Future XIFU observations of M87

Tomáš Plšek

Masaryk University

Galaxy clusters are surrounded by vast atmospheres of hot gas, reaching temperatures of 10-100 million Kelvin (corresponding thermal energy is 1-10 keV). This gas is continuously losing energy by emitting X-ray photons and should rapidly cool down to ~100 K, forming stars and feeding the central supermassive black hole (SMBH). However, observations indicate that this does not occur as efficiently as expected. We now know that active galactic nuclei (AGN) play a key role in preventing runaway cooling by injecting energy through powerful relativistic jets that inflate bubbles in the hot atmosphere (see this previous Nugget).

Despite this heating, relatively cool gas (~0.5 keV, compared to ~2 keV in the hot atmosphere) is commonly observed in cluster cores. This gas can form through thermal instabilities in the hot atmosphere, and in some cases, as is likely in M87, from low-entropy gas entrained by rising bubbles. It can cool even further, eventually becoming observable in the optical and radio bands. Detecting this cooling gas and measuring its extent and total amount across different wavelengths will enable us to better understand how AGN feedback and star formation operate.

However, spatially mapping this cooling gas (≤0.5 keV) remains challenging for current X-ray instruments. Chandra and XMM-Newton can reveal temperature variations and identify relatively cool (~0.8–1 keV) structures, but this requires very deep exposures (~2 megaseconds) combined with careful spectral modelling. Evidence for even colder gas, traced by emission lines such as O VII and Fe XVII, has so far been obtained mainly through high-resolution spectroscopy with XMM-Newton/RGS, but its spatial distribution remains unknown. The Resolve detector on board the recently launched XRISM telescope provides the required spectral resolution, but due to the gate valve issue, it cannot detect these soft X-ray photons (<1.7 keV), and its spatial resolution is insufficient for mapping these structures.

In this Nugget, we demonstrate that NewAthena’s X-IFU instrument, with its unprecedented spectral resolution and large effective area in the soft X-ray band, will revolutionise our ability to study cooling gas in clusters of galaxies. The X-IFU will separate individual emission lines from different gas components, characterise their motions through line shifts and broadenings, and even enable producing narrow-band images mapping the extent of cold gas filaments through lines such as O VII and Fe XVII. We show that with only 100 kiloseconds of exposure time, X-IFU will reveal the structure of ~0.8 keV cooling gas currently requiring megasecond Chandra observations, while opening a new window for spatially resolved studies of even colder gas phases.