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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:

  1. 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?
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.

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