Abstract
We explore the circumgalactic metal content traced by commonly observed low ion absorbers, including C II, Si II, Si III, Si IV, and Mg II. 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* (M_200=10^11.7-10^12.3 Msol) haloes hosting star-forming galaxies and group-sized (M_200=10^12.7-10^13.3 Msol) 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 10^4 K clouds with a small volume filling factor, and 3) a declining covering fraction as impact parameter increases from 20-160 kpc. Simulated Si II, Si III, Si IV, C II, and C III column densities show good agreement with observations, while Mg II is under-predicted. Low ions trace a significant metal reservoir, ~10^8 Msol, residing primarily at 10-100 kpc from star-forming and passive central galaxies. These clouds tend to 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 two-phase structure describes the inner CGM (<0.5 R_200) with low-ion metal clouds surrounded by a hot, ambient medium. This cool phase is separate from the O VI observed by COS-Halos, which arises from the outer CGM (>0.5 R_200) tracing virial temperature gas around L* galaxies. Physical parameters derived from standard photo-ionization modelling of observed column densities (e.g. aligned Si II/Si III absorbers) are validated against our simulations. Our simulations therefore support previous ionization models indicating that cloud covering factors decline while densities and pressures show little variation with increasing impact parameter.
Highlights
The circumgalactic medium (CGM) is thought to contain significant reservoirs of baryons and metals outside of galaxies, extending to the virial radius and beyond (e.g. Tumlinson et al 2011; Stocke et al 2013; Johnson, Chen & Mulchaey 2015)
W13 showed that the three main differences between low ions and O VI, which we studied in O16, are that the former have 1) little dependence on sSFR, 2) a larger range in column density indicating a patchier covering fraction, and 3) a more strongly declining covering fraction at large impact parameters for blue galaxies
Listed for each observation–simulation pair is the number of observations and the average column density deviation in dex of the simulation from the observation. δlogN is calculated at each step in the observed K–M function corresponding to each uncensored data point
Summary
The circumgalactic medium (CGM) is thought to contain significant reservoirs of baryons and metals outside of galaxies, extending to the virial radius and beyond (e.g. Tumlinson et al 2011; Stocke et al 2013; Johnson, Chen & Mulchaey 2015). W13 argued that the large dispersion in low-ion absorption strengths suggests that the cool CGM is patchy in nature and spans a large range of densities and/or ionization conditions This contrasts with O VI, which shows a significantly smaller spread in column densities around the blue star-forming galaxies (Peeples et al 2014). O16 ran a set of 20 zoom simulations of individual galactic haloes, 10 of which host blue, star-forming galaxies and have virial masses ∼1012M and another 10 haloes at ∼1013M most of which host red, passive galaxies They turned on the NEQ module at low redshift to follow non-equilibrium effects over the redshift range of COS–Halos galaxies. Resolution and non-equilibrium effects are explored in the Appendix, as well as statistical methods
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