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We present the KODIAQ-Z survey aimed to characterize the cool, photoionizedgas at 2.2<z<3.6 in 202 HI-selected absorbers with 14.6<log N(HI)<20, i.e., thegaseous interface between galaxies and the intergalactic medium (IGM). We findthat the 14.6<log N(HI)<20 gas at 2.2<z<3.6 can be metal-rich gas(-1.6<[X/H]<-0.2) as seen in damped Ly-alpha absorbers (DLAs); it can also bevery metal-poor ([X/H]<-2.4) or even pristine gas ([X/H]<-3.8) not observed inDLAs, but commonly observed in the IGM. For 16<log N(HI)<20 absorbers, thefrequency of pristine absorbers is about 1%-10%, while for 14.6<log N(HI)<16absorbers it is 10%-20%, similar to the diffuse IGM. Supersolar gas isextremely rare (<1%) in this gas. The factor of several thousand spread fromthe lowest to highest metallicities and large metallicity variations (a factorof a few to >100) between absorbers separated by less than 500 km/s imply thatthe metals are poorly mixed in 14.6<log N(HI)<20 gas. We show that thesephotoionized absorbers contribute to about 10% of the cosmic baryons and 30% ofthe cosmic metals at 2.2<z<3.6. We find the mean metallicity increases withN(HI), consistent with what is found in z<1 gas. The metallicity of gas in thiscolumn density regime has increased by a factor ~8 from 2.2<z<3.6 to z<1, butthe contribution of the 14.6<log N(HI)<19 absorbers to the total metal budgetof the universe at z<1 is half that at 2.2<z<3.6, indicating a substantialshift in the reservoirs of metals between these two epochs. We compare theKODIAQ-Z results to FOGGIE cosmological zoom simulations. The simulations showan evolution of [X/H] with N(HI) similar to our observational results. Verymetal-poor absorbers with [X/H]<-2.4 at z~2-3 in these simulations areexcellent tracers of inflows, while higher metallicity absorbers are a mixtureof inflows and outflows.

KODIAQ-Z: Metals and Baryons in the Cool Intergalactic and Circumgalactic Gas at 2.2