Measuring bulk flows of the intracluster medium in the Perseus and Coma galaxy clusters using XMM-Newton (1911.13108v1)

Abstract: We demonstrate a novel technique for calibrating the energy scale of the XMM EPIC-pn detector, which allows us to measure bulk flows in the intracluster medium (ICM) of the Perseus and Coma clusters. The procedure uses the instrumental lines present in all observations, in particular, Cu-Ka. By studying their spatial and temporal variations, in addition to incorporating calibration observations, we refined the absolute energy scale to better than 150 km/s at the Fe-K line, a large improvement over the nominal accuracy of 550 km/s. We then mapped the bulk motions over much of the central 1200 and 800 kpc of Perseus and Coma, respectively, in spatial regions down to 65 and 140 kpc size. We cross-checked our procedure by comparing our measurements with those found in Perseus by Hitomi for an overlapping region, finding consistent results. For Perseus, there is a LoS velocity increase of 480+-210 km/s (1sigma) 250 kpc east of the nucleus. This region is associated with a cold front, providing direct evidence of the ICM sloshing in the potential well. Assuming the intrinsic distribution of bulk motions is Gaussian, its width is 214+-85 km/s, excluding systematics. Removing the sloshing region, this is reduced to 20-150 km/s, which is similar in magnitude to the Hitomi line width measurements in undisturbed regions. In Coma, the line-of-sight velocity of the ICM varies between the velocities of the two central galaxies. Maps of the gas velocity and metallicity provide clues about the merger history of the Coma, with material to the north and east of the cluster core having a velocity similar to NGC 4874, while that to the south and west has velocities close to NGC 4889. Our results highlight the difference between a merging system, such as Coma, where we observe a ~1000 km/s range in velocity, and a relatively relaxed system, such as Perseus, with much weaker bulk motions. [abridged]

• The paper demonstrates that precise calibration of XMM-Newton's EPIC-pn detector reduces systematic uncertainties to approximately 150 km s⁻¹, enhancing ICM velocity measurements.
• It reveals distinct flow patterns in Perseus, including a high-velocity region (480 ± 210 km s⁻¹) likely linked to gas sloshing, and varied merger-driven dynamics in Coma.
• These findings emphasize the value of refined detector calibration for future high-resolution studies of galaxy cluster dynamics, supporting upcoming missions like XRISM and Athena.

Overview of the Techniques and Findings in Galaxy Cluster Observational Analysis

The paper presented in the paper details a technique for the precise measurement of bulk flows within the intracluster medium (ICM) of galaxy clusters, specifically focusing on the Perseus and Coma clusters, using the XMM-Newton observatory. The method primarily involves calibrating the energy scale of the EPIC-pn detector by leveraging background instrumental fluorescent emission lines, notably Cu-K$\alpha$, to achieve an accuracy significantly exceeding existing standards.

Key Methodological Advances

The paper introduces a multi-step calibration of the detector's energy scale. This comprises:

1. Average Gain Correction: Using the Cu-K$\alpha$ line’s redshift averaged over each observation.
2. Spatial Gain Correction: Addressing position-dependent variations by creating spatial maps of Cu-K$\alpha$ line shifts over the mission's lifetime, refining the energy scale corrections for each observation.
3. Non-Linearity Correction: Implementing a model to correct a systematic non-linearity found in the energy scale, compensated across time and detector position to linearize the spectral data interpretation effectively.

These calibrations reduced systematic uncertainties to approximately 150 km s$^{-1}$ at the energies pertinent to Fe-K emission, a substantial improvement from prior instrumentation capabilities.

Results on Galactic Clusters

Perseus Cluster

The investigation of the Perseus cluster revealed that the ICM's bulk velocity aligns closely, in many regions, with previous high-resolution spectroscopic observations by Hitomi. However, a specific region east of the nucleus showed significantly increased line-of-sight velocities of about 480 ± 210 km s$^{-1}$, possibly indicating gas sloshing within the cluster’s potential well. This discovery aligns with theoretical models predicting turbulent and random motions instigated by cluster interactions. The velocity distribution map further corroborates this analysis, indicating correlation with temperature and metallicity structures customarily attributed to sloshing motions.

Coma Cluster

In the Coma cluster, findings differ, reflecting the cluster's more complex merger history. The ICM's velocity varies distinctly between the central components, approximately matching the velocities of the central galaxy subclusters, NGC 4874 and NGC 4889, indicating the presence of significant bulk flows. This could imply ongoing or post-merger dynamics, evidenced by material velocities consistent with past subclusters.

Implications and Future Prospects

These measurements offer insights into the dynamic processes governing galaxy clusters and provide a novel approach to mapping bulk velocities across clusters. The accuracy afforded by the calibrated energy scale of XMM-Newton’s EPIC-pn detector serves as a precursor for future, more comprehensive studies of cluster dynamics.

The paper suggests areas for improvement, including the incorporation of double-pixel events and further calibration of lower-energy lines, which could extend the utility of this technique. The broader implication is a better understanding of the balance between gravitational dynamics and non-thermal pressures in the ICM, contributing to models of cluster formation and evolution.

Given the broader field’s trajectory, this paper provides a methodological lens through which future space missions could achieve greater precision in ICM diagnostics, paving the way for more detailed examination of large-scale cosmic structures. The integrity and detail provided by this methodology could particularly augment the scientific outcomes from upcoming missions such as XRISM and Athena, which are poised to further investigate the ICM dynamics with high spectral resolution instruments.

In summary, by enhancing the calibration accuracy of X-ray observations, researchers can obtain more precise velocity measurements within galaxy clusters, effectively contributing to the astrophysical understanding of large-scale structures and the physics governing their evolution.