They had the European Xmm-Newton space telescope at their disposal for three million seconds. Objective: to observe and study an ultra-selected sample of 118 galaxy clusters. They are the astronomers of the Chex-Mate project. Now that the observations draw to a close, their first scientific articles are starting to come out. We talk about it with Stefano Ettori of INAF of Bologna, principal investigator of the project together with Monique Arnaud of Cea
Astronomers are somewhat archaeologists, it is often said: the farther they look, the more they go back in time. But astronomers are also somewhat demographers: they study populations of stars and observe how they live, how they are distributed, how they congregate. In small or large cities – the galaxies – with a bustling center of activity, sometimes surrounded by small satellite cities and with arteries of gas – the filaments – connecting them to each other. Galaxies, in turn, congregate into larger structures, the largest in the universe: clusters of galaxies. And it is to study the properties of these clusters that the astronomers of the Chex-Mate projecthave obtained, since 2018, three million seconds of observing time with one of the best space telescopes for X-rays, the Xmm – Newton satellite of the European Space Agency.
“Three million seconds that we are running out in these days; by the summer we should finish all the observations “, Stefano Ettori of INAF OAS of Bologna told Media Inaf , who together with Monique Arnaud of the Cea of the scientific pole of Paris-Saclay, in France, is leading the team of about eighty astronomers (about a quarter of which from INAF, distributed among the offices of Bologna, Florence, Milan, Trieste, Padua), from eleven different states, participating in the project. With the observations now at the end, the first works are in production. Among these an article coming out on Astronomy & Astrophysics in which the demographic characteristics – in fact – of the population of clusters under observation are described in detail.
Hector, why are you astronomers so interested in clusters?
“First of all because they represent the largest structures that have collapsed in the universe. They collapsed under the action of gravity on megaparsec scales, or dimensions of a few million light years. There the action of gravity manifests its full effectiveness in assembling the most extreme – that is, the most massive – bound structures that our theories tell us can be formed. And representing the “extreme” population they are “extremely” sensitive to the initial ingredients that describe these theories. In other words, if the total mass had been different, or if the expansion of the universe was due to dark energyhad it been slowed down or occurred at different times, this would have altered the balance between the collapsing mass and the expanding universe, producing, for example, a different number of galaxy clusters. Therefore, simply counting these clusters already offers direct information on what are the fundamental ingredients of the universe and on their “doses”: how much baryon matter (ie gas and stars), how much dark matter, and how much dark energy ».
So counting how many have been able to reconstruct what the universe is like?
“How it is made and what its initial conditions were, when it started to evolve and form structures. But there’s more: being the largest structures, the clusters are easy to spot. Not only that: being relatively rare – very rare compared to galaxies – it is much easier to count the clusters than to count the single galaxies in the universe. And counting them, doing some statistics on their distribution in the sky and on how many have formed at different masses, we can go back to the initial conditions of the universe when the law that regulates the formation of these structures came into action. “.
Is it a bit like what cosmologists do when they go to count and measure the size of the dots on the CMB map, the radiation of the cosmic microwave background?
“Exactly. On the other hand, the “dots” of the Cmb are fluctuations, in the order of a few tens of microkelvins, with respect to the average temperature of 2.7 kelvins. Some of these points, among the best defined, are clusters of galaxies, which can sometimes appear linked together in even larger structures, called superclusters , which are expected to collapse in the distant future. But at the present time, among the structures that have energy within them distributed in a balanced way, the largest are the clusters of galaxies. This is why they are important to study. Not only that: once formed, they are like small universes ».
What do you mean?
“Having isolated themselves from the expansion of the universe, they have made a home to themselves. If we equated galaxies with cities, clusters would be states. It therefore becomes interesting to study how a state governs itself – and not all states are the same. This is the sense of Chex-Mate : in practice it is a “demographic” champion. We reconstructed the geography of the clusters and went to sample the population of the various “states” – something that had never been done in such a homogeneous way ».
And you did it, I see, by looking at its X-ray emission. How do clusters generate X-rays?
“What happens is that in ionized plasma – therefore with electrons and protons separated from each other – when an electron passes next to a proton its trajectory is deflected and its speed slowed down, thus giving rise to the phenomenon of bremsstrahlung , or braking radiation. . If the energy of the electrons is high enough, as is the case with baryonic matter falling into clusters – this radiation is emitted in the X-band. What we see, then, is hot plasma, at temperatures on the order of 100 million. of degrees. By tracking this hot gas we know how to reconstruct the temperature of the plasma and therefore the potential hole of the cluster – that is, how deep it is. And therefore its mass ».
What mass can a cluster reach?
“Let’s say that if we identified clusters larger than two million billion solar masses we would have problems. Our theories do not contemplate the formation of clusters of any size. Considering the mass and the time available – cosmic time, I mean – for the evolution of these structures, we cannot get to clusters larger than two million billion solar masses. The largest in our sample reach one million and six hundred billion, one million and eight hundred billion… In short, we have clusters that are close to the border ».
Let’s come to your project, Chex-Mate. You have chosen 118 of clusters. Why exactly those?
“We made the selection using data collected by the Planck Space Telescope, in particular the catalogs of objects selected for their Sunyaev-Zel’dovich signal. This is an effect due to the motion of the photons of the Cmb in a gravitational field with high energy electrons, such as that of clusters. The stronger this signal, the more the electrons have energy and the cluster has a high expected mass. Selecting a sample through this effect is almost like making a selection on the mass, so what we get is a representative sample not of the showy properties but of the intrinsic properties of clusters. On a few thousand clusters present in Planck’s Sunyaev-Zel’dovich signal maps, we selected the 118 with the best signal-to-noise ratio and which were distributed in cosmic time with an expected mass useful for the study of their “demography”. And for all these clusters we have obtained, thanks to the three million seconds of observation time with Xmm-Netwon assigned to our project, deep and homogeneous X-band exposures ».
That means? Have you observed all 118 of them for the same time?
“No, on the contrary, it’s like with a camera: being objects with different redshift , therefore at different distances, to obtain homogeneous exposures it is necessary to observe each of them for the required time – those further away require longer exposures, with the same mass. – so as to then be able to compare the signals obtained. And this is what we have done in recent years ».
The acronym of your project, Chex-Mate, alludes to the game of chess, and in particular to checkmate. What game are you playing?
“The game concerns a big problem that has remained unresolved, especially after seeing the data obtained from the Planck satellite. Planck’s observations allowed us to measure the cosmic background radiation and, from this, to arrive at very accurate estimates of cosmological parameters – in the order of one hundred: fantastic results. The same estimates can be made from clusters of galaxies. Scientists from the Planck community tried, but found that the mass of the clusters obtained through observations in X accounted for only 60 percent of the total mass, if the results obtained from the cosmic background radiation were to be reproduced. So we said to ourselves: well, let’s select a sample through the Sunyaev-Zel’dovich effect and let’s see in detail – in a homogeneous way,gravitational lensing in optical band – what is the exact contribution of the X-rays to the current mass ».
Is there any reason why it could only be a partial contribution?
«Yes, the underestimation in X could be due to the so-called hydrostatic bias , that is to say the presence of a component of kinetic energy that we cannot trace well. In order for the estimate of the mass through the measurement of the X-radiation to be reliable, all the kinetic energy of the matter falling into the potential well must be transformed into thermal energy. But if the baryons keep falling towards the potential well there is residual motion, and we can’t measure this from X observations – at least not with current instruments, we will need the future Esa Athena space telescope.to succeed. In short, the suspicion is that, through the X measurements, we are underestimating the mass of the clusters. Dedicated observations on a homogeneous sample, such as the one we are doing, will allow us to measure this discrepancy – the difference between the mass measured by observing the X properties and mass, instead, measured through gravitational lensing . Thus giving “checkmate” to the problem of bias in mass estimation. But at the same time we would also like to arrive at a definitive word – before the next generation of instruments for the measurement and characterization of X-rays – on how much we can really know about the plasma that emits X-rays and which contains the bulk of the baryon matter ».
To know more:
- Go to the Chex-Mate project site
- Read on Astronomy & Astrophysics the article “ The Cluster HEritage project with XMM-Newton: Mass Assembly and Thermodynamics at the Endpoint of structure formation. I. Program overview“, By M. Arnaud, S. Ettori, GW Pratt, M. Rossetti, D. Eckert, F. Gastaldello, R. Gavazzi, ST Kay, L. Lovisari, BJ Maughan, E. Pointecouteau, M. Sereno, I. Bartalucci, A. Bonafede, H. Bourdin, R. Cassano, RT Duffy, A. Iqbal, S. Maurogordato, E. Rasia, J. Sayers, F. Andrade-Santos, H. Aussel, DJ Barnes, R. Barrena, S. Borgani, S. Burkutean, N. Clerc, P.-S. Corasaniti, J.-C. Cuillandre, S. De Grandi, M. De Petris, K. Dolag, M. Donahue, A. Ferragamo, M. Gaspari, S. Ghizzardi, M. Gitti, CP Haines, M. Jauzac, M. Johnston-Hollitt, C. Jones, F. Kéruzoré, AMC Le Brun, F. Mayet, P. Mazzotta, J.-B. Melin, S. Molendi, M. Nonino, N. Okabe, S. Paltani, L. Perotto, S. Pires, M. Radovich, J.-A. Rubino-Martin, L. Salvati, A. Saro, B. Sartoris, G. Schellenberger, A. Streblyanska, P. Tarrio, P. Tozzi, K. Umetsu, RFJ van der Burg, F. Vazza, T.
Featured image: Stefano Ettori, astrophysicist at Inaf Oas in Bologna and principal investigator of the Chex-Mate project
Provided by INAF