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A multi-wavelength approach to investigate biases affecting galaxy clusters mass

Final Report Summary - GALCLU _ASTRO_COSMO (A multi-wavelength approach to investigate biases affecting galaxy clusters mass)

“A multi-wavelength approach to investigate biases affecting galaxy cluster mass”

Observations collected in the last two decades constrained the amount of "darkness" that characterises our Universe. About three quarters of its total density is ascribed to a component with a negative pressure that causes the accelerated expansion of the universe: the dark energy. In addition, about 80% of the remaining, accounts for a matter that we have not observed yet, hence called dark matter. Unveiling the mysterious nature of these two (dark) components is, currently, the primary challenge for cosmologists, astrophysicists, and particle physicists. This worldwide effort is embraced by the main international space agencies that join forces to fund future space and ground missions devoted to this goal.

Clusters of galaxies are among the astrophysical classes of objects with the most remarkable impact on cosmological investigations. Indeed, their physical properties (e.g. gas mass distribution within the systems) as well as the statistical ones (e.g. their counting in specific mass bins) carry important information on the hierarchical process of structure formation that, first and foremost, depends on the co-existence and co-evolution of baryons, dark matter and dark energy. Clusters are as massive as some quadrillion (million of billions) solar masses. They assemble material from a large-scale region through slow accretion or violent mergers with other smaller objects. In this picture of hierarchical block building, clusters of galaxies are the latest gravitationally bounded systems to form. Their counting at different epochs can, therefore, inform on how much matter was available to build them and on the level of expansion contrasting their gravitational collapse. To be specific, the adopted statistical measure for the goal is the ‘mass function’ which expresses the counting of clusters in fixed mass bins and epochs.

It is, however, challenging to obtain a precise mass estimate for a large sample of objects.
Difficulties lye on the sample selection, on the correction of the measurements for simplistic assumptions such as sphericity or equilibrium of the gas and on the presence of disturbances on the measurements such as interlopers or inhomogeneities of the gas.

The Marie Curie project "Gal_Clu_ASTRO_COSMO" proposed (1) to investigate possible biases in the mass determination derived from X-ray analyses performed under the assumption of hydrostatic equilibrium; (2) to establish the optimal way to select a cluster sample.

My methods of investigation are theoretical based even if the procedural steps of my analysis are closely mirroring the observational methods. At the Observatory of Trieste, I joined the numerical group. Soon after my arrival, we produced a new suite of cosmological simulations zoomed into cluster regions. We generated three separate sets of the same cluster collection of about one hundred objects by diversifying the physics of the gas component implemented in the code. The simplest version involves a gas which does not radiate. In the second set, the high dense gas initiates the process of radiative cooling, it cools down and under precise conditions produces stars. The stellar population evolves accordingly to the models proposed in Trieste and can eventually explode in supernovae (either of type Ia or type II). These pollute the medium with the metals produced by the stars during their life or during the explosion. The more complex description included in our simulations accounts for these sophisticated prescriptions as well as for the presence of black holes and the gas accretion into them. The process induces a released of energy which is radiated from the central region to the surrounding diffuse gas.
The three runs were carried out with a modern version of the hydrodynamical code which includes a prescription for numerical diffusion which contrasts the notorious problem, typical of particle-based codes like ours, of reduced mixing of gases with different entropies.
Owing to the new numerical implementation of the hydrodynamical code and to the effective description of the feedback produced by the activity around the black hole and by supernovae, the latter sample reproduces several observational features. In particular, the excellent agreement with the entropy distribution of observed clusters has been a remarkable and unexpected outcome. For year, simulators were seeking to reproduce systems formed via gravitational collapse in a cosmological context that had the same physical characteristics of observed clusters in their core. X-ray observations, indeed, clearly show a simultaneous presence of cool core and non cool core objects, meaning, respectively, clusters with either low or high entropy gas in the centremost regions. Our result has been achieved thanks to the simultaneous effect of the feedback by the active galactic nuclei that heats the gas and thus balances the radiative cooling which is present in the densest central parts. This dynamical balance is maintained in our numerical models by the artificial numerical diffusion which more efficiently mixes the intra-cluster medium.
By comparing these simulations to X-ray and millimetric observations, we found agreement in terms of profiles of various thermodynamical and chemo-dynamical quantities: gas density, temperature, pressure, globals metallicity as well as distribution of specific chemical elements such as oxygen, iron, and silicon.

This unique set allowed us to investigate the solidity of the hypothesis of hydrostatic equilibrium, the main assumption behind the derivation of the total mass from X-ray observations which implies equilibrium between the thermal pressure and the gravitational forces. We found that our numerical models generically lack of equilibrium apart in the cores of the cool-core systems where there is high thermal pressure. We, therefore, suggest to measure the mass from X-ray observations in the central spatial range and in systems that have a clear X-ray luminous peak, typical of cool-core objects. Beside that region, we found that the thermal pressure alone does not suffice and an extra term (due, for example, to gas motion or turbulence or presence of non-thermal plasma) needs to be added. As confirmation, the core properties are inefficient at providing a good strategy to identify objects whose mass is bias-free at large radii. The mass bias in the outskirts is, however, reduced for another class of objects, those that are dynamically relaxed, identified as such from their clumpiness level. This indicator can be derived from X-ray or millimetric observations and it is a quantification of the amount of inhomogeneities present in the gas density or in the gas pressure distribution.

With upcoming missions and thanks to these precautions, we can accurately measure the mass for a subsample of objects selected to calibrate the scaling relations, or in other words to calculate the normalisation and the slope of the power law relations that connect the total mass of the systems with other easily-derived observational properties. Theoretical arguments and numerical simulations support the existence of such relations originated as consequence of structure formation: while the gravitational potential generated by the density instabilities grows, cosmic baryons assemble from the field and become warmer thanks to the adiabatic compression, induced by mergers shocks. They reach such high temperature to emit via bremsstrahlung processes in X-ray. We studied the relations between the total mass and the gas mass, the X-ray temperature, and the X-ray luminosity and considered their evolution in the last ten billion years. After showing that our theoretical relations are statistically consistent with observations present in literature, we enlightened that the most reliable relation for cosmological studies is that between the total mass and the product of the gas mass by its X-ray temperature and that there is no advantage to carry the analysis to epoch that is antecedent of six billions years after the Big Bang. In that time frame, indeed, the activities of stars and black holes were so intense to induce a noticeable evolution on the scaling relations.

Our results were presented in ten papers published in peer-reviewed journals. I presented them in twenty-two events such as conferences, seminars, and colloquia. At the Observatory of Trieste, I have also entertained outreach activities devoted to elementary, middle, and high schoolers and supervised undergraduate and graduate students.
Elena Rasia (