Pith. Sign in

REVIEW

The PAU Survey: Active galactic nucleus galaxies in galaxy pairs

Astrophysics of Galaxies · Submitted 21 Oct 2026 · Published version · 210 references

Pith's verdict: PAUS photo-z allow a well-controlled study of AGN in pairs at z~0.5-1.0, showing a reddening trend and excess morphological disturbances; the results are suggestive but rest on a pair catalogue whose contamination estimate is acknowledged as non-representative.

T0 · AI review

How this review was made: An AI referee (deepseek/deepseek-v4-flash) read the paper on October 23, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

  1. T0 AI review An AI read the paper and answered a fixed list of questions.
  2. T1 Audit Automatic checks that the references and files the paper relies on exist and match.
  3. T2 Compute The paper's code or calculations re-run with the same results.
  4. T3 Formal statement The claims written out in a formal math language a computer can read.
  5. T4 Proof Those claims proved, and the proof checked by computer.

How Pith checks papers →

· 2 major objections · 8 minor · Ask about this paper · The compiled record · Read the full review →
Challenge this review Comment (0) Read on arXiv ↗
Cite ▾
arXiv ID2610.30309 (version 1)
Pith IDPQSST3UX, Pith's permanent record of this paper and its review
CitationA. L. O'Mill, C. Padilla, D. G. Lambas, D. Navarro-Giron\'es, E. Gazta\~naga, E. J. Gonzalez, E. S\'anchez, F. Duplancic, F. J. Castander, F. Rodriguez, H. Hildebrandt, I. Sevilla-Noarbe, I. V. Daza-Perilla, J. Carretero, J. Garc\'ia-Bellido, M. Eriksen, M. Mezcua, M. Siudek, P. Renard, P. Tallada-Cresp\'i, R. Casas, R. Miquel, S. Alonso, S. Serrano. "The PAU Survey: Active galactic nucleus galaxies in galaxy pairs." arXiv:2610.30309 (2026). https://pith.science/paper/PQSST3UX
Cite this reviewBibTeX for Pith's review of this paper
Claim paper

Why Pith says this

Highlight any text to comment on it.

Referee report
The paper

The paper claims

Using high-precision photometric redshifts from the PAU survey, this study finds that active galactic nuclei (AGNs) in galaxy pairs become progressively redder with redshift, especially beyond z=0.6, while isolated AGNs and non-AGN pairs…

The reading

This paper aims to establish a redshift-dependent link between galaxy interactions and AGN activity by exploiting the high-precision photometric redshifts of the PAU survey (σ68 ≤ 0.02). The authors identify 11,635 galaxy pairs out to z~1 and cross-match with multi-wavelength AGN catalogues to find 214 AGN hosts in pairs. They construct carefully matched control samples of isolated AGNs, non-AGN pairs, and isolated non-AGNs. The central finding is that AGNs in pairs show progressively redder rest-frame colours with increasing redshift (especially z > 0.6), whereas control samples show flat colour trends. Additionally, pairs hosting AGNs are preferentially morphologically disturbed and have smaller projected separations and velocity offsets. These results suggest that the efficiency of interactions in triggering nuclear activity increases at intermediate redshifts (z ~ 0.5–1.0), a regime where previous higher-redshift studies often failed to detect a connection. The work matters because it provides observational evidence that the interaction–AGN connection evolves with cosmic time, constraining models of black hole–galaxy co-evolution.

What carries the argument

The central enabler is the PAU survey's narrow-band photometric redshifts (40 filters, 4500–8500 Å, FWHM 130 Å) achieving σ68 ≤ 0.02 for iAB < 23 galaxies over 43 deg², which allows reliable pair identification to z~1. The pair-finding algorithm (Rodriguez et al. 2020) uses thresholds r_p < 100 kpc, ΔV < 3500 km/s, and an isolation criterion to exclude richer structures. AGN selection combines four complementary catalogues: mid-infrared (WISE R90), optical emission-line (BPT from SDSS), X-ray (2SXPS/4XMM with Bayesian classification), and a quasar catalogue (Milliquas). Control samples are matched in redshift, i-band absolute magnitude, and local overdensity (δ) to isolate the pair environment effect. Visual morphological classification from DESI Legacy Survey images identifies 'morphologically disturbed' (MD) pairs, providing a direct interaction signature.

Extended reading notes

Core claim

AGNs hosted by galaxy pairs show a clear redshift evolution in rest-frame colours: both (Mg−Mr) and (Mg−Mz) redden progressively towards higher redshifts, particularly beyond z ~ 0.5–0.6. In contrast, isolated AGNs and non-AGNs in pairs maintain flat colour trends around Mg−Mr ~ 0.65 across the same redshift range. The colour difference between AGNs in pairs and isolated AGNs exceeds 3σ at z > 0.6, and persists at 2σ after mass re-weighting. Furthermore, the fraction of morphologically disturbed galaxies is consistently ~12 percentage points higher among AGN in pairs than among non-AGN pairs (24% vs 12%). AGN-hosting pairs also exhibit smaller projected separations and velocity offsets, and a proximity index that evolves with redshift: AGN pairs become progressively tighter at lower redshifts, while non-AGN pairs show no evolution. The authors interpret these combined trends as evidence that galaxy interactions play an increasingly important role in triggering nuclear activity at intermediate redshifts (z ~ 0.5–1.0), with a link that weakens towards the local Universe.

Load-bearing premise

The pair catalogue's purity depends on the assumption that the photo-z precision and thresholds (ΔV < 3500 km/s, r_p < 100 kpc) distinguish physical pairs from chance projections, but the paper's own contamination analysis (based on a small, non-representative spec-spec subsample) finds an upper limit of ~40% spurious pairs, and if contamination differs systematically between AGN hosts and non-AGN hosts, the reported colour and morphological trends could be biased.

Editorial extensions

If this is right

  • If the redshift-dependent colour and morphological trends are physical, then models of AGN triggering must incorporate a declining efficiency of interaction-driven fuelling from z~1 to the present.
  • Future hydrodynamic simulations with AGN feedback (e.g., IllustrisTNG, EAGLE) should reproduce the observed colour–redshift relation for AGN in pairs, providing a test of feedback and gas circulation prescriptions.
  • The redder colours of pair-hosted AGNs at z>0.6 imply either enhanced dust obscuration or a distinct AGN population in interacting systems at these epochs, which will affect completeness of optical AGN surveys.
  • The work demonstrates that narrow-band photometric surveys can rival spectroscopic pair studies to z~1, opening a new window for interaction studies in wide fields without fibre-collision biases.
  • Extending the analysis to deeper fields or higher redshifts (z>1) will test whether the colour trend continues or reverses as one approaches the peak of cosmic star formation and AGN activity.

Reading between the lines

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

  • The colour reddening could be driven by increased dust obscuration in the central regions of interacting galaxies at higher redshift, implying that pairs at z>0.6 preferentially host obscured or mildly obscured AGN that are more efficiently revealed by MIR and X-ray selection.
  • If the observed colour trend is partially due to enhanced star formation in the pair hosts (rather than solely AGN-related dust), then the star formation–AGN connection in mergers must also be redshift-dependent — a prediction that could be tested with far-IR/submm data for the same sample.
  • The visual morphological classification, while iteratively refined, remains subjective and limited by surface-brightness sensitivity at higher redshift; automated merger statistics (e.g., Gini, M20, asymmetry) applied to deeper imaging could confirm the trend without human bias.
  • The reported upper limit of ~40% contamination in the pair catalogue, based on a small spec-spec subsample, leaves open the possibility that systematic projection effects differ for AGN hosts versus non-AGN hosts, which could affect the colour comparisons; a larger spectroscopic follow-up of PAUS pairs would settle this.
Original abstract

Aims. Leveraging the high-precision photometric redshifts of the Physics of the Accelerating Universe Survey (PAUS), we aim to investigate the connection between active galactic nuclei (AGNs) and galaxy pairs up to redshifts of $\sim1.0$. Methods. We identified 16,824 galaxy pairs with projected separations of $r_p < 100$ kpc and radial-velocity differences of $\Delta V < 3500$ km s$^{-1}$. AGNs were selected by cross-matching PAUS galaxies with multi-wavelength AGN catalogues, yielding 270 AGNs in pairs out of a parent sample of 8,880 AGNs in PAUS. We constructed control samples of isolated AGNs; non-AGNs in pairs; and isolated non-AGNs matched in redshift, absolute magnitude, and environment. Results. Active galactic nuclei in pairs show progressively redder rest-frame colours with increasing redshift, particularly at redshifts of $z\gtrsim 0.5$, whereas isolated AGNs and non-AGNs in pairs show no significant colour evolution and remain at $\mathrm{M_g-M_r}\sim0.65$. Pairs hosting AGNs are also preferentially morphologically disturbed interacting systems found at smaller projected separations and velocity offsets, consistent with more dynamically advanced interactions. Conclusions. These results provide evidence of a redshift-dependent link between galaxy interactions and AGN activity, suggesting that interactions play an increasingly important role in triggering nuclear activity at intermediate redshifts. This study highlights the unique capability of PAUS to probe AGN-galaxy interaction connections beyond the local Universe with unprecedented statistical power.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-10-23: 1 request, 7,238 input tokens.

What the abstract itself states, as probabilities: claims first 0.06 · states limitation 0.07 · human participants 0.01 · controlled comparison 0.98 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.54 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Aims. Leveraging the high-precision photometric redshifts of the Physics of the Accelerating Universe Survey (PAUS), we aim to investigate the connection between active galactic nuclei (AGNs) and galaxy pairs up to redshifts of $\sim1.0$. Methods. We identified 16,824 galaxy pairs with projected separations of $r_p < 100$ kpc and radial-velocity differences of $\Delta V < 3500$ km s$^{-1}$. AGNs were selected by cross-matching PAUS galaxies with multi-wavelength AGN catalogues, yielding 270 AGNs in pairs out of a parent sample of 8,880 AGNs in PAUS. We constructed control samples of isolated AGNs; non-AGNs in pairs; and isolated non-AGNs matched in redshift, absolute magnitude, and environment. Results. Active galactic nuclei in pairs show progressively redder rest-frame colours with increasing redshift, particularly at redshifts of $z\gtrsim 0.5$, whereas isolated AGNs and non-AGNs in pairs show no significant colour evolution and remain at $\mathrm{M_g-M_r}\sim0.65$. Pairs hosting AGNs are also preferentially morphologically disturbed interacting systems found at smaller projected separations and velocity offsets, consistent with more dynamically advanced interactions. Conclusions. These results provide evidence of a redshift-dependent link between galaxy interactions and AGN activity, suggesting that interactions play an increasingly important role in triggering nuclear activity at intermediate redshifts. This study highlights the unique capability of PAUS to probe AGN-galaxy interaction connections beyond the local Universe with unprecedented statistical power.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Aims. background 0.55 0.03 0.01 unclear 1.00
s1 Leveraging the high-precision photometric redshifts of the Physics of the Accelerating Universe Survey (PAUS), we aim to investigate the connection between active galactic nuclei (AGNs) and galaxy pairs up to redshifts of $\sim1.0$. Methods. contribution 0.63 0.34 0.01 unclear 0.62
s2 We identified 16,824 galaxy pairs with projected separations of $r_p < 100$ kpc and radial-velocity differences of $\Delta V < 3500$ km s$^{-1}$. method 0.99 0.97 0.01 asserted 1.00
s3 AGNs were selected by cross-matching PAUS galaxies with multi-wavelength AGN catalogues, yielding 270 AGNs in pairs out of a parent sample of 8,880 AGNs in PAUS. method 0.98 0.96 0.02 asserted 1.00
s4 We constructed control samples of isolated AGNs; non-AGNs in pairs; and isolated non-AGNs matched in redshift, absolute magnitude, and environment. Results. method 0.99 0.95 0.01 asserted 0.79
s5 Active galactic nuclei in pairs show progressively redder rest-frame colours with increasing redshift, particularly at redshifts of $z\gtrsim 0.5$, whereas isolated AGNs and non-AGNs in pairs show no significant colour evolution and remain at $\mathrm{M_g-M_r}\sim0.65$. result 1.00 0.94 0.15 asserted 1.00
s6 Pairs hosting AGNs are also preferentially morphologically disturbed interacting systems found at smaller projected separations and velocity offsets, consistent with more dynamically advanced interactions. Conclusions. result 0.98 0.92 0.09 hedged possibility 0.52
s7 These results provide evidence of a redshift-dependent link between galaxy interactions and AGN activity, suggesting that interactions play an increasingly important role in triggering nuclear activity at intermediate redshifts. interpretation 1.00 0.87 0.96 hedged possibility 0.89
s8 This study highlights the unique capability of PAUS to probe AGN-galaxy interaction connections beyond the local Universe with unprecedented statistical power. contribution 0.55 0.51 0.07 asserted 0.99

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from analysis of existing data 0.92.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Distribution of pair photo-z (zpairs; top), i-band absolute magnitude (Mi) of the brightest and the faintest galaxy in the system (bottom). The inset shows the relation between Mi and stellar mass (log(M∗/M⊙)) for galaxies in the pair sample. be genuine. This MIR-based approach is highly efficient at iden￾tifying dust-obscured AGNs, since ultraviolet (UV) and optical emission from the accretion disc absorbed by circumnuclear dust is reprocessed and re-emitted at MIR wavelengths. Regarding optical emission-line diagnostics (BPT-AGNs), we used the SDSS Portsmouth emission-line…
Figure 2. Distributions of photo-z (zb; top) and i-band absolute magnitude (Mi ; bottom) for the sample of 7342 AGNs and for each AGN subsample. We also included the distribution of 7342 non-AGN PAUS galaxies. At the top of the panels we show the number of galaxies, median value, and Q1 and Q3 percentiles of each sample. It is worth noting that PAUS fields are fully covered by the different AGN catalogues. Integrating samples across MIR, X￾ray, optical, and quasar catalogues provides a comprehensive perspective on the AGN population. This multi-wavelength anal￾ysis enhances the reliability of…
Figure 3. Normalised distributions of redshift (zb; top), absolute i￾band magnitude (Mi ; middle), and overdensity (δ; bottom). For each parameter, the upper panels show the distributions of the original samples: AGNs in pairs (black line with grey shad￾ing), isolated AGNs (dashed blue line), non-AGNs in pairs (dot￾dashed magenta line), and isolated non-AGNs (dotted red line). The lower panels display the corresponding distributions for the control samples matched to the AGNs in pairs: control 1 (iso￾lated AGNs, dashed blue line), control 2 (non-AGNs in pairs, dot-dashed magenta line), and…
Figure 4. Confusion matrices showing the distribution of the three
Figure 5. shows the evolution of the (Mg − Mr) and (Mg − Mz) colour indices as a function of redshift. To characterize the un￾derlying trends in these galaxy properties, we employed a non￾parametric smoothing approach based on locally weighted scat￾terplot smoothing (LOWESS). This method allows us to model the median behaviour of the population without assuming a spe￾cific functional form. The degree of smoothing was controlled by a fractional bandwidth parameter f = 0.3, which determines the local bandwidth, h, as the distance to the f · N th nearest neigh￾bour. This adaptive bandwidth…
Figure 6. Examples of non-MD (top) and MD (bottom) pairs at different redshifts. Images are 30′′ × 30′′ cut-outs from https:// www.legacysurvey.org/viewer/, using data from DR10 (Credit: Legacy Surveys / D. Lang (Perimeter Institute)). The markers indicate the pair barycentre (open red circle), the positions of both pair members (black crosses), and a scale bar. We also overlaid the coordinates and redshift of the pair in the bottom right corner. the different control samples (controls 1, 2, and 3). In the stan￾dard relations (main panels), AGNs in pairs exhibit the reddest colours for…
Figure 7. Top: Fraction of galaxies in MD pairs (FMD) for the AGN pair sample (solid black line) and the control 2 non-AGN pair sample (dot-dashed magenta line), shown in different redshift bins. The total number of galaxies in each sample per bin is listed in the figure. Bottom: Absolute difference between the two frac￾tions, |d|, as a function of redshift. Errors were estimated using the beta distribution quantile technique (Cameron 2011). The visual classification was performed by the first author of this paper and refined iteratively until a 85% agreement rate was achieved. The final…
Figure 8. Normalised distributions of projected separation,
Figure 9. Normalised radial-velocity difference (∆V/∆Vmax) versus normalised projected distance (rp/rp,max) plane. The marked re￾gions (R1–R4) correspond to the R quantile segmentation and are colour-coded according to the average photo-z (⟨zb⟩) of the individual paired galaxies within each shell. The number N in￾dicates the total count of individual galaxies. Errors were cal￾culated using bootstrap resampling. The upper panel shows the AGN pair sample, while the lower panel displays the corre￾sponding control 2 sample of non-AGN pairs. Dots represent the subsamples of MD pairs. requirement…
Figure 10. Average value of PI as a function of mean photo-z ⟨zb⟩. The solid black line represents pairs hosting AGNs, while the dot-dashed magenta line corresponds to pairs hosting non-AGNs in the control 2 sample. The numbers of objects in each interval are listed. The errors were calculated using bootstrap resampling techniques. AGNs. These samples were carefully selected to match the red￾shift, absolute magnitude, and local overdensity distributions of AGNs in pair systems. Such control samples are crucial for reli￾ably identifying similarities and differences in galaxy properties when…

Figures reproduced from arXiv:2610.30309 by the authors.

References

Discussion

Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.