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Mapping the Nearest Ancient Sloshing Cold Front in the Sky with XMM-Newton

Sheng-Chieh Lin, Yuanyuan Su, Iraj Vaezzadeh, William Forman, Elke Roediger, Charles Romero, Paul Nulsen, Scott W. Randall, John ZuHone, Ralph Kraft, Christine Jones

TL;DR

This paper reports XMM-Newton mosaic observations of an ancient sloshing cold front in the Virgo Cluster at ~250 kpc, linking it to the cluster’s inner cold fronts through a common off-axis merger scenario likely involving M49. The analysis combines imaging, spectroscopy, and deprojection to reveal sharp single and double edge features, a temperature increase across the front, and near-continuous pressure with a potential non-thermal component and magnetic-field influence. A hydrodynamic binary-merger simulation provides qualitative agreement, suggesting the outer front formed ~2–3 Gyr ago and may be beginning to split via Kelvin-Helmholtz instabilities, with turbulence consistent with a 2D Kolmogorov cascade on the bright side. The results illuminate ICM transport and microphysics beyond cluster cores and demonstrate the science case for future wide-field, high-resolution X-ray observatories such as AXIS to map ancient sloshing fronts across clusters.

Abstract

The Virgo Cluster is the nearest cool core cluster that features two well-studied sloshing cold fronts at radii of $r \approx 30$ kpc and $r \approx 90$ kpc, respectively. In this work, we present results of XMM-Newton mosaic observations of a third, southwestern, cold front at a radius of $r \approx 250$ kpc, originally discovered with Suzaku. All three cold fronts are likely to be parts of an enormous swirling pattern, rooted in the core. The comparison with a numerical simulation of a binary cluster merger indicates that these cold fronts were produced in the same single event $-$ likely the infall of M49 from the northwest of Virgo and it is now re-entering the cluster from the south. This outermost cold front has probably survived for $2-3$ Gyr since the disturbance. We identified single sharp edges in the surface brightness profiles of the southern and southwestern sections of the cold front, whereas the western section is better characterized with double edges. This implies that magnetic fields have preserved the leading edge of the cold front, while its western side is beginning to split into two cold fronts likely due to Kelvin-Helmholtz instabilities. The slopes of the 2D power spectrum of the X-ray surface brightness fluctuations, derived for the brighter side of the cold front, are consistent with the expectation from Kolmogorov turbulence. Our findings highlight the role of cold fronts in shaping the thermal dynamics of the intracluster medium beyond the cluster core, which has important implications for cluster cosmology. Next-generation X-ray observatories, such as the proposed AXIS mission, will be ideal for identifying and characterizing ancient cold fronts.

Mapping the Nearest Ancient Sloshing Cold Front in the Sky with XMM-Newton

TL;DR

This paper reports XMM-Newton mosaic observations of an ancient sloshing cold front in the Virgo Cluster at ~250 kpc, linking it to the cluster’s inner cold fronts through a common off-axis merger scenario likely involving M49. The analysis combines imaging, spectroscopy, and deprojection to reveal sharp single and double edge features, a temperature increase across the front, and near-continuous pressure with a potential non-thermal component and magnetic-field influence. A hydrodynamic binary-merger simulation provides qualitative agreement, suggesting the outer front formed ~2–3 Gyr ago and may be beginning to split via Kelvin-Helmholtz instabilities, with turbulence consistent with a 2D Kolmogorov cascade on the bright side. The results illuminate ICM transport and microphysics beyond cluster cores and demonstrate the science case for future wide-field, high-resolution X-ray observatories such as AXIS to map ancient sloshing fronts across clusters.

Abstract

The Virgo Cluster is the nearest cool core cluster that features two well-studied sloshing cold fronts at radii of kpc and kpc, respectively. In this work, we present results of XMM-Newton mosaic observations of a third, southwestern, cold front at a radius of kpc, originally discovered with Suzaku. All three cold fronts are likely to be parts of an enormous swirling pattern, rooted in the core. The comparison with a numerical simulation of a binary cluster merger indicates that these cold fronts were produced in the same single event likely the infall of M49 from the northwest of Virgo and it is now re-entering the cluster from the south. This outermost cold front has probably survived for Gyr since the disturbance. We identified single sharp edges in the surface brightness profiles of the southern and southwestern sections of the cold front, whereas the western section is better characterized with double edges. This implies that magnetic fields have preserved the leading edge of the cold front, while its western side is beginning to split into two cold fronts likely due to Kelvin-Helmholtz instabilities. The slopes of the 2D power spectrum of the X-ray surface brightness fluctuations, derived for the brighter side of the cold front, are consistent with the expectation from Kolmogorov turbulence. Our findings highlight the role of cold fronts in shaping the thermal dynamics of the intracluster medium beyond the cluster core, which has important implications for cluster cosmology. Next-generation X-ray observatories, such as the proposed AXIS mission, will be ideal for identifying and characterizing ancient cold fronts.

Paper Structure

This paper contains 16 sections, 20 equations, 6 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. Analysis and Results
  4. Image Analysis
  5. Spectral Analysis
  6. Deprojections of Thermal Profiles
  7. Discussion
  8. An ancient sloshing cold front in Virgo
  9. The split in the ancient cold front
  10. Thermal and chemical properties across cold front
  11. Intrinsic width of the cold front width
  12. Surface Brightness Fluctuations
  13. Summary
  14. XMM- Newton background modeling
  15. Possible systematics in spectral analysis
  16. ...and 1 more sections

Figures (6)

  • Figure 1: Left: Adaptively-smoothed XMM- Newton mosaic image of the Virgo Cluster in the band of $0.5$-$2.0$ keV after exposure correction and background subtraction. Point sources are removed. Right: Residual image obtained by dividing the mosaic image by the best-fit beta model. The arrows mark the positions of three cold fronts. The two residing in the core region were studied by 2010MNRAS.405...91S; the outermost cold front is the main focus of this work. The region enclosing bright background sources, marked by the black circle, is excluded from the analysis.
  • Figure 2: Surface brightness, projected temperature and projected metallicity profiles along the S, SW, and W wedges. The radii of cold fronts are denoted by the dashed lines (W wedge exhibits two edges). The x-axis labels the projected distance R from M87 in both arcmin (black) and kpc (purple). Upper: surface brightness profiles together with the best-fit models in red. The inset shows the best-fit models of 3D density profiles. Middle: projected temperature profiles overlaid with the measurements made by Suzaku2017MNRAS.469.1476S. The error bars include both statistical and systematic uncertainties. The filled points show the best-fit temperatures when varying the metallicity in the apec model, and the open points are the results with metallicity fixed at $0.3$. The gray point at $R \approx 65'$ shows the result from an extraction region matching the Suzaku observation area at the same radius. Bottom: best-fit metallicity profiles when varying the metallicity in the apec model. The horizontal dashed-dotted line marks Z$=0.3\,$Z$_{\odot}$, corresponding to the open points in the middle panel.
  • Figure 3: The metal abundance and the abundance ratios of O, Si, S, with respect to Fe. The measurements are made for the bright side (blue) and the faint side (magenta). The circles show the abundance ratios, and the bars show the abundances. The error bars include statistical and systematic uncertainties (Appendix \ref{['sec:sys']}). The dashed-dotted line marks the ratio of $1$. The dashed line marks the Fe abundance of $Z\approx 0.32$.
  • Figure 4: Deprojected profiles of temperature, density, pressure, and entropy along the S (blue), SW (green), and W (orange) directions. Shaded areas are measurements made by Suzaku2017MNRAS.469.1476S. Dashed vertical lines mark the positions of SB edges in three directions. In the lower left panel, we include the empirical pressure profile given by Eq. 1 in 2017MNRAS.469.1476S.
  • Figure 5: The merging history inferred for the Virgo Cluster. Left: Snapshots of temperature maps of a binary cluster merger, derived from a numerical simulation 2022MNRAS.514..518V, featuring the development of sloshing cold fronts. Middle: Same as the left column but for the density map. Both temperature and density maps are X-ray emission weighted. The 4 panels from top to bottom: the first core passage of an infalling substructure occurs 2.2 Gyr ago; the first sloshing cold front (red arrow) appears 1.6 Gyr ago as the substructure is approaching apoapsis; the second sloshing cold front (white arrow) develops 1 Gyr ago when the substructure is turning around; all three sloshing cold fronts (red, white, and black arrows) are visible as the substructure, likely to be M49, is re-entering the virial radius of the main cluster, which provide the best match to the X-ray image of the Virgo Cluster. Right: Point source excised, adaptively smoothed eROSITA X-ray image of the Virgo Cluster. The white circle marks the r$_{200c}$ radius. M49 is visible to the south of the Virgo Cluster. This image is taken from 2024AA...689A.113M with permission.
  • ...and 1 more figures