4. CONCLUSIONS AND OUTLOOK
A common thread of this review is that there is no silver bullet in the
quest to test models of galactic accretion. Each observational
diagnostic that we discussed has not only some advantages, but also some
ambiguities. There is in general likely no simple criterion that can be
used to robustly identify galactic accretion in individual
measurements. This is because the CGM is a complex and dynamic
environment in which galactic accretion interacts with galactic winds,
satellite galaxies, as well as more quiescent ambient gas. Furthermore,
simulations predict that – like galaxies - the properties of the
CGM evolve significantly with redshift and halo mass. This complexity
underscores the crucial role that simulations will continue to play in
testing models of galactic accretion and feedback. Indeed, our current
understanding points toward systematic, statistical comparisons with
full-physics cosmological simulations as a necessary step to test the
models.
We conclude with a brief list of general areas in which progress is
likely to be particularly fruitful over the next few years:
-
Since cosmological simulations cannot resolve some of the fine-scale
structure apparent in CGM observations (see
§ 2.2), it will be important to
clarify which observational tracers can be robustly compared with simulation
predictions. For example, are the main observable characteristics of the
more massive and volume-filling CGM phases reasonably converged?
- Relatedly, a better understanding of how metals
returned by stellar evolution mix with ambient gas (both inside galaxies
and after being ejected into the CGM) will ultimately be essential to
make robust predictions for the metallicity distribution of CGM
gas. Current sub-grid models for metal mixing due to unresolved
turbulence rely on simplified schemes and do not account for the fact
that metals may be injected as compact clumps well below the resolution
limit.
- Non-ideal hydrodynamic effects, such as
magnetic fields and thermal conduction, affect the survival and phase
structure of CGM clouds, and should therefore be investigated.
- With the exception of a few recent studies,
most previous simulations used to study diagnostics of galactic
accretion either neglected galactic winds or used stellar feedback too
weak to reproduce observed outflows and galaxy stellar masses. To
develop reliable accretion diagnostics, it is critical to use feedback
models that reproduce observed galaxy properties.
- Some promising diagnostics of inflows and
outflows (e.g., azimuthal angle and kinematic diagnostics;
§ 2.4) have so far only been
studied using small samples of simulated halos and for limited redshift
ranges. Since galaxies and their CGM evolve strongly with redshift and
mass, it will be necessary to analyze larger simulation samples that
systematically cover relevant mass and redshift ranges to quantify the
statistical robustness and limitations of the diagnostics.
- Given the ambiguities of different
inflow/outflow diagnostics when applied in isolation, quantifying how
different diagnostics (e.g., metallicity, azimuthal angle, and
kinematics) could be jointly used to distinguish between
inflows and outflows would be very useful.
- Differential studies comparing observations at
epochs where inflows/outflows are predicted to be more/less prominent
could also help in breaking degeneracies in single observations.
We note that most of these issues are not specific to the CGM, but of
general importance to galaxy formation. It is thus clear that studies of
the CGM will remain a very active area at the forefront of research on
galaxy evolution for the foreseeable future.
Acknowledgement. We are grateful to many colleagues and
collaborators who have helped shape our views on galactic accretion,
including: Chuck Steidel, Gwen Rudie, Alice Shapley, Xavier Prochaska,
Joe Hennawi, Michele Fumagalli, Nicolas Lehner, Chris Howk, Lars
Hernquist, Joop Schaye, Freeke van de Voort, Andrey Kravtsov, Cameron
Liang, Mark Dijkstra, Norm Murray, Eliot Quataert, Dusan Kereš,
Phil Hopkins, Alexander Muratov, Daniel Anglés-Alcázar, and
Zach Hafen. Our research on galactic accretion has been supported by NSF
and NASA.