33 research outputs found
Cold fronts: probes of plasma astrophysics in galaxy clusters
The most massive baryonic component of galaxy clusters is the “intracluster medium” (ICM), a diffuse, hot, weakly magnetized plasma that is most easily observed in the X-ray band. Despite being observed for decades, the macroscopic transport properties of the ICM are still not well-constrained. A path to determine macroscopic ICM properties opened up with the discovery of “cold fronts”. These were observed as sharp discontinuities in surface brightness and temperature in the ICM, with the property that the denser side of the discontinuity is the colder one. The high spatial resolution of the Chandra X-ray Observatory revealed two puzzles about cold fronts. First, they should be subject to Kelvin-Helmholtz instabilities, yet in many cases they appear relatively smooth and undisturbed. Second, the width of the interface between the two gas phases is typically narrower than the mean free path of the particles in the plasma, indicating negligible thermal conduction. It was thus realized that these special characteristics of cold fronts may be used to probe the properties of the cluster plasma. In this review, we will discuss the recent simulations of cold fronts in galaxy clusters, focusing on those which have attempted to use these features to constrain ICM physics. In particular, we will examine the effects of magnetic fields, viscosity, and thermal conductivity on the stability properties and long-term evolution of cold fronts. We conclude with a discussion on what important questions remain unanswered, and the future role of simulations and the next generation of X-ray observatories
Mapping the particle acceleration in the cool core of the galaxy cluster RX J1720.1+2638
We present new deep, high-resolution radio images of the diffuse minihalo in
the cool core of the galaxy cluster RX J1720.1+2638. The images have been
obtained with the Giant Metrewave Radio Telescope at 317, 617 and 1280 MHz and
with the Very Large Array at 1.5, 4.9 and 8.4 GHz, with angular resolutions
ranging from 1" to 10". This represents the best radio spectral and imaging
dataset for any minihalo. Most of the radio flux of the minihalo arises from a
bright central component with a maximum radius of ~80 kpc. A fainter tail of
emission extends out from the central component to form a spiral-shaped
structure with a length of ~230 kpc, seen at frequencies 1.5 GHz and below. We
find indication of a possible steepening of the total radio spectrum of the
minihalo at high frequencies. Furthermore, a spectral index image shows that
the spectrum of the diffuse emission steepens with the increasing distance
along the tail. A striking spatial correlation is observed between the minihalo
emission and two cold fronts visible in the Chandra X-ray image of this cool
core. These cold fronts confine the minihalo, as also seen in numerical
simulations of minihalo formation by sloshing-induced turbulence. All these
observations favor the hypothesis that the radio emitting electrons in cluster
cool cores are produced by turbulent reacceleration.Comment: 16 pages, 11 figures, accepted for publication in The Astrophysical
Journa
Gas Clumping in the Outskirts of Galaxy Clusters, an Assessment of the Sensitivity of STAR-X
In the outskirts of galaxy clusters, entropy profiles measured from X-ray
observations of the hot intracluster medium (ICM) drops off unexpectedly. One
possible explanation for this effect is gas clumping, where pockets of cooler
and denser structures within the ICM are present. Current observatories are
unable to directly detect these hypothetical gas clumps. One of the science
drivers of the proposed STAR-X observatory is to resolve these or similar
structures. Its high spatial resolution, large effective area, and low
instrumental background make STAR-X ideal for directly detecting and
characterizing clumps and diffuse emission in cluster outskirts. The aim of
this work is to simulate observations of clumping in clusters to determine how
well STAR-X will be able to detect clumps, as well as what clumping properties
reproduce observed entropy profiles. This is achieved by using yt, pyXSIM,
SOXS, and other tools to inject ideally modeled clumps into three-dimensional
models derived from actual clusters using their observed profiles from other
X-ray missions. Radial temperature and surface brightness profiles are then
extracted from mock observations using concentric annuli. We find that in
simulated observations for STAR-X, a parameter space of clump properties exists
where gas clumps can be successfully identified using wavdetect and masked, and
are able to recover the true cluster profiles. This demonstrates that STAR-X
could be capable of detecting substructure in the outskirts of nearby clusters
and that the properties of both the outskirts and the clumps will be revealed.Comment: This is a pre-copyedited, author-produced PDF of an article accepted
for publication in RAS Techniques and Instruments (RASTI) following peer
review. The version of record is available online at:
https://academic.oup.com/rasti/article/doi/10.1093/rasti/rzad042/725882
Predictions for the X-ray circumgalactic medium of edge-on discs and spheroids
We investigate how the X-ray circumgalactic medium (CGM) of present-day
galaxies depends on galaxy morphology and azimuthal angle using mock
observations generated from the EAGLE cosmological hydrodynamic simulation. By
creating mock stacks of {\it eROSITA}-observed galaxies oriented to be edge-on,
we make several observationally-testable predictions for galaxies in the
stellar mass range M. The soft X-ray CGM of
disc galaxies is between 60 and 100\% brighter along the semi-major axis
compared to the semi-minor axis, between 10-30 kpc. This azimuthal dependence
is a consequence of the hot ( K) CGM being non-spherical: specifically
it is flattened along the minor axis such that denser and more luminous gas
resides in the disc plane and co-rotates with the galaxy. Outflows enrich and
heat the CGM preferentially perpendicular to the disc, but we do not find an
observationally-detectable signature along the semi-minor axis. Spheroidal
galaxies have hotter CGMs than disc galaxies related to spheroids residing at
higher halos masses, which may be measurable through hardness ratios spanning
the keV band. While spheroids appear to have brighter CGMs than discs
for the selected fixed bin, this owes to spheroids having higher
stellar and halo masses within that bin, and obscures the fact that
both simulated populations have similar total CGM luminosities at the exact
same . Discs have brighter emission inside 20 kpc and more steeply
declining profiles with radius than spheroids. We predict that the {\it
eROSITA} 4-year all-sky survey should detect many of the signatures we predict
here, although targeted follow-up observations of highly inclined nearby discs
after the survey may be necessary to observe some of our azimuthally-dependent
predictions.Comment: 12 pages, 11 figures, 1 table. Submitted to MNRAS. Comments welcom