.

What carries the argument

High-S/N quasar spectra that act as background lights to reveal weak absorption features from intervening gas at low column densities.

If this is right

  • Precision measurements of chemical abundances, ionization, and temperature in the CGM and IGM.
  • Tighter constraints on the overall baryon and metal budgets of these gaseous media.
  • Detailed mapping of high-velocity clouds and merger-related gas streams in the Milky Way and Local Group.
  • New diagnostics of continuum generation and outflow gas in AGN accretion disks using rest-frame EUV lines.

Where Pith is reading between the lines

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

  • The dataset could provide empirical benchmarks for planning UV spectroscopy with future space telescopes.
  • Detection of gas at [Z/H] near -2 might identify a previously unseen reservoir of metal-poor material.
  • The same sight lines could be cross-matched with galaxy surveys to link absorber properties directly to host environments.

Load-bearing premise

That the targeted quasars will supply enough independent lines of sight to reach the stated sensitivity to low-column-density absorbers.

What would settle it

Completion of the survey spectra followed by a check that the achieved noise floor still prevents secure identification of H I absorbers at log N=12.6.

Figures

Figures reproduced from arXiv: 2605.29949 by Andreea Petric, Andrew J. Fox, Annalisa De Cia, Annelia Anderson, Bart Wakker, Benjamin Oppenheimer, Brad Koplitz, Cameron Hummels, C\'eline P\'eroux, Christopher Churchill, David French, Debopam Som, Doyeon Avery Kim, Farhan Hasan, Frances Cashman, Gisella De Rosa, Glenn Kacprzak, Hsiao-Wen Chen, Jane Charlton, Jason Tumlinson, Jerry Kriss, Jessica Werk, J. Michael Shull, Joseph Burchett, Kate Rowlands, Kathleen Barger, Matilde Mingozzi, Max Pettini, Michelle Berg, Molly Peeples, Nahum Arav, Nicolas Lehner, Nicolas Tejos, Patrick Petitjean, Philipp Richter, Raghunathan Srianand, Ramona Augustin, Ravi Sankrit, Romeel Dav\'e, Rongmon Bordoloi, Ryan Cooke, Sanchayeeta Borthakur, Sapna Mishra, Sean Johnson, Sowgat Muzahid, Svea Hernandez, TalaWanda Monroe, Travis Fischer, Varsha Kulkarni, Vikram Khaire, Yakov Faerman, Zhijie Qu.

Figure 1
Figure 1. Figure 1: — Top: Schematic representation of the four astrophysical environments probed by each quasar spectrum. Lower Left: Distribution of S/N ratios in all (1040) COS/FUV G130M/G160M AGN spectra in the MAST archive as of December 2025. The mean S/N is 14.5 per resolution element, and there is a shortage of high S/N spectra (≳30), particularly for targets at 0.5<z <1.5 (inset panel). Lower Right: The highest-S/N C… view at source ↗
read the original abstract

Hubble is still in prime observing condition for making transformative discoveries in UV astronomy. In this white paper we describe the science case for a deep (S/N>30) UV spectroscopic survey with HST/COS targeting approximately 20 QSOs at 0.5<z<1.5 at good resolution (20 km/s). This survey would capitalize on our current UV capability, produce a legacy dataset enabling community science in many areas of galactic and extragalactic research, and pioneer a path for future UV science with the Habitable Worlds Observatory. Such high-S/N spectra are largely missing from the MAST archives, and would be analogous to the deep Hubble imaging fields (HDF, UDF, Frontier Fields) that have been enormously successful and far-reaching in their science impact. This legacy dataset would enable frontier science programs in several areas, including (1) studies of the CGM and IGM at unparalleled sensitivity, covering a wide range of UV metal lines and reaching very low H I column densities of log N=12.6 and low metallicities near [Z/H]=-2, enabling precision studies of the chemical abundances, ionization, temperature, and baryon and metal budgets of the CGM and IGM; (2) diffuse gas in the Milky Way and Local Group, including high-velocity clouds and gas streams from satellite mergers; (3) AGN outflows, which would be probed in the rest-frame extreme ultraviolet (EUV), covering continuum-generation mechanisms and diagnostics of gas in accretion-disk outflows.

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

1 major / 0 minor

Summary. The manuscript is a white paper proposing a deep (S/N > 30) UV spectroscopic survey with HST/COS targeting ~20 quasars at 0.5 < z < 1.5 and 20 km/s resolution. The survey is framed as a legacy dataset analogous to the HDF/UDF, enabling precision CGM/IGM studies down to log N(H I) = 12.6 and [Z/H] = -2, plus work on Milky Way high-velocity clouds and AGN outflows in the EUV.

Significance. If executed, the proposed high-S/N spectra would fill a clear gap in the MAST archive and could support community studies of low-column-density absorbers and metal budgets in the CGM/IGM. The paper correctly identifies the scientific value of such data but provides no quantitative simulations, exposure-time calculations, or error budgets to demonstrate that the stated sensitivity is achievable with the proposed sample and instrument settings.

major comments (1)
  1. [Abstract] Abstract: the central sensitivity claim (reaching log N(H I) = 12.6 and [Z/H] = -2) is asserted without any supporting calculation, simulation, or exposure-time estimate. This quantitative justification is load-bearing for the science case and is absent from the text.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review of our white paper on a proposed high-S/N HST/COS quasar survey. We address the major comment below and will incorporate revisions to strengthen the quantitative basis of the science case.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the central sensitivity claim (reaching log N(H I) = 12.6 and [Z/H] = -2) is asserted without any supporting calculation, simulation, or exposure-time estimate. This quantitative justification is load-bearing for the science case and is absent from the text.

    Authors: We agree that the sensitivity claims require explicit quantitative support to be fully convincing. The current manuscript presents these limits as the expected outcome of S/N > 30 spectra at 20 km/s resolution but does not include the supporting exposure-time estimates, COS ETC calculations, or error budgets. In the revised version we will add a dedicated subsection (likely in Section 2 or an appendix) that provides: (1) exposure-time calculations for representative quasars at 0.5 < z < 1.5 using the COS Exposure Time Calculator for the G130M and G160M gratings; (2) an error budget showing how S/N > 30 per resolution element is achieved for the proposed sample of ~20 targets; and (3) simple simulations or scaling arguments demonstrating that these data reach log N(H I) = 12.6 and [Z/H] = -2 for typical CGM/IGM absorbers. This addition will directly address the load-bearing nature of the claim. revision: yes

Circularity Check

0 steps flagged

No significant circularity; proposal document with no derivations

full rationale

This is a forward-looking white paper proposing an HST/COS survey. It contains no equations, fitted parameters, predictions, or derivation chains that could reduce to inputs by construction. Claims about sensitivity (log N=12.6) are stated as prospective outcomes conditional on execution, not derived results. No self-citations, uniqueness theorems, or ansatzes appear as load-bearing steps. The document is self-contained as a science case without internal circularity.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

As this is a proposal paper rather than a research paper with derivations, there are no free parameters, axioms, or invented entities introduced in the provided abstract.

pith-pipeline@v0.9.1-grok · 6041 in / 1203 out tokens · 37214 ms · 2026-06-29T06:36:22.246221+00:00 · methodology

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