Observations are beginning to be sensitive enough to see the outskirts of galaxy clusters, where theory predicts interesting features in the dark matter and gas profiles: the so-called splashback and the accretion shock. Scientists at MPA use an analytical model to compute the locations of these features, and shed new light on the underlying physics.
Providing a simple model for complex cosmic phenomena is one of the goals of theoretical astrophysics. When successful, such a model yields a deeper understanding of the underlying physics. Recently scientists at MPA have done just this for better understanding what happens at the outskirts of galaxy clusters.
The observation of galaxy clusters has played an important role in
convincing astronomers of the existence of dark matter: Galaxies inside
galaxy clusters move with high velocities; hot, X-ray emitting gas fills
the galaxy cluster region; often galaxy clusters show a gravitational
lensing effect on background galaxies. All these measurements have
consistently shown that visible matter is embedded in dark matter halos
which make up about 85% of the total gravitating mass of a galaxy
cluster.
Unlike a star, which is a 'halo' of gas with a clear boundary and a
finite mass, these dark matter halos are fuzzy, extended structures.
Their densities typically drop like ρ ∝ r-3 with radius in
the outer regions without a clear cutoff, which has prevented
astronomers to assign a definite boundary and mass to them. When you
talk to an astronomer about a galaxy cluster with a certain mass, he or
she will likely ask 'Within which radius is this mass defined?'
However, scientists recently realized that a particular feature at
the outskirts of galaxy clusters can serve as a natural boundary for
their dark matter halo. This feature - the so-called 'splashback' -
marks the position of a sudden steepening in the density profile.
Physically, this 'splashback' is caused by recently accreted dark matter
that is piling up near the first apocenter of its orbit through the
dark matter halo.
Hydrodynamical numerical simulations have found that the splashback
radius closely tracks the position of the accretion shock. The accretion
shock is located, where intergalactic plasma gets shock-heated when
accreting onto a galaxy cluster. Also the accretion shock associated
with a sudden steepening feature – this time in gas density and
temperature. Since the physics underlying the splashback and the
accretion shock is quite different, it is rather intriguing that they
seem to track each other.
While simulations are a powerful tool for studying complex
astrophysical systems, analytical models are also needed to simplify the
picture and to understand the underlying physics. Using an analytical
model called the 'self-similar spherical collapse model', scientists at
MPA computed the growth of galaxy clusters in an expanding universe. The
profile of a galaxy cluster and the history of its mass growth are
treated in a consistent manner.
This model can predict the radial locations of the splashback and the
accretion shock as a function of the rate of cluster mass growth.
Although both radii depend sensitively on the mass growth rate, they are
found to indeed track each other. For typical mass growth rates for
observed galaxy clusters, both quantities shrink at a higher mass growth
rates, caused by different physics: For the gas, the inflowing material
is associated with higher energy and momentum for a higher mass growth
rate. Whereas for the dark matter, a significantly increased mass adds
gravitational attraction to the splashback material.
A more intricate finding is that the locations of the splashback and
the accretion shock track each other best if the adiabatic index of the
gas is close to 5/3. Coincidentally, the intergalactic plasma, which is
dominated by single atoms, has approximately this adiabatic index. In
this sense, this tracking behaviour is not universal.
Improvements in the quality and quantity of observational data are
bringing the outskirts of galaxy clusters into our view. Potential
observational evidence for the splashback radius has already been found,
and we may expect a direct detection of the accretion shock from X-ray
and millimetre observations in the near future. Then, combining
analytical predictions with those observations may lead to a deeper
understanding and new discoveries of galaxy clusters and the structure
assembly in our Universe.
Original:
X. Shi
The outer profile of dark matter haloes: an analytical approach
Original:
X. Shi
Locations of accretion shocks around galaxy
clusters and the ICM properties: insights from self-similar spherical
collapse with arbitrary mass accretion rates
Author:
Email: xshi@mpa-garching.mpg.de
