Research Note: Ghostly DLAs in SDSS DR16

Patrick Petitjean

arxiv: 2606.06270 · v1 · pith:6COLU5TNnew · submitted 2026-06-04 · 🌌 astro-ph.GA

Research Note: Ghostly DLAs in SDSS DR16

Patrick Petitjean This is my paper

Pith reviewed 2026-06-28 00:30 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords proximate damped Lyman-alpha absorbersghostly systemsquasarsSDSS DR16absorption linesoutflowspartial covering

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The pith

Updated classification doubles known ghostly proximate damped Lyman-alpha systems and shows most but not all are bona fide DLAs.

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

This research note updates the classification of proximate damped Lyman-alpha absorbers near quasars using SDSS DR16 data. It delivers a revised catalogue of ghostly systems that more than doubles the number of previous detections. The work measures equivalent widths of absorption lines in both stacked spectra and individual systems. It establishes that while most ghostly systems qualify as true DLAs, this does not hold in every case. The findings refine understanding of dense gas in quasar environments and associated outflows or inflows.

Core claim

The paper provides an updated classification of PDLAs, including a revised catalogue of ghostly systems that more than doubles the number of previously known detections. It shows that although most ghostly systems are bona fide DLAs, this is not always the case. Ghostly systems arise in dense, compact gas that partially covers the quasar emission regions, with most linked to outflows up to -2000 km/s and some to inflows up to +1200 km/s.

What carries the argument

Ghostly systems, identified by strong absorption from excited levels of atomic ground states in dense, compact gas that partially covers the quasar emission regions.

If this is right

Most PDLAs trace gas in the AGN environment or quasar host galaxy rather than unrelated intervening material.
The majority of ghostly systems participate in outflows reaching velocities of -2000 km/s.
A minority of systems trace inflowing clouds extending to +1200 km/s.
Equivalent width measurements from stacked and individual spectra can distinguish true DLAs from other ghostly absorbers.

Where Pith is reading between the lines

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

The doubled sample size enables tighter constraints on the covering fraction and density distribution of gas near quasars.
Systems that fail to qualify as DLAs may trace lower-column-density gas with different excitation conditions.
The partial-covering geometry could be tested directly with high-resolution spectra that resolve velocity structure across the quasar emission regions.

Load-bearing premise

The classification treats detection of absorption from excited levels of atomic ground states as evidence that the gas is dense and compact enough to partially cover the quasar emission regions.

What would settle it

A spectroscopic follow-up that finds a system classified as ghostly yet shows no partial coverage of the quasar continuum or broad emission lines would challenge its placement in the catalogue.

Figures

Figures reproduced from arXiv: 2606.06270 by Patrick Petitjean.

**Figure 1.** Figure 1: — Mean spectra of the HighWSys (blue), HighWSys with Si II* (black), DLA-Cor with Si II* (orange), and ghostly DLA (green) systems, shown on a velocity scale for H i Lyman-α, β, and γ. The inferred H i column densities for the HighWSys and HighWSys* classes are comparable and lower than those measured for the other absorber classes. DLA-Cor systems exhibit the highest H i column densities. Consistently, t… view at source ↗

**Figure 3.** Figure 3: — Mean spectra of the HighWSys (blue), HighWSys Si II* (black), DLA-Cor with Si II* (orange), and ghostly DLA (green) systems, shown on a velocity scale for high-ionization species 4. GHOSTLY DLAS 4.1. List of Ghostly DLAs During the initial visual inspection of all ProxSys, we classified systems showing no or little H i Lyman-α absorption as ghostly DLAs (Ghost) or probable ghostly DLAs (Ghost?) when som… view at source ↗

**Figure 4.** Figure 4: — Mean spectra of the HighWSys (blue), HighWSys with Si II* (black), DLA-Cor with Si II* (orange), and ghostly DLA (green) systems, shown on a velocity scale for S ii and absorptions from excited atomic levels [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗

**Figure 5.** Figure 5: — Equivalent width of transitions observed in the mean spectra of DLA, DLA with SiII*, DLA-Cor with SiII* and ghostly DLAs mean spectra. by low signal-to-noise ratio and/or blending. Among the 220 systems initially classified as Ghost (90) or Ghost? (130), we confirmed 95 as Ghost systems and 92 as probable Ghosts, while rejecting 33 after a more detailed inspection based on all available absorption feat… view at source ↗

**Figure 6.** Figure 6: Interestingly, about one third of the systems have column densities robustly below 1020 cm−2 . The relatively high column density inferred from the median spectrum may be overestimated due to the smearing of absorption features associated with large Doppler parameters. 4.3. Absorption equivalent widths In [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗

**Figure 7.** Figure 7: — The equivalent width of Si ii*λ1264 versus the equivalent width of Si iiλ1260 in individual ghostly DLAs. but statistically significant correlation is observed for all three ions, with Pearson coefficients of 0.29, 0.25, and 0.23, and corresponding p-values of 8.8×10−5 , 6.3×10−4 , and 0.05, respectively. The presence of strong high-ionization absorption indicates that the gas is located in the vicinit… view at source ↗

**Figure 9.** Figure 9: — The equivalent width of Si ii*λ1264 as a function of the velocity of the system relative to the quasar redshift (in km s−1 ). 5. CONCLUSIONS In this research note, we have presented additional observational constraints on the nature of metal-selected proximate absorption systems identified in SDSS DR16 and provided a revised catalogue of ghostly DLAs. Our updated sample contains 187 ghostly DLA candidat… view at source ↗

**Figure 8.** Figure 8: — The equivalent widths of C ivλ1548 (bottom), N vλ1238 (middle) and O viλ1031 (top) as a function of the equivalent width of Si ii∗λ1264. spanning velocities between −1000 and −2000 km s−1 . The above anti-correlation thus supports our previous findings that ghostly DLAs trace outflowing gas resulting from the interaction between the interstellar medium of the host galaxy and the quasar-driven wind and r… view at source ↗

read the original abstract

In Petitjean (2026, arXiv:2605.12188)), we revisited the origin of proximate damped Lyman-alpha absorbers (PDLAs), which trace cold gas within 3000 km/s of the quasar redshift, and interpreted their kinematics and physical properties within a unified framework. We showed that most PDLAs are associated with the environment of the AGN and/or the quasar host galaxy. We also provided the first census and characterization of absorption systems exhibiting strong absorption from excited levels of atomic ground states among quasar-associated absorbers. Among these, ghostly and coronagraphic systems arise in dense, compact gas that partially covers the quasar emission regions. Most systems are associated with outflows reaching velocities up to -2000 km/s, while a smaller fraction of inflowing clouds extends to velocities of up to +1200 km/s. In the present work, we provide an updated classification of PDLAs, including a revised catalogue of ghostly systems that more than doubles the number of previously known detections. We investigate the properties of these systems by measuring and discussing the equivalent widths of the detected absorption lines in both stacked spectra and individual ghostly systems. In particular, we show that although most ghostly systems are bona fide DLAs, this is not always the case.

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.

Desk Editor's Note private letter to a colleague

This research note doubles the sample of ghostly DLAs from the author's prior work but adds no new methods or corrections for partial covering when classifying DLA status.

read the letter

The main takeaway is that this is a catalogue extension paper. It takes the ghostly systems defined in arXiv:2605.12188 and adds enough new detections to more than double the previous count, then reports equivalent-width measurements from both stacked and individual spectra. That is the concrete new content.

What the work does is straightforward observational bookkeeping in a narrow subfield. It supplies additional examples of systems showing excited-level absorption and notes that most qualify as bona fide DLAs while a few do not. The measurements themselves appear to be the standard EW approach.

The soft spot is the classification step. Ghostly systems are defined by partial covering, so the observed Ly-alpha profile is a flux-weighted mix of covered and uncovered sightlines. Equivalent widths do not directly yield N(HI); damping-wing fits without an explicit covering-fraction term will underestimate the true column density. The abstract gives no sign that such a correction was applied to the new systems when deciding DLA versus sub-DLA status. If that step is missing, the central claim rests on an uncorrected bias.

This is the kind of incremental data note that specialists in proximate absorbers might want to see for completeness, but it does not change the framework or resolve the physical questions raised in the earlier paper. A serious referee could check the new EW values and the covering-fraction handling; without that check the classification remains provisional.

I would send it to peer review rather than desk-reject, mainly because the added sample size is verifiable observational content, but the review would need to focus on whether the DLA assignments hold once partial covering is modeled.

Referee Report

1 major / 0 minor

Summary. The manuscript is a Research Note updating the classification of proximate damped Lyman-alpha absorbers (PDLAs) from SDSS DR16. It presents a revised catalogue of ghostly systems (more than doubling prior detections), measures equivalent widths of absorption lines (including from excited levels) in both stacked and individual spectra, and concludes that most ghostly systems are bona fide DLAs while noting that this is not always the case.

Significance. If the DLA classifications hold after proper accounting for partial covering, the expanded catalogue of these rare systems (associated with dense, compact gas in AGN outflows or inflows) would provide a substantially larger sample for studying cold gas kinematics and physical conditions near quasars, extending the unified framework from the companion paper (Petitjean 2026).

major comments (1)

[Abstract] Abstract: The central claim that 'most ghostly systems are bona fide DLAs, this is not always the case' requires reliable N(HI) >= 2e20 cm^-2 determinations. Ghostly systems are defined by partial covering, so equivalent-width measurements (the only analysis method described) do not suffice; Voigt-profile fits to the Ly-alpha damping wings will underestimate column density unless the covering fraction is modeled explicitly in the fits. No indication is given that such corrections were applied when classifying the newly added systems.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their detailed review and constructive feedback on our Research Note. We address the single major comment below regarding the robustness of our DLA classifications.

read point-by-point responses

Referee: [Abstract] Abstract: The central claim that 'most ghostly systems are bona fide DLAs, this is not always the case' requires reliable N(HI) >= 2e20 cm^-2 determinations. Ghostly systems are defined by partial covering, so equivalent-width measurements (the only analysis method described) do not suffice; Voigt-profile fits to the Ly-alpha damping wings will underestimate column density unless the covering fraction is modeled explicitly in the fits. No indication is given that such corrections were applied when classifying the newly added systems.

Authors: We agree that equivalent-width measurements provide only an indirect indication of column density and that partial covering must be explicitly modeled to obtain reliable N(HI) values from the damping wings. The present Research Note relies on equivalent widths measured in stacked and individual spectra (as described in the text) together with the detection of excited-level lines to classify systems, following the approach of earlier studies. However, this does not constitute a full Voigt-profile analysis that accounts for covering fraction. We will therefore add such modeling for the Ly-alpha profiles of the newly catalogued systems in the revised version to confirm which meet the bona-fide DLA threshold after correction. revision: yes

Circularity Check

0 steps flagged

No significant circularity; observational catalogue extension

full rationale

The paper performs an observational extension of a PDLA catalogue by measuring equivalent widths in stacked and individual SDSS spectra and applying an updated classification. It cites prior work (Petitjean 2026) only for the initial definition of ghostly systems and their physical interpretation; the new results consist of direct measurements on fresh data that do not reduce to those inputs by construction. No equations, fitted parameters, predictions, or self-referential derivations appear. The claim that most systems are bona fide DLAs rests on the new equivalent-width data rather than tautology.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

The paper is purely observational and relies on standard astronomical data and classification criteria established in the cited prior work; no free parameters, new axioms, or invented entities are introduced in the abstract.

pith-pipeline@v0.9.1-grok · 5750 in / 958 out tokens · 25602 ms · 2026-06-28T00:30:54.051721+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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M., Bluck, A

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2018