FOGGIE X: Characterizing the Small-Scale Structure of the CGM and its Imprint on Observables

TL;DR

This study uses high-resolution simulations to analyze the small-scale structure of the circumgalactic medium at redshift 1, revealing properties of gas clumps, their metallicity, and implications for observations and galaxy evolution.

Contribution

The paper provides detailed characterization of small-scale CGM structures using FOGGIE simulations, highlighting their physical properties and potential observational signatures.

Findings

Clumps peak at ~10^5 solar masses and 10^4 K temperature.

Metallicity in clumps is 1.5-2 times lower than surrounding gas.

Structures larger than 0.5 kpc tend to be long-lived.

Abstract

One of the main unknowns in galaxy evolution is how gas flows into and out of galaxies in the circumgalactic medium (CGM). Studies observing the CGM in absorption using multiple or extended background objects suggest a high degree of variation on relatively small ($\lesssim 1$ kpc) spatial scales. Similarly, high-resolution simulations generally exhibit small-scale substructure in the gas around galaxies. We examine the small-scale structure of the $z = 1$ CGM using simulations from the FOGGIE (Figuring Out Gas & Galaxies in Enzo) project. We select gaseous substructures ("clumps") by their local overdensity and investigate their physical properties, including temperature, metallicity, and kinematics with respect to the galaxy and the nearby surroundings. FOGGIE resolves clumps down to sphericalized radii $R \sim 0.25$ kpc at $z = 1$. The distribution of clumps peaks at $\sim 10^5$ $\rm M_{\odot}$ and $10^{4}$ K, consistent with relatively condensed, cool gas with a slight preference for inflow-like velocities. Many clumps show internal temperature and density variations, and thus internally varying ionization levels for key diagnostic ions such as HI, MgII, and OVI. The average metallicity in clumps is about a factor 1.5--2$\times$ lower in metallicity than nearby gas, suggesting that the metals are not well-mixed between structured and diffuse CGM, which may have implications for observational metallicity estimations of dense CGM clouds. We estimate the survivability of CGM clumps and find that structures larger than 0.5 kpc are generally long-lived. Finally, we qualitatively compare the simulated cloud properties to Milky Way high-velocity clouds.