# Intracluster stars in simulations with AGN feedback 

Ewald Puchwein $^{1}$, Volker Springel$^{1}$, Debora Sijacki$^{2}$, and Klaus Dolag1 

$^{1}$Max-Planck-Institut f¨ ur Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany 

$^{2}$Kavli Institute for Cosmology, Cambridge and Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, United Kingdom 

1 September 2018 

## ABSTRACT 

We use a set of high-resolution hydrodynamical simulations of clusters of galaxies to study the build-up of the intracluster light (ICL), an interesting and likely significant component of their total stellar mass. Our sample of groups and clusters includes AGN feedback and is of high enough resolution to accurately resolve galaxy popula- tions down to the smallest galaxies that are expected to significantly contribute to the stellar mass budget. We describe and test four different methods to identify the ICL in cluster simulations, thereby allowing us to assess the reliability of the measurements. For all of the methods, we consistently find a very significant ICL stellar fraction ( ∼ 45%) which exceeds the values typically inferred from observations. However, we show that this result is robust with respect to numerical resolution and integration accuracy, remarkably insensitive to changes in the star formation model, and almost in- dependent of halo mass. It is also almost invariant when black hole growth is included, even though AGN feedback successfully prevents excessive overcooling in clusters and leads to a drastically improved agreement of the simulated cluster galaxy population with observations. In particular, the luminosities of central cluster galaxies and the ages of their stellar populations are much more realistic when including AGN. In the light of these findings, it appears challenging to construct a simulation model that simultaneously matches the cluster galaxy population and at the same time produces a low ICL component. We find that intracluster stars are preferentially stripped in a cluster’s densest region from massive galaxies that fall into the forming cluster at z > 1. Surprisingly, some of the intracluster stars also form in the intracluster medium inside cold gas clouds that are stripped out of infalling galaxies. 

Key words: galaxies: clusters: general – galaxies: formation – cosmology: theory – methods: numerical – black hole physics 

## 1 INTRODUCTION 

In optical observations, galaxy clusters appear as concen- trations of galaxies on the sky. However, deep exposures reveal that not all the emission comes from stars that re- side in the cluster’s member galaxies. Instead, there is also a smoothly distributed stellar component which is typically peaked around the cluster’s central galaxy, but extends to large radii. Due to its low surface brightness, observations of this intracluster light (ICL) component are difficult, result- ing in a significant uncertainty in the current observational constraints of the amount of ICL present in clusters. 

Lin and Mohr (2004) give an overview of the fractions of intracluster stars reported in the literature for different clusters and groups (see their Table 2). The reported values span a huge range from ∼ 2% to ∼ 50%. Instead of analyz- ing very deep exposures of individual objects, Zibetti et al. (2005) stacked hundreds of cluster images from the Sloan Digital Sky Survey and masked all satellite galaxies, allow- 

ing them to obtain a very deep exposure of the brightest cluster galaxy (BCG) and the ICL of an average galaxy cluster. For a mean cluster mass of 7 − 8 × 10 $^{13}$M ⊙ they find that about ∼ 22% of the stars reside in the BCG, while roughly ∼ 11% are intracluster stars within a fixed aperture of 500 kpc. Gonzalez et al. (2005, 2007) describe another study for which a significant number of clusters and groups were observed. By analyzing the observed systems individu- ally, they found that both the total stellar fractions as well as the relative fractions of stars residing in the BCG+ICL component strongly increase with decreasing halo mass. It was also concluded that intracluster stars are important for the baryon budget of clusters and groups. 

The latter is particularly interesting in the light of the recent findings that the baryon fractions measured in clusters seem to be significantly lower than those inferred from the most current cosmological constraints (see e.g. McCarthy et al. 2007). However, no mechanism that can ef- ficiently segregate baryons from dark matter on the scale 

