Introduction

This website introduces and defines ExpCGM, a modeling framework for the gaseous atmospheres around galaxies, sometimes called the circumgalactic medium or CGM.

The name ExpCGM is short for Expandable CGM, reflecting the framework’s focus on how gains and losses of atmospheric energy govern a galaxy’s gas supply by driving either atmospheric expansion or contraction. Its goal is to bridge the gap between observations of the CGM and sophisticated numerical simulations of galaxy evolution by clarifying how differences among models affect observable predictions for galactic atmospheres.

ExpCGM pursues that goal by representing the most crucial features of galactic feedback with a simple and general system of differential equations. Various parametric models for both a galaxy’s atmosphere and its feedback output can be inserted into that simple system and integrated to yield observable predictions applying to both numerical simulations and the real universe. Different classes of models can then be compared and evaluated within a common framework.

The parametric atmosphere models that ExpCGM relies upon can also be used as purely descriptive fitting formulae applicable to both observations and simulations of galactic atmospheres. They do not assume hydrostatic equilibrium but rather a more general force-balance condition including non-thermal forms of atmospheric support energy. They also apply to all the baryons initially associated with a halo’s dark matter, including baryons that feedback has pushed beyond the halo’s virial radius, because the gravitational potential energy of the large-scale baryon distribution around a galaxy reflects its cumulative feedback energy output.

Newcomers should begin with the Essentials page.