[Paper Review] Astrophysics and cosmology with galaxy clusters: the WFXT perspective

S. Borgani, P. Rosati+3 more|arXiv (Cornell University)|Oct 29, 2010

Astronomy and Astrophysical Research1 references3 citations

TL;DR

This paper proposes that a Wide-Field X-ray Telescope (WFXT) is essential for advancing astrophysics and cosmology by enabling large-area, deep X-ray surveys of galaxy clusters. By combining high sensitivity and sharp angular resolution across wide fields, WFXT would resolve key questions on entropy injection, metal enrichment, cool cores, and early cluster formation, providing precise constraints on cosmological parameters and gravity models through cluster mass function evolution and growth rate measurements.

ABSTRACT

We discuss the central role played by the X-ray study of hot baryons within galaxy clusters to reconstruct the assembly of cosmic structures and to trace the past history of star formation and accretion onto supermassive Black Holes (BHs). We shortly review the progress in this field contributed by the current generation of X-ray telescopes. Then, we focus on the outstanding scientific questions that have been opened by observations carried out in the last years and that represent the legacy of Chandra and XMM: (a) When and how is entropy injected into the inter-galactic medium (IGM)? (b) What is the history of metal enrichment of the IGM? (c) What physical mechanisms determine the presence of cool cores in galaxy clusters? (d) How is the appearance of proto-clusters at z~2 related to the peak of star formation activity and BH accretion? (e) What do galaxy clusters tell us about the nature of primordial density perturbations and on the history of their growth? We show that the most efficient observational strategy to address these questions is to carry out a large-area X-ray survey, reaching a sensitivity comparable to that of deep Chandra and XMM pointings, but extending over several thousands of square degrees. A similar survey can only be carried out with a Wide-Field X-ray Telescope (WFXT), which combines a high survey speed with a sharp PSF across the entire FoV. We emphasize the important synergies that WFXT will have with a number of future ground-based and space telescopes, covering from the radio to the X-ray bands. Finally, we discuss the immense legacy value that such a mission will have for extragalactic astronomy at large.

Motivation & Objective

• To address unresolved questions in galaxy cluster astrophysics and cosmology using deep, wide-area X-ray surveys.
• To demonstrate that a Wide-Field X-ray Telescope (WFXT) is uniquely suited to achieve the sensitivity and angular resolution needed for large-scale cluster surveys.
• To quantify how WFXT surveys can constrain cosmological parameters, particularly σ₈ and the growth index γ, by tracing the evolution of the cluster mass function.
• To establish synergies between WFXT and future multi-wavelength facilities to enable comprehensive cluster characterization.
• To provide a legacy survey that will enable detailed studies of baryonic cycles, feedback mechanisms, and the growth of cosmic structures.

Proposed method

• Propose a large-area X-ray survey using WFXT, combining high grasp with a sharp and stable point-spread function (PSF) across the full field of view.
• Use X-ray luminosity and temperature measurements to derive cluster mass proxies such as YX = M_gas × T, minimizing core-region contamination.
• Apply statistical modeling to the cluster mass function and redshift evolution to constrain cosmological parameters like σ₈ and γ.
• Simulate WFXT survey performance across three survey areas (1000, 3000, and 10,000 deg²) to estimate cosmological constraints.
• Integrate multi-wavelength data from future missions (Euclid, JDEM, ALMA, JWST, ACT, SPT, CCAT) to cross-calibrate masses and validate physical models.
• Use the combination of X-ray, SZ, and weak lensing data to independently measure cluster mass profiles and reduce systematics.

Experimental results

Research questions

• RQ1 When and by what mechanisms is entropy injected into the intergalactic medium (IGM) in galaxy clusters?
• RQ2 What is the cosmic history of metal enrichment in the IGM, and how is it linked to star formation and AGN feedback?
• RQ3 What physical processes determine the presence and stability of cool cores in galaxy clusters?
• RQ4 How are proto-clusters at z ≈ 2 related to the peak of star formation and black hole accretion activity?
• RQ5 What do the growth and clustering of galaxy clusters reveal about the nature of primordial density perturbations and the validity of general relativity?

Key findings

• WFXT surveys covering 10,000 deg² would provide cosmological constraints on σ₈ and γ comparable to those from the full cosmic microwave background (CMB) dataset.
• The combination of WFXT surveys across 1000, 3000, and 10,000 deg² significantly improves constraints on the growth index γ and power-spectrum normalization σ₈.
• WFXT would detect approximately 2×10⁵ galaxy clusters at redshifts z > 0.5, enabling detailed study of their baryonic cycles and feedback processes.
• The synergy between WFXT and future missions like Euclid and JDEM would allow spectroscopic confirmation and high-resolution optical imaging of cluster members, enabling full characterization of stellar populations and star formation histories.
• Combining X-ray and Sunyaev-Zeldovich (SZ) data from WFXT and next-generation mm telescopes like CCAT would provide independent, non-spectroscopic mass profiles, reducing systematics in cluster mass calibration.
• WFXT would provide an exceptional target pool for follow-up with future X-ray observatories (e.g., IXO) and submillimeter facilities (e.g., ALMA), enabling detailed studies of metallicity and entropy structure in high-redshift clusters.