Ambient Column Densities of Highly Ionized Oxygen in Precipitation-limited Circumgalactic Media

Abstract

Many of the baryons associated with a galaxy reside in its circumgalactic medium (CGM), in a diffuse volume-filling phase at roughly the virial temperature. Much of the oxygen produced over cosmic time by the galaxy's stars also ends up there. The resulting absorption lines in the spectra of UV and X-ray background sources are powerful diagnostics of the feedback processes that prevent more of those baryons from forming stars. This paper presents predictions for CGM absorption lines (O VI, O VII, O VIII, Ne VIII, N V) that are based on precipitation-regulated feedback models, which posit that the radiative cooling time of the ambient medium cannot drop much below 10 times the freefall time without triggering a strong feedback event. The resulting predictions align with many different observational constraints on the Milky Way's ambient CGM and explain why N OVI ≈ 10 14 cm −2 over large ranges in halo mass and projected radius. Within the precipitation framework, the strongest O VI absorption lines result from vertical mixing of the CGM that raises low-entropy ambient gas to greater altitudes, because adiabatic cooling of the uplifted gas then lowers its temperature and raises the fractional abundance of O 5+ . Condensation stimulated by uplift may also produce associated lowionization components. The observed velocity structure of the O VI absorption suggests that galactic outflows do not expel circumgalactic gas at the halo's escape velocity but rather drive circulation that dissipates much of the galaxy's supernova energy within the ambient medium, causing some of it to expand beyond the virial radius.

Thermal instability in the CGM of L⋆ galaxies: testing ‘precipitation’ models with the FIRE simulations

“…As stated at the end of Section 3.2, the model of Sharma et al (2012b) -a variant of which was used in our analysis -differs significantly from the "precipitation limited" model of Voit (2019). In fact, these two models can make qualitatively different predictions in some regimes.…”

“…The model of Sharma et al (2012b) has been extended to both the steadily growing observational data on massive group and cluster systems from X-ray emission (Voit et al 2015a;Voit et al 2017) and the rapidly emerging absorption line data on the CGM of less massive galaxies (Voit 2019;Voit et al 2019). The version presented in the latter papers has also been used to model scaling relations in galaxy properties across the entire galaxy mass range (see also Voit et al 2015b).…”

“…However, we note that the model described in these papers is not identical to the one introduced in Sharma et al (2012b) or presented in our analysis. Specifically, the version of the model presented in Voit (2019) does not self-consistently combine the assumptions of a constant t cool /t ff and HSE to predict thermodynamic profiles. Instead, this version combines the number density profile Eq.…”

“…As the exploration of and debate on the physics of ICM cooling in groups and clusters continues, some recent investigations have extended the precipitation scenario to galaxy scales and argued that circum-galactic thermal instability may play a central role in the cycle of gas accretion, star formation, and feedback (Soker 2010;Sharma et al 2012b;Voit et al 2015aVoit et al , 2018Voit et al , 2019Voit 2019). Although the overall dynamics of gravitational collapse is approximately self-similar in the ΛCDM cosmology (e.g., Kravtsov & Borgani 2012), the interplay of gas shock heating and cooling during the formation of galaxy-sized systems is qualitatively different from that of cluster-scale haloes.…”

The warm-hot circumgalactic medium around EAGLE-simulation galaxies and its detection prospects with X-ray and UV line absorption

“…Previous theoretical studies of CGM X-ray absorption include analytical modelling, which tends to focus on the Milky Way. For example, Voit (2019) Nelson et al (2018) studied O vii and O viii in IllustrisTNG, but focused on a wider range of halo masses: two orders of magnitude in halo mass around L * .…”