Galaxy Properties due to their Environment

The environment of a galaxy strongly influences its properties. Here we give an overview of the work we have done in this area.

· · · · ·

Morphology–Density Relation

Over the past years the connection between galaxies and their host halos has been discussed in several studies. One tool that is used to estimate halo masses is to count satellite galaxies around massive host galaxies. We use the Magneticum Pathfinder Simulations to study the abundance of satellites around the host galaxies, focusing on the differences between disk and spheroidal galaxies. In agreement with recent observations, when using all galaxies in our simulations, we count more small galaxies within the vicinity of spheroids than disks with similar stellar masses. Interestingly, when restricting the host galaxy sample to central galaxies, i.e., galaxies which are at the center of the underlying halo, this difference disappears. However, for companion galaxies, i.e., galaxies which are large satellites of other massive galaxies within a common dark-matter potential, the signal is even stronger.

lambda dark matter

Thus, although we reproduce the observations, our findings do not support the idea that spheroidal galaxies reside in significantly more massive dark-matter halos than disk galaxies at fixed stellar mass. We conclude that this split-up seen in the abundance of satellites, which is mainly caused by companion galaxies, is simply a reflection of the density–morphology relation. Furthermore, we find that the density–morphology relation starts to build up at around z = 2 and is independent of the star formation properties of the host galaxies. The shape of the relation of centrals and companions is similar to each other, except that the number density of quiescent centrals peaks in low-density environments, while that of the quiescent companions peaks in high-density environments. This implies that environmental quenching is more important for satellite than for central galaxies.

For further details, see Teklu et al., 2017

· · · · ·

Galaxies in Clusters

The effect of galactic orbits on a galaxy’s internal evolution within a galaxy cluster environment has been the focus of heated debate in recent years. To disentangle this relationship, we investigate the phase space, the velocity anisotropy, and the orbital evolution of cluster satellites. Through the use of the hydrodynamic cosmological simulation Magneticum Pathfinder, we evaluate the orbits of subhalos associated with 20 clusters. Thus, we are able to achieve a statistically relevant sample of galaxies inside clusters, which we further split into quiescent and star-forming galaxies. This split allows us to observe the internal galactic evolution and study its dependence on the radial distance and anisotropy parameter. We then extend our investigation and consider the evolution from high redshift to present day. This allows us, among other considerations, to relate infalling galaxies with their progenitors, so as to understand the star formation history. To evaluate the validity of the simulation-based findings, we compare, where possible, with observations. We find that at redshifts z < 0.5 the vast majority of galaxies are quenched through ram-pressure stripping during their first passage.

Phase space overplot

Phase space of 20 stacked clusters at z = 0.03. The figure shows the normalised velocity, i.e., the radial velocity divided by the virial velocity (vrad/vvir), in dependence of the radius in units of the virial radius, Rvir. Any subhalos with a non-zero star formation rate are indicated by triangles, the remaining subhalos are indicated by red crosses. The colour of the triangles represents the degree of blueness, i.e., the specific star formation rate multiplied by the Hubble time, sSFR · tH.

Phase space overplot

Overlay of phase space diagrams of 20 stacked clusters from z = 1.7 to z = 0.03. The figure shows the normalised velocity, i.e., the radial velocity divided by the virial velocity (vrad/vvir), in dependence of the radius in units of the virial radius, Rvir. Any subhalos with a non-zero star formation rate are indicated by triangles, the remaining subhalos are indicated by red crosses. The colour of the triangles represents the degree of blueness, i.e., sSFR · tH.

Phase space overplot

Scaled temporal evolution of 121 subhalos with regard to their radial distance and blueness. Each line represents an individual subhalo tracked through time. The trajectories of the subhalos are shifted to the point where they pass below 1 Rvir. Top panel: radial distance to the centre of the cluster in dependence of time. Bottom panel: blueness, i.e., specific star formation rate multiplied by the Hubble time, dependent on time.

A number of distinct insights become apparent when considering the results obtained with the Magneticum Pathfinder simulation. Most importantly, the results, when compared, coincide with observations. With regard to the investigation into the effect of galactic orbits on internal evolution, we find that ram-pressure stripping is the dominant quenching mechanism in clusters at low redshifts. The vast majority of star forming subhalos are violently quenched within their first passage through the cluster.