The multiphase circumgalactic medium traced by low metal ions in EAGLE zoom simulations

We explore the circumgalactic metal content traced by commonly observed low ion absorbers, including C II, SiII, SiIII, SiIV, and MgII. We use a set of cosmological hydrodynamical zoom simulations run with the EAGLE model and including a non-equilibrium ionization and cooling module that follows 136 ions. The simulations of z ≈ 0.2 L* (M200= 1011.7- 1012.3M⊙) haloes hosting star-forming galaxies and group-sized (M200= 1012.7- 1013.3M⊙) haloes hosting mainly passive galaxies reproduce key trends observed by the COS-Halos survey - low ion column densities show 1) little dependence on galaxy-specific star formation rate, 2) a patchy covering fraction indicative of 104K clumps with a small volume filling factor, and 3) a declining covering fraction as impact parameter increases from 20-160kpc. Simulated Si II, Si III, Si IV, CII, and C III column densities show good (mostly within 0.3 dex) agreement with observations, while MgII is under-predicted. Low ions trace a significant metal reservoir, ≈108M⊙, residing primarily at 10-100kpc from star-forming and passive central galaxies. These clumps preferentially flow inwards and most will accrete onto the central galaxy within the next several Gyr, while a small fraction are entrained in strong outflows. A multiphase structure describes the inner CGM ( 0.5R200) tracing virial temperature gas around L* galaxies. Our simulations support previous ionization models indicating that cloud covering factors decline while densities and pressures show little decline with increasing impact parameter (typically < 0.3 dex from 40 to 160 kpc). We find the cool clumps have lower pressures than the ambient medium they are embedded in, and discuss that numerical effects within the hydrodynamic solver likely play a role. © 2018 The Author(s)