Why do galaxies that live in the enormous structures known as galaxy clusters look different from normal, isolated galaxies, such as our Milky Way? To answer this question, an international research team led by MPA has created the Hydrangea simulations, a suite of 24 high-resolution cosmological hydrodynamic simulations of galaxy clusters. Containing over 20,000 cluster galaxies in unprecedented detail and accuracy, these simulations provide astrophysicists with a powerful tool to understand how galaxies have formed and evolved in one of the most extreme environments of our Universe.
Galaxy clusters are giant associations of up to several thousand
galaxies, embedded in diffuse hot gas and invisible dark matter (see
Fig. 1). Observations have shown that these extreme environments
influence the properties of the galaxies within: while isolated galaxies
often contain star-forming discs where massive young stars shine in
blue, cluster galaxies are mostly yellow or red - indicating that they
stopped their star formation several billion years ago. Often, these
cluster galaxies present an apparently featureless “elliptical”
morphology. Understanding the origin of these differences has been a
major unsolved problem in astrophysics for decades.
One key reason for this is that galaxies evolve on timescales of
millions to billions of years. Astrophysicists therefore cannot directly
observe this process through the telescope, they have to rely on
computer simulations to “speed up time” and solve this mystery. Starting
from the observed tiny density fluctuations in the early Universe (see Planck CMB results),
such simulations calculate the growth of structure through the action
of gravity, hydrodynamics, and astrophysical processes such as star
formation and supernova explosions.
The latest generation of these simulations - for example, those
produced by the EAGLE collaboration that also involved participation
from MPA - have finally succeeded in producing galaxies that resemble
those found in the real Universe in key properties such as their mass,
size, and gas content (see here).
In principle, such simulations therefore provide an ideal tool to study
the physics of galaxy formation. However, galaxy clusters occupy only a
tiny fraction of the Universe by volume and are therefore not well
represented in the original EAGLE simulations.
The Hydrangea project, led by Yannick Bahé at MPA and involving
researchers in Germany, the UK, the Netherlands, and Spain, has filled
this gap with a large suite of 24 simulations of massive galaxy
clusters. The project name is derived from the flower “Hydrangea”, whose
petals change their colour between red and blue depending on their
environment – an analogy to the aforementioned colour difference between
field and cluster galaxies. These simulations employ the so-called
“zoom-in” technique, which focuses computing power on a relatively small
region (with a diameter approximately 100 million light years). This
core region was carefully selected to contain a massive galaxy cluster,
within a total volume that is many thousand times larger.
Even with this trick, the Hydrangea simulations constituted a major
computational effort. This is due to the vast range of scales involved
(see Fig. 2): a galaxy cluster exceeds an individual galaxy in mass by
more than a factor of 1000. This means that for adequately resolving
individual cluster galaxies, the simulations need to follow several
billion particles, which interact both gravitationally and
hydrodynamically.
The total computational cost of the suite thus exceeded 40 million
CPU hours, corresponding to a serial run time of more than 4500 years -
as long as the time since the construction of the great pyramids of
Giza. Access to large supercomputing facilities, including the “Hazel
Hen” system of HLRS (Stuttgart) and “Hydra” at MPCDF (Garching), where
the simulations could be run on more than 10,000 CPUs simultaneously,
was therefore crucial for completing the project in less than one year.
Fig. 2 presents a visualization of one of the simulated galaxy clusters.
The video below shows its formation from an initially nearly
structureless “blob” over the course of 13.5 billion years.
In total, the Hydrangea simulations contain more than 20,000
galaxies. When the researchers compared them to the existing EAGLE
simulations, they found a surprising difference: galaxies are, on
average, more massive in the vicinity of galaxy clusters than those
formed in more typical, lower density regions of the Universe. At least
in part, this difference is likely due to the fact that dark matter
haloes (into which all galaxies are embedded) form earlier in the
vicinity of clusters. As a consequence, a larger fraction of the gas is
concentrated into the star-forming centre, leading to a higher total
mass of stars formed. This is an important prediction, not least because
astronomers often use stellar mass to compare “similar” galaxies in
different environments. Systematic variations in stellar mass fractions
with environment could therefore cause biases in such comparisons and
must be carefully taken into account.
The full analysis of the simulations is an ongoing effort that will
take several years to complete. As well as testing the accuracy of the
EAGLE model in the essentially unchartered regime of massive galaxy
clusters, this effort will allow astrophysicists to gain ground-breaking
new insight into how galaxies interact with their cluster environment.
This will significantly improve our understanding of how the structures
we see in the Universe formed and evolved over the last 13.7 billion
years.
The formation of a galaxy cluster in the Hydrangea simulations
This movie shows the formation of the galaxy cluster shown in
Fig. 2 over a period of 13.5 billion years. Starting from a nearly
homogeneous structure a few hundred million years after the Big Bang,
this first collapses to a sponge-like web. Several “proto-clusters”
(yellow-white blobs in the movie) crystallize out of this web, and then
successively merge into one massive cluster. These mergers drive shocks,
which heat the gas to ever higher temperatures. Also visible are
violent outbursts of hot gas from proto-clusters, which are driven by
supermassive black holes in their centres. Credit: Yannick Bahé / MP.
Acknowledgement: The simulations presented in this article were in part
performed on the German federal maximum performance computer “HazelHen”
at the maximum performance computing centre Stuttgart (HLRS), under
project GCS-HYDA / ID 44067 financed through the large scale project
“Hydrangea” of the Gauss Center for Supercomputing. Further computing
resources were provided by the Max Planck Computing and Data Facility in
Garching, and by the DiRAC system “Cosma5” hosted by Durham University
(UK).
Bahe, Yannick
Postdoc
Phone:
2236
Fax:
2235
Email: ybahe@mpa-garching.mpg.de
