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
Why Pith says this
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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
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
- 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.
The abstract, sentence by sentence
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
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| 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 was examined, and what was not
- arXiv listing abstract v1
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Figures
Figures reproduced from arXiv:2610.30309 by the authors.
References
Discussion
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