Investigation of the Enigma of the Iron Routes (Astronomy)

If you want to understand how the universe evolves, follow the iron. And it is precisely by reconstructing the quantity and distribution of iron in 12 clusters of galaxies that a team led by Simona Ghizzardi of INAF in Milan managed to discover three significant anomalies. One above all: to explain the abundance of iron estimated by astronomers, there should be seven times more stars, than those observed, capable of producing it. Or they should be seven times more efficient

In the end there is iron. It is there, in box 26 of the periodic table, that you can push the pieces of that goose game that is the stellar nucleosynthesis to the maximum. Elements beyond iron cannot form through the normal nuclear fusion processes that occur in stars: they require special processes triggered by extreme events – such as supernova explosions or neutron star collisions . But the life of a star cannot go beyond that threshold – iron. And it is precisely this dead end, not being able to go beyond iron, which – with the sudden cessation of nuclear fusion – leads to the collapse of the nucleus of large-mass stars, therefore to the dizzying increase in its density andexplosion as a supernova – resulting in the scattering of iron all around in interstellar space and beyond to intergalactic space.

In the end there is iron, then. And about the process that leads to its production – this rough summary above – there is now little doubt. But by studying its distribution in the universe – and in particular by estimating how much iron there is in the clusters of galaxies and on their periphery – the astronomers realized that the picture is not complete. And that there are still many open questions.

«What is known for sure is who are responsible for the production of iron and metals in general: the ‘iron makers’ are the most massive stars that will then explode as supernovae. But how and when they enriched the gas of the intracluster medium is still to be understood “, tells Media Inaf Simona Ghizzardi , astrophysicist at INAF in Milan and first author of a study , published last February in Astronomy & Astrophysics , which investigates the distribution of iron in 12 clusters of galaxies. “Furthermore, preliminary estimates published in recent years in the literature show that the stars contained in a cluster appear not to be able to produce all the iron measured.”

And it is precisely on the mystery of this excess of iron – as well as on its strangely uniform distribution along the entire extension of galaxy clusters – that the census undertaken by the team led by Ghizzardi tries to clarify. Observing the 12 clusters with the Xmm-Newton European space telescope , sensitive to X emission, and from the ground with the Canada-France-Hawaii Telescope and Wide Isaac Newton Telescope , Ghizzardi and colleagues surveyed the relative abundance of iron. 56 up to what in jargon is called R500. That is to say, pushing to the periphery of the 12 clusters of their sample until the density of matter falls below the threshold of 500 times thecritical density of the entire universe – roughly 3 or 4 million light years from the center. Thanks to their investigation, three significant anomalies were confirmed.

As can be seen from the graph, moving away from the center of the masses (therefore moving to the right along the horizontal axis) the profile of the iron abundance curve is flat, a sign that the relative quantity of iron remains more or less constant.  The shower head in the background represents an analogy with the mechanisms of dispersion of iron, evidently very efficient, since they have managed to spread it over enormous distances. Graphic credits: S. Ghizzardi et al., A&A, 2021. Image credits: Pixabay

“First of all, we observed that iron abundance profiles are – apart from the most central regions – extremely flat,” says Ghizzardi. In other words, the abundance of iron is uniform over most of the volume of the clusters, and moreover it varies very little from one cluster to another. This result leads to a very interesting scenario: it is plausible that enrichment essentially occurred in “ancient” times, at about 2 redshifts , when the clusters were not yet formed. Supernovae would have produced most of the iron and expelled it out of galaxies already in the protocluster phase, enriching the surrounding gas which will subsequently be increased to form the cluster. The iron would thus have had time to spread and dilute in a homogeneous way, so as to provide a similar metallicity for all the clusters and uniform within each cluster ».

In addition to this unexpected uniformity in its distribution, iron was then surprisingly abundant in the apparently “empty” spaces, those outside the galaxies that populate the clusters. Spaces that are not empty, however: the space between galaxies is in fact permeated by a very hot gas (millions of degrees), called intracluster gas , invisible to the eye but emitting X-rays. It is mainly made up of hydrogen and helium but contains traces heavier elements – “metals”, in the jargon of astronomers – including iron. “The amount of iron in the intracluster gas  is about ten times greater than the amount of iron that is trapped inside galaxies. This implies that the feedback mechanisms(Agn, stellar winds …) that expel iron from galaxies must be extremely efficient “, explains Ghizzardi,” because 90 percent of the iron produced inside galaxies is expelled and disperses in the intracluster gas “. To make an analogy, it is a bit like coming across a nation in which only one in ten inhabitants live in the city, while all the others have been expelled from urban centers and live scattered among rural, mountainous or desert areas.

The graph highlights the difference between the efficiency that stars should have in producing iron to reach the levels found in the study (pink band at the top) compared to the efficiency expected from stellar evolution models and data. 
(yellow band at the bottom).  In the background, an image of Xmm-Newton with the mosaic of the Abell 2319 cluster, one of the twelve in the X-Cop sample analyzed in the study.  Graphic credits: S. Ghizzardi et al., A&A, 2021. Image credits: The Xmm Cluster Outskirts Project (X-Cop)

But the most intriguing result is perhaps what concerns the amount of iron, which resulted – as we said at the beginning – inexplicably high. «Our work confirms the preliminary measurements in the literature: apparently the stars present in the galaxies that form the clusters are not able to manufacture all the revealed iron: to do this they would have to be 7 times more, or 7 times more efficient. But obviously there are no other “iron makers”: supernovae are the only ones able to synthesize it. So the enigma of who produced the iron is not solved », concludes Ghizzardi.

Among the hypotheses to explain this last anomaly, one of the most intriguing is that the stars responsible for the enrichment that are missing are today in regions even more peripheral than those observed in the course of the study – or perhaps even beyond of what astronomers call the virial radius of the cluster . In practice, beyond its borders, that is to say in regions where gas and stars are already affected by the gravitational attraction of the cluster but have not yet been captured within its potential well. In short, the investigation is not yet closed.

Featured image: Simona Ghizzardi, astrophysicist at the INAF IASF in Milan and first author of the study on the distribution of iron in 12 galaxy clusters published in Astronomy & Astrophysics © INAF

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