Ph.D. Position in Astrophysics (M/F)

Job Description

Thesis Subject

Title: Probing gas motions in the intracluster medium by combining X-ray and Sunyaev-Zel'dovich observations

1. Context and objectives

Galaxy clusters are unique astrophysical laboratories for studying the formation of the Universe's large-scale structures and powerful cosmological probes [1]. They grow through the merging of subclusters and the continuous accretion from the surrounding cosmic web, while their cores are shaped by Active Galactic Nuclei (AGN) feedback. These processes generate shocks that heat the intracluster medium (ICM). A non-negligible fraction of energy, however, is converted into turbulent motions. They generate non-thermal pressure support that is considered a primary source of the hydrostatic mass bias (HSE bias [2]), currently limiting cluster cosmology. Turbulence should also drive cosmic ray (re)acceleration and amplify magnetic fields, producing diffuse radio synchrotron emission. Yet, the physical processes driving this emission remain poorly understood [3]. Understanding the nature, role, and evolution of ICM gas motions across cosmic time, mass scales, and environments is thus a cornerstone question in modern cluster astrophysics.

Direct measurements of ICM turbulence are now feasible with high-resolution X-ray spectroscopic missions such as XRISM [4]. However, such observations are telescope-time intensive and spatially limited, probing only the cores of nearby massive clusters. Alternatively, turbulence can be inferred from fluctuations in thermodynamic quantities, assuming these variations trace the turbulent velocity field [5]. Previous studies have primarily relied on X-ray surface brightness fluctuations [6], but the resulting estimates show some scatter, highlighting the value of complementary kinematic measurements.

The Sunyaev-Zel'dovich (SZ) effect offers an independent and complementary probe of the ICM. Arising from inverse Compton scattering of cosmic microwave background photons by hot ICM electrons, it directly traces thermal pressure. Unlike X-ray emission, the SZ signal is redshift-independent (thus powerful for distant cluster studies), provided high angular resolution and sensitivity are achieved. SZ surface brightness fluctuations are therefore promising tracers of turbulence, although systematic studies are still limited [7]. With the advent of large arrays of sensitive millimeter detectors on large telescopes (e.g., IRAM30m/NIKA2 [8]), high-resolution SZ imaging has become routine. The quality of the SZ data is now sufficient to investigate turbulence.

The project aims to quantify ICM turbulence properties, as traced by the fluctuations of thermodynamic quantities, as a function of mass, redshift, and environment, leveraging high-quality SZ and X-ray observations.

2. Methodology and expected results

The approach proposed in this thesis combines X-ray and SZ imaging to characterize turbulence. Their complementarity enables joint analyses that disentangle the physical nature of ICM perturbations and reduce systematic uncertainties. The PhD candidate will use our NIKA2 SZ samples comprising over 50 clusters up to z∼1, complemented by public X-ray data from XMM-Newton and Chandra. The project is structured as follows.

1 – Pressure fluctuations from SZ imaging.
We recently developed tools to constrain ICM fluctuations from SZ maps. Application to the NIKA2-LPSZ sample has yielded the first high-z measurements of average turbulent pressure support. Nevertheless, SZ fluctuation studies have not reached full maturity yet. The PhD candidate will incorporate simulation-based inference (SBI) methods that account for the stochastic nature of the turbulence. Enhanced modeling will also allow us to probe turbulence from cluster cores to outskirts. Applying this framework to our data will provide robust constraints on the pressure fluctuation power spectrum.

2 – Nature of ICM perturbations from joint X-ray and SZ modeling.
Comparing the power spectra of density (X-ray) and pressure (SZ) fluctuations reveals the thermodynamic regime of turbulence that is related to the source mechanism (AGN feedback, mergers). Joint analyses also help identify contaminating signals (e.g. ICM clumping, background noise) and reduce systematics in the unperturbed model. Using our analysis framework, the PhD candidate will simulate coherent X-ray and SZ observables, including instrumental responses, to generate cross-spectra directly comparable to what we will extract from the data. Application to our sample will enable constraints on the power spectra of the fluctuations and their physical source.

3 – Evolution.
The PhD candidate will study how turbulence properties vary with cluster mass, redshift, radius, and physical scale. They will also examine the links between turbulence, shocks, AGN activity, and particle acceleration by correlating thermal and non-thermal ICM tracers. Turbulence measurements across environments will be compared with radio properties—power, spectral index, source class—to connect turbulence regimes and acceleration mechanisms.

Ultimately, the project will deliver a) statistical characterization of ICM thermodynamic fluctuations as a tracer of turbulence, b) measurements of the thermodynamic regime, telling us about the source mechanism, c) quantification of the evolution of turbulence properties as a function of redshift, mass, and environment. The thesis will advance our understanding of the HSE bias and provide new insights into particle acceleration mechanisms.

References
[1] Pratt et al. (2019), SSR, 215, 25, arXiv:1902.10837
[2] Angelinelli et al. (2020), MNRAS, 495, 864, arXiv:1905.04896
[3] Van Weeren et al. (2019), SSR, 215, 1, 16, arXiv:1901.04496
[4] XRISM collaboration (2025), submitted to Nature, arXiv:2509.04421
[5] Zhuravleva et al. (2018), MNRAS, 520, 4, pp.5157-5172, arXiv:2210.11544
[6] A. Heinrich, I. Zhuravleva et al. (2024), MNRAS, 528, 4, pp.7274-7299, arXiv:2401.15179
[7] R. Adam et al. (2025), A&A, 694, A182, arXiv:2409.14804
[8] R. Adam et al. (2018), A&A, 609, A115, arXiv:1707.00908

Your Work Environment

The Ph.D. will be supervised by R. Adam within the Galaxies and Cosmology team (Lagrange Laboratory, Côte d'Azur Observatory), in close collaboration with I. Zhuravleva (University of Chicago), an expert in X-ray data analysis, density fluctuations, and turbulence. The project will also benefit from the complementary expertise of additional collaborators in France (Toulouse, Saclay, Grenoble), contributing to a multidisciplinary approach for joint SZ–X analysis. Regular meetings and a structured work plan with clear milestones will ensure continuous supervision. The candidate will spend about four months in total in Chicago to develop the X-ray analysis within the joint SZ–X-ray framework.

Constraints and risks

Planned research stay(s) totaling several months at the University of Chicago (USA) during the Ph.D.

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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