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

Abstract

We present the KODIAQ-Z survey aimed to characterize the cool, photoionized gas at 2.2 ≲ z ≲ 3.6 in 202 H i-selected absorbers with 14.6 ≤ log N H I < 20 that probe the interface between galaxies and the intergalactic medium (IGM). We find that gas with 14.6 ≤ log N H I < 20 at 2.2 ≲ z ≲ 3.6 can be metal-rich (−1.6 ≲ [X/H] ≲ − 0.2) as seen in damped Lyα absorbers (DLAs); it can also be very metal-poor ([X/H] < − 2.4) or even pristine ([X/H] < − 3.8), which is not observed in DLAs but is common in the IGM. For 16 < log N H I < 20 absorbers, the frequency of pristine absorbers is about 1%–10%, while for 14.6 ≤ log N H I ≤ 16 absorbers it is 10%–20%, similar to the diffuse IGM. Supersolar gas is extremely rare (<1%) at these redshifts. 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 Δv < 500 km s−1 imply that the metals are poorly mixed in 14.6 ≤ log N H I < 20 gas. We show that these photoionized absorbers contribute to about 14% of the cosmic baryons and 45% of the cosmic metals at 2.2 ≲ z ≲ 3.6. We find that the mean metallicity increases with N H i , 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 H I < 19 absorbers to the total metal budget of the universe at z < 1 is a quarter of that at 2.2 ≲ z ≲ 3.6. We show that FOGGIE cosmological zoom-in simulations have a similar evolution of [X/H] with N H i , which is not observed in lower-resolution simulations. In these simulations, very metal-poor absorbers with [X/H] < − 2.4 at z ∼ 2–3 are tracers of inflows, while higher-metallicity absorbers are a mixture of inflows and outflows.

The Cosmic Ultraviolet Baryon Survey: Empirical Characterization of Turbulence in the Cool Circumgalactic Medium

“…This exercise shows that our resolved CGM absorbing component sample is not biased against broad components with N H exceeding 10 18 cm −2 or components with N H ≈ 17 cm −2 and a modest chemical enrichment level of Z  0.1 Z e . While we cannot rule out the possibility of missing broad, low-columndensity, and metal-poor clumps, random absorber surveys have shown that broad components with b > 50 km s −1 are rare (e.g., Danforth et al 2016;Rudie et al 2019;Lehner et al 2022). Therefore, we argue that the strong correlation between b NT and l cl is not driven by survey incompleteness.…”

The Bimodal Absorption System Imaging Campaign (BASIC). I. A Dual Population of Low-metallicity Absorbers at z < 1

“…22 Similar properties are also observed at higher redshifts (e.g., Fumagalli et al 2016; Lehner et al 2016Lehner et al , 2022.…”

“…These high-resolution simulations show that the IGM can be clustered in small-scale, dense-cloud structures that are not observed in lower-resolution simulations (Mandelker et al 2021). In these simulations, the metallicity of this denser gas is [X/H] < − 3, at z ; 3, but according to the metallicity evolution of these absorbers (Lehner et al 2022), the metallicity of such structures is expected to increase at lower redshift. Future high-resolution simulations should shed light on the possibility of such dense IGM structures being common at z < 1.…”

“…At higher redshifts, the KODIAQ-Z survey characterizes H I absorbers with N log H 14.5 Lehner et al 2016Lehner et al , 2022. This survey covers a similar H I column density regime as CCC, but thanks to the higher S/N of the High Resolution Echelle Spectrometer (HIRES) spectra taken on the Keck I telescope, they are able to explore even lower N(H I) absorbers.…”

“…The lowmetallicity systems could be associated with the IGM or the CGM. (Systems with [X/H] < −2 imply no redshift evolution in their metal content since z ≈ 2-3, since a majority of the MDF for pLLSs/LLSs lies below this value at these redshifts; Lehner et al 2016Lehner et al , 2019Lehner et al , 2022 Here we present the galaxy associations we find for each absorber and an overview of the associated galaxy sample characteristics. There are no apparent correlations between the absorber column densities and host galaxy properties discussed in this section.…”