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Published March 2023 | Published
Journal Article Open

Characterizing the circumgalactic medium of quasars at z ∼ 2.2 through H α and Ly α emission


The discovery of giant quasar Ly α nebulae at z > 2 has opened up the possibility to directly study in emission the Circumgalactic and Intergalactic Medium (CGM/IGM). However, the resonant nature of the Ly α line and its different emission mechanisms hamper the ability to constrain both the kinematics and physical properties of the CGM/IGM. Here, we present results of a pilot project aiming at the detection of CGM H α emission, a line which does not suffer from these limitations. To this end, we first used KCWI to detect Ly α emission around three bright quasars with 2.25 < z < 2.27, a range which is free from bright IR sky lines for H α, and then selected the most extended nebula for H α follow-up with MOSFIRE. Within the MOSFIRE slit, we detected H α emission extending up to 20 physical kpc with a total H α flux of F_(H α) = (9.5 ± 0.9) × 10⁻¹⁸ erg s⁻¹, cm⁻². Considering the Ly α flux in the same region, we found F_(Ly α)/F_(H α) = 3.7 ± 0.3 consistent with that obtained for the Slug Nebula at z = 2.275 and with recombination radiation. This implies high densities or a very broad density distribution within the CGM of high-redshift quasars. Moreover, the H α line profile suggests the presence of multiple emitting components overlapping along our line of sight and relatively quiescent kinematics, which seems incompatible with either quasar outflows capable of escaping the potential well of the host halo or disc-like rotation in a massive halo (>10¹² M_⊙).

Additional Information

© 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 864361, PI: S. Cantalupo). SC gratefully acknowledges additional support from the Swiss National Science Foundation grant PP00P2_190092 and from Fondazione Cariplo. GP acknowledges support from the Netherlands Research School for Astronomy (NOVA). SG acknowledges support from the Swiss National Science Foundation grant P2EZP2_199856. DATA AVAILABILITY. The data underlying this article will be shared on reasonable request to the corresponding author.

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August 22, 2023
August 22, 2023