Solar abundance ratios of the iron-peak elements in the Perseus cluster


The metal abundance of the hot plasma that permeates galaxy clusters represents the accumulation of heavy elements produced by billions of supernovae1. Therefore, X-ray spectroscopy of the intracluster medium provides an opportunity to investigate the nature of supernova explosions integrated over cosmic time. In particular, the abundance of the iron-peak elements (chromium, manganese, iron and nickel) is key to understanding how the progenitors of typical type Ia supernovae evolve and explode2,3,4,5,6. Recent X-ray studies of the intracluster medium found that the abundance ratios of these elements differ substantially from those seen in the Sun7,8,9,10,11, suggesting differences between the nature of type Ia supernovae in the clusters and in the Milky Way. However, because the K-shell transition lines of chromium and manganese are weak and those of iron and nickel are very close in photon energy, high-resolution spectroscopy is required for an accurate determination of the abundances of these elements. Here we report observations of the Perseus cluster, with statistically significant detections of the resonance emission from chromium, manganese and nickel. Our measurements, combined with the latest atomic models, reveal that these elements have near-solar abundance ratios with respect to iron, in contrast to previous claims. Comparison between our results and modern nucleosynthesis calculations12,13,14 disfavours the hypothesis that type Ia supernova progenitors are exclusively white dwarfs with masses well below the Chandrasekhar limit (about 1.4 times the mass of the Sun). The observed abundance pattern of the iron-peak elements can be explained by taking into account a combination of near- and sub-Chandrasekhar-mass type Ia supernova systems, adding to the mounting evidence that both progenitor types make a substantial contribution to cosmic chemical enrichment5,15,16.

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Figure 1: The Hitomi SXS spectra of the Perseus cluster.
Figure 2: Elemental abundances of the Perseus cluster.
Figure 3: Comparison between the observed and theoretically calculated abundances of the Fe-peak elements.


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Acknowledgements are provided in the Supplementary Information.

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H.Y. wrote the manuscript. H.Y., S.N., A. Simionescu, E.B. and M.L. analysed the data. H.Y., K. Matsushita, M.L., A. Simionescu, S.N., K.S. and R.M. discussed the results. Y. Ishisaki confirmed the reliability of the observed results using his expertise in the SXS signal processing system. The science goals of Hitomi were discussed and developed over more than 10 years by the ASTRO-H Science Working Group (SWG), all members of which are authors of this manuscript. All the instruments were prepared by the joint efforts of the team. Calibration of the Perseus dataset was carried out by members of the SXS team. The manuscript was subject to an internal collaboration-wide review process. All authors reviewed and approved the final version of the manuscript.

Corresponding authors

Correspondence to Kyoko Matsushita or Hiroya Yamaguchi.

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Extended data figures and tables

Extended Data Figure 1 The SXS field of view overlaid on a Chandra image in the 1.8–9.0 keV band.

The corresponding sequence IDs of the Hitomi observations are given. Each side of the SXS field of view has an angular size of 3′ (about 64 kpc).

Extended Data Figure 2 Additional gain correction.

ac, The data points indicate the difference ΔE = E′ − E0 between the measured (E′) and theoretical (E0) energies of each detected line at the given X-ray energy. The best-fitting parabolic functions are given as solid curves. The error bars correspond to the 1σ confidence level. a, b and c show the results from sequence 100040020, 100040030–50 (combined) and 100040060, respectively.

Extended Data Figure 3 Elemental abundances calculated with different model assumptions.

‘A’ and ‘S’ indicate the results obtained using the atomic databases AtomDB v.3.0.8 and SPEX v.3.03, respectively, and ‘S′’ represents an old atomic model (SPEX v.2.05, which does not contain Cr and Mn line data). Numerical designations are as follows: ‘1’, one-temperature fit with the Fe xxv resonance-scattering effect; ‘2’, one-temperature fit without the resonance-scattering effect; ‘3’, two-temperature fit with the resonance-scattering effect; ‘4’, two-temperature fit without the resonance-scattering effect. The error bars correspond to the 1σ confidence level.

Extended Data Figure 4 Elemental abundances of the Perseus cluster core from different X-ray measurements.

Relative abundances with respect to Fe (X/Fe, X = Si, S, Ar, Ca, Cr, Mn, Ni) normalized to the corresponding solar abundances25 (dashed line). The red circles are identical to those in Fig. 2 and represent the SXS measurements, with error bars that include both the 1σ statistical uncertainty and systematic uncertainty. The red diamonds show the same SXS measurements analysed with an outdated atomic model that was used in the XMM-Newton study. The blue triangles represent the XMM-Newton results11, as in Fig. 2. The green squares are abundances obtained from Suzaku observations of the innermost 2′ region of the Perseus cluster39, converted using the updated solar abundance table25 for direct comparison with the other measurements. The error bars are also converted to represent the statistical uncertainty at a 1σ confidence level.

Extended Data Figure 5 Weak emission lines at different energy resolutions.

a, SXS spectrum of the Perseus cluster around the Cr and Mn emission. The red line is the best-fitting model (model A1) with the Cr and Mn abundances set to zero. The bottom panel shows the ratio between the data and the model results. The error bars correspond to the 1σ confidence level. b, Simulated spectrum at the typical energy resolution of the XMM-Newton CCD data (MOS1 detector), assuming the best-fitting model for the SXS data and sufficiently long exposure time (4 Ms). This comparison demonstrates the robustness of our measurements of weak emission lines with high-resolution spectroscopy (see Methods for details).

Extended Data Figure 6 Effect of potential bias in the continuum-level estimate on the abundance measurement using weak emission lines.

a, Abundances of Cr (red), Mn (blue) and Fe (black) determined by intentionally adding a small offset to the continuum normalization (within ±3% of the measured value). The solid and dashed lines are obtained from our test analysis of the simulated CCD spectrum (Extended Data Fig. 5b) and the Hitomi spectrum, respectively. This illustrates that the CCD measurement of Cr and Mn abundances is sensitive to the accuracy of the continuum-level determination because of the weakness of the emission and the low spectral resolution. The Fe abundance is less subject to such uncertainty, even in the CCD measurement, owing to the much larger equivalent width of the emission. b, Abundance ratios of Cr/Fe (red) and Mn/Fe (blue), calculated using the values in a as a function of offset in the continuum level.

Extended Data Table 1 Summary of the observations
Extended Data Table 2 Solar abundance table used in this work
Extended Data Table 3 Mass ratios of the Fe-peak elements in type Ia supernova models
Extended Data Table 4 Example calculations of supernova nucleosynthesis models

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Hitomi Collaboration., Aharonian, F., Akamatsu, H. et al. Solar abundance ratios of the iron-peak elements in the Perseus cluster. Nature 551, 478–480 (2017).

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