An azimuthally resolved study of sloshing cold fronts in three nearby galaxy clusters

TL;DR

This study performs an azimuthally resolved X-ray analysis of sloshing cold fronts in three nearby clusters (A496, A2029, A1644) using Chandra residual maps to map edges along the sloshing spiral. By fitting a three-component density model to sector-based surface brightness profiles and combining with deprojected spectroscopy, it derives azimuthal profiles of density, temperature, pressure, and entropy contrasts across the fronts, denoted $j_n$, $j_T$, $j_p$, and $j_K$. The authors find that, in general, $j_p$ is below unity, indicating non-thermal pressure support beyond bulk flows; the azimuthal velocity gradients inferred from $j_p$ vary by cluster and do not align with a universal bulk-motion–driven mechanism. They conclude that magnetic fields, viscosity, or turbulence likely contribute to the front sharpness, and advocate for future XRISM measurements to directly probe ICM motions and test these non-thermal processes.

Abstract

We present a detailed analysis of sloshing cold fronts in a sample of three nearby galaxy clusters (Abell 496, Abell 2029, and Abell 1644) observed with the Chandra X-ray Observatory. Cold fronts manifest as sharp edges in the X-ray surface brightness of the intracluster medium (ICM) in galaxy clusters. In the residual X-ray surface brightness maps, where the global ICM distribution has been subtracted, cold fronts generated by gas sloshing are observed at the boundaries of the spiral excesses. We perform a systematic and comprehensive study of the surface brightness edges along the spiral excesses. We find the deficit of the thermal pressure radially inward of the brightness edges, in contrast to stripping cold fronts that typically exhibit higher thermal pressure in brightness edges. Assuming that the sharp edges in the X-ray surface brightness distributions are sustained entirely by the gas bulk motions, we estimate the velocity gradients across the edges that are required to compensate for the deficit of the thermal pressure. We do not find statistically significant velocity gradients along the azimuthal direction. Our results suggest that alternative mechanisms such as magnetic fields and viscosity are necessary to maintain the sharpness of sloshing cold fronts.

Figures (17)

• Figure 1: This schematic illustration represents substructures seen in a residual X-ray surface brightness map. The spiral excess (the yellow region) is the primary focus of this work. For each detected surface brightness edge ($r_{\rm{in}}$ and $r_{\rm{out}}$), we denote the side closer to the cluster center with the subscript ${\rm{i}}$, and the farther side with the subscript ${\rm{o}}$. Note that the subscripts "$\rm{i}$" and "$\rm{o}$" strictly refer to the radial position relative to the edge. For the outer spiral boundary, the spiral excess lies in region "$\rm{i}$", whereas for the inner spiral boundary, the spiral excess lies in region "$\rm{o}$".
• Figure 2: The Chandra X-ray surface brightness images (first and third rows) and residual maps (second and fourth rows) of A496 (left), A2029 (middle), and A1644 (right). The zoom-in version of the first and second rows are shown as the third and fourth rows, respectively. The surface brightness images and residual maps are smoothed by a Gaussian kernel with $3$ and $10$ FWHM, respectively. Point sources and backgrounds are masked by black ellipses in the X-ray surface brightness maps. The bars in the lower-left corner of each panel show the corresponding spatial scales. The numbers in yellow denote the indices of the sectors defined in Table \ref{['tab:AnalysesSxsectors']}. The sectors in cyan with an opening angle of $30^{\circ}$ show the regions defined for the azimuthal study. The arcs within the sectors denote the positions of X-ray surface brightness edges, determined by the X-ray surface brightness fitting described in Section \ref{['sec:AnalysesSx']}.
• Figure 3: The radial profiles of the X-ray surface brightness extracted from sectors in A496. The blue vertical bars show the $1\,\sigma$ confidence range of the X-ray surface brightness. The black dashed lines show the best-fit X-ray surface brightness profiles. The pink vertical lines denote the best-fit positions of the surface brightness edges (i.e., $r_{\rm{in}}$ and $r_{\rm{out}}$). In addition, the panels below the profiles show the residuals with respect to the best-fit models, computed as $\left(S_{\rm{X}} - S_{\rm{X}, m}\right)/S_{\rm{X}}$, where $S_{\rm{X}}$ is the observed surface brightness and $S_{\rm{X}, m}$ is the model prediction.
• Figure 4: Same as Figure \ref{['fig:AnalysesSxA496']}, but for A2029.
• Figure 5: Same as Figure \ref{['fig:AnalysesSxA496']}, but for A1644.