Accretion of galaxy groups into galaxy clusters

Laura Sales 1

  • 1 UC Riverside

Abstract

We study the role of group infall in the assembly and dynamics of galaxy clusters in ΛCDM. Using 10 Virgo-like clusters in the cosmological simulation Illustris, we find a median of ~38% surviving galaxies at z = 0 are accreted as part of groups and did not infall directly from the field, albeit with significant cluster-to-cluster scatter. These substructure plays a dominant role in fostering the conditions for galaxy mergers to happen, even within the cluster environment. We argue that such mergers, which are typically gas rich for dwarfs and a combination of gas rich and gas poor for M* ~ 1e11 Msun, may contribute significantly within ΛCDM to the formation of specific morphologies, such as lenticulars (S0) and blue compact dwarfs in groups and clusters.

Motivation -- Benavides et al. (2020)

Original work by: Benavides J. A., Sales L. V. & Abadi M. G. MNRAS (2020), 498, pp.3852.

Paper here

Simulated Galaxy Cluster with M200 = 2.3x1014 M⨀

Substructures of galaxies and merger remnants have been detected observationally in clusters. The typically large velocity dispersion associated with galaxy clusters, σ≈ 500-1000 km/s, makes it very unlikely for such associations to spontaneously occur after the galaxies are embedded in the gravitational potential of the clusters. These galaxy associations must therefore occur before infall, process called ``pre-processing", as the movie below shows.

Infall of galaxy group into the galaxy cluster in formation, the magente  circle indicates the growth of r200 to the host halo.

Pre-processing signatures weaken quickly

We use data from Illustris simulations to analyze the evolution of groups of galaxies falling into clusters. We found that 38% of surviving galaxies at z = 0 are pre-processed, meaning that they are accreted as part of groups and did not infall directly from the field.

Projected stellar map at redshift z = 0 of the most massive cluster in our Illustris sample (FoF No0). This object has a virial mass M200 = 2.32 x 1014 M⨀ and 232 satellite galaxies with M* ≥ 1.5 x 108 M⨀ inside the virial radius r200 = 1.27 Mpc, indicated by the magenta circle. We highlight with small colour symbols a set of galaxies that were part of a group before joining the cluster at z = 1.67. The blue circle shows to central of the group and the red circles its surviving satellites. Despite their common origin, little evidence of the past association for these galaxies remains at present-day.

The median size of groups increases a factor ∼ x4.4 average from infall to z = 0 and the velocity dispersion between galaxy members also increases by a factor ∼ ×2.5 in the same time period.

 

Orbit versus time for the infalling group of galaxies after their infall into FoF No0. The central galaxy in the group is shown in black, and the satellites are colour-coded according to their maximum stellar mass (see bar on the right). Note the propelling of three ``escapees" after the first pericenter passage, that takes them well beyond the virial radius of the host cluster (magenta curve). At $z=0$ there is little coherence left among these orbits.

The time τd that it takes for a given group to double its size occurs over short timescales, ∼ 1-3 Gyr.

Mergers inside galaxy clusters

The lower velocities between galaxies associated with the infalling groups may play a vital role in facilitating the conditions for mergers to occur within (or in the outskirts) of massive clusters.

 

We find an average of ∼ 17 ± 9 mergers per cluster above our resolution limits, of these ∼ 40%, happen within r200.

 

The gas ratios of these mergers depend critically on galaxy mass: they are strongly gas-rich dominated for M* ≤ 109 M⨀ ,while a combination of gas rich and gas poor is found for more massive galaxies with M* ≥ 1010 M⨀.

 

Gas fraction fgas = Mgas/M* in mergers in and around clusters as a function of the stellar mass of the primary galaxy M*1$. Triangle/squares are used for Sat-Sat and Sat-Cen mergers, colour coded according to the location of the merger (see colour bar). Histograms show the distribution along each axis for Sat-Sat and Sat-Cen events. Mergers are gas rich for the dwarfs domain, but transition into a mix of gas rich and gas poor for more massive galaxies. These mergers may provide a natural avenue for the formation of S0s and blue compact dwarf galaxies in galaxy clusters.

Take home points

A significant fraction of galaxies infall into clusters as part of smaller groups and galaxy associations. About 40% of galaxies of any mass today in clusters are therefore ``pre-processed".  

Cluster environments erase the dynamical signatures of these galaxy substructures in a few Gyrs, making it challenging to reconstruct, in particular for galaxies with early infall times.

The environments of groups allow for lower relative velocities between members favoring the occurence of mergers, which might happen well within the cluster. This may be an important chanel for the formation of S0 galaxies and compact dwarfs. 

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