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XRISM Observations of The Prototypical Cold Front in Abell 3667

Yuki Omiya, Yuto Ichinohe, Kazuhiro Nakazawa, Hisamitsu Awaki, Dominique Eckert, Yutaka Fujita, Isamu Hatsukade, Maxim Markevitch, François Mernier, Ikuyuki Mitsuishi, Naomi Ota, Aurora Simionescu, Yuusuke Uchida, Shutaro Ueda, Irina Zhuravleva, John Zuhone

TL;DR

This study uses XRISM/Resolve high-resolution spectroscopy to measure line-of-sight velocities and velocity dispersions across the cold front in Abell 3667, addressing whether the front results from an offset, sloshing merger. Two deep pointings (core and front) yield spatially resolved kinematics via a bapec model, with a notable LoS velocity change of about $535^{+167}_{-154}$ km s$^{-1}$ across the front and a separate ~400 km s$^{-1}$ contrast between interior and central ICM, indicating rotation of the sloshing core in the plane perpendicular to the sky. The results support an offset-merger scenario, reveal regional turbulence (notably $\sigma_z \approx 420$ km s$^{-1}$ inside the front), and constrain magnetic stabilization of the front to $B_{ m crit} \sim 7 \mu$G (lower limit $>4.8 \mu$G). These measurements demonstrate the power of nondispersive microcalorimeter spectroscopy to characterize ICM dynamics and microphysics, with implications for merger physics and magnetic-field configurations in clusters.

Abstract

We present high-resolution X-ray spectroscopy of the merging galaxy cluster Abell 3667 with \textit{XRISM}/Resolve. Two observations, targeting the cluster X-ray core and the prototypical cold front, were performed with exposures of 105 ks and 276 ks, respectively. We find that the gas in the core is blueshifted by $v_z\sim-200$ km s$^{-1}$ relative to the brightest cluster galaxy, while the low-entropy gas inside the cold front is redshifted by $v_z\sim 200$ km s$^{-1}$. As one moves further off-center across the front, the line-of-sight (LoS) velocity changes significantly, by $Δv_z=535^{+167}_{-154}$ km s$^{-1}$, back to the value similar to that in the core. There are no significant LoS velocity gradients perpendicular to the cluster symmetry axis. These features suggest that the gas forming the cold front is flowing in the plane oriented along the LoS, supporting an offset merger scenario in which the main cluster has passed in front of the subcluster and induced rotation of the core gas in the plane perpendicular to the sky. The region just inside the front exhibits the largest LoS velocity dispersion seen across two pointings, $σ_z\sim420$ km s$^{-1}$, which can be interpreted as a developing turbulence or a projection of the LoS velocity shear within the front. The large LoS velocity jump across the cold front, combined with the lack of Kelvin-Helmholtz instability on the surface of the front, suggests some mechanism to suppress it. For example, a magnetic field with $B>5\,μ$G is required if the cold front is stabilized by magnetic draping.

XRISM Observations of The Prototypical Cold Front in Abell 3667

TL;DR

This study uses XRISM/Resolve high-resolution spectroscopy to measure line-of-sight velocities and velocity dispersions across the cold front in Abell 3667, addressing whether the front results from an offset, sloshing merger. Two deep pointings (core and front) yield spatially resolved kinematics via a bapec model, with a notable LoS velocity change of about km s across the front and a separate ~400 km s contrast between interior and central ICM, indicating rotation of the sloshing core in the plane perpendicular to the sky. The results support an offset-merger scenario, reveal regional turbulence (notably km s inside the front), and constrain magnetic stabilization of the front to G (lower limit G). These measurements demonstrate the power of nondispersive microcalorimeter spectroscopy to characterize ICM dynamics and microphysics, with implications for merger physics and magnetic-field configurations in clusters.

Abstract

We present high-resolution X-ray spectroscopy of the merging galaxy cluster Abell 3667 with \textit{XRISM}/Resolve. Two observations, targeting the cluster X-ray core and the prototypical cold front, were performed with exposures of 105 ks and 276 ks, respectively. We find that the gas in the core is blueshifted by km s relative to the brightest cluster galaxy, while the low-entropy gas inside the cold front is redshifted by km s. As one moves further off-center across the front, the line-of-sight (LoS) velocity changes significantly, by km s, back to the value similar to that in the core. There are no significant LoS velocity gradients perpendicular to the cluster symmetry axis. These features suggest that the gas forming the cold front is flowing in the plane oriented along the LoS, supporting an offset merger scenario in which the main cluster has passed in front of the subcluster and induced rotation of the core gas in the plane perpendicular to the sky. The region just inside the front exhibits the largest LoS velocity dispersion seen across two pointings, km s, which can be interpreted as a developing turbulence or a projection of the LoS velocity shear within the front. The large LoS velocity jump across the cold front, combined with the lack of Kelvin-Helmholtz instability on the surface of the front, suggests some mechanism to suppress it. For example, a magnetic field with G is required if the cold front is stabilized by magnetic draping.

Paper Structure

This paper contains 14 sections, 3 equations, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Data treatment
  3. Observations
  4. Data reduction and Spectral modeling
  5. Results
  6. Spectra from large regions
  7. Resolving the cluster velocity structure
  8. Discussion
  9. Velocities and kinetic pressure
  10. Galaxy–ICM velocity comparison
  11. Implications for the offset merger scenario
  12. Constraints on magnetic field strength
  13. Conclusion
  14. Robustness of the SSM analysis in region A

Figures (6)

  • Figure 1: (Top) Chandra image of A3667 in the $0.5-7.0$ keV band. White squares show fields of view of the two XRISM/Resolve observations. White pixellated subregions within the white squares represent detector regions used for spectral extraction. The dashed orange regions represent the sky regions for which the ICM parameters are derived. A black "x" marks the position of the BCG, and cyan arrows indicate the prominent cold front seen in the Chandra image. (Bottom) Soft-to-hard ratio map of A3667, constructed from the ratio of the 0.5–2.0 keV to $2.0–7.0$ keV Chandra images. This map shows spatial variations of the ICM temperature, with brighter regions corresponding to cooler gas. Alt text: The top panel shows an X-ray image of Abell 3667, with right ascension on the horizontal axis and declination on the vertical axis. The bottom panel shows a hardness-ratio map, where brighter areas correspond to cooler gas.
  • Figure 2: (Top left) Resolve spectra in the 2.0--10.0 keV energy band for the cold front (CF_IN, black) and central (Center, red) pointings. The best-fit models are overlaid on each spectrum together with NXB models (Top right) Zoom-in of the Resolve spectra in the 6.2--6.7 keV energy band. Data are binned by 2 eV for display purposes. (Bottom) The spectra in region a--g used for the SSM analysis in the 6.2--6.7 keV energy band. Each colored curve shows the modeled emission from the corresponding sky region: region A (red), B (orange), C (blue), D (green), E (magenta), F (cyan), and G (light gray). The spectrum of region a is also shown with the sum of all models (black) except for the region A model, highlighting the blueshifted contribution of region A relative to the others. Alt text: Ten line graphs. In the upper left panel, the x axis shows the energy from 2.0 to 10.0 kilo electron volt. The y axis shows the count from 0.0002 to 0.3 counts per second and per kilo electron volt. In the upper right panel, the x axis shows the energy from 6.2 to 6.7 kilo electron volt. The y axis shows the count from 0.0002 to 0.3 counts per second and per kilo electron volt. In the bottom panels, the x axis shows the energy from 6.2 to 6.7 kilo electron volt. The y axis shows the count from 0.000002 to 0.2 counts per second and per kilo electron volt.
  • Figure 3: Maps of temperature (top left), abundance (top right), LoS bulk velocity relative to the BCG (bottom left), and LoS velocity dispersion (bottom right) for the sky regions defined in Figure \ref{['fig:chandra_image']}. The black star indicates the position of the BCG. Contours represent the X-ray surface brightness from the Chandra image. The green boxes indicate regions covered by the XRISM/Resolve observations. Alt text: Four color maps arranged in two rows and two columns, each with right ascension on the horizontal axis and declination on the vertical axis. The upper left map shows temperature in kilo–electron volts ranging from 3.0 to 9.0. The upper right map shows abundance in solar units ranging from 0.1 to 0.7. The lower left map shows bulk velocity in kilometers per second ranging from minus 400 to plus 400. The lower right map shows velocity dispersion in kilometers per second ranging from 0 to 500.
  • Figure 4: A schematic illustration of the gas flows in A3667. The cool, low-entropy gas associated with the displaced main core (or, alternatively, the core of the disturber subcluster) is shown in blue, while the hot, high-entropy ICM of the main cluster is shown in red. White arrows indicate the inferred directions of the bulk gas motion, including the rotation of the cool gas. The viewing direction is from the bottom of the figure. Alt text: A color schematic of the merging scenario in Abell 3667.
  • Figure 5: Same as Figure \ref{['fig:Resolve_map_ssm']}, but with the temperature and abundance parameters in region A left free. The values shown in parentheses correspond to the results obtained with these parameters fixed ($kT=8.0$ keV and $Z=0.41$$Z_{\odot}$).
  • ...and 1 more figures