Although we only have candidate detections of gas accretion, the community has provided a body of evidence indicating that cosmic accretion exists and is occurring over a large range of redshifts. Individual studies on their own are only suggestive that this accretion is occurring, but taking all the aforementioned aspects together does paint a nice picture of low metallicity gas accreting along co-planar co-rotating disk/filaments. The picture seems nice and simple but understanding how previously ejected gas is also recycled back onto the galaxy is also important since it can possibly mimic gas accretion signatures (Oppenheimer et al., 2012; Ford et al., 2014).
Simulations predict the cross-section of cold flows to be as low as 5% (Faucher-Giguère & Kereš, 2011; Fumagalli et al., 2011b; Kimm et al., 2011; Goerdt et al., 2012) and that the metal-poor cold flow streams signatures should be overwhelmed by the metal-rich outflows signatures detected in absorption spectra. However, signatures of intergalactic cold gas accretion seem to be quite frequent. Thus, either the simulations are under-producing gas accretion cross-sections due to various reasons such as dust, resolution effects, self-shielding and/or magnetic fields, or the observations are selecting more than just outflowing and accreting gas. Trying to address how recycled winds fit into the picture, and understanding how we can differentiate them from gas accretion, will possibly aide in resolving this issue. It is great to see however, how the simulations and observations have been working together in our community to try and understand the gas cycles of galaxies.
Aside from observing accretion directly, the way forward now is to combine geometry, kinematics and metallicity at a range of epochs to try to understand how gas accretion occurs. Hopefully in the near future, we will be able to directly image gas accretion, putting to rest one of the most debated issues in the circumgalactic field.
Acknowledgements GGK acknowledges the support of the Australian Research Council through the award of a Future Fellowship (FT140100933). Thanks to Nikole Nielsen for providing comments, and to Nikole Nielsen, Stephanie Ho and Rongmon Bordoloi for providing additional new plots for this Chapter.