Highly ionized oxygen: signatures of galactic feedback

July 01, 2018

Oxygen, after hydrogen and helium, is the most abundant element in the universe. It is a fundamental tracer to learn more about the formation of single stars as well as entire galaxies. Understanding the origin of highly excited states of oxygen in the circumgalactic medium (CGM) around galaxies has proven difficult, and past theoretical models have had difficulty matching observational constraints. Using cosmological simulations from the IllustrisTNG suite, researchers at MPA have demonstrated how the feedback from supernovae and supermassive black holes can shape the heavy element content of the CGM, bringing it into agreement with data from the local universe. The amount of highly ionized oxygen around blue, star-forming galaxies is predicted to be noticeably higher than around red, quenched systems of exactly the same mass.

As the dominant “metal” (i.e. element heavier than hydrogen or helium) in the interstellar, circumgalactic, and intergalactic media, oxygen is one of the most important elements in astrophysics. Observations of oxygen in its various forms underpin much of our understanding of galaxy formation and evolution. In its three highest observable ionization states – OVI, OVII, and OVIII – oxygen traces gas which is either hot, at temperatures above 100,000 Kelvin, or at low densities, with less than 100 atoms per cubic meter. All three ions can arise in the rarefied plasmas which surround galaxies and extend out to large distances, into their hot gaseous halos, which are commonly referred to as the intra-cluster medium (ICM) or circumgalactic medium (CGM), depending on the mass of the dark matter halo. These highly ionized states of oxygen also emerge in the low gas density structures which make up the topology of the cosmic web of large-scale structure: the intergalactic medium (IGM).

Theoretically, MPA researchers study the relationship between these three states of oxygen using the IllustrisTNG project (first presented here in February), a new set of computer simulations based on the basic laws of physics and modeling the formation and evolution of galaxies across cosmic time. In addition to the evolution of dark matter under the influence of gravity, IllustrisTNG simultaneously solves for the hydrodynamical evolution of cosmic gas. As gas collapses to form stars, these stars produce heavy elements including oxygen, which are then released back into the nearby interstellar medium and can even be ejected outside of galaxies entirely. Oxygen is thus an abundant metal throughout cosmic space.

The simulations show that the ratio between different ionization states of oxygen can vary widely, depending on environment, see Figure 1. In under-dense cosmic voids, the two ions are roughly in equipartition. The elongated, dense filaments of the cosmic web are dominated by the quintuplely ionized oxygen (that is, an oxygen atom which has lost five of its outermost electrons, commonly labeled as OVI); the gas here is quite “cold”, only 10,000 Kelvin. In contrast, the hot gas halos around massive groups and clusters, where gas temperatures can exceed 10 million Kelvin, are dominated by highly ionized oxygen (OVIII). This also holds for the localized IGM around these high mass halos, as well as in the largest cosmic web filaments bridging them.

While detailed observations of the two highest observable states of oxygen (OVII and OVIII) are not yet available – they are among the challenging goals for the next generation of space-based X-ray telescope missions, such as Athena and Lynx – quintuplely ionized oxygen (OVI), has been the subject of many recent observational campaigns, such as COS-Halos and others using the Cosmic Origins Spectrograph instrument on the Hubble Space Telescope. As this ion can absorb light at a frequency of 103nm, i.e. at ultraviolet wavelengths, space observatories are required to get above the Earth’s atmosphere. These observations have concluded that OVI is nearly ubiquitous around galaxies which have a similar mass as our own Milky Way – it is found essentially 100% of the time, even out to distances as large as the entire dark matter halo, i.e. ten or twenty times larger than the galaxy itself.

Hydrodynamical simulations have long had trouble reproducing this trend, generally finding too little OVI in the circumgalactic media around galaxies of similar mass. Not so IllustrisTNG: Producing ‘mock’ (or synthetic) surveys of this simulation, the MPA scientists showed that it is fully consistent with the OVI content of the CGM from data in the local universe – for the first time, producing as much absorption as observed. This success is primarily because of the updated feedback model used in these simulations, where supernovae as well as energetic winds driven by supermassive blackholes generate strong outflows of metal from galaxies.

In fact, an interesting prediction emerges from IllustrisTNG about the properties of OVI surrounding galaxies. Using the simulation, we find a correlation between the color of a galaxy and the total amount of oxygen in its CGM. The culprit is feedback energy from the central black hole: in the process of quenching, high-velocity outflows from this central engine physically push some oxygen out of the halo and to larger distances, while at the same time increasing the temperature and lowering the density of the remaining halo gas. Both effects combine to systematically lower the amount of OVI found around red galaxies, a theoretical prediction which will require future observations to either confirm or disprove.

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