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

José M Solanes,Evangelia Athanassoula,Carlos García-Gómez,Laura Darriba,Jaime D Perea,Albert Bosma

doi:10.1093/mnras/stw1278

Abstract

We have developed a computationally competitive N-body model of a previrialized aggregation of galaxies in a flat Lambda cold dark matter 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,M- (gr) similar to 10(13) h(-1)M(circle dot)) demonstrates that the gravitational dynamics involved in their hierarchical collapse is capable of creating realistic first-ranked 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 S ` ersic 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 by giant early-type galaxies in the local Universe reinforces confidence in the results obtained.

Highlights

The brightest group galaxies (BGGs), as well as their cluster counterparts (BCGs; throughout the paper, we use both terms interchangeably), are among the largest known single structures in the Universe
The latter are extremely large elliptical galaxies sitting near the centres of some rich galaxy clusters which are surrounded by a distinct dynamical stellar subsystem in the form of a very extended low-surface brightness envelope of excess light over and above the R1/4 profile defined by the inner regions made of old stars (Kormendy 1989). cDs are purely a rich cluster phenomenon, unlike fossil groups (Ponman et al 1994), which are isolated and likely relaxed early-formed galaxy aggregations dominated by a central object, sometimes as bright as a rich-cluster cD, surrounded by substantial numbers of
As pointed out by Hearin et al (2013), Bayes’ Theorem implies that if BGG dominance were purely statistical, i.e. resulting from N random draws from a given luminosity function (LF) function, the magnitude gap between the two brightest galaxies would contain no information about mass that would not already be provided by the knowledge of richness

Summary

Introduction

The brightest group galaxies (BGGs), as well as their cluster counterparts (BCGs; throughout the paper, we use both terms interchangeably), are among the largest known single structures in the Universe. Some BGGs/BCGs are classified as D (gE possessing a large, diffuse envelope) or cD (extra-large D) galaxies The latter are extremely large elliptical galaxies sitting near the centres of some rich galaxy clusters which are surrounded by a distinct dynamical stellar subsystem in the form of a very extended low-surface brightness envelope of excess light over and above the R1/4 profile defined by the inner regions made of old stars (Kormendy 1989). This has led some to theorize that cD clusters may result from the creation first of a fossil group and the accumulation via secondary infall of new galaxies around the fossil remnant (e.g. Jones et al 2003)

Objectives

Methods

Results