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The handle http://hdl.handle.net/1887/49237 holds various files of this Leiden University dissertation
Author: Mernier, François
Title: From supernovae to galaxy clusters : observing the chemical enrichment in the hot intra-cluster medium
Issue Date: 2017-05-31
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Where do we come from? This question is of course very broad, as it con- cerns many disciplines (physics, biology, astronomy, philosophy, etc.), and it is difficult (if not impossible) to provide one clear and trivial answer.
One of the most extraordinary astronomical discoveries of the 20th cen- tury, however, has revolutionised our view of the Universe regarding this question. Sixty years ago, we understood that the building blocks of plan- ets and life have been formed in the core of stars and in their powerful end-of-life explosions, namely supernovae. In other words, we are noth- ing else than ”stardust”.
The origin of chemical elements
These elemental building blocks are named chemical elements. They as- semble into molecules to form stars, planets, rock, water, ice, cells, plants, animals, etc. They are the essence of matter and life. Thanks to the remark- able work of several generations of astrophysicists, we know the basic his- tory of the production of chemical elements in the Universe. About 13.7 bil- lion years ago, the extreme conditions following the first minutes of the Big Bang created all the hydrogen, and almost all the helium that are present in today’s Universe. However, heavier elements (or ”metals”, including for example carbon, nitrogen, oxygen, silicon, iron, etc.) could not have been synthesised during these first minutes. Instead, they formed in the very hot and dense core of stars and, especially when these stars explode as su- pernovae.
Not all supernovae are the same, and different supernovae may pro- duce elements in different amounts. In fact, supernovae can be broadly
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Figure 1: The Tycho supernova is a remnant of a (Type Ia) supernova whose explosion was observed in the year 1572. The remains of this former white dwarf, including a large amount of freshly produced metals, are being dispersed into the surrounding interstellar medium (Credit:
NASA/SAO/CXC,JPL-Caltech/MPIA).
classified into two main categories.
1. Core-collapse supernovae (SNcc) are massive stars (more than ten times the mass of the Sun) undergoing a dramatic collapse of their core when they reach the end of their life. This results in an ultimate explosion that ejects most of the stellar material into space. The core remnant of the star becomes then either a neutron star (if the star was less than 30 times the mass of the Sun) or a black hole (if the star was more than 30 times the mass of the Sun). These supernovae are thought to produce almost all the oxygen, neon, and magnesium present in the Universe. Because the lifetime of massive stars is very short on astronomical scales (a few million years at most), these su- pernovae explode ”quickly” relative to other stars.
2. Type Ia supernovae (SNIa) are formed in a double system of low- mass stars (less than eight times the mass of the Sun each). They are the result of the explosion of a white dwarf (the core remnant
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of a low-mass star). This explosion is caused by the interaction of the white dwarf with its companion object. If that companion is a normal star, its material is progressively sucked by the white dwarf, until the temperature of the latter becomes too extreme and leads to a vi- olent explosion. Alternatively, the companion object can be another white dwarf. In that case, the explosion may come from a violent col- lision between the two white dwarfs. Even today, it is unclear to as- tronomers which of these two scenarios is the correct one. In any case, these supernovae are thought to produce and eject heavier elements, in particular chromium, manganese, iron, and nickel. Compared to SNcc, SNIa take much more time to explode, because low-mass stars live much longer than massive stars (up to several billion years).
Intermediate elements, such as silicon, sulfur, argon, and calcium, are prob- ably produced by SNIa and SNcc in comparable proportions. Finally, car- bon and nitrogen, which are also essential for life on Earth, are thought to be produced by low- and intermediate-mass stars during their lifetime.
Nowadays, supernovae are far from being completely understood. For example, what is the precise nature of the companion of the exploding SNIa? What is the precise physical mechanism driving its explosion? Also, there are many unsolved questions left about the massive stars that turned into SNcc. How many very massive stars were typically formed with re- spect to less massive stars? Were these massive stars previously enriched by a former generation of stars?
The number of heavy elements produced by each supernova type are very sensitive to all these unknowns. This means that if we can measure the relative amounts, namely abundances, of all these elements in SNIa and/or SNcc, we will be able to better understand the physics and the environmen- tal conditions of these fascinating objects (Fig. 1). However, studying the metals released by a couple of supernovae only would not give us a good picture of all the supernovae in the Universe. If we want to understand their general properties, it is necessary to zoom out to Universal scales.
From supernovae to galaxy clusters
On Universal scales, clusters of galaxies are the largest ”bound” objects. In fact, galaxies are not randomly distributed in space. They are instead often found within groups (a few tens of galaxies) or larger clusters (100 to 1000
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Figure 2: The galaxy cluster Abell 1689. At optical wavelengths (here in yellow), the individual galaxies can be seen. However, most of the ”normal” matter of the cluster is present in the form of a hot gas, visible in X-ray (here in purple). This gas is also rich in metals, which are produced by SNIa and SNcc over the last billion years (Credit: NASA, ESA, E. Jullo, P.
Natarajan, and J-P. Kneib).
galaxies). All the stars, planets, and the interstellar gas and dust belonging to the galaxies accounts for only 10 to 20 percent of the total visible (or
”normal”) matter in a galaxy cluster. The major ”normal” component of galaxy clusters is, in fact, in the form of a very hot, diffuse gas. Because of the very large gravity in clusters, this intra-cluster medium (ICM) falls rapidly towards the centre, interacts and collides with itself, and is thus heated up to 10 to 100 million degrees. This extreme heating makes that gas visible in X-ray light (Fig. 2). The most recent generation of X-ray satellites, in particular the European mission XMM-Newton (Fig. 3 left), is well suited to observe the ICM and study its properties via X-ray spectroscopy.
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What is X-ray spectroscopy?
Like many other telescopes on Earth or in orbit around the Earth, the cur- rent X-ray space telescopes can do much more than simply ”see” astro- physical sources in the sky. Exactly like a rainy cloud is able to decompose sunlight into a wide range of colours (or more specifically, wavelengths), the instruments onboard the most recent X-ray satellites are able to decom- pose the X-ray light coming from the hot ICM. By analysing the relative amounts of all these X-ray ”colours” we get (in other words, what its X-ray spectrum looks like), we can determine various features of that gas, such as its temperature or its density.
Metals in the hot intra-cluster medium
About forty years ago, astronomers discovered the presence of emission lines in the X-ray spectra of this intra-cluster gas. These emission lines are a characteristic imprint of the presence of heavy elements. This means that the ICM contains a significant fraction of metals. Since only supernovae can produce heavy elements, these metals must originate from SNIa and SNcc within the individual galaxies. Metals are thus not only located in the vicinity of supernovae, but also in the ICM, beyond galaxies. In other words, even the largest scales of the Universe are chemically enriched by stars and supernovae.
Luckily, the X-ray emission of the ICM is easy to model with comput- ers, and the metal abundances of this gas can be accurately measured, by analysing their corresponding lines in the X-ray spectra of galaxy clusters (Fig. 3 right). In turn, because they trace the total yields of billions of su- pernovae over cosmic times, the abundances of these elements measured in the ICM can be directly compared to the yields predicted by the cur- rently competing SNIa and SNcc theoretical models. This helps to favour some specific scenarios for supernovae, and to rule out some others. Even- tually, measuring the amount of metals in galaxy clusters enables us to better understand supernovae.
How about the spatial distribution of these metals in galaxy clusters?
Are they concentrated rather in the core of clusters, or rather in the out- skirts? Are they distributed uniformly through the intra-cluster gas, or are they present in some specific regions only? Answering these questions may provide valuable information to understand how and when stars and su-
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0.100.10.100.1
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O VIII (Ly) FeL complex (incl. Ne) Mg XII (Ly) Si XIII (He) Si XIV (Ly) S XV (He)S XVI (Ly) Ar XVII (He)Ar XVIII (Ly) Ca XIX (He) Ca XX (Ly) Ca XIX / Ca XX Cr XXIII (He) Mn XXIV (He)FeK complex Fe XXVI (Ly) Fe XXV (He⇥)
(/ Fe XXIV) Ni XXVII / Ni XXVIII / Fe XXV (He⇤)
> SNIa
> SNcc
> SNIa & SNcc
Figure 3: Left: Artist impression of the XMM-Newton satellite in orbit around the Earth (Credit: ESA). Right: This plot shows the typical emission lines that can be found in the X-ray spectra of the central ICM regions (Chapter 3). Each line, or unresolved line complex, corresponds to the imprint of a specific heavy element. Together, they provide a robust determination of the abundances of these elements in the ICM.
pernovae enriched the ICM.
This thesis
In this thesis, I have compiled the XMM-Newton observations of 44 nearby and relaxed galaxy clusters, groups, and giant ellipticals (the CHemical Enrichment Rgs Sample, or CHEERS). These observations represent a total of almost two months of uninterrupted observing time.
I have started by summarising our current knowledge of the ICM en- richment, as well as the most recent progress achieved in this field of re- search (Chapter 1). Using both the high-resolution RGS and the moder- ate resolution EPIC instruments on board XMM-Newton, I have devoted Chapter 2 to the extensive study of the temperature and abundances in the ICM of one galaxy cluster, Abell 4059. I have extended this study to the whole CHEERS sample (Chapter 3, Fig. 3 right), for which I could measure the average abundances of 11 key elements (oxygen, neon, magnesium, silicon, sulfur, argon, calcium, chromium, manganese, iron, and nickel). I have compared these abundance measurements to the yields predicted by the best SNIa and SNcc theoretical models, in order to better understand how SNIa explode, and how massive and enriched were the massive stars that gave birth to SNcc (Chapter 4). I have also devoted a lot of attention to
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all the uncertainties that may affect the final results. In particular, I have in- vestigated the effects that the latest major update of the spectral code SPEX, which is used to model the X-ray emission of the ICM, has on the average abundance measurements (Chapter 5). Finally, the EPIC instruments on board XMM-Newton also allow for a study of the radial distribution of the different elements in the ICM (Chapter 6). In turn, this provides important clues on the main epoch and the dynamics driving the ICM enrichment.
The conclusions of this thesis are various, but can be summarised as fol- lows.
• In some cases, the metal distribution in galaxy clusters is far from being symmetric. Abell 4059 is a textbook example, where a dense, metal-rich region of the hot gas is found outside of the cluster centre.
This suggests that galaxies can enrich their surroundings with metals when they travel so fast that their gas gets stripped by the ambient ICM pressure.
• I have probably obtained the most accurate ICM abundance measure- ments that are ever possible to obtain with XMM-Newton. Further significant improvements of these measurements cannot be achieved without better instruments on board future X-ray missions, such as XARM or Athena (Chapter 7), or without a substantial reduction of the systematic uncertainties (for example a better calibration of the XMM-Newton instruments).
• The average ICM abundance measurements I have obtained are valu- able to better understand supernovae. In particular, they suggest that the burning flame driving SNIa explosions propagates first below the speed of sound, then reaches a supersonic speed before ejecting the stellar material into space. They also suggest that most of the SNcc having enriched galaxy clusters come from massive stars that had been already enriched by a former generation of stars. Finally, it is possible that a specific sub-class of SNIa, namely the Ca-rich gap tran- sients, which produce and release calcium in very large quantities, play an important role in enriching galaxy clusters.
• In the hot gas of relaxed galaxy clusters and groups, the radial dis- tribution of oxygen, magnesium, silicon, sulfur, argon, calcium, iron, and nickel are all peaked: there is more of these metals in the centre than in the outskirts of clusters. On average, these profiles are all very
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similar to each other. This strongly suggests that both SNIa and SNcc enrich clusters in a very similar way. Given that SNIa take longer to explode than SNcc, this probably means that the bulk of the ICM en- richment occurred at early times, before the Universe was half of its current age.
