Voyage through the Hidden Physics of the Cosmic Web (1908.01778v2)

Abstract: The majority of the ordinary matter in the local Universe has been heated by strong structure formation shocks and resides in a largely unexplored hot, diffuse, X-ray emitting plasma that permeates the halos of galaxies, galaxy groups and clusters, and the cosmic web. We propose a next-generation "Cosmic Web Explorer" that will permit a complete and exhaustive understanding of these unseen baryons. This will be the first mission capable to reach the accretion shocks located several times farther than the virial radii of galaxy clusters, and reveal the out-of-equilibrium parts of the intra-cluster medium which are live witnesses to the physics of cosmic accretion. It will also enable a view of the thermodynamics, kinematics, and chemical composition of the circumgalactic medium in galaxies with masses similar to the Milky Way, at the same level of detail that $Athena$ will unravel for the virialized regions of massive galaxy clusters, delivering a transformative understanding of the evolution of those galaxies in which most of the stars and metals in the Universe were formed. Finally, the proposed X-ray satellite will connect the dots of the large-scale structure by mapping, at high spectral resolution, as much as 100% of the diffuse gas hotter than $10^6$ K that fills the filaments of the cosmic web at low redshifts, down to an over-density of 1, both in emission and in absorption against the ubiquitous cosmic X-ray background, surveying at least 1600 square degrees over 5 years in orbit. This requires a large effective area (~10 m$^2$ at 1 keV) over a large field of view ($\sim1$ deg$^2$), a megapixel cryogenic microcalorimeter array providing integral field spectroscopy with a resolving power $E/\Delta E$ = 2000 at 0.6 keV and a spatial resolution of 5 arcsec in the soft X-ray band, and a low and stable instrumental background ensuring high sensitivity to faint, extended emission.

• The paper introduces a mission concept with a 10 m² effective area and high spectral resolution to detect faint baryons in the cosmic web.
• It details innovative X-ray observational techniques to map the circumgalactic and warm-hot intergalactic medium with unprecedented sensitivity.
• The mission aims to advance our understanding of cosmic structure formation by revealing the thermodynamics, kinematics, and chemical composition of diffuse gas.

Overview of "Voyage through the Hidden Physics of the Cosmic Web"

The paper "Voyage through the Hidden Physics of the Cosmic Web" by Simionescu et al. proposes an ambitious mission concept aimed at exploring the largely uncharted domain of the cosmic web. This concept builds on the scientific groundwork laid by current and forthcoming X-ray observatories like Athena and explores the potential for a next-generation "Cosmic Web Explorer." The mission seeks to uncover the elusive baryons that reside in the cosmic web—particularly those heated by shock phenomena—and to detail the thermodynamics, kinematics, and chemical composition of the circumgalactic medium (CGM) and warm-hot intergalactic medium (WHIM).

Scientific Motivation

The authors underscore a significant gap in our understanding of the large-scale structure of the universe due to the limited sensitivity and field of view of existing X-ray observatories. Advanced knowledge of the cosmic web is crucial because it hosts the majority of baryons, yet escaping detection due to the diffuse nature and low density of these regions.

Mission Design and Capabilities

The proposed mission concept outlines an X-ray satellite with unique specifications:

• A significant effective area (~10 m² at 1 keV) to capture faint signals.
• A large field of view (~1 deg²) enabled by a megapixel cryogenic microcalorimeter array.
• High spectral resolution with resolving power $E/\Delta E \approx 2000$ at 0.6 keV.
• Spatial resolution of 5 arcseconds in the soft X-ray band.
• A low and stable instrumental background for detecting faint extended emissions.

These capabilities are designed to achieve unprecedented sensitivity to diffuse gas hotter than $10^6$ K throughout the cosmic web, and to map large areas of sky in both emission and absorption.

Key Scientific Objectives

The paper identifies key objectives for the proposed mission, including:

1. Mapping the hot gas distributions in galaxy clusters, particularly beyond their virial radii, to understand infall processes and structure formation shocks.
2. Investigating the circumgalactic medium in galaxies similar to the Milky Way to decipher metal enrichment and energy feedback processes.
3. Fully mapping the warm-hot intergalactic medium components filling the cosmic filaments to derive a comprehensive 3D model of baryon distribution.

Implications and Future Prospects

The potential impact of this mission spans a broader comprehension of the baryonic components that evade current detectors, thereby enriching our knowledge of cosmic evolution and structure formation processes. This mission could provide critical cross-correlation with forthcoming surveys across other wavelengths, enhancing the holistic nature of astrophysical investigations. It's a key stepping stone towards uncovering the cosmic web's role in the universe's baryonic matter budget.

Conclusion

The proposed Cosmic Web Explorer is ambitious but driven by vital scientific questions that aim to extend our understanding of cosmic structure formation. By significantly improving the sensitivity and spectral resolution of X-ray observations, this mission concept offers a path toward addressing outstanding questions in cosmology and astrophysics, potentially reshaping our comprehension of the invisible baryonic structures that underpin galaxies and the cosmic web itself.