KODIAQ-Z: Metals and Baryons in the Cool Intergalactic and Circumgalactic Gas at 2.2<z<3.6

TL;DR

The KODIAQ-Z survey investigates the metallicity, distribution, and evolution of cool, photoionized gas at redshifts 2.2 to 3.6, revealing diverse metal contents and their role in cosmic baryon and metal budgets.

Contribution

This study provides detailed measurements of metallicity variations and their evolution in intergalactic and circumgalactic gas at high redshift, comparing observations with cosmological simulations.

Findings

Metal-rich and pristine gas coexist in the studied absorbers.

Pristine absorbers constitute 1-20% depending on column density.

Metallicity increases with N(HI) and evolves over cosmic time.

Abstract

We present the KODIAQ-Z survey aimed to characterize the cool, photoionized gas at 2.2<z<3.6 in 202 HI-selected absorbers with 14.6<log N(HI)<20, i.e., the gaseous interface between galaxies and the intergalactic medium (IGM). We find that 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 be very metal-poor ([X/H]<-2.4) or even pristine gas ([X/H]<-3.8) not observed in DLAs, but commonly observed in the IGM. For 16<log N(HI)<20 absorbers, the frequency of pristine absorbers is about 1%-10%, while for 14.6<log N(HI)<16 absorbers it is 10%-20%, similar to the diffuse IGM. Supersolar gas is extremely rare (<1%) in this gas. The factor of several thousand spread from the lowest to highest metallicities and large metallicity variations (a factor of a few to >100) between absorbers separated by less than 500 km/s imply that the metals are poorly mixed in 14.6<log N(HI)<20 gas. We show that these photoionized absorbers contribute to about 10% of the cosmic baryons and 30% of the cosmic metals at 2.2<z<3.6. We find the mean metallicity increases with N(HI), consistent with what is found in z<1 gas. The metallicity of gas in this column density regime has increased by a factor ~8 from 2.2<z<3.6 to z<1, but the contribution of the 14.6<log N(HI)<19 absorbers to the total metal budget of the universe at z<1 is half that at 2.2<z<3.6, indicating a substantial shift in the reservoirs of metals between these two epochs. We compare the KODIAQ-Z results to FOGGIE cosmological zoom simulations. The simulations show an evolution of [X/H] with N(HI) similar to our observational results. Very metal-poor absorbers with [X/H]<-2.4 at z~2-3 in these simulations are excellent tracers of inflows, while higher metallicity absorbers are a mixture of inflows and outflows.