| In the universe,whether it is in galaxy or galaxy cluster,there exists a significant reservoir of hot(mainly in X-ray band)gas,and it is of great significance to study the properties of these hot gas.Galaxy clusters host a large reservoir of diffuse plasma with radially-varying temperature profiles.The efficiency of thermal conduction in the in-tracluster medium(ICM)is complicated by the existence of turbulence and magnetic fields,and has received a lot of attention in the literature.Previous studies suggest that the magnetothermal instability developed in outer regions of galaxy clusters would drive magnetic field lines preferentially radial,resulting in efficient conduction along the radial direction.Using a series of spherically-symmetric simulations,here we in-vestigate the impact of thermal conduction on the observed temperature distributions in outer regions of three massive clusters,and find that thermal conduction substantially modifies the ICM temperature profile.Within 3 Gyr,the gas temperature at a represen-tative radius of 0.3r500 typically decreases by?10-20%and the average temperature slope between 0.3r500 and r500 drops by?30-40%(here r500 is the radius enclosing a mean matter density of 500 times the critical density of the Universe at the cluster redshift),indicating that the observed ICM would not stay in hydrostatic equilibrium in the presence of thermal conduction.However,X-ray observations show that the outer regions of massive clusters have remarkably similar radially-declining temperature pro-files,suggesting that they should be quite stable.Our study thus suggests that the effec-tive conductivity along the radial direction must be suppressed below the Spitzer value by a factor of 10 or more,unless additional heating sources offset conductive cooling and maintain the observed temperature distributions.Our study may provide the first smoking-gun evidence for the suppression of parallel conduction along magnetic field lines in low-collisionality plasmas by kinetic mirror or whistler instabilities.For Milky Way,theoretical and observational arguments suggest that there is a large amount of hot(?106 K),diffuse gas residing in the Milky Way's halo,while its total mass and spatial distribution are still unclear.In this work,we present a general model for the gas density distribution in the Galactic halo,and investigate the gas evolution under radiative cooling with a series of 2D hydrodynamic simulations.We find that the mass inflow rate in the developed cooling flow increases with gas metallicity and the total gas mass in the halo.For a fixed halo gas mass,the spatial gas distribution affects the onset time of the cooling catastrophe,which starts earlier when the gas distribution is more centrally-peaked,but does not substantially affect the final mass inflow rate.The gravity from the Galactic bulge and disk affects gas properties in inner regions,but has little effect on the final inflow rate either.We confirm our results by investi-gating cooling flows in several density models adopted from the literature,including the Navarro-Frenk-White(NFW)model,the cored-NFW model,the Maller&Bullock model,and the ? model.Typical mass inflow rates in our simulations range from?5 M?yr-1 to?60M? yr-1,and are much higher than the observed star formation rate in our Galaxy,suggesting that stellar and active galactic nucleus feedback processes may play important roles in the evolution of the Milky Way(MW)and MW-type galaxies. |
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