The Circumgalactic Medium in Massive Halos

TL;DR

This review summarizes recent advances in understanding the properties, origins, and distribution of cool circumgalactic gas around massive galaxies at intermediate redshifts, highlighting observational findings and open questions.

Contribution

It provides a comprehensive synthesis of observational data on the cool circumgalactic medium in massive halos, emphasizing the role of external infall, supernova winds, and quasar activity.

Findings

Infalling clouds are the main source of cool gas at large distances from quiescent galaxies.

Supernova-driven winds significantly contribute to cool gas within 100 kpc.

Cool gas in quasar halos correlates with quasar luminosity, but its origin remains uncertain.

Abstract

This chapter presents a review of the current state of knowledge on the cool (T ~ 1e4 K) halo gas content around massive galaxies at z ~ 0.2-2. Over the last decade, significant progress has been made in characterizing the cool circumgalactic gas in massive halos of Mh ~ 1e12-1e14 Msun at intermediate redshifts using absorption spectroscopy. Systematic studies of halo gas around massive galaxies beyond the nearby universe are made possible by large spectroscopic samples of galaxies and quasars in public archives. In addition to accurate and precise constraints for the incidence of cool gas in massive halos, detailed characterizations of gas kinematics and chemical compositions around massive quiescent galaxies at z ~ 0.5 have also been obtained. Combining all available measurements shows that infalling clouds from external sources are likely the primary source of cool gas detected at d >~ 100 kpc from massive quiescent galaxies. The origin of the gas closer in is currently less certain, but SNe Ia driven winds appear to contribute significantly to cool gas found at d < 100 kpc. In contrast, cool gas observed at d <~ 200 kpc from luminous quasars appears to be intimately connected to quasar activities on parsec scales. The observed strong correlation between cool gas covering fraction in quasar host halos and quasar bolometric luminosity remains a puzzle. Combining absorption-line studies with spatially-resolved emission measurements of both gas and galaxies is the necessary next step to address remaining questions.