We study the components of cool and warm/hot gas in the circumgalactic medium (CGM) of simulated galaxies and address the relative production of OVI by photoionization versus collisional ionization, as a function of halo mass, redshift, and distance from the galaxy halo center. This is done utilizing two different suites of zoom-in hydro-cosmological simulations, VELA (6 halos; z > 1) and NIHAO (18 halos; to z = 0), which provide a broad theoretical basis because they use different codes and physical recipes for star formation and feedback. In all halos studied in this work, we find that collisional ionization by thermal electrons dominates at high redshift, while photoionization of cool or warm gas by the metagalactic radiation takes over near z ∼ 2. In halos of ∼ 10 12 M and above, collisions become important again at z < 0.5, while photoionization remains significant down to z = 0 for less massive halos. In halos with M v > 3 × 10 11 M , at z ∼ 0 most of the photoionized OVI is in a warm, not cool, gas phase (T 3 × 10 5 K). We also find that collisions are dominant in the central regions of halos, while photoionization is more significant at the outskirts, around R v , even in massive halos. This too may be explained by the presence of warm gas or, in lower mass halos, by cool gas inflows.

We analyse the distribution and origin of O vi in the Circumgalactic Medium (CGM) of dark-matter haloes of ∼1012 M⊙ at z ∼ 1 in the VELA cosmological zoom-in simulations. We find that the O vi in the inflowing cold streams is primarily photoionized, while in the bulk volume it is primarily collisionally ionized. The photoionized component dominates the observed column density at large impact parameters (≳0.3Rvir), while the collisionally ionized component dominates closer in. We find that most of the collisional O vi, by mass, resides in the relatively thin boundaries of the photoionized streams. Thus, we predict that a reason previous work has found the ionization mechanism of O vi so difficult to determine is because the distinction between the two methods coincides with the distinction between two significant phases of the CGM. We discuss how the results are in agreement with analytic predictions of stream and boundary properties, and their compatibility with observations. This allows us to predict the profiles of O vi and other ions in future CGM observations and provides a toy model for interpreting them.

We present a suite of high-resolution cosmological zoom-in simulations to z = 4 of a 10 12 M e halo at z = 0, obtained using seven contemporary astrophysical simulation codes (ART-I, ENZO, RAMSES, CHANGA, GADGET-3, GEAR, and GIZMO) widely used in the numerical galaxy formation community. The physics prescriptions for gas cooling and heating and star formation are the same as the ones used in our previous Assembling Galaxies of Resolved Anatomy (AGORA) disk comparison but now account for the effects of cosmological processes such as the expansion of the universe, intergalactic gas inflow, and the cosmic ultraviolet background radiation emitted by massive stars and quasars. In this work, we introduce the most careful comparison yet of galaxy formation simulations run by different code groups, together with a series of four calibration steps each of which is designed to reduce the number of tunable simulation parameters adopted in the final run. In the first two steps, we methodically calibrate the gas physics, such as cooling and heating, in simulations without star formation. In the third step, we seek agreement on the total stellar mass produced with the common star formation prescription used in the AGORA disk comparison, in stellar-feedback-free simulations. In the last calibration step, we activate stellar feedback, where each code group is asked to set the feedback prescription to as close to the most widely used one in its code community as possible, while aiming for convergence in the stellar mass at z = 4 to the values predicted by semiempirical models. After all the participating code groups successfully complete the calibration steps, we achieve a suite of cosmological simulations with similar mass assembly histories down to z = 4. With numerical accuracy that resolves the internal structure of a target halo (100 physical pc at z = 4), we find that the codes overall agree well with one another, e.g., in gas and stellar properties, but also show differences, e.g., in circumgalactic medium (CGM) properties. We argue that, if adequately tested in accordance with our proposed calibration steps and common parameters, high-resolution cosmological zoom-in simulations can have robust and reproducible results. New code groups are invited to join and enrich this comparison by generating equivalent models or to test the code's compatibility on their own, by adopting the common initial conditions, the common easy-to-implement physics package, and the proposed calibration steps. Further analyses of the zoom-in

Most studies of highly ionized plasmas have historically assumed ions are either in photoionization equilibrium, PIE, or collisional ionization equilibrium, CIE, sometimes including multiple phases with different relevant mechanisms. Simulation analysis packages, on the other hand, tend to use precomputed ion fraction tables which include both mechanisms, among others. Focusing on the low-density, high temperature phase space likely to be most relevant in the circumgalactic medium, in this work we show that most ions can be classified as ‘PI’ or ‘CI’ on an ion-by-ion basis. This means that for a cloud at a particular point in phase space, some ions will be created primarily by PI and others by CI, with other mechanisms playing only very minor roles. Specifically, we show that ions are generally CI if the thermal energy per particle is greater than ∼6 per cent of their ionization energy, and PI otherwise. We analyse the accuracy of this ansatz compared to usual PIE/CIE calculations, and show the surprisingly minor dependence of this conclusion on redshift and ionizing background.

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