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Register a new userProbing the Warm-Hot Circumgalactic Medium with broad OVI and X-rays
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Most of the baryonic mass in the circumgalactic medium (CGM) of a spiral galaxy is believed to be warm-hot, with temperature around $10^6$K. The narrow OVI absorption lines probe a somewhat cooler component at $log rm T(K)= 5.5$, but broad OVI absorbers have the potential to probe the hotter CGM. Here we present 376 ks Chandra LETG observations of a carefully selected galaxy in which the presence of broad OVI together with the non-detection of Lya was indicative of warm-hot gas. The strongest line expected to be present at $approx 10^6$K is OVII $lambda 21.602$. There is a hint of an absorption line at the redshifted wavelength, but the line is not detected with better than $2sigma$ significance. A physical model, taking into account strengths of several other lines, provides better constraints. Our best-fit absorber model has $log rm T(K) =6.3pm 0.2$ and $log rm N_{H} (cm^{-2})=20.7^{+0.3}_{-0.5}$. These parameters are consistent with the warm-hot plasma model based on UV observations; other OVI models of cooler gas phases are ruled out at better than $99$% confidence. Thus we have suggestive, but not conclusive evidence for the broad OVI absorber probing the warm-hot gas from the shallow observations of this pilot program. About 800ks of XMM-Newton observations will detect the expected absorption lines of OVII and OVIII unequivocally. Future missions like XRISM, Arcus and Athena will revolutionize the CGM science.
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We estimate the detectability of X-ray metal-line emission from the circumgalactic medium (CGM) of galaxies over a large halo mass range ($mathrm{M}_{mathrm{200c}} =10^{11.5}$-$10^{14.5},mathrm{M}_{odot}$) using the EAGLE simulations. With the XRISM Resolve instrument, a few bright (K-$alpha$ or Fe L-shell) lines from $mathrm{M}_{mathrm{200c}} gtrsim 10^{13},mathrm{M}_{odot}$ haloes should be detectable. Using the Athena X-IFU or the Lynx Main Array, emission lines (especially from O$,$VII and O$,$VIII) from the inner CGM of $mathrm{M}_{mathrm{200c}} gtrsim10^{12.5},mathrm{M}_{odot}$ haloes become detectable, and intragroup and intracluster gas will be detectable out to the virial radius. With the Lynx Ultra-high Resolution Array, the inner CGM of haloes hosting $mathrm{L}_{*}$ galaxies is accessible. These estimates do assume long exposure times ($sim 1,$Ms) and large spatial bins ($sim1$-$10,mathrm{arcmin}^{2}$). We also investigate the properties of the gas producing this emission. CGM emission is dominated by collisionally ionized (CI) gas, and tends to come from halo centres. The gas is typically close to the maximum emissivity temperature for CI gas ($mathrm{T}_mathrm{peak}$), and denser and more metal-rich than the bulk of the CGM at a given distance from the central galaxy. However, for the K-$alpha$ lines, emission can come from hotter gas in haloes where the virialized, volume-filling gas is hotter than $mathrm{T}_mathrm{peak}$. Trends of emission with halo mass can largely be explained by differences in virial temperature. Differences between lines generally result from the different behaviour of the emissivity as a function of temperature of the K-$alpha$, He-$alpha$-like, and Fe~L-shell lines. We conclude that upcoming X-ray missions will open up a new window onto the hot CGM.
We use the EAGLE (Evolution and Assembly of GaLaxies and their Environments) cosmological simulation to study the distribution of baryons, and far-ultraviolet (O VI), extreme-ultraviolet (Ne VIII) and X-ray (O VII, O VIII, Ne IX, and Fe XVII) line absorbers, around galaxies and haloes of mass $mathrm{M}_{200c}=10^{11}$-$10^{14.5},mathrm{M}_{odot}$ 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, photoionization 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 $mathrm{M}_{200c}=10^{12.0}$-$10^{13.5},mathrm{M}_{odot}$. We predict that the X-IFU will detect O VII (O VIII) in 77 (46) per cent of the sightlines passing $mathrm{M}_{star}=10^{10.5}$-$10^{11.0},mathrm{M}_{odot}$ galaxies within 100 pkpc (59 (82) per cent for $mathrm{M}_{star}>10^{11.0},mathrm{M}_{odot}$). Hence, the X-IFU will probe covering fractions comparable to those detected with the Cosmic Origins Spectrograph for O VI.
We use adaptive mesh refinement cosmological simulations to study the spatial distribution and covering fraction of OVI absorption in the circumgalactic medium (CGM) as a function of projected virial radius and azimuthal angle. We compare these simulations to an observed sample of 53 galaxies from the Multiphase Galaxy Halos Survey. Using Mockspec, an absorption line analysis pipeline, we generate synthetic quasar absorption line observations of the simulated CGM. To best emulate observations, we studied the averaged properties of 15,000 mock samples each of 53 sightlines having a distribution of $D/R_{vir}$ and sightline orientation statistically consistent with the observations. We find that the OVI covering fraction obtained for the simulated galaxies agrees well with the observed value for the inner halo ($D/R_{vir} leq 0.375$) and is within $1.1sigma$ in the outer halo ($D/R_{vir} > 0.75$), but is underproduced within $0.375 < D/R_{vir} leq 0.75$. The observed bimodal distribution of OVI covering fraction with azimuthal angle, showing higher frequency of absorption along the projected major and minor axes of galaxies, is not reproduced in the simulations. Further analysis reveals the spatial-kinematic distribution of OVI absorbing gas is dominated by outflows in the inner halo mixed with a inflowing gas that originates from further out in the halo. Though the CGM of the individual simulated galaxies exhibit spatial structure, the flat azimuthal distribution occurs because the individual simulated galaxies do not develop a CGM structure that is universal from galaxy to galaxy.
We use hydrodynamical simulations of two Milky Way-mass galaxies to demonstrate the impact of cosmic-ray pressure on the kinematics of cool and warm circumgalactic gas. Consistent with previous studies, we find that cosmic-ray pressure can dominate over thermal pressure in the inner 50 kpc of the circumgalactic medium (CGM), creating an overall cooler CGM than that of similar galaxy simulations run without cosmic rays. We generate synthetic sightlines of the simulated galaxies CGM and use Voigt profile fitting methods to extract ion column densities, Doppler-b parameters, and velocity centroids of individual absorbers. We directly compare these synthetic spectral line fits with HST/COS CGM absorption-line data analyses, which tend to show that metallic species with a wide range of ionization potential energies are often kinematically aligned. Compared to the Milky-Way simulation run without cosmic rays, the presence of cosmic-ray pressure in the inner CGM creates narrower OVI absorption features and broader SiIII absorption features, a quality which is more consistent with observational data. Additionally, because the cool gas is buoyant due to nonthermal cosmic-ray pressure support, the velocity centroids of both cool and warm gas tend to align in the simulated Milky Way with feedback from cosmic rays. Our study demonstrates that detailed, direct comparisons between simulations and observations, focused on gas kinematics, have the potential to reveal the dominant physical mechanisms that shape the CGM.
Large reservoirs of cold (~ 10^4 K) gas exist out to and beyond the virial radius in the circumgalactic medium (CGM) of all types of galaxies. Photoionization modeling suggests that cold CGM gas has significantly lower densities than expected by theoretical predictions based on thermal pressure equilibrium with hot CGM gas. In this work, we investigate the impact of cosmic ray physics on the formation of cold gas via thermal instability. We use idealized three-dimensional magnetohydrodynamic simulations to follow the evolution of thermally unstable gas in a gravitationally stratified medium. We find that cosmic ray pressure lowers the density and increases the size of cold gas clouds formed through thermal instability. We develop a simple model for how the cold cloud sizes and the relative densities of cold and hot gas depend on cosmic ray pressure. Cosmic ray pressure can help counteract gravity to keep cold gas in the CGM for longer, thereby increasing the predicted cold mass fraction and decreasing the predicted cold gas inflow rates. Efficient cosmic ray transport, by streaming or diffusion, redistributes cosmic ray pressure from the cold gas to the background medium, resulting in cold gas properties that are in-between those predicted by simulations with inefficient transport and simulations without cosmic rays. We show that cosmic rays can significantly reduce galactic accretion rates and resolve the tension between theoretical models and observational constraints on the properties of cold CGM gas.
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