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All of the aforementioned efforts are possibly direct observations of gas accretion, yet the scientific community is not satisfied since it is difficult to prove conclusively that we have detected cold gas accretion onto galaxies. The only way to conclusively do so is by direct spectral imaging of cosmic flows onto galaxies. Obviously this is quite difficult due to the faintness and low gas column densities of these cold-flow filaments. However, we may be on the verge of being able to detect these cold gas-flows using the new generation of ultra sensitive instruments such as the Keck Comic Web Imager.

It is also possible that we have already observed comic accretion onto two separate quasar hosts (Martin et al., 2015; Martin et al., 2016) with one example shown in Figure 7. The figure shows a narrow-band image obtained with the Palomar Comic Web Imager. The extended nebula and filaments are likely illuminated by the nearby quasar. The streams, as indicted in the figure, extend out to ∼ 160−230 kpc. It was determined that this nebula/disk and filaments are well fit to a rotating disk model with are dark matter halo mass of ∼ 1012.5 M with a circular velocity of ∼ 350 km s−1 at the viral radius of 125 kpc (Martin et al., 2016). They further found that by adding gas accretion with velocities of 80−100 km s−1 improved their kinematic model fit. Furthermore, they estimate the baryonic spin parameter is 3 times higher than that of the dark matter halo and has an orbital period of 1.9 Gyr. The high-angular momentum and the well fitted inflowing stable disk is consistent with the predictions of cold accretion from cosmological simulations (Stewart et al., 2011; Danovich et al., 2012; Stewart et al., 2016; Danovich et al., 2015; Stewart et al., 2013).

Figure 7

Figure 7. a) Mean velocity obtained with an intensity-weighted velocity moment within a narrow 7 Å band image shown in panel b). The data were obtained using Palomar cosmic web imager. The disk and filamentary candidates are indicated. – b) A pseudo slit is placed over the narrow-band image covering the disk and filaments 1 and 3 indicated by the curved white lines with tick marks indicating the distance in arcseconds along the slit. – c) The narrow-band image produced using the sheared velocity window slit indicating the distance along the slit as in panel b) – d) The mean velocity (green) and velocity dispersion (red) from the above panels. Note observed rotation with high velocity dispersions seen along the filament. This is also seen for the remaining filaments (not shown here). This is potentially one of the first direct images of cold gas accretion. Image courtesy of Chris Martin and modified from Martin et al. (2016).

It is possible that we have direct evidence of cold accretion already, but it has yet to be observed and quantified for typical star-forming galaxies (which is much harder). These types of ultra-sensitive instruments will potentially allow us to directly image cosmic accretion in the near future.

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