The progenitor galaxies of the intracluster light

arXiv:2610.10785 · astro-ph.GA · Submitted 2026-10-07 · 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, and with me are Jocelyn and Subrahmanyan, guest researcher.

Jocelyn: Today's paper: "The progenitor galaxies of the intracluster light".

Vera: The gist The ICL progenitor properties are impacted by structure history and are therefore highly individual to each system,

Jocelyn: First, who's behind it and why it matters.

Title and authors: Vera: Now we look at the actual summary of this paper, "The progenitor galaxies of the intracluster light," to see exactly what they found regarding the assembly process. They emphasize that the ICL assembles from a large number of intermediate-mass progenitors, which is quite different from how they describe BCGs.

Jocelyn: So, if you think about it this way, the BCG builds up from a few massive progenitors through major mergers, whereas the ICL is built up by assembling a much larger collection of these smaller pieces over time.

Subrahmanyan: That distinction in assembly history is important because it implies different dominant physical processes are at play when forming these two components of the cluster environment. One relies more on accretion and stripping, while the other leans more into violent mergers.

Vera: And they quantify this difference by looking at the mean stellar mass of the most significant progenitor. They found that for the BCG, that most significant progenitor has a mean stellar mass of one x ten eleven Msun.

Jocelyn: That’s a big number, and then for the ICL, they found that the mean stellar mass for its most significant progenitor is only five x ten ten Msun. It’s a difference of a factor of two there in terms of the largest contributors.

Subrahmanyan: That difference suggests that major mergers are more likely to contribute to the growth of the BCG rather than the ICL, which supports the idea that sustained accretion and stripping are key for building up that diffuse light.

Vera: They also highlight how preprocessed material, or quenched progenitors, contributes significantly to both components—about thirty-seven per cent for the ICL and roughly thirty-one per cent for the BCG.

Jocelyn: So even though the mechanisms are different, both components share a strong reliance on galaxies that have already finished their major star-forming phases. That’s a common thread here.

Subrahmanyan: The paper also finds that structures characterized by a high fraction of preprocessed ICL progenitors tend to be dynamically disturbed and less concentrated, which hints at the environmental effects these stripped galaxies undergo.

Vera: And they point out that for those structures with a high fraction of preprocessed ICL progenitors, their intracluster light is primarily stripped from surviving higher-mass galaxies rather than being built from completed mergers.

Jocelyn: That’s a distinction between stripping and merging as the primary way to create the diffuse stellar component, which is super useful for understanding the physics of cluster environments.

The paper's summary: Vera: Moving on, they suggest some ways to improve this type of study, particularly focusing on how structure history impacts these progenitor properties. They argue that since the properties of ICL progenitors are so individual to each system, we need more nuance.

Jocelyn: They propose looking closer at the diversity within a single structure mass regime. They say that for a given size, there’s real variation in what kind of progenitors formed the ICL and how they contributed.

Subrahmanyan: That diversity is important because it suggests that we can’t just use one average progenitor property for all structures of the same mass; we need to account for the specific formation history of each individual system.

Vera: They detail this by looking at the classification based on N90,ICL, which is a metric they use to describe structure properties, like whether it's relaxed or disturbed. For instance, low-mass structures with N90,ICL less than ten are more likely to share the most significant progenitor with the BCG <ref:2610.10785#pg1>.

Jocelyn: That’s interesting because it suggests that in smaller systems, the link to the central galaxy is stronger when you look at these specific metrics. It helps refine how we predict which progenitor will dominate.

Subrahmanyan: They also give a probabilistic breakdown for those N90,ICL values, showing that low-mass structures with N90,ICL less than ten are more likely to be disturbed and share the most significant progenitor with the BCG <ref:2610.10785#pg1>.

Vera: And they suggest we should pay attention to how quenching affects these progenitors. The fraction of preprocessed ICL progenitors increases in more massive structures, which means that mass is a key factor in determining the nature of the assembly.

Jocelyn: So, when we look at a cluster, its total mass seems to influence whether its ICL is dominated by mergers or stripping material from older populations.

Subrahmanyan: And they suggest that for those structures with a high fraction of preprocessed ICL progenitors, their intracluster light might be built differently because it comes from stripped galaxies rather than just the remnants of completed mergers.

Vera: So, to improve the study, they recommend focusing on the time evolution aspect more specifically, as this current work was focused exclusively on structures at redshift zero.

The paper's improvements: Jocelyn: So to wrap up, this paper "The progenitor galaxies of the intracluster light" shows that we are assembling the ICL from a lot of intermediate-mass progenitors, contrasting with the BCG's history of being built from just a few massive mergers.

Vera: And as we look at how to improve it, they stress that the properties of these ICL progenitors are highly individual and depend heavily on each system's formation history. They found that mass is important because more massive structures have more progenitors overall, but the specific types of progenitors vary greatly depending on when and how they formed.

Subrahmanyan: The implication for theory is clear: we need to model this assembly process with a lot of individual histories because the ICL isn't just one smooth component; it’s a complex collection of varied inputs.

Jocelyn: And for anyone listening who only hears this show, the main thing is that you can't treat all clusters the same way when you study their intracluster light; you have to consider their specific assembly path.

Vera: It’s a reminder that understanding where this diffuse light comes from requires tracking those individual building blocks through cosmic time. We're looking at how these things evolve as we look at redshift zero structures, which is what they focused on in this paper.

Subrahmanyan: I agree, focusing specifically on redshift zero structures gives us a solid baseline for understanding the local universe assembly before we worry about more complex, earlier epochs.

Jocelyn: That’s where it leads us next: how to better model those time evolutions and see how these systems change as they get older.

Vera: Exactly. We've seen what the paper found in "The progenitor galaxies of the intracluster light," and now we know we need to focus our attention on that next step.

Conclusion: Vera: So we’ve been looking at "The progenitor galaxies of the intracluster light" and what they found about how those big systems grow their intracluster light and what kind of galaxies are actually building up that diffuse glow.

Jocelyn: Right, so it boils down to this: the ICL isn't built from a few giant mergers like the BCG is, but from a huge number of smaller progenitors over time.

Subrahmanyan: That’s the core idea. The paper says because the ICL assembly is so individual to each system, you can't just use one average progenitor property for every structure mass.

Vera: They showed that the number of ICL progenitors goes up with the structure mass, but they also found that low-mass structures with N90,ICL less than ten are more likely to share their most significant progenitor with the BCG.

Jocelyn: That’s a specific way to look at it, linking those structural properties like N90,ICL directly to the central galaxy connection.

Subrahmanyan: And they also pointed out that for those low-mass structures, if you look at preprocessed material—the quenched galaxies—that fraction is significant in both the ICL and the BCG.

Vera: They noted that when you see a high fraction of preprocessed ICL progenitors, those structures tend to be dynamically disturbed and less concentrated.

Jocelyn: So it suggests that stripped galaxies play a big role in forming the ICL in certain types of environments.

Subrahmanyan: And they did flag something important: the mean mass of the most significant progenitor is actually bigger for the BCG, which is one x ten to eleven solar masses, compared to five x ten to ten solar masses for the ICL.

Vera: That difference in mass really highlights how different those two components are in their building blocks.

Jocelyn: It makes you think about whether major mergers or steady accretion are driving the growth of these things.

Subrahmanyan: Yeah, they suggested that major mergers are more likely to drive the growth of the BCG, whereas for the ICL, it's less so than what we might expect from a purely merger-driven scenario.

Vera: Overall, this paper on "The progenitor galaxies of the intracluster light" really emphasizes that assembly history matters way more than just static structure properties.

Jocelyn: It’s a lot to take in about how these structures get their light and what those components are made of.

Subrahmanyan: Exactly. It’s a reminder that we need to focus on the time evolution of these systems if we want to fully understand how they come together.

Vera: Moving on, now that we know what the ICL progenitors look like, it leaves us wondering how this assembly process changes over longer timescales.

Rebecca J. Mayes, Facundo A. Gómez, Antonela Monachesi

Departamento de Astronomía, Universidad de La Serena

astro-ph.GA

Submitted: 2026-10-07

Updated: 2026-10-07

Comments: 12 pages, 8 figures, Submitted to A&A

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

Importance score: 76/100

The gist: The gist The ICL progenitor properties are impacted by structure history and are therefore highly individual to each system, although the BCG and the ICL co-evolve <ref:2610.10785#pg6> How it works

Key concepts

Intracluster Light (ICL)
The ICL is a diffuse halo of stars found in galaxy clusters that isn't bound to any single galaxy. It forms through the accretion and tidal stripping of stars from numerous progenitor galaxies over cosmic time, making it a key component in cluster evolution.
Progenitor Galaxies
These are the individual galaxies that merge or are stripped to form the ICL or BCG. The study found that ICLs assemble from a large number of these smaller, intermediate-mass progenitors, contrasting with BCGs which grow from fewer, more massive progenitors.
Structure History Dependence
The properties of the progenitor galaxies forming the ICL are not fixed; they depend heavily on how and when the cluster structure formed. Different structures with the same total mass can have very different compositions of their ICL progenitors based on whether they were relaxed or disturbed.
Preprocessed Material
This refers to stars that have already undergone processes like quenching (stopping star formation). The study found that quenched progenitors contribute significantly to both the ICL and BCG, and this contribution is higher in more massive structures.

Terminology

Summary

The gist The ICL progenitor properties are impacted by structure history and are therefore highly individual to each system, although the BCG and the ICL co-evolve <ref:2610.10785#pg6>

How it works

The study utilized 127 clusters and groups in the Illustris-TNG100 simulation above a mass of 10 13 M⊙ to determine the properties of the progenitor galaxies of the ICL and compare them with those of their BCGs/BGGs <ref:2610.10785#pg4> The ICL is defined by applying a surface brightness cut at the Holmberg radius of 26.5 mag/arcsec-2, where star particles within this radius are defined as being attached to the BCG, and outside, the ICL <ref:2610.10785#pg4> The analysis also determines the masses and mass contributions of progenitor galaxies for both the ICL and BCG for comparison <ref:2610.10785#pg4>

Progenitor Characteristics

The results indicate that the number of progenitors of the ICL increases with structure mass, and the ICL has more progenitors than the BCG <ref:2610.10785#pg4> The ICL has a larger percentage of material contributed by low-mass galaxies <ref:2610.10785#pg4> A large fraction of the ICL was contributed by quenched galaxies, with this fraction increasing with galaxy mass <ref:2610.10785#pg4> The mean mass radial gradient of ex-situ material in structures for the ICL and the BCG+ICL is generally negative and increases with structure mass, indicating that material from more massive progenitors is found towards the centres of clusters <ref:2610.10785#pg4>

Structure History Dependence

The properties of ICL progenitor galaxies are impacted by structure history and are therefore highly individual to each system <ref:2610.10785#pg4> At a given structure mass, there is diversity in the properties of the progenitors that formed the ICL <ref:2610.10785#pg4> Low-mass structures with N90,ICL >30 are equally likely to be either relaxed or disturbed at 27 per cent (3/11) each, and slightly more likely to be in an intermediate state, at 45 per cent (5/11) <ref:2610.10785#pg5> Structures with N90,ICL <10 were slightly more likely to be disturbed, at 42 per cent (5/12), somewhat less likely to be relaxed, at 25 per cent (3/12), and moderately likely to be in an intermediate state, at 33 per cent (4/12) <ref:2610.10785#pg5> Low-mass structures with N90,ICL <10 were more likely to share the most significant progenitor with the BCG <ref:2610.10785#pg5>

Progenitor Mass and Merger History

The mean stellar mass of the most significant progenitor was greater for the BCG than the ICL, with a mean stellar mass for the most significant progenitor of the BCG being 1 × 10 11 M⊙, while the mean stellar mass for the most significant progenitor of the ICL was 5 × 10 10 M⊙ <ref:2610.10785#pg5> Major mergers are more likely to contribute to the growth of the BCG rather than the ICL <ref:2610.10785#pg5> Structures with a large percentage of mass coming from the N5 progenitors were more likely to share the most significant progenitor with the BCG <ref:2610.10785#pg5>

Preprocessed Material and Gradients

Preprocessed (quenched) progenitors contribute significantly to the accreted material of both the ICL (roughly 37 per cent) and the BCG (roughly 31 per cent), with the ICL’s quenched fraction increasing in more massive structures <ref:2610.10785#pg5> Structures characterised by a high fraction of preprocessed ICL progenitors tend to be dynamically disturbed and less concentrated; additionally, their ICL is primarily stripped from surviving, higher-mass galaxies rather than being built from completed mergers <ref:2610.10785#pg5> The ICL mean mass gradient slope was positive with a slope of 0.66 ± 0.17, indicating that towards higher masses, structures have less negative slopes <ref:2610.10785#pg5>

Conclusion

The goal of this paper was to determine the properties of the progenitor galaxies of the ICL and to compare them with those of the BCG <ref:2610.10785#pg4> The ICL assembles from a large number of intermediate-mass progenitors, in contrast to the BCG, which has a few massive progenitors <ref:2610.10785#pg4> Progenitor properties can vary strongly for a given structure mass, with the assembly of the ICL being highly individual and dependent on a structure’s formation history <ref:2610.10785#pg4> Future work should focus more specifically on how the ICL may build up and evolve over time, as this work focused exclusively on redshift 0 structures <ref:2610.10785#pg4>

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Rebecca J. Mayes, Facundo A. Gómez, Antonela Monachesi: Coevolution of ICL and BCGs <ref:2610.

Improvements for AI systems

  1. Bold header: Increased structural mass dependency in ICL progenitor identification. This allows AI systems to predict how the number of progenitors of the ICL increases with structure mass, enabling better classification of cluster environments based on predicted progenitor diversity and assembly history.

  2. Bold header: Discrimination between merger-dominated and stripping-dominated ICL growth pathways. AI can use the finding that "low-mass structures with N90,ICL <10 are those that have more recently undergone completed mergers versus those with many significant progenitors have not experienced a recent accretion of a single massive galaxy," to predict the dominant formation mechanism for any given structure.

  3. Bold header: Quantification of the BCG's progenitor mass dominance. AI systems can now model BCG growth more accurately by knowing that the most significant progenitor of the BCG is usually a major merger and has a mean stellar mass of 1 × 10 11 M⊙, allowing for targeted predictions about which specific mergers drive BCG evolution.

  4. Bold header: Predictive modeling of ICL material origin via preprocessing. The AI can estimate the contribution of stripped versus quenched galaxies by using the finding that the fraction of the ICL from progenitor galaxies that have completed their mergers was more significant in low-mass structures with N90,ICL <10, enabling refined predictions for ICL assembly in specific mass regimes.

  5. Bold header: Dynamic state classification based on mass gradient and progenitor distribution. The AI can classify cluster dynamical states by analyzing the mean mass radial gradient, which is generally negative and increases with structure mass, linking this to whether a structure is likely disturbed or relaxed.

Abstract

Intracluster Light (ICL) is a diffuse stellar component of galaxy groups and clusters, bound to the structure's potential rather than to individual galaxies and primarily formed through the stripping and mergers of intermediate-mass galaxies. Its formation and evolution is strongly linked to the central galaxies of groups and clusters, typically the Brightest Cluster Galaxies (BCGs) in clusters. To determine the properties of the progenitor galaxies of the ICL of clusters and groups at redshift z = 0 and compare them with the progenitors of their BCGs. We select 127 clusters and groups in the hydrodynamic Illustris-TNG100 simulation above a mass of 10 13 M. We divide the ICL from the BCG by applying a surface brightness cut at the Holmberg radius of 26.5 mag/arcsec-2, where star particles within this radius are defined as attached to the BCG, and outside, the ICL. We then determine the masses and contributions of progenitor galaxies of the ICL and BCG. We find that the number of ICL progenitors increases with structure mass, and that the ICL has more progenitors than the BCG. The ICL has a larger percentage of material from low-mass galaxies. Quenched galaxies contributed a large fraction of the ICL, and this fraction increases with galaxy mass. The mean mass radial gradient of ex-situ material in structures for the ICL and the BCG+ICL is generally negative and increases with structure mass, indicating that material from more massive progenitors is found towards the centres of clusters. The mean mass radial gradient is significantly impacted by structure formation history. ICL progenitor properties are impacted by structure history and are therefore highly individual to each system. At a given structure mass, there is diversity in the properties of the progenitors that formed the ICL. Although the BCG and the ICL co-evolve, the BCG has fewer, more massive progenitors.

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