The distribution of chemical elements in the hot intracluster medium (ICM) retains valuable information about the enrichment and star formation histories of galaxy clusters, and on the feedback and ...dynamical processes driving the evolution of the cosmic baryons. In the present study we review the progresses made so far in the modelling of the ICM chemical enrichment in a cosmological context, focusing in particular on cosmological hydrodynamical simulations. We will review the key aspects of embedding chemical evolution models into hydrodynamical simulations, with special attention to the crucial assumptions on the initial stellar mass function, stellar lifetimes and metal yields, and to the numerical limitations of the modelling. At a second stage, we will overview the main simulation results obtained in the last decades and compare them to X-ray observations of the ICM enrichment patterns. In particular, we will discuss how state-of-the-art simulations are able to reproduce the observed radial distribution of metals in the ICM, from the core to the outskirts, the chemical diversity depending on cluster thermo-dynamical properties, the evolution of ICM metallicity and its dependency on the system mass from group to cluster scales. Finally, we will discuss the limitations still present in modern cosmological, chemical, hydrodynamical simulations and the perspectives for improving the theoretical modelling of the ICM enrichment in galaxy clusters in the future.
Four decades ago, the firm detection of an Fe-K emission feature in the X-ray spectrum of the Perseus cluster revealed the presence of iron in its hot intracluster medium (ICM). With more advanced ...missions successfully launched over the last 20 years, this discovery has been extended to many other metals and to the hot atmospheres of many other galaxy clusters, groups, and giant elliptical galaxies, as evidence that the elemental bricks of life—synthesized by stars and supernovae—are also found at the largest scales of the Universe. Because the ICM, emitting in X-rays, is in collisional ionisation equilibrium, its elemental abundances can in principle be accurately measured. These abundance measurements, in turn, are valuable to constrain the physics and environmental conditions of the Type Ia and core-collapse supernovae that exploded and enriched the ICM over the entire cluster volume. On the other hand, the spatial distribution of metals across the ICM constitutes a remarkable signature of the chemical history and evolution of clusters, groups, and ellipticals. Here, we summarise the most significant achievements in measuring elemental abundances in the ICM, from the very first attempts up to the era of
XMM-Newton
,
Chandra
, and
Suzaku
and the unprecedented results obtained by
Hitomi
. We also discuss the current systematic limitations of these measurements and how the future missions
XRISM
and
Athena
will further improve our current knowledge of the ICM enrichment.
The hot intra-cluster medium (ICM) permeating galaxy clusters and groups is not pristine, as it has been continuously enriched by metals synthesised in Type Ia (SNIa) and core-collapse (SNcc) ...supernovae since the major epoch of star formation (z ≃ 2–3). The cluster/group enrichment history and mechanisms responsible for releasing and mixing the metals can be probed via the radial distribution of SNIa and SNcc products within the ICM. In this paper, we use deep XMM-Newton/EPIC observations from a sample of 44 nearby cool-core galaxy clusters, groups, and ellipticals (CHEERS) to constrain the average radial O, Mg, Si, S, Ar, Ca, Fe, and Ni abundance profiles. The radial distributions of all these elements, averaged over a large sample for the first time, represent the best constrained profiles available currently. Specific attention is devoted to a proper modelling of the EPIC spectral components, and to other systematic uncertainties that may affect our results. We find an overall decrease of the Fe abundance with radius out to ~0.9 r500 and ~0.6 r500 for clusters and groups, respectively, in good agreement with predictions from the most recent hydrodynamical simulations. The average radial profiles of all the other elements (X) are also centrally peaked and, when rescaled to their average central X/Fe ratios, follow well the Fe profile out to at least ~0.5 r500. As predicted by recent simulations, we find that the relative contribution of SNIa (SNcc) to the total ICM enrichment is consistent with being uniform at all radii, both for clusters and groups using two sets of SNIa and SNcc yield models that reproduce the X/Fe abundance pattern in the core well. In addition to implying that the central metal peak is balanced between SNIa and SNcc, our results suggest that the enriching SNIa and SNcc products must share the same origin and that the delay between the bulk of the SNIa and SNcc explosions must be shorter than the timescale necessary to diffuse out the metals. Finally, we report an apparent abundance drop in the very core of 14 systems (~32% of the sample). Possible origins of these drops are discussed.
We present the results of deep Chandra, XMM–Newton and Suzaku observations of the nearby galaxy cluster Abell 85, which is currently undergoing at least two mergers, and in addition shows evidence ...for gas sloshing which extends out to r ≈ 600 kpc. One of the two infalling subclusters, to the south of the main cluster centre, has a dense, X-ray bright cool core and a tail extending to the south-east. The northern edge of this tail is strikingly smooth and sharp (narrower than the Coulomb mean free path of the ambient gas) over a length of 200 kpc, while towards the south-west the boundary of the tail is blurred and bent, indicating a difference in the plasma transport properties between these two edges. The thermodynamic structure of the tail strongly supports an overall north-westward motion. We propose, that a sloshing-induced tangential, ambient, coherent gas flow is bending the tail eastwards. The brightest galaxy of this subcluster is at the leading edge of the dense core, and is trailed by the tail of stripped gas, suggesting that the cool core of the subcluster has been almost completely destroyed by the time it reached its current radius of r ≈ 500 kpc. The surface-brightness excess, likely associated with gas stripped from the infalling southern subcluster, extends towards the south-east out to at least r
500 of the main cluster, indicating that the stripping of infalling subclusters may seed gas inhomogeneities. The second merging subcluster appears to be a diffuse non-cool-core system. Its merger is likely supersonic with a Mach number of ≈1.4.
Abstract
We present a robust representation of the chemical and thermal structure in the galaxy group NGC 5813 using archival, deep X-ray observations, and employing a multi-temperature spectral ...model based on up to date atomic line emission databases. The selection of our target is motivated by the fact that NGC 5813 has a very relaxed morphology, making it a promising candidate for the study of the AGN feedback’s influence in the intra-group medium (IGrM). Our results showcase a prominent, extended distribution of cool gas along the group’s NE-SW direction, correlating with the direction along which the supermassive black hole in the group’s central galaxy is known to interact with the IGrM. Our analysis indicates gas being uplifted from the group’s centre as the probable origin of the cool gas, although alternative scenarios, such as in-situ cooling can not be explicitly ruled out. Regarding the chemical structure of the IGrM, and unlike previous findings in massive clusters, we find no evidence for recent metal transport by jets/lobes from the central AGN. Instead, elemental abundances remain near Solar on average across the group. The distribution of elements appears to be independent of galactocentric radius, azimuth and the thermodynamics of the gas, suggesting that the IGrM has been efficiently mixed. The large scale uniformity of the abundance distribution implies the presence of complex dynamical processes in NGC 5813, despite its overall relaxed morphology. Past events of extreme AGN feedback or sloshing could be the primary mechanisms behind this.
ABSTRACT
Hot atmospheres pervading galaxy clusters, groups, and early-type galaxies are rich in metals, produced during epochs and diffused via processes that are still to be determined. While this ...enrichment has been routinely investigated in clusters, metals in lower mass systems are more challenging to probe with standard X-ray exposures and spectroscopy. In this paper, we focus on very deep XMM–Newton (∼350 ks) observations of NGC 1404, a massive elliptical galaxy experiencing ram-pressure stripping of its hot atmosphere while infalling towards the centre of the Fornax cluster, with the aim to derive abundances through its hot gas extent. Importantly, we report the existence of a new fitting bias – the ‘double Fe bias’ – leading to an underestimate of the Fe abundance when two thermal components cannot realistically model the complex temperature structure present in the outer atmosphere of the galaxy. Contrasting with the ‘metal conundrum’ seen in clusters, the Fe and Mg masses of NGC 1404 are measured 1–2 orders of magnitude below what stars and supernovae could have reasonably produced and released. In addition, we note the remarkable Solar abundance ratios of the galaxy’s halo, different from its stellar counterpart but similar to the chemical composition of the ICM of rich clusters. Completing the clusters regime, all these findings provide additional support towards a scenario of early enrichment, at play over two orders of magnitude in mass. A few peculiar and intriguing features, such as a possible double metal peak as well as an apparent ring of enhanced Si near the galaxy core, are also discussed.
ABSTRACT X-ray measurements find systematically lower Fe abundances in the X-ray emitting haloes pervading groups (kT ≲ 1.7 keV) than in clusters of galaxies. These results have been difficult to ...reconcile with theoretical predictions. However, models using incomplete atomic data or the assumption of isothermal plasmas may have biased the best-fitting Fe abundance in groups and giant elliptical galaxies low. In this work, we take advantage of a major update of the atomic code in the spectral fitting package spex to re-evaluate the Fe abundance in 43 clusters, groups, and elliptical galaxies (the CHEERS sample) in a self-consistent analysis and within a common radius of 0.1r500. For the first time, we report a remarkably similar average Fe enrichment in all these systems. Unlike previous results, this strongly suggests that metals are synthesized and transported in these haloes with the same average efficiency across two orders of magnitude in total mass. We show that the previous metallicity measurements in low-temperature systems were biased low due to incomplete atomic data in the spectral fitting codes. The reasons for such a code-related Fe bias, also implying previously unconsidered biases in the emission measure and temperature structure, are discussed.
Abstract The hot intracluster medium (ICM) pervading galaxy clusters and groups is rich in metals, which were synthesized by billions of supernovae and have accumulated in cluster gravitational wells ...for several gigayears. Since the products of both Type Ia and core-collapse supernovae – expected to explode over different time-scales – are found in the ICM, constraining accurately the chemical composition of these hot atmospheres can provide invaluable information on the history of the enrichment of large-scale structures. Recently, Hitomi observations reported solar abundance ratios in the core of the Perseus cluster, in tension with previous XMM–Newton measurements obtained for 44 cool-core clusters, groups, and massive ellipticals (the CHEERS sample). In this work, we revisit the CHEERS results by using an updated version of the spectral code used to fit the data (spexact v3), the same that was used to obtain the Hitomi measurements. Despite limitations in the spectral resolution, the average Cr/Fe and Ni/Fe ratios are now found to be remarkably consistent with unity and in excellent agreement with the Hitomi results. Our updated measurements suggest that the solar composition of the ICM of Perseus is a common feature in nearby cool-core systems.
Chemical enrichment of the Universe at all scales is related to stellar winds and explosive supernovae phenomena. Metals produced by stars and later spread throughout the intracluster medium (ICM) at ...the megaparsec scale become a fossil record of the chemical enrichment of the Universe and of the dynamical and feedback mechanisms determining their circulation. As demonstrated by the results of the soft X-ray spectrometer onboard Hitomi, high-resolution X-ray spectroscopy is the path to differentiating among the models that consider different metal-production mechanisms, predict the outcoming yields, and are a function of the nature, mass, and/or initial metallicity of their stellar progenitor. Transformational results shall be achieved through improvements in the energy resolution and effective area of X-ray observatories, allowing them to detect rarer metals (e.g. Na, Al) and constrain yet-uncertain abundances (e.g. C, Ne, Ca, Ni). The X-ray Integral Field Unit (X-IFU) instrument onboard the next-generation European X-ray observatory
Athena
is expected to deliver such breakthroughs. Starting from 100 ks of synthetic observations of 12 abundance ratios in the ICM of four simulated clusters, we demonstrate that the X-IFU will be capable of recovering the input chemical enrichment models at both low (
z
= 0.1) and high (
z
= 1) redshifts, while statistically excluding more than 99.5% of all the other tested combinations of models. By fixing the enrichment models which provide the best fit to the simulated data, we also show that the X-IFU will constrain the slope of the stellar initial mass function within ∼12%. These constraints will be key ingredients in our understanding of the chemical enrichment of the Universe and its evolution.
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