A Dynamically Driven, Universal Thermal Profile of Galaxy Groups and Clusters

TL;DR

This paper demonstrates that a spiral flow model can naturally produce the universal thermal entropy profile observed in galaxy groups and clusters, challenging previous explanations based on shock accretion or conduction.

Contribution

It introduces a dynamical spiral flow mechanism as the origin of the universal thermal profile in galaxy clusters, supported by a generalized model matching observations.

Findings

A power-law entropy profile $K(r) \,\propto\, r^{0.96}$ fits 28 galaxy systems.

A spiral flow model reproduces the observed thermal profile across the virial radius.

A convective layer is necessary in spiral patterns to sustain the thermal structure.

Abstract

Large scale structures such as groups and clusters of galaxies show a universal, nearly linear entropy radial profile $K(r)$. Using deprojected 16 clusters and 12 groups from the literature, we find that $K\propto r^{0.96\pm0.01}$, consistent with the mean power-law index $(0.9\mbox{--}1.1)$ of previous studies. A similarly good fit is given by a $\tau\propto r^{0.72\pm0.01}$ ratio between cooling and free-fall times. Both profiles slightly flatten at small radii, as $\tau$ becomes of order unity. The entropy profile is usually attributed to self-similar shock accretion (shown to be inconsistent with the data), to non-standard heat conduction, or to turbulent heating. We argue that a dynamical mechanism is needed to sustain such a universal 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 active galactic nucleus (AGN) activity. In particular, we show that such a profile can be naturally obtained in a spiral flow, which is likely to underlie most galaxy aggregates according to the ubiquitous spiral patterns and cold fronts observed. Generalizing a two-phase spiral flow model out to the virial radius surprisingly reproduces the thermal profile. A generalized Schwarzschild criterion indicates that observed spiral patterns must involve a convective layer, which may regulate the thermal profile.