How Galaxies treat their Globular Clusters

After almost 2 years of development and testing, Michael Brockamp (University of Bonn) and I have finally published our MUESLI code for the simulation of globular star clusters orbiting in elliptical galaxies. Together with Holger Baumgardt from Brisbane (Australia), Ingo Thies, and Pavel Kroupa (both from the University of Bonn), we have put together a really exciting study of systems of globular clusters and how they erode over time in the gravitational field of their host galaxies.

This figure shows the number of globular star clusters at a given radius of the elliptical galaxy they were simulated in. The black line gives their initial distribution at the beginning of the simulations, the blue dashed line shows their distribution after just one billion years of evolution. As you can see, the distribution develops a core, as clusters in the center of the galaxy get destroyed more quickly. These cores are observed for giant elliptical galaxies, hence we suggest that they formed via ‘tidal erosion’, that is, the clusters in the center got disrupted by the galaxy.

In short, all we do is, we set up 4 different galaxies, which represent the full observed mass range of elliptical galaxies in the universe, and study the survival rates of globular clusters within them. We find that very early in the life of a galaxy, most globular clusters get disrupted, leaving only the ones behind that are on orbits around the galaxy on which the tidal forces are not too extreme. This is especially important in the centers of galaxies, which is why these central parts get quickly depleted in globular clusters. In this way we can explain the cored distributions of globular clusters which are observed for giant elliptical galaxies.

What does that mean in detail? Elliptical galaxies are common spherically, or elliptically shaped galaxies, which have masses between a few billion times the mass of the sun, up to a few trillion times the solar mass. Within these galaxies we find basically only stars. Most of them don’t show any or very little gas. But we also find globular clusters, which are compact configurations of thousands to millions of stars that orbit within the elliptical galaxy as a group. The galaxy tries to rip these balls of stars apart, but the gravitational attraction of the group within itself binds it together and shields it from the influence of the host galaxy.

This process is quite balanced for many globular clusters in elliptical galaxies, so that they survived from their formation about 13 billion years ago till today (like we recently showed in detail for the Milky Way globular cluster Palomar 4). These are the clusters we observe today. But for how many clusters was this process unbalanced such that they got eroded by the gravitational field of their host galaxy? This is the question we answered in this study. We find that the fraction of globular clusters depends crucially on the shape of the galaxy, meaning that concentrated galaxies like M 32 (the companion to our neighbor spiral galaxy Andromeda) are able to destroy all their globular clusters very quickly. Giant elliptical galaxies like NGC 4889, on the other hand, keep many of their clusters. For these galaxies we see that they preferentially destroy clusters in the center, which leaves their distribution of clusters with radius to have a core. These cores are observed for many giant elliptical galaxies, suggesting that ‘tidal erosion’ is in fact the underlying mechanisms for how these cores are formed.

Since globular clusters are small compared to elliptical galaxies, we had to develop a whole new code for treating this problem numerically. Details on the code can be found in the paper. The code is available upon request, but will be freely available at some point. Yay to open science! MUESLI, by the way, stands for MUlti-purpose Elliptical-galaxy SCF + time-transformed Leapfrog Integrator, which totally speaks for itself, right?


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