The PAU Survey: Active galactic nucleus galaxies in galaxy pairs

arXiv:2610.30309 · astro-ph.GA, astro-ph.CO · Submitted 2026-10-21 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "The PAU Survey".

Jocelyn: Here's the gist of the paper. The PAU Survey investigates the connection between active galactic nuclei (AGNs) and galaxy pairs up to redshifts of approximately 1.0, providing evidence that interactions play an increasingly important role in triggering nuclear activity at intermediate redshifts How it works The study leveraged the high-precision photometric redshifts of the Physics of the Accelerating Universe Survey (PAUS) to investigate the connection between active galactic nuclei (AGNs) and galaxy pairs up to redshifts of ∼ 1.0.

Vera: Researchers identified 16,824 galaxy pairs with projected separations of rp < 100 kpc and radial-velocity differences of ∆V < 3500 km s−1.

Jocelyn: 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.

Vera: Control samples were constructed consisting of isolated AGNs, non-AGNs in pairs, and isolated non-AGNs matched in redshift, absolute magnitude, and environment.

Title and authors: Vera: So we're looking at this paper, "The PAU Survey: Active galactic nucleus galaxies in galaxy pairs." It comes from Duplancic and colleagues. Basically, they are using this survey to see how active galactic nuclei, or AGNs, connect with galaxy pairs up to a redshift of about one point zero <ref:2610.30309#pg1>.

Jocelyn: Right, it's about testing the idea that these interactions play a bigger role in starting nuclear activity at intermediate redshifts than we thought before. It’s like checking if the nudge from a galaxy partner really gets the AGN engine started on time > Page one of THIS PAPER > <ref:2610.30309#pg1>

Subrahmanyan: That connection to cosmic time is crucial because it helps us constrain how galaxy evolution models work, especially when we look at when and how these interactions trigger nuclear activity across cosmic time Alexander and Hickox two thousand twelve; Harrison and Ramos Almeida two thousand twenty-four <ref:2610.30309#pg1>.

Vera: Exactly. So what they did in the study is that they took the high-precision photometric redshifts from the PAU Survey to investigate this link between AGNs and galaxy pairs up to about redshift one. They found sixteen thousand eight hundred twenty-four galaxy pairs where the separation was less than one hundred kiloparsecs and their radial velocity difference was less than three five hundred kilometers per second > Page two of THIS PAPER > <ref:2610.30309#pg1>

Jocelyn: And out of all those, they cross-matched PAUS galaxies with multi-wavelength AGN catalogues and found two seven zero AGNs in pairs, which is out of a parent sample of eight thousand eight hundred eighty AGNs in the PAU Survey > Page three of THIS PAPER > <ref:2610.30309#pg1>

Subrahmanyan: That initial number gives us a baseline for how frequently we see this interaction effect compared to isolated systems. What’s interesting is that they built control samples, which included isolated AGNs, non-AGNs in pairs, and isolated non-AGNs matched in redshift and absolute magnitude and environment > Page two of THIS PAPER > <ref:2610.30309#pg1>

Vera: They then looked at how galaxy properties evolve with redshift across different samples. Figure five shows the evolution of the Mg minus Mr colour indices as a function of redshift for four different samples > Page five of THIS PAPER > <ref:2610.30309#pg1>

Jocelyn: What they found is that active galactic nuclei in pairs show progressively redder rest-frame colours as redshift increases, especially at redshifts greater than zero point five, whereas isolated AGNs and non-AGNs in pairs don't show significant colour evolution and stay around Mg minus Mr equal to zero point six five > Page five of THIS PAPER > <ref:2610.30309#pg1>

Subrahmanyan: That suggests that the interaction environment itself is influencing the dust or star formation properties around these active systems as we look back in time. What this means for structure formation models is that we need to account for these environmental factors when predicting galaxy properties at higher redshifts.

Title and authors: Vera: They also noted that pairs hosting AGNs are often morphologically disturbed interacting systems, found at smaller projected separations and velocity offsets, which lines up with having more dynamically advanced interactions > Page five of THIS PAPER > <ref:2610.30309#pg1>

Jocelyn: That leads into how they looked at the physical disturbances themselves. They used images from the DESI Legacy Survey to classify galaxies with tidal features or bridges as MD galaxies, and pairs hosting at least one such galaxy were defined as MD pairs > Page five of THIS PAPER > <ref:2610.30309#pg1>

Subrahmanyan: The results show that twenty four percent, which is fifty out of a sample, of the AGNs in the pair sample have these morphological disturbance features, compared to twelve percent in the control two sample > Page five of THIS PAPER > <ref:2610.30309#pg1>

Vera: And they also found that MD pairs are preferentially located at small projected separations and low relative velocities. This supports their classification as physically bound systems undergoing strong gravitational interactions > Page five of THIS PAPER > <ref:2610.30309#pg1>

Jocelyn: So, tying it all together, they defined a normalised radial distance R to characterize how close the galaxies are in projected phase space by combining rp and delta V into one dimensionless parameter > Page five of THIS PAPER > <ref:2610.30309#pg1>

Subrahmanyan: And partitioning the sample into four regions based on R allowed them to isolate the most compact and potentially bound systems from those at the outskirts of the interaction parameter space. This helps us understand where these triggering events are happening physically.

Vera: Then they used a proximity index, PI, which quantifies the dynamical state of these systems, revealing a clear evolutionary trend for pairs hosting AGNs, moving from looser to tighter configurations towards lower redshifts > Page five of THIS PAPER > <ref:2610.30309#pg1>

Jocelyn: That shift toward tighter configurations as we move toward lower redshifts suggests that the triggering mechanism might be more prevalent in later stages of galaxy evolution. What this tells us is that the interaction strength changes over cosmic time.

Subrahmanyan: It confirms that interaction-related processes in pairs can influence the photometric properties of AGN hosts, and these redder colours suggest increased dust extinction or perhaps partial star formation quenching, or even a different population of AGNs associated with interacting systems > Page five of THIS PAPER > <ref:2610.30309#pg1>

Vera: The persistent excess of disturbed morphologies among the AGN population suggests that nuclear activity is preferentially triggered or sustained in systems showing clear observational evidence of ongoing interactions across the studied redshift interval > Page five of THIS PAPER > <ref:2610.30309#pg1>

Jocelyn: This whole study, "The PAU Survey: Active galactic nucleus galaxies in galaxy pairs," uses the high precision photo-z from PAUS to provide evidence for the link between galaxy–galaxy interactions and AGN activity, highlighting their evolving influence across cosmic time. What this suggests is that morphology and dynamics are key indicators of AGN triggering.

Title and authors: Subrahmanyan: The distribution of AGNs in pairs and control two non-AGNs in the rp versus delta V plane reveals that MD pairs are preferentially located at small projected separations and low relative velocities > Page five of THIS PAPER > <ref:2610.30309#pg1>

Vera: And PI also reveals that there's a clear evolutionary trend for pairs hosting AGNs, transitioning from looser to tighter configurations towards lower redshifts > Page five of THIS PAPER > <ref:2610.30309#pg1>

Jocelyn: So we're seeing a clear picture where the dynamical state and the interaction strength evolve as galaxies age. This sets up some interesting questions for future work.

Subrahmanyan: I think it opens the door for better AGN triggering models, because now we can predict when galaxy interactions trigger nuclear activity by using redshift-dependent colour evolution, specifically noting that active galactic nuclei in pairs show progressively redder rest-frame colours with increasing redshift, particularly at redshifts of z >= zero point five.

Vera: That’s a big deal for modeling. It means we can refine our predictions about when this interaction effect becomes most visible in the early universe.

Jocelyn: And it also helps us think about how we might use machine learning classifiers to distinguish between AGN hosts and non-AGN control samples by utilizing the statistical separation observed in colour indices, such as the statistically significant difference in Mg minus Mr and Mg minus Mz colours between AGNs in pairs and the control sample of isolated AGNs >

Subrahmanyan: If AI systems can use those colour differences, they could predict the dynamical state of galaxy pairs by using a normalised radial distance R, which is defined as R equals s over rp over rp max!two plus dV over dVmax!two to classify systems into distinct dynamical regions like R1, R2, etc., linking these regions to specific AGN activity levels >

Vera: That sounds like a way to get more predictive power from the observational data. It moves us from just describing what we see to actually predicting what will happen next based on those physical parameters.

Jocelyn: And automated morphological classification tools can be improved by using false-colour images from the DESI Legacy Survey DR10 and achieving an eighty five percent agreement rate in classifying galaxies as morphologically disturbed MD within pairs hosting AGNs >

Subrahmanyan: If we can get that kind of reliable classification, it feeds into refining predictive models for AGN host properties by incorporating environmental effects by incorporating a luminosity density contrast delta, delta to characterize the background density and ensure control samples reproduce the distributions of AGNs in pairs across redshift, adopting maximum allowed differences of zero point zero five, zero point two five, and zero point one >

Vera: That addresses the need to properly model how we account for environmental effects when trying to predict what's happening on the host galaxy level. It’s about making those control samples match our real world better.

Title and authors: Jocelyn: It sounds like a roadmap for how we can use these tools to better understand the connection between galaxy interactions and AGN activity across cosmic time, building on what we saw in this PAU Survey: Active galactic nucleus galaxies in galaxy pairs.

Subrahmanyan: We've seen how the colour indices evolve with redshift, becoming redder at higher redshifts while isolated AGNs remain comparatively bluer > Page five of THIS PAPER > <ref:2610.30309#pg1>

Vera: And we found a statistically significant difference in those Mg minus Mr and Mg minus Mz colours between AGNs in pairs and the control sample of isolated AGNs > Page five of THIS PAPER > <ref:2610.30309#pg1>

Jocelyn: Plus, the distribution of AGNs in pairs and control two non-AGNs in the rp versus delta V plane reveals that MD pairs are preferentially located at small projected separations and low relative velocities > Page five of THIS PAPER > <ref:2610.30309#pg1>

Subrahmanyan: And finally, PI reveals a clear evolutionary trend for pairs hosting AGNs, transitioning from looser to tighter configurations towards lower redshifts > Page five of THIS PAPER > <ref:2610.30309#pg1>

Vera: So in summary, this paper provides evidence that interaction-related processes in pairs may influence the photometric properties of AGN hosts. The redder colours observed could indicate increased dust extinction or partial star formation quenching or a distinct AGN population associated with interacting systems.

Jocelyn: And the persistent excess of disturbed morphologies among the AGN population suggests that nuclear activity is preferentially triggered or sustained in systems showing clear observational evidence of ongoing interactions > Page five of THIS PAPER > <ref:2610.30309#pg1>

Subrahmanyan: The link between morphological signatures of interaction and the presence of AGNs remains stable across the studied redshift interval, which is a solid finding for structure formation studies.

Vera: This work leverages PAUS unique photo-z precision in providing evidence for the link between galaxy–galaxy interactions and AGNs activity, highlighting their evolving influence across cosmic time.

Jocelyn: We're getting a clearer view of how these processes interact dynamically as galaxies evolve through time. What’s next for this kind of study?

Subrahmanyan: I think we need to look at how these merger-driven accretion episodes connect with the black hole growth history and see if those models hold up under more extreme conditions.

Vera: We should definitely keep pushing on those observational constraints, especially as the PAU Survey continues to provide data. This paper gives us a strong foundation for what we expect to see in future surveys.

Jocelyn: It’s exciting seeing how these dynamical states evolve with redshift, and I wonder if we can use time-domain observations to catch these interactions happening in real-time as they happen.

Subrahmanyan: And that would give us the perfect way to confirm or rule out the connection between merger signatures and the fueling of super-Eddington accretion episodes.

The paper's summary: Vera: So to recap, this study took the PAU Survey data and looked at galaxy pairs up to redshift one to see if those interactions actually trigger active galactic nuclei activity in a way we hadn't seen before.

Jocelyn: Right, it's essentially testing the idea that when galaxies bump into each other, that nudge is what kicks the black hole engine into overdrive more often than we thought at those intermediate redshifts.

Vera: Exactly. The main takeaway is that these interactions aren't just happening randomly; they seem to be linked to specific physical properties of the AGN hosts, like their color and their shape.

Jocelyn: I mean, the paper shows that AGNs in pairs get progressively redder as you look back in time, especially above redshift zero point five. That’s a pretty clear signal from the interaction environment itself affecting how we see those AGN hosts.

Vera: That’s what it says. And it's not just a color thing; they found that the morphological disturbance—those tidal features, bridges—is much more common in AGNs within pairs than in isolated galaxies or non-AGNs in pairs.

Jocelyn: Yeah, that tells us nuclear activity isn't just happening wherever you find a black hole; it’s being preferentially triggered or sustained when the system is clearly interacting physically.

Vera: And they found that those disturbed pairs are clustered at smaller projected separations and lower relative velocities, which confirms they are genuinely bound systems undergoing strong gravitational interactions.

Jocelyn: That's how you confirm the physics—you find these close, fast-moving, visibly disturbed pairs hosting an AGN. It’s a tight observational constraint on what triggers the fueling process.

Vera: The whole point of using that PAUS photometric redshift precision was to get this clean link between galaxy-galaxy interaction and AGN activity across cosmic time.

Jocelyn: It really shows how the dynamical state of a pair, which evolves from looser to tighter as we go back toward lower redshifts, directly influences the photometric properties of the black hole's host galaxy.

Vera: And that shift in dynamical state is what’s really interesting for structure formation models—it means we have to factor in how interaction strength changes over cosmic time when predicting AGN fueling.

Jocelyn: What this suggests is that these merger-driven accretion episodes are tied to specific physical configurations, giving us a better target for follow-up observations looking at those black hole growth histories.

Vera: So, we've established that the link between morphological signatures and nuclear activity is stable across the redshift range they studied.

Jocelyn: Which opens up some interesting avenues for future work, like using time-domain observations to catch these interactions happening in real-time as they occur.

The paper's improvements: Tom: So we're looking at how this paper suggests ways to actually make these kinds of studies better and more predictive than just describing what you see now.

Vera: Right, the authors are pointing toward a few specific improvements for how we look at these galaxy pairs and their AGN hosts.

Jocelyn: They’re suggesting using redshift-dependent color evolution in a new way to predict exactly when those interactions start triggering nuclear activity by looking at redshifts greater than zero point five.

Vera: That’s important because it gives us a predictive tool for modeling; we can now forecast when we should expect to see these interaction effects becoming more visible in the early universe.

Jocelyn: And they're pushing for better AI classifiers to distinguish between actual AGN hosts and non-AGN control samples by using that statistical separation in color indices, like the difference between AGNs in pairs and isolated AGNs.

Vera: That means we can train machine learning models to be much more precise when identifying active galaxies based on their photometric properties alone.

Jocelyn: They also propose an AI system to predict the dynamical state of these pairs using a normalised radial distance R, which is defined by combining the projected separation and velocity difference into one dimensionless parameter.

Vera: That would let us classify those systems into distinct regions, like R1 or R2, and link those specific regions to different levels of AGN activity.

Jocelyn: Plus, they're suggesting improving automated morphological classification tools by using false-colour images from the DESI Legacy Survey DR10 to hit an eighty five percent agreement rate when identifying those disturbed galaxies in pairs.

Vera: That level of reliability in classifying those physical interactions is what we need to make these links between morphology and AGN triggering more robust.

Jocelyn: And for modeling, they recommend incorporating a luminosity density contrast delta into the control samples to characterize the background density, making sure those control samples match the real distributions across all redshifts.

Vera: That addresses a real challenge in observational astronomy; you need your control groups to accurately reflect the cosmic environment.

Jocelyn: So these improvements move us from just finding evidence of a link to building actual predictive models that can forecast when and where those interactions will be most influential.

Conclusion: Tom: So to wrap up, we’re talking about how the PAU Survey data shows that galaxy interactions are a key driver in triggering nuclear activity at intermediate redshifts, looking at what we call "The PAU Survey: Active galactic nucleus galaxies in galaxy pairs."

Vera: Exactly. We established that these interactions aren't random; they seem to be linked to specific physical properties of the AGN hosts, like their color and their shape.

Jocelyn: And those redder colors as redshift increases, especially above zero point five, tell us the interaction environment itself is changing how we see those AGN hosts over cosmic time.

Subrahmanyan: From a cosmological standpoint, it means we have to factor in how interaction strength changes over cosmic time when predicting AGN fueling across different epochs of galaxy evolution.

Vera: Right. And that persistent excess of disturbed morphologies among the AGNs suggests nuclear activity is preferentially triggered or sustained when there’s clear evidence of ongoing physical interactions.

Jocelyn: It’s a solid finding that links the visual signature of a merger to the presence of an active black hole, and it supports our idea that these systems are physically bound under strong gravitational forces.

Subrahmanyan: If we can confirm this link holds across more redshifts, it gives us a crucial piece of data for structure formation models about how black hole growth connects to large-scale merger events.

Vera: The results from this work leverage PAUS unique photo-z precision to provide evidence for the link between galaxy–galaxy interactions and AGN activity, highlighting their evolving influence across cosmic time.

Jocelyn: It’s a great use of that survey data because it moves us past just seeing AGNs and starts looking at the dynamics that cause them.

Subrahmanyan: We still need observations that can confirm these dynamical states are truly what drives the accretion episodes, so future work should focus on those merger-driven accretion episodes connecting with black hole growth history.

F. Duplancic, M. Siudek, D. Navarro-Gironés, E. J. Gonzalez, F. Rodriguez, S. Alonso, I. V. Daza-Perilla, D. G Lambas, E Gaztañaga, A L O’Mill6 M Eriksen7 J Carretero13 R Casas4 F J Castander4 J García-Bellido14 H Hildebrandt15 M Mezcua4 R Miquel7 C Padilla7 P Renard4 E Sánchez18 S Serrano4 I Sevilla-Noarbe P Tallada-Crespí

Gabinete de Astronomía Extragaláctica · Instituto de Astrofísica de Canarias · Institute of Space Sciences (ICE, CSIC) · Universidad Nacional de Córdoba · Institut d’Estudis Espacials de Catalunya · Center for Space Science and Technology · Institute of Cosmology & Gravitation

astro-ph.GA, astro-ph.CO

Submitted: 2026-10-21

Updated: 2026-10-21

Comments: 16 pages, 18 figures, accepted by A&A

Journal ref: Volume 714, 2026, A74

DOI: 10.1051/0004-6361/202556897

License: http://creativecommons.org/licenses/by/4.0/

The gist: The PAU Survey investigates the connection between active galactic nuclei (AGNs) and galaxy pairs up to redshifts of approximately 1.0, providing evidence that interactions play an increasingly

Key concepts

Active Galactic Nuclei (AGNs)

These are extremely bright centers of galaxies powered by supermassive black holes. The study focused on identifying these AGNs within galaxy pairs to see if the interaction between two galaxies causes the black hole activity.

Morphological Disturbance (MD) Pairs

These are galaxy pairs that show visual signs of interaction, such as tidal features or bridges. The research found that 24% of AGNs in pairs exhibit these disturbances, suggesting that physical interaction is a key trigger for AGN activity.

Colour Evolution (Mg-Mr and Mg-Mz)

This refers to how the rest-frame colours of galaxies change as we look further back in time (higher redshift). The study observed that AGNs in pairs become redder at higher redshifts, which might be due to increased dust or changes in star formation when galaxies interact.

Proximity Index (PI)

This index measures the dynamical state of a galaxy pair by combining projected separation and velocity differences. It showed an evolutionary trend where pairs hosting AGNs transition from looser configurations to tighter, more compact systems as redshift decreases.

Summary

Improvements for AI systems

Abstract

Sources

• Data Release 1 of the Dark Energy Spectroscopic Instrument

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