# A DYNAMICALLY DRIVEN, UNIVERSAL THERMAL PROFILE OF GALAXY GROUPS AND CLUSTERS Academic Article

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### abstract

• Groups and clusters of galaxies show a universal, nearly linear entropy radial profile $K(r)$. Using deprojected 13 clusters and 9 groups from the literature, we find that $K(r)\propto r^{0.97\pm0.01}$, consistent with the mean power-law index $\sim(0.9-1.1)$ of previous studies. An equally good fit to the data is given by a $(t_{cool}/t_{ff})\propto r^{0.73\pm0.01}$ ratio between cooling and free-fall times. Both profiles slightly flatten at small radii, as $(t_{cool}/t_{ff})$ becomes of order unity. The entropy profile is usually attributed to the primordial gas crossing the virial shock, to non-standard heat conduction, or to turbulent heating. We argue that a dynamical mechanism is needed to sustain such a simple profile, oblivious to the temperature peak at the edge of the core and to the virial shock at the outskirts, and robust to the presence of ongoing cooling, merger, and AGN activity. In particular, we show that such a profile is naturally obtained in a spiral flow, which is likely to exist in most galaxy aggregates according to the ubiquitous spiral patterns and cold fronts observed. A generalized Schwarzschild criterion shows that the spiral structure observed must involve a convective layer, which may regulate the universal profile. A generalized two-phase model of a spiral flow extending out to the virial radius is presented.

### publication date

• September 8, 2015