The previous sections describe the individual geometric, kinematic and metallicity indicators as evidence for cold-mode accretion. On their own, they are quite suggestive that we have detected signatures of gas accretion, however, combining all of these accretion indicators together can provide quite compelling evidence.
There are a few of such examples that exist where some point to their circumgalactic medium originating from metal enriched outflows along the galaxy minor axis (Kacprzak et al., 2014; Muzahid et al., 2015) or the circumgalactic medium is kinematically consistent with gas arising from tidal/streams or interacting galaxy groups (Kacprzak et al., 2011b; Muzahid et al., 2016; Péroux et al., 2016) or even enriched gas that is being recycled along the galaxy major axis (Bouché et al., 2016).
Bouché et al. (2013) presented a nice example of a moderately inclined z = 2.3 star-forming galaxy where the quasar sight-line is within 20 degrees of the galaxy's projected major axis. They derived the galaxy rotation field using IFU observations and found that the kinematics of circumgalactic medium at 26 kpc away could be reproduced by a combination of an extended rotating disk and radial gas accretion. The metallicity of the circumgalactic medium is −0.72, which is typically for gas accretion metallicities from cosmological simulations at these redshifts (van de Voort & Schaye, 2012; Kacprzak et al., 2016). The mass inflow rate was estimated to be between 30−60 M⊙yr−1, which is similar to the galaxy star-formation rate of 33 M⊙yr−1 and suggestive that there is a balance between gas accretion and star-formation activity. This particular system is described in detail in the chapter by Nicolas Bouché.
In a slightly different case, a star-forming galaxy at z = 0.66 was examined where the quasar sightline is within 3 degrees of the minor axis at a distance of 104 kpc (Kacprzak et al., 2012b). Contrary to expectations, they identify a cool gas phase with metallicity ∼ −1.7 that has kinematics consistent with a accretion or lagging halo model. Furthermore, they also identify a warm collisionally ionized phase that also has low metallicity ( ∼ −2.2). The warm gas phase is kinematically consistent with both radial outflows or radial accretion. Given the metallicities and kinematics, they conclude that the gas is accreting onto the galaxy, however this is contrary to the previously discussed interpretations that absorption found along a the projected minor axis is typically associated outflows. This could be a case where there is a miss-alignment with the accreting filaments and the disk or is an example of the three filaments typically predicted by simulations (e.g., Dekel et al., 2009; Danovich et al., 2012), thus making it unlikely that all accreting gas is co-planer.
Detailed studies like these are one of the best ways of constraining the origins of the circumgalactic medium and to help us understand how gas accretion works. Larger samples are required to build up a statistical sample of systems to negate cosmic variance. Large IFU instruments like the Multi Unit Spectroscopic Explorer (MUSE) will provide ample full field imaging/spectra datasets and help us build up these larger samples with less observational time. Gas accretion studies using IFUs are further discussed in the chapter by Nicolas Bouché.