Deep Chandra observation and numerical studies of the nearest cluster cold front in the sky

Abstract

We present the results of a very deep (500 ks) Chandra observation, along with tailored numerical simulations, of the nearest, best resolved cluster cold front in the sky, which lies 90 kpc (19 ) to the northwest of M 87. The northern part of the front appears the sharpest, with a width smaller than 2.5 kpc (1.5 Coulomb mean free paths; at 99 per cent confidence). Everywhere along the front, the temperature discontinuity is narrower than 4-8 kpc and the metallicity gradient is narrower than 6 kpc, indicating that diffusion, conduction and mixing are suppressed across the interface. Such transport processes can be naturally suppressed by magnetic fields aligned with the cold front. Interestingly, comparison to magnetohydrodynamic simulations indicates that in order to maintain the observed sharp density and temperature discontinuities, conduction must also be suppressed along the magnetic field lines. However, the northwestern part of the cold front is observed to have a nonzero width. While other explanations are possible, the broadening is consistent with the presence of Kelvin-Helmholtz instabilities (KHI) on length scales of a few kpc. Based on comparison with simulations, the presence of KHI would imply that the effective viscosity of the intra-cluster medium is suppressed by more than an order of magnitude with respect to the isotropic Spitzer-like temperature dependent viscosity. Underneath the cold front, we observe quasi-linear features that are ∼ 10 per cent brighter than the surrounding gas and are separated by ∼ 15 kpc from each other in projection.Comparison to tailored numerical simulations suggests that the observed phenomena may be due to the amplification of magnetic fields by gas sloshing in wide layers below the cold front, where the magnetic pressure reaches ∼ 5-10 per cent of the thermal pressure, reducing the gas density between the bright features.

On the Nature of X-Ray Surface Brightness Fluctuations in M87

“…The dominance of isothermal fluctuations of these scales in this region is consistent with the presence of several bubbles (see, e.g., Owen et al 2000;Forman et al 2007). In addition to these bubbles, at distances of about 6 7 from the center the gas is likely disturbed by largescale sloshing (e.g., Figure 2 in Simionescu et al 2010;Werner et al 2016). The most prominent cold front presumably associated with sloshing is located ∼20′ north of M87.…”

Cold fronts: probes of plasma astrophysics in galaxy clusters

“…XMM-Newton EPIC/MOS mosaic image of the central r ∼ 150 kpc region of the Virgo cluster (Simionescu et al 2010). The white square indicates the Chandra ACIS-I pointing of Werner et al (2016) shown in figure 6. Reproduced from Werner et al (2016).…”

“…The white square indicates the Chandra ACIS-I pointing of Werner et al (2016) shown in figure 6. Reproduced from Werner et al (2016).…”

Applications for edge detection techniques usingChandraandXMM–Newtondata: galaxy clusters and beyond

“…Often the variation of this width is very subtle, and not obvious from the raw Chandra images, instead requiring a lengthy process of dividing the edge into numerous sectors, measuring surface brightness profiles across each sector, and fitting the profiles with a model (e.g. Werner et al 2016, Sanders et al 2016a.…”

“…Markevitch et al 2002, Russell et al 2012, and cavities in the intracluster medium (ICM) resulting from powerful AGN feedback (Boehringer et al 1993, McNamara et al 2000, Fabian et al 2006) leave a myriad of such surface brightness features in X-ray images. Galaxy clusters also feature X-ray surface brightness features which are much more subtle, such as sound waves (Fabian et al 2006) and substructure behind cold fronts (Werner et al 2016, Sanders et al 2016a, with Email: swalker@ast.cam.ac.uk variations of only a few percent. Our ability to unravel the physics of the intracluster medium is limited by our ability to resolve features which have low contrast in X-ray images.…”