# Revealing the Gas Recycling in the Circumgalactic Medium (CGM) Utilizing a Luminous Ly$α$ Nebula Around a Type-II Quasar at z=2.6 with the Keck Cosmic Web Imager (KCWI)

**Authors:** Shiwu Zhang, Zheng Cai, Dandan Xu, Andrea Afruni, Yunjing Wu, Wuji Wang, Fabrizio Arrigoni Battaia, Mingyu Li, Sen Wang, Xianzhi Bi  
**arXiv:** [2307.14224](https://arxiv.org/abs/2307.14224)  
**Date:** 2023-07-26  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.14224v1/

## Key points

- Pilot study using KCWI to investigate gas recycling process in CGM and its implications for galaxy evolution
- Focus on type-II quasar Q1517+0055 at redshift z = 2.65
- Observations reveal diffuse emissions of Lyα 1216, HeII 1640, and CIV 1549 on physical scales of 122 kpc, 45 kpc, and 79 kpc respectively
- Total Lyα luminosity measured to be L_Lyα = (3.04 ± 0.02) × 10^44 erg s^-1
- Line ratio diagnostics indicate HeII/Lyα ≈ 0.08 and CIV/Lyα ≈ 0.28, consistent with photoionization processes involving recombination and photon pumping
- Identification of associated HI and CIV absorption features in spectra
- Column density and velocity profiles derived from fitting spectra
- Absorption and HeII emission exhibit increasing trends in velocity
- Analysis confirms cool gas metallicity ≥ Z_☉ (solar metallicity)
- Gas recycling may be a more plausible interpretation compared to powerful outflows
- Insights into how galaxies acquire material from CGM through gas recycling processes
- Presence of diffuse emissions and associated absorptions in various spectral lines support role of gas recycling in star formation and galaxy evolution at high redshifts

## Summary

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This paper presents a pilot study using the Keck Cosmic Web Imager (KCWI) to investigate the gas recycling process in the circumgalactic medium (CGM) and its implications for galaxy evolution. The study focuses on a type-II quasar, Q1517+0055, at a redshift of z = 2.65, which is part of a sample of Lyα nebulae at z ≈ 2. The observations reveal diffuse emissions of Lyα 1216, HeII 1640, and CIV 1549 on projected physical scales of 122 kpc, 45 kpc, and 79 kpc respectively. The total Lyα luminosity is measured to be L_Lyα = (3.04 ± 0.02) × 10^44 erg s^-1. Line ratio diagnostics indicate that HeII/Lyα ≈ 0.08 and CIV/Lyα ≈ 0.28, consistent with photoionization processes involving recombination and photon pumping. The study also identifies associated HI and CIV absorption features in the spectra. By fitting the spectra, both the column density and velocity profiles are derived. Notably, both the absorption and HeII emission exhibit increasing trends in velocity. Furthermore, analysis of line ratios from the emission and column density ratios from the absorption confirm that the cool gas metallicity is ≥ Z_☉ (solar metallicity). This finding suggests that gas recycling may be a more plausible interpretation compared to a scenario involving powerful outflows. In conclusion, this study provides valuable insights into how galaxies acquire material from the CGM through gas recycling processes. The observations highlight the presence of diffuse emissions and associated absorptions in various spectral lines, supporting the idea that gas recycling plays an important role in star formation and galaxy evolution at high redshifts.