The Hot Universe

The X-IFU on board Athena will be vital to characterise the Hot Universe, by measuring the mechanical energy stored in gas bulk motions and turbulence in groups and clusters of galaxies, the distribution of its metal abundances across cosmic time, the effect of Active Galactic Nuclei (AGN) in the intra-cluster medium (ICM) and by obtaining the distribution and properties of the warm/hot baryonic filaments in the intergalactic medium.

Cluster bulk motions and turbulence

The hierarchical growth of large-scale structures happens through continuous accretion of material and successive merging events (Refs. 3-5). These processes heat the gas filling the massive halos by adiabatic compression and by countless shocks they generate at all scales (e.g., Ref. 6).

Part of the gravitational energy released at the formation of a halo is channeled through bulk motions and turbulent flows. It cascades down to smaller scales where it is dissipated, thereby contributing to the virialisation of their hot gaseous atmosphere (Refs. 7-9). Turbulence in the ICM is also related to the weak magnetic fi eld bathing the cluster gas and further links to the viscosity, convection and conduction of the gas (Ref. 10). The connection between these micro-scale physical processes, as well as their impact on the larger scales is still to be unveiled in order to fully understand the overarching process of assembling large scale structures.

To date this has been investigated mostly by establishing a connection between the statistics of surface brightness fluctuations and of the turbulent velocity field (e.g., Refs. 11-13). Direct measurement of the turbulent velocity and bulk motions can be obtained from the respective measurement of the broadening and shift they induce on the atomic emission lines from the intracluster medium (ICM). X-ray grating spectroscopy has only provided upper limits (e.g., Refs. 14, 15). Thanks to the capabilities of the SXS calorimeter spectrometer (Ref. 16) Hitomi (Ref. 17) has provided an unprecedented view of the Perseus cluster (Ref. 2), showing for the first time what spatially-resolved-high-resolution X-ray spectroscopy can deliver. These unique observations have shown that the level of turbulence close to the core of Perseus is rather modest (< 164 +/-10 km/s), despite the highly structured spatial distribution of hot gas resulting from AGN feedback. This is only a small glimpse of what X-IFU will be able to do, but on 12 times smaller spatial scales and with a spectroscopic throughput which will be 25 times larger than that of Hitomi/SXS.

The direct measurement of the bulk motions and turbulence velocity field thus requires spatially-resolved, high resolution spectroscopy. The X-IFU on board the Athena observatory will be the first integral fi eld unit operating at X-ray wavelengths that will allow, thanks to its joint spatial (5″) and spectral (2.5 eV) resolution, to map the velocity field of the hot gas in groups and clusters of galaxies down to a precision of 10-20 km/s for velocities ranging between 100-1000 km/s (see Fig. below).

The joint high spatial and spectral resolution of the X-IFU shall also allow to resolve line complexes (e.g., Iron around 6.7 keV) and measure line ratios to further constrain thermodynamics or ionisation state of the gas far out into the clusters outskirts (see e.g. Ref. 18). All these diagnostics will provide us with a comprehensive view of how the dark and baryonic matter assemble and evolve into large scale potential wells.

Bulk motions in clusters


Reconstructed bulk motion induced velocity field (in km/s) of the hot intra-cluster gas for a 50 kilo-seconds X-IFU observation of the central parts of a Perseus like cluster from the numerical simulations in Ref. 20. The cluster has the luminosity of Perseus but is considered at a redshift of 0.1.

Chemical enrichment

With masses up to and exceeding 1015 M⊙ , the deep potential wells of galaxy clusters retain all the information regarding the chemical enrichment of their ICM across cosmic time (this is not the case for galaxies, which lose their gaseous haloes and eject metals into the inter-galactic medium through stellar winds and stellar explosions). Just 40 years ago a feature corresponding to Fe XXV and Fe XXVI transitions was discovered in the X-ray spectrum of the Perseus cluster (Ref. 19). Since then, it has been recognised that the hot gas of the ICM is continuously enriched with heavy elements generated in type Ia (SN1a) and core-collapse supernova (SNcc) explosions in the cluster member galaxies. Elements from O to Si and S are mainly produced in massive stars and ejected in SNcc at the end of their lifetime. White dwarfs in binary systems give rise to SNIa explosions that produce elements up to Si, Fe and Ni. X-ray observations of emission lines from highly-ionized elements are the only way to access information on the abundances of the hot gas, its evolution to high redshift, and the processes by which heavy elements are redistributed into the surrounding ICM. The combination of Athena’s large effective area and the 2.5 eV spectral resolution of the X-IFU will allow the abundances of many common heavy elements to be measured to unprecedented precision. Abundance ratios are a powerful method for constraining the contribution of SN1a, SNcc and AGB stars to the total heavy element abundance, as each source produces different heavy element yields. Information on the Initial Mass Function (IMF), the stellar populations, and the star formation history of the galaxies in the cluster can also be gleaned from the evolution of abundance ratios across time.

The exceptional spectral resolution of the X-IFU will allow accurate abundance ratios to be determined to high redshift (z > 1) for the first time (see Fig. below). An example application is the measurement of the evolution of the ratios of O/Fe and Si/Fe for an ensemble of clusters. This will allow discrimination between models where recent enrichment is caused by SNIa ejection, which produces relatively more Fe than O and Si at low redshift, and models where the enrichment is due to stripping of already pre-enriched member galaxies, which do not show evolution in redshift. The heavy elements are ejected and redistributed into the ICM by a number of processes, including outflows and jets from active galactic nuclei (e.g., Ref. 21), galactic winds and starbusts (e.g., Ref. 22), and ram-pressure stripping of the galaxies (e.g., Ref. 23); in addition, it is also possible that intracluster stars may also contribute to the ICM enrichment (e.g., Ref. 24). Spatially-resolved measurements such as abundance pro files provide insight into the different enrichment mechanisms and their spatial distribution, their timescales, and how the gas is mixed by gas-dynamical processes. Detailed abundance mapping is a powerful tracer of the jet energy distribution and can supply constraints on entrainment of enriched gas by the jets, the jet power itself (which is correlated with the radial range of the metal-enriched outflows). It can also put strong constraints on gas physics through ram-pressure stripping of enriched plasma from infalling sub-clumps. The X-IFU will enable all of these measurements to be obtained to unprecedented precision.

Chemical enrichment in clusters


Simulated X-IFU spectrum of a z = 1 galaxy group with kT = 3 keV and LX = 1044 erg/s for 50 ks. Emission lines from elements which are key to understand chemical evolution can be clearly seen.

AGN feedback on cluster scales

Active galactic nuclei (AGN) at the centres of galaxy groups and clusters play a critical role in shaping the properties of the central galaxy and the surrounding ICM. Mechanical feedback from AGN jets is thought to be one of the best candidates for suppression of star formation in the massive central galaxies and for heating the gas inside and beyond the cluster core. Observations of X-ray cavities surrounding radio lobes in systems from massive elliptical galaxies to galaxy clusters (e.g., Refs. 25,26) provide observational support for the existence of feedback from AGN jets at all scales. In the centre of bright, nearby clusters, X-IFU measurements of the X-ray line pro files and variations of the line centroid will allow estimation of the characteristic spatial scales of the turbulent motions induced by AGN jets on scales of tens of kpc, and mapping of the velocity field of the hot gas to an accuracy of ~20 km/s.

This will give unprecedented insights into how power from the initially highly collimated jets is distributed into the surrounding ICM. The thermal and non-thermal energy content of the X-ray cavities will be measured accurately for the first time, helping to establish their composition. X-IFU will help to detect directly the shocked gas surrounding expanding radio lobes, with a spectral resolution sufficient to resolve shock expansion speeds for the fi rst time. The dynamics of the hot gas in the vicinity of cool filaments will yield essential clues to the cooling and mixing process, and enable for the first time a measurement of the amount of material cooling out of the hot phase and relate this to the fuel available to power the AGN. The X-IFU will enable understanding of the entire cycle of heating and cooling in the cores of nearby clusters and groups. Robust jet power measurements for large samples can be compared to accretion rates of hot and cold material, enabling insights into the accretion process and black hole growth to be obtained.

The missing baryons and the Warm-Hot Intergalactic Medium

The number of visible baryons in the local (z < 2) Universe (stars, cold atomic gas and molecular gas in galaxies) adds up to only about 15% of the total number of baryons inferred through a number of independent measurements of cosmological parameters, and recent radio observations have shown the evidence that these invisible baryons mostly reside in the gaseous phase. However, not only the detailed state of the baryons remains unclear, but at least half of these baryons are still elusive, and thought to lie in a tenuous web of warm-hot intergalactic medium (WHIM). X-IFU studies of the WHIM will provide unprecedented information on the hot phase of the baryons in large scale structures, complementing the COS/HST observations which are sensitive to the luke-warm phase at 105-5.5 K.

X-IFU will pursue three independent approaches to the problem: 

  • Studies of WHIM in absorption against bright AGN at z < 0.5, probing low-z LSS and their interplay with the surrounding intergalactic medium; 
  • Studies of WHIM in absorption (and simultaneously in emission at z < 0.1) against Gamma-Ray Burst (GRB) afterglows, probing the WHIM up to redshift of ~2 or higher and strongly constraining the physics (i.e., density and temperature) and kinematics (turbulence and bulk motions) of the WHIM for those filaments detected both in emission and absorption; 
  • WHIM in emission in the outskirts of galaxy concentrations, probing the kinematics of the warm-hot gas near large structures.

The main advances compared to previous instrumentation are the unique combination of large collecting area and spectral resolution. Accurate simulations based on theoretical predictions, show that the X-IFU will be able to fully understand the WHIM baryon budget with a set of Athena observations of bright nearby (z < 0.5) AGN and more distant (z < 2) GRB afterglows. The figure below (top panel) displays a simulated spectrum of a GRB afterglow among the 10% brightest in the sky, going through a random WHIM line of sight extracted from the hydrodynamical simulations in Ref. 27 (spanning the redshift range z = 0-0.85). Four WHIM filaments are detected at redshifts 0.108, 0.350, 0.444 and 0.753, one of them with 3 lines. Taking the Swift/BAT GRB monitor as a reference, there are 25 such GRB afterglows per year, and therefore with a moderate ToO efficiency several such targets could be observed each year. Obtaining higher quality spectra of high-z objects will be rather infrequent in terms of GRB afterglow availability. Alternatively (Figure below, bottom panel) a bright BL Lac like 3C 454.3, (z = 0.86) could provide a high-quality spectrum along the same simulated line of sight. The challenge in that approach to sample sufficiently long lines of sight is the scarcity of such bright objects at z > 0.5. An optimised combination of both approaches as a function of the various performance parameters will be necessary.

Revealing the missing baryons

The two panels show a simulated X-IFU spectrum of a bright background source at z > 0.8 crossing the same random patch of the WHIM as predicted by the simulations in Ref. 27. Four WHIM filaments are clearly detected in both spectra, in some cases through several absorption lines, enabling direct measurements of the gas temperature and even turbulence.


The background source is a GRB afterglow at z > 0.8 with an effective intrinsic column density of 1022 cm2 yielding about 1.5 x 106 counts in the 0.3-2 keV band. This corresponds to the brightest 10% of the GRB afterglows expected in the sky.


The background spectrum is that of the brightest known BL Lac at z > 0.8 3C454.3 (z = 0.86), with a flux F(0.3-2 keV) = 2.7 10-11 erg cm2/s and integrated for 100 ks yielding about 7 x 106 counts.


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