Clusters of galaxies are the most massive gravitationally bound objects in the Universe and are still forming. They are thus important probes1 of cosmological parameters and many astrophysical processes. However, knowledge of the dynamics of the pervasive hot gas, the mass of which is much larger than the combined mass of all the stars in the cluster, is lacking. Such knowledge would enable insights into the injection of mechanical energy by the central supermassive black hole and the use of hydrostatic equilibrium for determining cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50-million-kelvin diffuse hot plasma filling its gravitational potential well. The active galactic nucleus of the central galaxy NGC 1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These bubbles probably induce motions in the intracluster medium and heat the inner gas, preventing runaway radiative cooling—a process known as active galactic nucleus feedback2,3,4,5,6. Here we report X-ray observations of the core of the Perseus cluster, which reveal a remarkably quiescent atmosphere in which the gas has a line-of-sight velocity dispersion of 164 ± 10 kilometres per second in the region 30–60 kiloparsecs from the central nucleus. A gradient in the line-of-sight velocity of 150 ± 70 kilometres per second is found across the 60-kiloparsec image of the cluster core. Turbulent pressure support in the gas is four per cent of the thermodynamic pressure, with large-scale shear at most doubling this estimate. We infer that a total cluster mass determined from hydrostatic equilibrium in a central region would require little correction for turbulent pressure.
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We acknowledge all the JAXA members who have contributed to the ASTRO-H (Hitomi) project. All US members gratefully acknowledge support through the NASA Science Mission Directorate. Stanford and SLAC members acknowledge support via DoE contract to SLAC National Accelerator Laboratory DE-AC3-76SF00515 and NASA grant NNX15AM19G. Part of this work was performed under the auspices of the US DoE by LLNL under contract DE-AC52-07NA27344 and also supported by NASA grants to LLNL. Support from the European Space Agency is gratefully acknowledged. French members acknowledge support from CNES, the Centre National d’Etudes Spatiales. SRON is supported by NWO, the Netherlands Organization for Scientific Research. The Swiss team acknowledges support of the Swiss Secretariat for Education, Research and Innovation SERI and ESA’s PRODEX programme. The Canadian Space Agency is acknowledged for the support of Canadian members. We acknowledge support from JSPS/MEXT KAKENHI grant numbers 15H02070, 15K05107, 23340071, 26109506, 24103002, 25400236, 25800119, 25400237, 25287042, 24540229, 25105516, 23540280, 25400235, 25247028, 26800095, 25400231, 25247028, 26220703, 24105007, 23340055, 15H00773, 23000004, 15H02090, 15K17610, 15H05438, 15H00785 and 24540232. H. Akamatsu acknowledges support of NWO via a Veni grant. M. Axelsson acknowledges a JSPS International Research Fellowship. C. Done acknowledges STFC funding under grant ST/L00075X/1. P. Gandhi acknowledges a JAXA International Top Young Fellowship and UK Science and Technology Funding Council (STFC) grant ST/J003697/2. H. Russell, A. C. Fabian and C. Pinto acknowledge support from ERC Advanced Grant Feedback 340442. We thank contributions by many companies, including, in particular, NEC, Mitsubishi Heavy Industries, Sumitomo Heavy Industries and Japan Aviation Electronics Industry. Finally, we acknowledge strong support from the following engineers. JAXA/ISAS: C. Baluta, N. Bando, A. Harayama, K. Hirose, K. Ishimura, N. Iwata, T. Kawano, S. Kawasaki, K. Minesugi, C. Natsukari, H. Ogawa, M. Ogawa, M. Ohta, T. Okazaki, S.-i. Sakai, Y. Shibano, M. Shida, T. Shimada, A. Wada, T. Yamada; JAXA/TKSC: A. Okamoto, Y. Sato, K. Shinozaki, H. Sugita; Chubu U: Y. Namba; Ehime U: K. Ogi; Kochi U of Technology: T. Kosaka; Miyazaki U: Y. Nishioka; Nagoya U: H. Nagano; NASA/GSFC: T. Bialas, K. Boyce, E. Canavan, M. DiPirro, M. Kimball, C. Masters, D. Mcguinness, J. Miko, T. Muench, J. Pontius, P. Shirron, C. Simmons, G. Sneiderman, T. Watanabe; Noqsi Aerospace Ltd: J. Doty; Stanford U/KIPAC: M. Asai, K. Gilmore; ESA (Netherlands): C. Jewell; SRON: D. Haas, M. Frericks, P. Laubert, P. Lowes; U of Geneva: P. Azzarello; CSA: A. Koujelev, F. Moroso.
Extended data figures and tables
Extended Data Figure 1 SXS spectrum of the full field overlaid with a CCD spectrum of the same region.
The CCD is the Suzaku X-ray imaging spectrometer (XIS) (red line); the difference in the continuum slope is due to differences in the effective areas of the instruments.
a–c, These have been obtained from various emission-line databases typically used in the literature. The spectra were modelled as a single-temperature, optically thin plasma in collisional ionization equilibrium using either APEC/ATOMDB 3.0.3 (ref 16; red) or SPEX 3.0 (ref. 17; blue). We determined the best-fit model by fitting the Hitomi spectrum from the outer 23 pixels in the energy range 6.4–8 keV, excluding the Fe Heα resonance line and Ni Heα line complex. We obtain consistent best-fit parameters, with both APEC and SPEX predicting a temperature of 4.1 ± 0.1 keV. The iron-to-hydrogen abundances are 0.62 ± 0.02 from APEC and 0.74 ± 0.02 from SPEX, relative to solar values31. The line broadening obtained from APEC, 146 ± 7 km s−1, is smaller than the best-fit SPEX value of 171 ± 7 km s−1, although both values are consistent with the line broadening obtained by fitting a set of Gaussians (the result presented in the main body of the paper). Apart from the Fe Heα w line affected by resonance scattering (a), both emission line models presented here currently have difficulty reproducing the measured Fe Heα intercombination lines (a) as well as the exact position of the Fe Heβ line (c). This motivates the model-independent approach adopted in the manuscript for determining the line widths. Error bars are 1 s.d.
The 5.0–8.5-keV spectrum was modelled with an isothermal, optically thin plasma in collisional ionization equilibrium using either APEC/ATOMDB 3.0.3 (red) or SPEX 3.0 (blue), with an additional power-law component accounting for emission from the central AGN. During the fit we excluded the Fe Heα resonance line because this can be affected by resonant scattering of photons by the intracluster gas in the line of sight. The two spectral codes provide similar results with an average temperature of 3.8 ± 0.1 keV and metallicity consistent with the solar value. We obtain a velocity broadening of 156 ± 12 km s−1 from APEC and 178 ± 9 km s−1 from SPEX. Both models suggest that the resonant line has been suppressed in the central region. Error bars are 1 s.d.
Extended Data Figure 4 Confidence contours for joint fits of redshift z and velocity broadening σv are compared.
The three line complexes have been fitted independently. The contours are plotted at (68%, two parameters) and (95%). The three fits give consistent redshifts (with the one to which the data were self-calibrated) and broadening.
The total broadband counts (colour scale) seen across the detector array (left), Fe Heα line counts (centre) that come mostly from the diffuse cluster plasma, and a model response of a point source centred in the pixel coincident with the nucleus of NGC 1275 (right) are compared. Brightness is normalized to the same peak value.
The Chandra image is from ref. 32. The contours increase by a factor of 1.5. The numbers in the larger font indicate the velocity in each region (see also the colour scale). The 90% errors in the figure (numbers in the smaller font) are statistical only; our estimate of the calibration uncertainty in individual pixels is 50 km s−1. Heliocentric correction has been applied. Velocities are shown relative to that of NGC 1275, whose redshift is z = 0.01756 (ref. 33).
a, The composite spectrum of all pixels (excluding the calibration pixel) when they were exposed to the 55Fe source on the filter wheel. The blue line shows the expected natural line shape and the red line shows the observed profile (error bars are 1 s.d.). b, A histogram of pixel resolution. N is the number of pixels sharing that resolution.
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Hitomi Collaboration. The quiescent intracluster medium in the core of the Perseus cluster. Nature 535, 117–121 (2016). https://doi.org/10.1038/nature18627
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