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Recent studies have revealed a dynamic interplay between the galaxy ecosystemand the circumgalactic medium (CGM). We investigate the CGM at high redshifts(z $\gtrsim$ 2) by using bright afterglows of gamma-ray bursts (GRB) asbackground sources. We compiled a sample of medium-resolution ($\delta$v $<$ 50km/s) and high signal-to-noise (typical SNR $\sim$ 10) spectra from 27 GRBafterglows covering z $\sim$ 2 - 6, with six of them at z $\gtrsim$ 4. Weanalyzed the high- and low-ionization absorption features within $\pm$400 km/sto extract the CGM and ISM kinematics. In the stacked spectra, high-ionizationabsorption profiles (e.g. C IV, Si IV) present significant absorption excess intheir blue wings (v $<$ -100 km/s) relative to the red wings (v $>$ 100 km/s).The stronger blue wings in high-ionization species are indicative of thepresence of ubiquitous warm outflows in the GRB hosts at high redshifts. Weused simple toy models to kinematically distinguish the CGM and ISM absorptionand estimate the CGM mass and outflow velocity. We find tentative evidence ofthe evolution of the CGM metal mass by $\sim$ 0.5 dex between two redshiftbins, each spanning 1 Gyr, z1: 2-2.7 and z2: 2.7-5. By comparing with paststudies, we find that over the course of evolution of present-day galaxies with$M_{*} > 10^{10}M_{\odot}$, the ratio of C IV mass in the CGM to the stellarmass remains fairly uniform, with $log(M_{CIV}/M_{*}) \sim -4.5$ within$\pm$0.5 dex from z $\sim$ 4 to z $\sim$ 0, suggesting CGM-galaxy co-evolution.

The CGM-GRB Study. I. Uncovering The CircumGalactic Medium around GRB hosts at redshifts 2-6