The Circumgalactic Medium
--- Drummond Fielding, Eliot Quataert, Michael McCourt, and Todd Thompson
The Impact of Star Formation Feedback on the
Circumgalactic Medium. MNRAS, 466, 3810 (2017).
In the modern paradigm for the formation of structure in our universe,
matter over-densities imprinted during the big bang collapse to form
‘halos’ of dark matter. These dark matter halos are initially filled
with hot gas heated to the virial temperature by an accretion shock.
Though the gravitational process of structure formation is self-similar,
pioneering work in the 1970’s recognized that the cooling of halo
gas ultimately differentiates galaxies from more massive systems such as
galaxy groups or clusters. In galaxies with sufficiently low mass, halo
gas can in principle
cool, accrete, and play a role in star formation. In galaxy clusters, on
the other hand, halo gas cannot cool and instead persists for the age of
the universe, essentially independent of the galaxy formation taking
place at smaller radii. The transition between these two outcomes for halo
gas occurs around a total mass of ∼1012 M⊙,
relatively independent of redshift. Thus, a seemingly small distinction
related to atomic physics ultimately drives all of the marked observational
differences between galaxies and galaxy clusters. The basic physics of halo
gas therefore plays a primary, but relatively unexplored, role in our
history of the universe and the types of structures that form within it.
Until recently, we had only indirect measurements of halo gas in galaxies.
As a result, current cosmological simulations have been tuned to reproduce
the stellar properties of galaxies, which are much better constrained from
observations. As new observational techniques begin to probe the halo gas
in galaxies, however, it will become increasingly important to improve the
prescriptions for gas physics in our models. Many of the new
observations—particularly the HST COS quasar absorption spectra observations
(e.g., Werk et al. 2014)—are at odds with the predictions from state-of-the-art
cosmological simulations. Predicting, observing, and understanding the properties
of halo gas represents the new frontier in galaxy formation.
In our first paper on this topic, we use idealized three-dimensional hydrodynamic
simulations to study the dynamics and thermal structure of the circumgalactic
medium (CGM) resulting from the interplay of cooling and stellar feedback in the
presence of cosmological accretion of gas. As in the pioneering calculations
without stellar feedback (e.g., Birnboim & Dekel 2003; Keres et al 2005), we find
that above a critical halo mass (∼1012 M⊙) halo gas is
supported by thermal pressure created in the halo virial shock. The thermal
properties of the halo gas at small radii (near any central galaxy) are regulated
by feedback triggered when tcool/tff ~ 1−10 in the hot halo
gas. Below a critical halo mass, ∼1012 M⊙, however, there is
no thermally supported halo. Instead, the halo gas properties are determined by the
interaction between inflowing gas and galactic winds. The halo gas is not in
thermal pressure balance, but is instead effectively supported by random motions
and outflow ram pressure. This may explain the failure of pressure equilibrium to
account for the properties of cool halo gas inferred from COS observations.
In future work on this subject we will use a similar numerical setup to study
the effect of filamentary accretion, magnetic fields, anisotropic conduction and
viscosity, and halo growth rate.