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The local interstellar medium (ISM) is suffused with "dark" gas, identifiedby excess infrared and gamma ray emission, yet undetected by standard ISMtracers such as neutral hydrogen (HI) or carbon monoxide emission. Based onobserved dust properties from Planck, recent studies have argued that HI mixedwith dust is strongly saturated and that dark gas is dominated byoptically-thick HI. We test this hypothesis by reproducing this model usingdata from Planck and new 21 cm emission maps from GALFA-HI -- the firstlarge-area 21cm emission survey with comparable angular resolution to Planck.We compare the results with those from a large sample of HI column densitiesbased on direct observations of HI optical depth, and find that the inferredcolumn density corrections are significantly lower than those inferred by thePlanck-based model. Further, we rule out the hypothesis that the pencil-beam HIabsorption sight lines preferentially miss opaque "blobs" with small coveringfraction, as these structures require densities and pressures which areincompatible with ISM conditions. Our results support the picture that excessdust emission in the local ISM is not dominated by optically-thick HI, but israther a combination of intrinsic changes in dust grain emissivities and H2missed by CO observations.

Optically thick HI does not dominate dark gas in the local ISM