franco_vazza

#astroph:
interesting new look at using X-ray fluctuations in clusters of galaxy as possible proxy for turbulent motions, using 80 clusters observed with @ChandraScience
https://arxiv.org/abs/2401.15179 by Heinrich et al.

#astronomy #astrodon

A very short #AstroPhysicsFactlet and explainer for the curious:

Merger-driven multi-scale ICM density perturbations: testing cosmological simulations and constraining plasma physics

The hot intracluster medium (ICM) provides a unique laboratory to test multi-scale physics in numerical simulations and probe plasma physics. Utilizing archival Chandra observations, we measure density fluctuations in the ICM in a sample of 80 nearby (z<1) galaxy clusters and infer scale-dependent velocities within regions affected by mergers (r<R2500c), excluding cool-cores. Systematic uncertainties (e.g., substructures, cluster asymmetries) are carefully explored to ensure robust measurements within the bulk ICM. We find typical velocities ~220 (300) km/s in relaxed (unrelaxed) clusters, which translate to non-thermal pressure fractions ~4 (8) per cent, and clumping factors ~1.03 (1.06). We show that density fluctuation amplitudes could distinguish relaxed from unrelaxed clusters in these regions. Comparison with density fluctuations in cosmological simulations shows good agreement in merging clusters. Simulations underpredict the amplitude of fluctuations in relaxed clusters on length scales <0.75 R2500c, suggesting these systems are most sensitive to missing physics in the simulations. In clusters hosting radio halos, we examine correlations between gas velocities, turbulent dissipation rate, and radio emission strength/efficiency to test turbulent re-acceleration of cosmic ray electrons. We measure a weak correlation, driven by a few outlier clusters, in contrast to some previous studies. Finally, we present upper limits on effective viscosity in the bulk ICM of 16 clusters, showing it is systematically suppressed by at least a factor of 8, and the suppression is a general property of the ICM. Confirmation of our results with direct velocity measurements will be possible soon with XRISM.

arXiv.org
Jan 30, 2024 at 10:20amWeb
Show threadfranco_vazzaJan 30, 2024
If you shake a compressible fluid at rest, the fluid starts to move, and the density fluctuations induced by the shaking trace the gas velocity fluctuations to some degree. The exact relation between density and velocity fluctuations depends on the gas equation of state, however this is the basic idea:
from a δρ/ρ fluctuation one can get a δv/cs flucutation, where cs=sounds speed
Show threadfranco_vazzaJan 30, 2024

So the idea is that we can use an X-ray observable, like the map of X-ray surface brightness in clusters, as tool to infer the amount of density fluctuations (because δSx \propto δρ^2 ), and from this use a relation to also guess the amount of turbulence in the intracluster plasma.

This idea started, I guess, from this early work by the late P.Shuecker, which used XMM Newton on the Coma cluster to try this:
https://arxiv.org/abs/astro-ph/0404132

Probing Turbulence in the Coma Galaxy Cluster

Spatially-resolved gas pressure maps of the Coma galaxy cluster are obtained from a mosaic of XMM-Newton observations in the scale range between a resolution of 20 kpc and an extent of 2.8 Mpc. A Fourier analysis of the data reveals the presence of a scale-invariant pressure fluctuation spectrum in the range between 40 and 90 kpc and is found to be well described by a projected Kolmogorov/Oboukhov-type turbulence spectrum. Deprojection and integration of the spectrum yields the lower limit of $\sim 10$ percent of the total intracluster medium pressure in turbulent form. The results also provide observational constraints on the viscosity of the gas.

arXiv.org
Show threadfranco_vazzaJan 30, 2024
However, this is no easy task: the density fluctuations induced by turbulence are a tiny perturbation on top of the large scale density distribution of gas in clusters, which can also be asymmetric and irregular for reasons different than turbulence. Removing the turbulent from the non-turbulent part of density fluctuations, and relating it to the velocity field, is difficult.
Here is a picture from a paper by former student M. Simonte, where we studied this in detail:
Show threadfranco_vazzaJan 30, 2024
So people have been trying extracting this signal in different ways and with different approaches to "mask" or "filter out" the non-turbulent component of density fluctuations. In this latest paper by Heinrich, the emphasis also was on using many objects in a uniform way (80 in total!) and to investigate the various dependencies on the inferred turbulent properties, with the dynamical state of the host cluster (i.e. relaxed vs post-merger)
Show threadfranco_vazzaJan 30, 2024
Some interesting findings:
- unrelaxed clusters have just a bit more density fluctuations across the entire range of analysed spatial scales (in the plot: the power spectrum of them). this suggests that turbulence is not the main contributor to these fluctuations, but the presence of gas substructures due to accretions is.
Show threadfranco_vazzaJan 30, 2024
- overall, the observed pattern of density fluctuations follow to a zero order the distribution you would expect in case Kolmogorov turbulence, i.e. a power-law decreasing spectrum at large k (inverse spatial scale) -as predicted ~20 yr ago my simulations (including mines! 😱 )
Show threadfranco_vazzaJan 30, 2024
- there is a tendency of systems hosting diffuse radio emission to show a larger amount of density fluctuations, which is compatible with the idea that large "radio halos" from thanks to the re-acceleration by turbulent Fermi II mechanisms.
Show threadfranco_vazzaJan 30, 2024

So, why trying to capture turbulent motions with this convolved technique, and not directly through the "classical" Doppler line shift or broadening techniques?

Well..this was the idea since at least a couple of decades, but one needs high spectroscopic resolution to see the broadening by subsonic turbulent motions (v<cs in clusters) in the hot plasma between galaxies.

..And all X-ray satellites with enough spectral resolution to do that, failed one way or another (AstroE, Suzaku, Hitomi). 😩

Show threadfranco_vazzaJan 30, 2024

However, now the hopes are high for the next one on the list, the XRISM satellite who was successfully deployed a few weeks ago, and has already started taking spectra.

https://heasarc.gsfc.nasa.gov/docs/xrism/

(it is suffering of some technical issues too, but it should be able to measure the velocity dispersion of gas in the innermost region of clusters via line broadening at 6keV, and hopefully cross-check the estimates obtained so far with the X-ray fluctuations method.
🤞

[EoT]

X-Ray Imaging and Spectroscopy Mission (XRISM)

The X-ray Imaging and Spectroscopy Mission