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 explode2–6. Recent X-ray studies of the intracluster medium found that the abundance ratios of these elements differ substantially from those seen in the Sun7–11, 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, highresolution 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 calculations12–14 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 nearand 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. The Soft X-ray Spectrometer (SXS) on board the Hitomi observatory has achieved unprecedented spectral resolution in orbit (Δ E ≈ 5 eV in the 2–10 keV band)17. Figure 1 shows the SXS spectrum of the core of the Perseus cluster (radius r smaller than about 2′ , approximately 40 kpc) in the 1.8–9.0 keV band. This was obtained from the same series of observations as that used in our previous work, which constrained turbulent velocities in the intracluster medium (ICM)17, but with 25% longer exposure times, which totalled 290 ks. The refined calibration of the effective area of the telescope and the aperture window transmission of the SXS allow the flux measurement of each individual line in the 1.8–9.0 keV band, which encompasses the Hand He-like transitions from Si to Ni. The excellent performance of the SXS also makes possible the detection of weak He-like resonance lines from Cr, Mn and Ni, with statistical significance of 6σ, 4σ and 12σ, respectively (Fig. 1b, c). Flux measurements of these lines in celestial sources have been extremely challenging with traditional non-dispersive X-ray detectors (such as charge-coupled devices, or CCDs) because such weak features blend into the bremsstrahlung continuum under lower spectral resolution and the Ni xxvii Heα and Fe xxv Heβ lines cannot be resolved (see Fig. 1c). The hot ICM, confined in the deep gravitational potential well of the cluster, contains the largest fraction (about 80%) of metals in the cluster1. Among these, the Fe-peak elements (Cr, Mn, Fe and Ni) are thought to be created predominantly by type Ia supernovae over a cosmological time period18,19. Therefore, the abundance of these elements provides crucial information about the integrated nucleosynthesis in type Ia supernova and its physics. Despite the importance of type Ia supernovae as distance indicators in cosmology20,21, many of their basic features remain elusive. One important open question is whether the mass of an exploding white dwarf is close to the Chandrasekhar limit (MCh ≈ 1.4M, where M is the mass of the Sun), regardless of whether it originates from a single white dwarf accreting mass from a non-degenerate companion22 or a violent merger of two white dwarfs23. Recent hydrodynamical simulations show that both delayed-detonation explosions of near-MCh white dwarfs4,12 and full detonations of sub-MCh white dwarfs13,14,24 can reproduce the observed properties (such as optical light curves and spectra) of type Ia supernovae. Therefore, it is difficult to distinguish the two scenarios from optical observations of individual explosions alone. From the point of view of type Ia supernova nucleosynthesis, the main difference between near-MCh and sub-MCh explosions is whether the core of the white dwarf is dense enough for electron capture (p + e− → n + νe) to take place during the initial phase of the explosion. The threshold density for this reaction (ρc ≈ 108 g cm−3) is achieved only when the white dwarf mass is close to MCh. A distinguishing characteristic of the two models is therefore the production efficiency of neutron-rich species, such as Ni and Mn, which is higher in the nearMCh scenario4–6. We can use this distinction to identify the dominant system of type Ia supernova progenitor in galaxy clusters by measuring the abundance of the Fe-peak elements in the ICM. The results may apply globally because the large scale of rich galaxy clusters makes them representative of the Universe as a whole. Here we model the SXS spectrum of the Perseus cluster in the 1.8–9.0 keV band (Fig. 1a) with an optically thin thermal plasma in collisional ionization equilibrium using the latest atomic databases (AtomDB v.3.0.8 and SPEX v.3.03). The emission from the active galactic nucleus (AGN) of the cD galaxy NGC 1275 is taken into account by adding a power law and redshifted Fe i Kα 1 and Kα 2 lines. Details about the analysis and systematic uncertainty assessment are provided in Methods. Our constraints on the elemental abundances with respect to Fe are shown in Fig. 2 (red circles). These are fully consistent with the latest measured solar abundance ratios25. Figure 2 also displays previously measured ICM abundances of the core of the Perseus cluster as well as the average abundance of 44 objects, including galaxy clusters, groups and elliptical galaxies, from XMMNewton (X-ray Multi Mirror Mission) observations (blue triangles and squares)11. This plot highlights some important differences between the measurements. First, the SXS-measured abundances have statistical uncertainties comparable to those of the XMM-Newton results from the combined data of the 44 objects, despite a 15-times-shorter exposure time and a much smaller field of view. Second, while the abundances of Si, S, Ar and Ca obtained in the two studies are consistent, the earlier measurements systematically obtained supersolar abundances of the Fe-peak elements from both the Perseus cluster and the 44-object average.