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

Abstract

We use the EAGLE cosmological simulation to study the distribution of baryons, and FUV (O vi), EUV (Ne viii) and X-ray (O vii, O viii, Ne ix, and Fe xvii) line absorbers, around galaxies and haloes of mass $\, \rm {M}_{\rm {200c}}= 10^{11}$–$10^{14.5} \, \rm {M}_{\odot }$ at redshift 0.1. EAGLE predicts that the circumgalactic medium (CGM) contains more metals than the interstellar medium across halo masses. The ions we study here trace the warm-hot, volume-filling phase of the CGM, but are biased towards temperatures corresponding to the collisional ionization peak for each ion, and towards high metallicities. Gas well within the virial radius is mostly collisionally ionized, but around and beyond this radius, and for O vi, photo-ionization becomes significant. When presenting observables we work with column densities, but quantify their relation with equivalent widths by analysing virtual spectra. Virial-temperature collisional ionization equilibrium ion fractions are good predictors of column density trends with halo mass, but underestimate the diversity of ions in haloes. Halo gas dominates the highest column density absorption for X-ray lines, but lower-density gas contributes to strong UV absorption lines from O vi and Ne viii. Of the O vii (O viii) absorbers detectable in an Athena X-IFU blind survey, we find that 41 (56) per cent arise from haloes with $\, \rm {M}_{\rm {200c}}= 10^{12.0 \rm {-}13.5} \, \rm {M}_{\odot }$. We predict that the X-IFU will detect O vii (O viii) in 77 (46) per cent of the sightlines passing $\, \rm {M}_{\star }= 10^{10.5 \rm {-}11.0} \, \rm {M}_{\odot }$ galaxies within $100 \, \rm {pkpc}$ (59 (82) per cent for $\, \rm {M}_{\star }> 10^{11.0} \, \rm {M}_{\odot }$). Hence, the X-IFU will probe covering fractions comparable to those detected with the Cosmic Origins Spectrograph for O vi.

Simulating Groups and the IntraGroup Medium: The Surprisingly Complex and Rich Middle Ground between Clusters and Galaxies

“…Figure 17 from Wijers et al (in prep) demonstrates that XRISM should be able to detect O VII and Mg XII emission tracing the approximate temperatures of virialized IGrM gas. This prediction follows on from the "virial temperature thermometer" model of Oppenheimer et al (2016) and Wijers et al (2020) that specific metal ions should trace the volume-filling virialized halo gas corresponding to the temperature of the ion's peak collisional ionization fraction. While (UV-band) O VI traces outer virialized galactic halo gas at 3 × 10 5 K, O VII and O VIII in the X-ray spectrum are predicted to trace ≥ 10 6 K IGrM in groups.…”

“…While (UV-band) O VI traces outer virialized galactic halo gas at 3 × 10 5 K, O VII and O VIII in the X-ray spectrum are predicted to trace ≥ 10 6 K IGrM in groups. Figure 8 of Wijers et al (2020) predict that O VIII absorption is strongest in poor groups with M 500 ∼ 10 13 M and Fe XVII in intermediate groups with M 500 ∼ 10 13.5 M .…”

The Metal Content of the Hot Atmospheres of Galaxy Groups

“…According to cosmological simulations [335], the IGrM has the greatest potential to show a rich variety of ion species. O VIII should be the most detected species within group virial radii, while O VII should be detected in lower mass groups at M 500 10 13.5 M .…”

The Cosmic Ultraviolet Baryon Survey (CUBS) – I. Overview and the diverse environments of Lyman limit systems at z < 1

“…Because the gas temperature in a massive group is expected to be high, T ∼ 6 × 10 6 K, heavy elements, including oxygen, are expected to be ionized to higher states (e.g., Oppenheimer et al 2016). Indeed, Wijers et al (2020) 13) implies a relatively low gas metallicity. It is therefore likely that the LLS originate in dense gaseous streams accreted from the IGM (e.g., Hafen et al 2017).…”