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Localizing the accretion shock and constraining gaseous conditions in galaxy cluster outskirts with UV absorption spectroscopy

Presentation #427.02 in the session Intergalactic Medium, QSO Absorption Line Systems.

Published onJun 29, 2022
Localizing the accretion shock and constraining gaseous conditions in galaxy cluster outskirts with UV absorption spectroscopy

Galaxy clusters play host to multiphase gaseous environments whose transformations over time are essential for understanding halo gas physics and galaxy evolution. Accretion shocks are uniformly predicted in formation models of the intracluster medium, but their location (1-5 times the virial radius - r200) varies from model to model. The location of the shock front has important implications for infalling galaxies and gas physics in cluster outskirts, such as the quenching of satellite galaxies. I am carrying out a study to detect these accretion shocks with a novel application of QSO absorption line spectroscopy. Using a statistical sample of HST/COS-observed quasar sightlines that probe the outskirts of >30 foreground, optically selected galaxy clusters, I have mapped diffuse H I and O VI absorption as a function of projected clustocentric distance from within r200 to 5 r200. My results for H I reveal consistency with the ambient intergalactic medium (IGM) density in clusters’ far outskirts, a sudden peak between 2–3 r200, consistent with the accretion shock location in some models, and a decline to the IGM density within r200. This suggests a scenario where circumcluster H I arises from infalling gas that is then heated by the accretion shock front, ionizing the dense gas to a greater degree than is photoionized in the IGM. Once the gas is exceedingly shock-heated, the neutral fraction plummets, causing a depression in H I following the shock front. We also trace the high ion O VI and find it is enhanced above the ambient IGM density in the far outskirts and falls to the IGM density by 3-4 r200, with an indication of a slight enhancement around 1-2 r200. Therefore, O VI is preferentially detected in the far outskirts. This result may be alluding to the potential presence of the warm-hot intergalactic medium, which, coupled with our statistical detection of the accretion shock front, sheds new light on the formation and environmental conditions of these massive structures.

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