2016/04/05 by Shota Inoue, Kiyoshi Hayashida, Shutaro Ueda +3 · 1 citation
Physics and Astronomy · #Galaxies: Formation, Evolution, Phenomena #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations
paper · pdf · doi:10.1093/pasj/psw027
Abstract Abell 754 is a galaxy cluster in which an ongoing merger is evident on the plane of the sky, from the southeast to the northwest. We study the spatial variation of the X-ray spectra observed with Suzaku along the merging direction, centering on the Fe Ly α/Fe He α line ratio to search for possible deviation from ionization equilibrium. Fitting with a single-temperature collisional non-equilibrium plasma model shows that the electron temperature increases from the southeast to the northwest. The ionization parameter is consistent with that in equilibrium (net > 1013 s cm−3) except for the specific region with the highest temperature (kT=13.3-1.1+1.4 keV) where n\rm et=10^11.6-1.7+0.6 s cm−3. The elapsed time from the plasma heating estimated from the ionization parameter is 0.36–76 Myr at the 90% confidence level. This timescale is quite short but consistent with the traveling time of a shock to pass through that region. We thus interpret that the non-equilibrium ionization plasma in Abell 754 observed is a remnant of the shock heating in the merger process. However, we note that the X-ray spectrum of the specific region where the non-equilibrium is found can also be fitted with a collisional ionization plasma model with two temperatures, low kT=4.2+4.2-1.5 keV and very high kT >19.3 keV. The very high temperature component is alternatively fitted with a power-law model. Either of these spectral models is interpreted as a consequence of the ongoing merger process as in the case of the non-equilibrium ionization plasma.