Forming first-ranked early-type galaxies through hierarchical dissipationless merging

Abstract

We have developed a computationally competitive N -body model of a previrialized aggregation of galaxies in a flat ΛCDM universe to assess the role of the multiple mergers that take place during the formation stage of such systems in the configuration of the remnants assembled at their centres. An analysis of a suite of 48 simulations of low-mass forming groups (M tot,gr ∼ 10 13 h −1 M ) demonstrates that the gravitational dynamics involved in their hierarchical collapse is capable of creating realistic firstranked galaxies without the aid of dissipative processes. Our simulations indicate that the brightest group galaxies (BGGs) constitute a distinct population from other group members, sketching a scenario in which the assembly path of these objects is dictated largely by the formation of their host system. We detect significant differences in the distribution of Sérsic indices and total magnitudes, as well as a luminosity gap between BGGs and the next brightest galaxy that is positively correlated with the total luminosity of the parent group. Such gaps arise from both the grow of BGGs at the expense of lesser companions and the decrease in the relevance of secondranked objects in equal measure. This results in a dearth of intermediate-mass galaxies which explains the characteristic central dip detected in their luminosity functions in dynamically young galaxy aggregations. The fact that the basic global properties of our BGGs define a thin mass fundamental plane strikingly similar to that followed giant early-type galaxies in the local universe reinforces confidence in the results obtained.

The Impact of Environment on the Stellar Mass–Halo Mass Relation

“…As we have shown, in agreement with Solanes et al (2016), the clusters with the largest magnitude gaps and stellar masses have likely undergone more mergers than small gap, low magnitude gap clusters. If a similar dynamical timescale for the mergers that occur in each cluster exists, then the clusters with the largest stellar mass and magnitude gaps would likely be the clusters that formed first.…”

“…Recently, Solanes et al (2016) used dissipationless simulations of young and pre-virialized groups to show that the magnitude gap between the BCG and second brightest cluster galaxy correlates with the initial stellar mass fraction of the parent cluster halo. This correlation suggests that the observed magnitude gap can inform us about the underlying normal mass (both stellar and baryonic) of a cluster.…”

“…For example, by identifying clusters with large magnitude gaps, we may simultaneously be identifying clusters with high stellar mass fractions during the epoch of BCG formation. Additionally, Solanes et al (2016) found that the magnitude gap contains information about the BCG's merger history. In agreement with hierarchical growth, they found that a BCG's stellar mass increases with the number of progenitor galaxies (i.e., the number of mergers).…”

Ten billion years of brightest cluster galaxy alignments

“…The primary factor that appears to determine whether a galaxy is aligned with its host cluster is that it must be the brightest cluster member, which suggests that there is something special about the birth and evolution of those galaxies. Other studies likewise support the view of BCGs as distinct from other cluster galaxies rather than just the statistical extreme of a single population [21][22][23] , with alignments yet another piece of evidence that they are the product of a unique formation history. Figure 4 shows the relative orientations of the brightest member galaxies with respect to their host clusters for the ten most distant clusters in our sample, all at z > 1.3.…”

The Impact of Environment on Late-time Evolution of the Stellar Mass–Halo Mass Relation

“…For the purpose of this paper, we use the 4th brightest member. Based on dissipationless simulations of young and pre-virialized groups, Solanes et al (2016) find that the stellar mass of the central galaxy linearly increases with the number of progenitor galaxies, in agreement with hierarchical growth. Furthermore, BCGs grow at the expense of the 2nd brightest galaxy.…”