Constraining the density distribution of the cold Circumgalactic Medium of quasars at z>2 through Lya, HeII and Ha emission
Year: 2026
Type: preprint
Abstract: Extended Lya emission is routinely detected around high-redshift quasars, revealing large reservoirs of cool circumgalactic medium (CGM). Its resonant nature complicates interpretation of gas density, ionization state, and kinematics, making non-resonant tracers such as HeII and Ha essential. Extending previous analytical models, we use CLOUDY photo-ionization simulations to predict HeII/Ha in quasar-illuminated gas over a broad parameter range, including log-normal density distributions. The ratio constrains density fluctuations, commonly quantified by the clumping factor C. We compare the models with extended Lya, HeII, and Ha observations around quasars at z~2.1-4.5. At z~2.2, we observed 13 quasars with Keck/KCWI and targeted the systems with the most prominent extended Lya and HeII emission with Keck/MOSFIRE, providing the first statistical quasar-CGM sample with combined Lya, HeII, and Ha measurements. Within matched apertures out to ~35 pkpc, stacked spectra give Lya/Ha=9.1, HeII/Ha=0.54, and HeII/Lya=0.06. Single-density models require very high CGM densities (n>10 cm^-3) or implausibly soft AGN spectra to reproduce the low HeII/Ha ratio. Broad log-normal density distributions instead reproduce it at lower median densities but require a substantial emissivity-weighted high-density tail, corresponding formally to C~10^3-10^4 in our parameterization. We also analyze 26 VLT/MUSE quasars at 3.1
Source: arXiv (Cornell University)
Authors: Travascio, Andrea, Cantalupo, Sebastiano, Lazeyras, Titouan, Reddy, Naveen A., Steidel, Charles C., Liang, Yongming, Pezzulli, Gabriele, Gaspari, Massimo, Fossati, Matteo, Fumagalli, Michele, Feruglio, Chiara, Fiore, Fabrizio, Mao, Huiyang, Galbiati, Marta, Wang, Weichen, Ledos, Nicolas, Mobasher, Bahram, Pensabene, Antonio, Quadri, Giada
Institutions: Trieste Astronomical Observatory, University of Milano-Bicocca, University of California, Riverside, California Institute of Technology, Institute for Cosmic Ray Research, The University of Tokyo, University of Groningen, University of Modena and Reggio Emilia, Brera Astronomical Observatory, Chinese Academy of Sciences, Purple Mountain Observatory, DTU Space, Technical University of Denmark
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7216077523