Runaway Merger Shocks in Galaxy Cluster Outskirts and Radio Relics. (arXiv:1904.03052v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_C/0/1/0/all/0/1">Congyao Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Churazov_E/0/1/0/all/0/1">Eugene Churazov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Forman_W/0/1/0/all/0/1">William R. Forman</a>
Moderately strong shocks arise naturally when two subclusters merge. For
instance, when a smaller subcluster falls into the gravitational potential of a
more massive cluster, a bow shock is formed and moves together with the
subcluster. After pericenter passage, however, the subcluster is decelerated by
the gravity of the main cluster, while the shock continues moving away from the
cluster center. The aim of this paper is to explore the fate of such shocks
when they travel to the cluster outskirts, far from the place where the shocks
were initiated. In a uniform medium, such a “runaway” shock should weaken with
distance. However, as shocks move to large radii in galaxy clusters, the shock
is moving down a steep density gradient that helps the shock to maintain its
strength over a large distance. Observations and numerical simulations show
that, beyond $R_{500}$, gas density profiles are as steep as, or steeper than,
$sim r^{-3}$, suggesting that there exists a “Habitable zone” for moderately
strong shocks in cluster outskirts where the shock strength can be maintained
or even amplified. A characteristic feature of runaway shocks is that the
strong compression, relative to the initial state, is confined to a narrow
region just behind the shock. This makes these shocks promising candidates for
“igniting” radio relics via adiabatic compression of pre-existing populations
of relativistic particles.
Moderately strong shocks arise naturally when two subclusters merge. For
instance, when a smaller subcluster falls into the gravitational potential of a
more massive cluster, a bow shock is formed and moves together with the
subcluster. After pericenter passage, however, the subcluster is decelerated by
the gravity of the main cluster, while the shock continues moving away from the
cluster center. The aim of this paper is to explore the fate of such shocks
when they travel to the cluster outskirts, far from the place where the shocks
were initiated. In a uniform medium, such a “runaway” shock should weaken with
distance. However, as shocks move to large radii in galaxy clusters, the shock
is moving down a steep density gradient that helps the shock to maintain its
strength over a large distance. Observations and numerical simulations show
that, beyond $R_{500}$, gas density profiles are as steep as, or steeper than,
$sim r^{-3}$, suggesting that there exists a “Habitable zone” for moderately
strong shocks in cluster outskirts where the shock strength can be maintained
or even amplified. A characteristic feature of runaway shocks is that the
strong compression, relative to the initial state, is confined to a narrow
region just behind the shock. This makes these shocks promising candidates for
“igniting” radio relics via adiabatic compression of pre-existing populations
of relativistic particles.
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