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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 nonequilibrium ionization and cooling module that follows 136 ions. The simulations of z ≈ 0.2 L * (M 200 = 10 11.7 −10 12.3 M ) haloes hosting star-forming galaxies and groupsized (M 200 = 10 12.7 − 10 13.3 M ) 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 M , 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 (< 0.5R 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 (> 0.5R 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.

We explore the origin of fast molecular outflows that have been observed in Active Galactic Nuclei (AGN). Previous numerical studies have shown that it is difficult to create such an outflow by accelerating existing molecular clouds in the host galaxy, as the clouds will be destroyed before they can reach the high velocities that are observed. In this work, we consider an alternative scenario where molecules form in-situ within the AGN outflow. We present a series of hydro-chemical simulations of an isotropic AGN wind interacting with a uniform medium. We follow the timedependent chemistry of 157 species, including 20 molecules, to determine whether molecules can form rapidly enough to produce the observed molecular outflows. We find H 2 outflow rates up to 140 M ⊙ yr −1 , which is sensitive to density, AGN luminosity, and metallicity. We compute emission and absorption lines of CO, OH and warm (a few hundred K) H 2 from the simulations in post-processing. The CO-derived outflow rates and OH absorption strengths at solar metallicity agree with observations, although the maximum line of sight velocities from the model CO spectra are a factor ≈ 2 lower than is observed. We derive a CO (1−0) to H 2 conversion factor of α CO (1−0) = 0.13 M ⊙ (K km s −1 pc 2 ) −1 , 6 times lower than is commonly assumed in observations of such systems. We find strong emission from the mid-infrared lines of H 2 . The mass of H 2 traced by this infrared emission is within a few per cent of the total H 2 mass. This H 2 emission may be observable by JWST.

Recent results indicate a correlation between nuclear radio loudness of active galaxies and their central stellar surface‐brightness profiles, in that ‘core’ galaxies (with inner logarithmic slope γ≤ 0.3) are significantly more radio loud than ‘power‐law’ galaxies (γ≥ 0.5). This connection, which indicates possible links between radio loudness and galaxy formation history (e.g. through black hole spin), has so far only been confirmed for a radio‐selected sample of galaxies. Furthermore, it has since been shown that the Nuker law, which was used to parametrize the brightness profiles in these studies, gives a poor description of the brightness profile, with its parameters varying systematically with the radial fitted extent of the profile. Here, we present an analysis of the central surface brightness profiles of the active galaxies of Hubble type T≤ 3, that were identified by the optically selected Palomar spectroscopic survey of nearby galaxies. We fit the brightness profiles using Sérsic, Core‐Sérsic and, where necessary, Double‐Sérsic models, which we fit to the semimajor axis brightness profiles extracted from high‐resolution images of the galaxies from the Hubble Space Telescope. We use these fits to classify the galaxies as ‘Core’, ‘Sérsic’ or ‘Double‐Sérsic’. We compare the properties of the active galactic nuclei (AGN) and their host galaxies with this classification, and we recover the already established trend for Core galaxies to be more luminous and contain a higher mass supermassive black hole. Defining the radio loudness of an AGN as the ratio of the nuclear radio luminosity to [O iii] line luminosity, which allows us to include most of the AGN in our sample and prevents a bias against dim nuclei that are harder to extract from the brightness profiles, we find that AGN hosted in Core galaxies are generally more radio loud than those hosted in Sérsic galaxies, although there is a large overlap between the two subsamples. The correlation between radio loudness and brightness profile can partly be explained by a correlation between radio loudness and black hole mass. Additionally, there is a significant (99 per cent confidence) partial correlation between radio loudness and the Core/Sérsic classification of the host galaxy, which lends support to the previous results based on the radio‐selected sample, although it is possible that this partial correlation arises because AGN in core galaxies tend to have a lower accretion rate as well as a higher central black hole mass.

Proximity zone fossils (PZFs) are ionization signatures around recently active galactic nuclei (AGN) where metal species in the circumgalactic medium remain overionized after the AGN has shut-off due to their long recombination timescales. We explore cosmological zoom hydrodynamic simulations using the EAGLE model paired with a non-equilibrium ionization and cooling module including time-variable AGN radiation to model PZFs around star-forming, disk galaxies in the z ∼ 0.2 Universe. Previous simulations typically under-estimated the O vi content of galactic haloes, but we show that plausible PZF models increase O vi column densities by 2 − 3× to achieve the levels observed around COS-Halos star-forming galaxies out to 150 kpc. Models with AGN bolometric luminosities 10 43.6 erg s −1 , duty cycle fractions 10%, and AGN lifetimes 10 6 yr are the most promising, because their supermassive black holes grow at the cosmologically expected rate and they mostly appear as inactive AGN, consistent with COS-Halos. The central requirement is that the typical star-forming galaxy hosted an active AGN within a timescale comparable to the recombination time of a high metal ion, which for circumgalactic O vi is ≈ 10 7 years. H i, by contrast, returns to equilibrium much more rapidly due to its low neutral fraction and does not show a significant PZF effect. O vi absorption features originating from PZFs appear narrow, indicating photo-ionization, and are often well-aligned with lower metal ion species. PZFs are highly likely to affect the physical interpretation of circumgalactic high ionization metal lines if, as expected, normal galaxies host flickering AGN.