The progenitor galaxies of the intracluster light

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.