Editors: Dr Mukul Kumar Baruah, Dr Rahul Kanti Nath & Dr Joyobrato Nath (2025)

ISBN: 97978-93-94174-62-7

Ne VIII Absorbers in the Low-Redshift Universe: Physical Conditions, Ionization Models, and Implications for the Cosmic Baryon Census

Assistant Professor, Department of Physics, Patharkandi College, Sribhumi, Assam, India

The detection of Ne VIII (seven-times ionized neon) absorption lines in quasar spectra provides a unique window into the warm-hot phase of the intergalactic medium (IGM) and circumgalactic medium (CGM), particularly in the temperature range of 10⁵–10⁶ K. As an elusive tracer of low–density, highly ionized gas, Ne VIII plays a central role in efforts to identify the reservoir of baryons unaccounted for in the low-redshift Universe. However, its detection remains observationally challenging due to the intrinsic weakness of the lines, contamination from blending features, and the limited redshift range accessible to current ultraviolet spectrographs. Advances in far-ultraviolet spectroscopy, especially with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST), have led to a growing number of confirmed Ne VIII absorbers in quasar sightlines. Ionization modelling has shown that both collisional ionization and photoionization contribute to the observed Ne VIII population, with hybrid scenarios often necessary to explain the full set of ionic column densities. In many cases, Ne VIII appears to trace warm-hot, metal-enriched gas in the extended haloes of galaxies, reinforcing its role in probing the interaction between galaxies and their surrounding medium. This review presents a comprehensive synthesis of current research on Ne VIII absorbers, including detection methods, ionization mechanisms, statistical properties, and the physical environments they trace. Recent observational and theoretical developments are examined to assess the role of Ne VIII in characterizing the multiphase structure of the CGM and IGM, and in constraining the distribution of baryons in the low-redshift Universe. Key challenges in detection and interpretation are discussed, along with the prospects for future ultraviolet spectroscopic facilities to advance the study of this important ion..

*Corresponding author; e-mail: 10tanvir.h88@gmail.com

Cite this Chapter: Tanvir Hussain. 2025. Ne VIII Absorbers in the Low-Redshift Universe: Physical Conditions, Ionization Models, and Implications for the Cosmic Baryon Census. In: Mukul Kumar Baruah, Rahul Kanti Nath and Joyobrato Nath (Eds.), Transdisciplinary Science: Mapping the Future of Research. Excellent Publishers, India. pp. 219-249. doi: https://doi.org/10.20546/978-93-94174-62-7_20