The large differences noted in Figure 9 are surely related to the different approximations underlying both the complex hydrodynamical simulations of ILLUSTRIS-1 and our simplified approach. As recently discussed by Chua et al. (2017), the distribution of noncollisional matter (i.e., stars and DM) can effectively cool down in the cores of galaxies formed in the ancient epochs, at variance with those born more recently. This provides them a much better chance of survival, or at least stronger resilience against potentially disruptive interactions that they would surely undergo throughout the cluster environment. A key role is also played by the host cluster itself, because the mass profile of this latter becomes steeper and steeper in the core due to dynamical relaxation. Consequently, the environment gets denser with time; this trend is found to be quantifiable in terms of the number of dynamical times elapsed since the cluster formation (e.g., Jiang & van den Bosch 2016). Therefore, each galaxy has to attain a sufficiently cold core in order to first survive in the cluster environment and then grow by accretion of matter from the ICM and mergers with other substructures. Given these preliminary considerations, which explain the evaporation of galaxies while falling into more massive halos, the inclusion of the BM physics in this context is crucial, and its effects are still not fully explored. Based on a systematic comparison of the full-physics and DM-only ILLUSTRIS suites, Chua et al. (2017) claimed that galactic winds and photoionization from UV radiation may effectively inhibit mass aggregation, at least at the low-mass end and in certain regimes of redshifts (see also Despali & Vegetti 2017 for similar reasoning for DM subhalos of intermediate masses inside ILLUSTRIS and EAGLE simulations; Schaye et al. 2015). In particular, photometric data inside ILLUSTRIS are bound to whether stellar particles, i.e., stellar populations, are found inside a bound subhalo. Recalling that in ILLUSTRIS-1, the average gas cell mass is 8.86 × 105 M⊙, from which integrated stellar populations might stem, and that a prescription for subgrid physics is always needed to follow up the integrated properties of all kinds of particles, these could be a substantial limit on state-of-the-art hydrodynamical simulations in describing photometry, especially for galaxies inside more recently formed DM halos of lower masses. On the other side, our numerical approach, at the basis of the buildup of samples inside clusters, provides a population of galaxies that should reflect the observational evidence of the mass–radius relation at z ∼ 0 (Chiosi et al. 2012). If observed galaxies are merely those that survived all disrupting interactions they underwent, then at least a minimum fraction of the sample (not as close to zero as the one of ILLUSTRIS-1) should have formed in an evidently large range of look-back time. However, acknowledging that our approach might not be the most refined one, we feel that the best interpretation might be a middle ground between the two models.
Citation: Jiang & van den Bosch 2016
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