Intracluster stars in simulations with AGN feedback 11 This is illustrated for the same cluster in more detail in Fig. 11, which shows the distance of newly formed stars from the centres of their host halos. Stars that at z = 0 are part of satellite galaxies or the BCG exclusively form in halo centres, roughly within 10 h − $^{1}$kpc. However, a sig- nificant fraction ( ∼ 30%) of stars that are part of the ICL at z = 0 form at much larger distances between 10 h − $^{1}$kpc and 1 h − $^{1}$Mpc. We found a similar fraction also in our other clusters. We have investigated the nature of this ‘intracluster star formation’ in more detail, finding that it happens in small gas clouds that are distributed throughout the cluster, which can in fact be easily spotted in gas surface density maps. For most of these clouds, however, an associated halo can not be identified in maps of the dark matter particle distribution. We also checked whether these clouds form by thermal in- stability from the intracluster medium or whether they are stripped from infalling halos. We did this by tracing both the star-forming gas particles in these clouds, as well as all gas particles in the clusters main halo back to earlier times. This revealed that contrary to other gas particles, the vast ma- jority of star-forming gas particles fell into the cluster very recently. For example, less than 5% of the star-forming gas particles in these clouds in the progenitor of a 10 $^{14}$h − $^{1}$M ⊙ cluster at z = 1 are already part of this progenitor’s main halo at z = 1 . 4, while this fraction is over 50% when look- ing at all main halo gas particles at the same cluster-centric radius. Also, at this earlier time most of the gas particles that later form intracluster stars can be unambiguously as- sociated with infalling halos. If our simulations strongly overestimate the amount of such ‘intracluster star formation’, part of the discrepancy between our simulation results and the amount of ICL in- ferred from observational studies like Zibetti et al. (2005) and Gonzalez et al. (2007) could be explained. In the most extreme case all star formation outside halo centres may be spurious, in which case we would expect about ∼ 30% less ICL than reported above. On the other hand, it is also worth noting that if real clusters contain as many intracluster stars as predicted by our simulations, this would significantly re- lax the tension between constraints on the cosmic baryon fraction and baryon fractions measured in galaxy clusters as, e.g., reported in McCarthy et al. (2007). Given the diffi- cult observational challenge to determine the amount of ICL In other words, this means that most of these star- forming gas clouds consist of material that was stripped out of small infalling halos. Curiously, some of the gas clouds survived this stripping basically intact and continue to form stars, which then become part of the ICL. Note that there is observational evidence that some star formation does hap- pen in gas stripped from infalling galaxies (e.g. Sun et al. 2010). Nevertheless, it is not entirely clear how realistic this mode of ‘intracluster star formation’ in our simulations is, and whether it is in part occurring due to numerical effects. For example, SPH simulations are known to poorly resolve fluid instabilities (e.g. Agertz et al. 2007) that could act to disrupt such gas clouds after they are stripped from an in- falling halo. Also, thermal conduction in the hot intracluster medium is not included in our simulations and might play some role in reality, unless it is very efficiently suppressed by magnetic fields. For each star particle that was not formed in the cluster’s main progenitor we try to find the galaxy in which it fell into the forming cluster. We do this by looking for the most massive subhalo that a star particle was part of before be- coming part of the main cluster’s FoF group. This allows a determination of the properties of these halos before tidal stripping in the cluster potential strongly affects them. Fig. 12 shows the distribution of the stars in satellite galaxy, BCG, and ICL components that were not formed in the cluster’s main progenitor according to the time of their infall into the cluster. To reduce noise in the distributions we averaged them over four clusters, assigning the same weight to each of them. The simulations that were used for this figure included AGN feedback, but we note that simulations without AGN yield very similar results. The most obvious difference between the distributions is that almost all stars that end up in the BCG or ICL fall into the cluster before redshift z ∼ 1, while the vast majority of stars that fall into the cluster later remain bound to their host galaxies. This suggests that galaxies falling into the cluster after redshift z ∼ 1 do usually not have enough time to sink towards the cluster centre by dynamical friction and are thus unlikely to merge with the BCG or get tidally disrupted close to accurately, it certainly appears possible that the present ob- servations have still missed a significant amount of the ICL. Figure 10. Distribution of the masses of the halos in which the stars in the satellite galaxies, the BCG, and the ICL of a 10 14 h − 1 M$_{⊙}$ cluster formed. Results are shown for simulations with and without AGN feedback. Method 4 was used for mak- ing a distinction between BCG and ICL. 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