O vi traces photoionized streams with collisionally ionized boundaries in cosmological simulations of z ∼ 1 massive galaxies

Abstract

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.

The CGM$^2$ Survey: Circumgalactic O VI from dwarf to massive star-forming galaxies

“…We find that about 60% of the O VI mass is found within R 200c and 95% is found within 2R 200c of the galaxy. This extent is approximately consistent with analyses of the O VI distribution in simulations (Oppenheimer et al 2016;Strawn et al 2021;Ho et al 2021).…”

“…Stern et al (2018) instead assume that gas on the outskirts of the CGM has a low density and is in thermal equilibrium with the ultraviolet background, conditions that are favorable for the production of O VI through photoionization. Strawn et al (2021) find that O VI is produced through different mechanisms in different parts of the CGM: an ambient, outflow-heated phase at small radii, photoionized inflows at larger radii. Qu & Bregman (2018) and Roy et al (2021) find that there is a transition from cool, photoionized O VI production at low M * to collisional O VI production in ambient gas at higher M * .…”

“…However, the value of f O VI used in this expression is maximal and applies over a narrow range in density and temperature. Even in simulations in which O VI is mostly found in photoionized inflows, f O VI is at least 5 times smaller than this maximal value at large radii and declines steeply at smaller radii (Strawn et al 2021). The metallicity may also be lower than 0.3Z , depending on the origin of the inflow: Strawn et al…”

“…Our measurements tentatively favor a superposition of an inner and an outer O VI generation mechanism. The large extent of the CGM suggests that the "cool outer inflow" scenario of Stern et al (2018) and Strawn et al (2021) is plausible. The need to reproduce the inner half of the O VI mass suggests a need for an ambient phase with some non-hydrostatic temperature structure.…”

The AGORA High-resolution Galaxy Simulations Comparison Project. III. Cosmological Zoom-in Simulation of a Milky Way–mass Halo

“…In this section, however, we demonstrate how multiplatform studies like AGORA could be useful to better understanding the thermal and kinematic states of the CGM, in which disparities exist between contemporary cosmological simulations carried out with different codes, by presenting the first analysis of gas kinematics in four different temperature bins at z = 4. The temperature bins are defined following the observationally motivated temperature thresholds proposed in Roca-Fàbrega et al (2019) and in Strawn et al (2021).…”