Abstract
Clusters of galaxies are the most massive gravitationally bound objects in the Universe and are still forming. They are thus important probes 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 feedback. 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.
Highlights
Doppler shifts and broadening of the emission lines with unprecedented accuracy
Gain values were pinned to an absolute scale via extrapolation of a subsequent calibration of the whole array 10 days later using illumination by another 55Fe source mounted on the filter wheel. (For more detail, see Methods.) We used a subset of the Perseus data closest to that calibration to derive the velocity map
We adopt a minimally model-dependent method for spectral fitting and represent the iron He-α, He-β and H-like Lyman α complexes in the spectrum with a set of Gaussians with free normalizations and energies fixed at redshifted laboratory energies in the case of He-like Fe and theory in the case of H-like Fe (Extended Data Table 1)
Summary
Gain scales for each pixel were measured in ground calibration using a series of fiducial x-ray lines at several detector heat sink temperatures (a single spectral energy reference is sufficient to determine the effective detector temperature and the appropriate gain curve to use). The full dataset was corrected using this timedependent gain function, and the fit errors were incorporated into the error analysis To validate this approach, we compared the first observation, which required a substantial gain correction, to the second, for which the instrument was much closer to thermal equilibrium and required much less correction. We applied additional scale factors for each SXS pixel to match the apparent energies of the cluster Fe He-α complex in order to remove any residual gain errors at the relevant energy. This removes the effect of true bulk shear. Any comparison with simulations will have to take these into account
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