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DESI Mg II Absorbers: Extinction Characteristics & Quasar Redshift Accuracy

T0 review · 4 major / 6 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read Associated Mg II absorbers bias DESI quasar redshifts at $z>1.5$, and masking the doublet pixels before re-fitting recovers velocity-offset distributions like those at lower redshift, with typical shifts of $\Delta z \approx \pm 0.005$.

desk verdict A plausible redshift bias from associated Mg II absorbers, supported by a large sample, but the masking demonstration is under-controlled and needs a cleaner comparison. read the letter →

arxiv 2412.15383 v3 pith:OLAB36A4 submitted 2024-12-19 astro-ph.GA

Lucas Napolitano , Adam D. Myers , Vicky Fawcett , Jessica Aguilar , Steven Ahlen , Davide Bianchi , David Brooks , Todd Claybaugh
show 27 more authors
Shaun Cole Axel de la Macorra Biprateep Dey Andreu Font-Ribera Jaime E. Forero-Romero Enrique Gaztañaga Satya Gontcho A Gontcho Gaston Gutierrez Klaus Honscheid Stephanie Juneau Andrew Lambert Martin Landriau Laurent Le Guillou Aaron Meisner Ramon Miquel John Moustakas Jeffrey A. Newman Francisco Prada Ignasi Pérez-Ràfols Graziano Rossi Eusebio Sanchez David Schlegel Michael Schubnell David Sprayberry Gregory Tarlé Benjamin Alan Weaver Hu Zou
This is my paper · ORCID
classification astro-ph.GA
keywords MgIIabsorbersquasarredshiftsextinctionE(B-V)DESIassociatedabsorptionsystemsvelocityoffsetredshiftbias
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

Using 50,674 DESI quasar spectra that each contain a single Mg II absorption system, together with a matched control sample, the paper sets out to establish two things: that Mg II absorbers redden quasar light in a way that depends on their velocity offset, and that associated absorbers (velocity offset below 3500 km/s) corrupt the quasar redshifts measured by DESI. It reports an average color excess of $E(B-V) = 0.04$ magnitudes for intervening absorbers, rising to roughly $0.15$ magnitudes at velocity offset close to zero for associated absorbers. At $z>1.5$ the velocity-offset distribution of associated absorbers broadens and bifurcates in a way the authors argue is nonphysical, and masking the Mg II doublet pixels and re-running the redshift fitter narrows those distributions and shifts quasar redshifts by typically $\Delta z \approx \pm 0.005$. This identifies a redshift-dependent systematic in DESI quasar redshifts that can be at least partially removed by line masking.

What carries the argument

The central diagnostic is the velocity offset $v_{\rm off} = c(z_{\rm QSO} - z_{\rm ALS})/(1+z_{\rm QSO})$, which separates associated from intervening absorbers and exposes the redshift bias through its distribution. The corrective machinery is a masking-and-refit procedure: using each absorber's fitted redshift and line width from the catalog, the authors flag every pixel within $5\sigma$ of either line of the Mg II doublet, exclude those pixels, and rerun Redrock — DESI's template-matching redshift code — with updated quasar templates on the masked spectrum. Comparing $v_{\rm off}$ before and after masking isolates the contribution of the absorption lines to the quasar redshift measurement.

What would settle it

Take a sample of associated absorbers at $1.5<z<2.1$ and remeasure each quasar's redshift from narrow emission lines such as [O II] or [Ne V]; if those narrow-line redshifts put most systems near $v_{\rm off}=0$ without Mg II masking, the claim is confirmed, whereas if the narrow-line redshifts preserve the broad, bifurcated $v_{\rm off}$ distribution, the absorber catalog redshifts are the culprit.

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Extended reading notes

Core claim

The paper's central claim is that associated Mg II absorbers (velocity offset below $3500\,\mathrm{km\,s^{-1}}$) do not merely redden a quasar; they systematically change its measured redshift. The authors find that at $z>1.5$ the $v_{\rm off}$ distribution of associated absorbers broadens, its median moving from roughly 140–240 km/s at lower redshift to 700–1000 km/s, and then splits into two peaks near $\pm1500$ km/s, while the most reddened systems pile up at strongly negative $v_{\rm off}$. They interpret this as the Mg II doublet distorting the Mg II emission line that the DESI redshift pipeline fits. Masking the doublet pixels and re-running the fitter with updated quasar templates shifts typical redshifts by $\Delta z \approx \pm 0.005$, most effectively at $1.6<z<2.0$, and brings the $v_{\rm off}$ distributions back toward $v_{\rm off}=0$, though the recovery is incomplete at $z>2$.

Load-bearing premise

The load-bearing premise is that the catalogued Mg II absorber redshifts are accurate, so all nonphysical broadening of $v_{\rm off}$ at $z>1.5$ is blamed on errors in the quasar redshift; if absorber redshifts are themselves biased at high $z$, the bifurcation and the correction from masking are misattributed.

Editorial extensions

If this is right

  • DESI quasar redshifts at $z>1.5$ that host associated Mg II absorbers carry a systematic offset of order $\Delta z \approx \pm 0.005$, peaking near $z\approx 1.8$, which will propagate into any cosmology or clustering analysis that uses those redshifts.
  • Masking the Mg II doublet before redshift fitting is a practical correction: it recovers velocity-offset distributions that look like the $z<1.5$ population for most of the $1.5<z<2.1$ range.
  • The $E(B-V)$ behavior supports keeping 3500 km/s as the boundary between associated and intervening absorbers, and confirms that dust content grows with time and with Mg II line strength.
  • Because the masking leaves residual broadening at $z>2$, where C IV, C III, and Lyman-$\alpha$ dominate the fits, further corrections that handle those broad lines will be needed before quasar redshifts in that regime are fully cleaned.

Reading between the lines

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

  • A step the authors do not take, but that follows naturally, would be to use the measured $\Delta z$ as a calibration prior: build a correction map in $z$ and $v_{\rm off}$ and apply it to the full DESI DR1 quasar sample, including systems without detected absorbers, since the bias may also affect weak or undetected absorbers.
  • The bifurcation pattern suggests the Mg II doublet is being fit as if it were velocity-shifted emission; injecting synthetic absorbers into spectra at known redshifts and observing the fitted $z$ would directly test this mechanism and could predict masking performance from line strength alone.
  • Cross-checking absorber redshifts with other transitions (Fe II $\lambda2600$, the C IV doublet, or Mg I) in the same systems would determine whether the residual high-$z$ broadening is a quasar-side or absorber-side problem, a test the paper does not include.
  • The reddish control-sample behavior at $z<1$ is left unexplained; if real, it means even DESI quasars without detected Mg II have dust or template mismatches that could produce extinction and redshift errors of the same class.
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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

4 major / 6 minor

Summary. This paper studies Mg II absorption-line systems in DESI EDR/DR1 QSO spectra. The sample consists of 50,674 QSOs with a single detected Mg II absorber at voff < 20,000 km/s, plus a matched control sample of 50,674 QSOs with no detected Mg II absorbers. The authors fit each spectrum with a blue QSO composite template and an SMC extinction curve to estimate E(B-V), finding that intervening absorbers have an average E(B-V) of about 0.04 magnitudes, with E(B-V) increasing at lower absorber redshift and higher rest-frame equivalent width. Associated absorbers (voff < 3500 km/s) show a strong rise in E(B-V) toward voff = 0, peaking near 0.15 magnitudes. The paper then shows that the voff distribution of associated absorbers broadens and bifurcates at z > 1.5, and proposes that this is caused by Mg II absorption biasing the QSO redshifts. It attempts to mitigate this by masking the Mg II doublet pixels and rerunning Redrock, reporting that this narrows the voff distributions and shifts QSO redshifts by Delta z of roughly +/-0.005.

Significance. If the causal attribution is correct, this paper identifies a redshift-dependent systematic in DESI QSO redshifts that could affect both absorber-host studies and cosmological analyses using DESI QSOs. The paper's strengths include the large sample, the explicit control sample, the public data release on Zenodo, and a straightforward and reproducible extinction-fitting procedure. The authors also honestly flag limitations, including the unexpected low-redshift control behavior in Figure 5 and the reduced effectiveness of masking at z > 2.0. However, the central mitigation claim--that masking Mg II pixels, rather than the concurrent Redrock template update or the post-hoc removal of large-shift systems, recovers the low-redshift voff population--is not yet demonstrated by a controlled same-version comparison.

major comments (4)
  1. [Section 3.3, Figures 8 and 9] The demonstration that masking Mg II pixels corrects DESI QSO redshifts is not a controlled masking experiment. The plotted comparison is between the original catalog redshifts (Redrock versions 0.15.3 and 0.17.0) and a masked re-run with Redrock v0.20.0, whose QSO templates were trained on a substantially larger sample. The authors state that they also re-ran v0.20.0 without masking 'to ensure we can separate the effect of our masking strategy from that of the new QSO templates,' but no unmasked v0.20.0 version of Figures 7, 8, or 9 is shown. Without that same-version unmasked control, the improvement in Figure 8 could be caused by the template update rather than by masking. Please add the unmasked v0.20.0 voff distributions and a direct masked-vs-unmasked difference (Delta z) computed with an identical selection.
  2. [Section 3.3, paragraph on excluded systems] The post-hoc exclusion of systems whose re-run redshift moved outside voff < 3500 km/s (784 for the masked run and 1052 for the unmasked run) removes the objects with the largest redshift shifts, and dropping them mechanically narrows the remaining voff distribution. The 268-system difference between the masked and unmasked exclusions may itself account for part of the apparent improvement. Please show the full distributions before exclusion (for example, by retaining all systems at their new voff values or by displaying them as a separate population), and quantify how the trimming changes the 16th, 50th, and 84th percentiles reported in Figures 7 and 8.
  3. [Sections 2.1 and 3.2] The analysis assumes that the Mg II absorber redshifts in the EDR/DR1 catalog are correct and that all voff broadening at z > 1.5 arises from errors in the QSO redshift. The paper notes that Mg II falls beyond 8400 Angstroms at z > 2.3 where DESI spectra are noisier, but it never validates absorber redshifts against other transitions (for example, C IV, Fe II, or [O II]) or against an independent line list. Because the same catalog defines both the mask and the reference redshift z_ALS in the definition of voff, any systematic error in the absorber redshifts would directly produce an apparent QSO-redshift bias. A cross-check of a subset of z > 1.5 absorbers with other metal lines would substantially strengthen the causal interpretation.
  4. [Sections 2.2 and 3.1] Negative E(B-V) fits are treated as physical measurements in the reported medians and trends, although the paper recognizes that negative values likely reflect the spread in intrinsic QSO power-law slopes relative to the assumed blue composite. Since the fit has only two parameters (normalization and E(B-V)) and the SMC extinction curve is assumed, the absolute scale of the reported E(B-V) values and the shape of the E(B-V)-voff trend depend on the distribution of intrinsic spectral indices in the fitted sample. The claim that associated absorbers show E(B-V) rising to about 0.15 at voff = 0 would be strengthened by a test that either marginalizes over spectral index, restricts the sample to a narrow color or luminosity range, or compares with a composite-stacking approach at the same voff.
minor comments (6)
  1. [Section 3.3] Please state explicitly whether the unmasked v0.20.0 rerun exists as a machine-readable table or figure; if so, include a direct comparison, since the current text only summarizes the number of systems (1052 versus 784) that leave the associated sample.
  2. [Figure 5 caption] The caption says 'Bins are 0.04 units wide'; please specify that this is in absorber redshift and, for the control sample, the matched absorber redshift.
  3. [Section 3.1] The sentence 'We can note that the associated absorber sample contains 28,178 systems with E(B-V) = 0.1092 magnitudes' does not state whether this is the mean or median; please state the estimator and use consistent terminology with the abstract and Figure 3.
  4. [Equation (1)] Equation (1) uses the speed of light c without explicitly defining it; please add a definition.
  5. [Section 5] In the conclusion, the reported median E(B-V) = 0.052 appears without the asymmetric 16th/84th percentile uncertainties given in Section 3; please include them for consistency.
  6. [Data availability] The data-release statement should include the exact Redrock version and the pixel-masking recipe used for the masked re-run, so that Figure 8 is reproducible from the Zenodo data alone.

Circularity Check

0 steps flagged · score 2.0 of 10

No circular reduction is exhibited: E(B-V) is a fitted parameter against an external template, and the redshift-masking claim is an empirical intervention, though a key unmasked v0.20.0 control is omitted and large-shift systems are excluded.

full rationale

The paper's derivation chain does not reduce any prediction to its inputs by construction. The extinction analysis fits an external VLT/X-shooter blue-QSO composite (Fawcett et al. 2022) with an assumed SMC extinction curve, so E(B-V) is a fitted parameter anchored by a matched non-absorbed control sample; no E(B-V) trend is encoded in the fit. The voff analysis starts from a separately line-fit Mg II absorber catalog (Napolitano et al. 2023), and the inference that Mg II absorption biases z_QSO is drawn empirically from the broadening and bifurcation of the voff distribution at z>1.5 (Section 3.2, Figure 7). The masking test (Section 3.3) is an intervention: Mg II doublet pixels are flagged, Redrock is rerun, and the resulting redshift shifts are measured rather than imposed. The low-redshift panels of Figure 8 are essentially unchanged, providing an internal control that masking does not mechanically narrow every distribution. However, the paper omits a load-bearing control: it states, 'In order to ensure we can separate the effect of our masking strategy from that of the new QSO templates, we have calculated redshifts for our sample of associated absorbers both with and without the masking of MgII absorption line pixels,' but never presents the unmasked v0.20.0 voff distribution; Figure 8 compares masked v0.20.0 redshifts to the original v0.15.3/v0.17.0 catalog redshifts. It also excludes the 784 (masked) and 1052 (unmasked) systems whose rerun redshifts left the voff<3500 km/s associated window, a trim that removes the largest shifts and can narrow the displayed distribution independently of masking. These are significant validity gaps in the causal attribution, but they are not circular reductions: no equation or fitted parameter is reused as its own prediction. The self-citations (Napolitano et al. 2023 catalog; Fawcett et al. 2022 template) are method and data references with external content, not load-bearing uniqueness theorems. Score 2 reflects the mild self-referential design (the same absorber catalog defines the mask and the success metric) and the missing unmasked control, while the central extinction and redshift-shift measurements retain independent content.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

All central claims are empirical fits rather than derivations; the free parameters are the per-spectrum extinction and normalization, plus hand-chosen masking and classification boundaries. The analysis leans on external templates and an assumed extinction curve.

free parameters (4)
  • Color excess E(B-V) = distribution with median 0.052 (+0.120, -0.058) for absorbed sample
    Fitted as the extinction amplitude in a two-parameter fit of a reddened blue QSO template to each of 50,674 spectra (Section 2.2); all central reddening claims depend on these fits.
  • Template normalization = unbounded
    Second free parameter in the per-spectrum fit; absorbs continuum flux differences between the X-shooter composite and DESI quasars.
  • 5-sigma masking width = 5 sigma
    Chosen by hand for the MgII doublet masking in Section 3.3; the results of the redshift-recovery test depend on this width.
  • voff boundary = 3500 km/s
    Adopted as the associated/intervening division, based on prior literature rather than fitted; used to split the sample.
assumptions (5)
  • domain assumption The X-shooter blue QSO composite (Fawcett et al. 2022) is an unbiased intrinsic template for the DESI quasars in the sample.
    Invoked in Section 2.2; if DESI quasars have different intrinsic continuum slopes, fitted E(B-V) values are biased. The paper acknowledges this via negative E(B-V) fits.
  • domain assumption The SMC Bar extinction curve with R(V) = 2.74 from Gordon et al. (2003) applies to all MgII absorber dust.
    Adopted in Section 2.2 following prior studies; no test of alternative extinction curves is performed.
  • domain assumption The MgII absorber redshifts from the EDR/DR1 catalog are accurate enough to define voff and the mask.
    Used throughout Sections 2.1 and 3.3; no validation against other metal lines is presented, especially at z > 1.5 where MgII falls in noisier spectral regions.
  • domain assumption Masking the MgII doublet pixels does not itself bias the Redrock fitted redshift toward the absorber redshift.
    The masking test in Section 3.3 assumes that removing the doublet pixels does not artificially pull the fitted redshift; no independent redshift check is provided at z > 1.5.
  • domain assumption The z < 1.5 voff distribution of associated absorbers is the physically correct expectation for z > 1.5.
    Used in Section 3.2 to label the high-z bifurcation as nonphysical; if AAS kinematics evolve strongly with redshift, the inferred redshift bias is partly astrophysical.

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Cite this review

Pith. "Pith review of DESI Mg II Absorbers: Extinction Characteristics & Quasar Redshift Accuracy." pith.science (2026). https://pith.science/paper/OLAB36A4

@misc{pith2026241215383,
  author       = {Pith},
  title        = {Pith review of: DESI Mg II Absorbers: Extinction Characteristics & Quasar Redshift Accuracy},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/OLAB36A4}},
  note         = {Machine review of arXiv:2412.15383}
}
read the original abstract

In this paper, we study how absorption-line systems affect the spectra and redshifts of quasars (QSOs), using catalogs of Mg II absorbers from the early data release (EDR) and first data release (DR1) of the Dark Energy Spectroscopic Instrument (DESI). We determine the reddening effect of an absorption system by fitting an un-reddened template spectrum to a sample of 50,674 QSO spectra that contain Mg II absorbers. We find that reddening caused by intervening absorbers (voff > 3500 km/s) has an average color excess of E(B-V) = 0.04 magnitudes. We find that the E(B-V) tends to be greater for absorbers at low redshifts, or those having Mg II absorption lines with higher equivalent widths, but shows no clear trend with voff for intervening systems. However, the E(B-V) of associated absorbers, those at voff < 3500 km/s, shows a strong trend with voff , increasing rapidly with decreasing voff and peaking (approximately 0.15 magnitudes) around voff = 0 km/s. We demonstrate that Mg II absorbers impact redshift estimation for QSOs by investigating the distributions of voff for associated absorbers. We find that at z > 1.5 these distributions broaden and bifurcate in a nonphysical manner. In an effort to mitigate this effect, we mask pixels associated with the Mg II absorption lines and recalculate the QSO redshifts. We find that we can recover voff populations in better agreement with those for z < 1.5 absorbers and in doing so typically shift background QSO redshifts by delta_z approximately equal to plus or minus 0.005.

Figures

Figures reproduced from arXiv: 2412.15383 by the authors.

Figure 1
Figure 1. Histogram of absorber redshift values for the 50,674 absorption line systems in our sample. Bins are 0.1 redshift wide. to z = 1.5 which reflects the QSO redshift distribu￾tion in DESI (e.g. Chaussidon et al. 2023). At redshifts 1.5 < z < 2.3 the distribution somewhat flattens, and beyond z = 2.3 the number of detected absorbers sig￾nificantly decreases. This is a result of the noise level of DESI spectra increasing… view at source ↗
Figure 2
Figure 2. Visualization of our extinction-fitting process. Top: An example QSO spectrum with an absorber plotted alongside the blue QSO template, both shifted into the rest frame of the absorption system. Masked regions, including the edges of the spectrum as well as the emission and absorption lines, are shown in gray. Note that the template spectrum has been scaled arbitrarily. Bottom: The result of the fitting process, sho… view at source ↗
Figure 4
Figure 4. E(B-V) values in bins of voff . Error bars are the standard error of the mean in each bin. Note that at voff < 5000 km s−1 we have drawn samples every 200 km s−1 , whereas at voff > 5000 km s−1 they are drawn every 500 km s−1 . Black lines are overlaid at voff = 0 km s−1 and voff = 3500 km s−1 [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figures from the paper (5 more)
Figure 5
Figure 5. Figure 5: E(B-V) calculated for bins of absorber redshift. Bins are 0.04 units wide and the results are plotted for as￾sociated (voff < 3500 km s−1 ) and intervening (voff > 3500 km s−1 ) absorption systems, as well as for the sample of con￾trol QSOs [PITH_FULL_IMAGE:figures/fu…
Figure 6
Figure 6. Figure 6: E(B-V) calculated for bins of Wλ2796 0 . Bins are 0.2 ˚A wide and the results are plotted for both associated (voff < 3500 km s−1 ) and intervening (voff > 3500 km s−1 ) systems. 3.2. Redshift evolution of voff distributions We can now consider the evolution of E(B-V) …
Figure 7
Figure 7. Figure 7: Histogram distributions of voff values, drawn every 100 km s−1 and shown in light orange, overlaid with E(B-V) values, drawn every 1000 km s−1 and shown in dark purple, as calculated using redshift subsamples of associated absorbers. The redshift range of each panel is…
Figure 8
Figure 8. Figure 8: A recreation of [PITH_FULL_IMAGE:figures/full_fig_p010_8.png]
Figure 9
Figure 9. Figure 9: We have divided the sample into those sys￾tems initially redshifted or blueshifted relative to the background QSO, and in doing so it becomes clear that when the masking resulted in a change in the QSO red￾shift, the typical effect was to reduce the difference be￾tween…

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