{
  "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<z<4.5, where Ha is inaccessible from the ground, finding 14 systems with co-spatial extended Lya and HeII emission. The detected, emission-weighted HeII/Lya component shows no evidence for strong evolution between z~2.2 and z~3.7, though this does not imply an invariant underlying density PDF. These constraints provide a basis for comparison with theoretical models and hydrodynamical simulations of the physical processes shaping quasar CGM at high redshift.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.01454",
  "author_total": 19,
  "authors": [
    {
      "name": "Andrea Travascio"
    },
    {
      "name": "Sebastiano Cantalupo"
    },
    {
      "name": "Titouan Lazeyras"
    },
    {
      "name": "Naveen A. Reddy"
    },
    {
      "name": "Charles C. Steidel"
    },
    {
      "name": "Yongming Liang"
    },
    {
      "name": "Gabriele Pezzulli"
    },
    {
      "name": "Massimo Gaspari"
    },
    {
      "name": "Matteo Fossati"
    },
    {
      "name": "Michele Fumagalli"
    },
    {
      "name": "Chiara Feruglio"
    },
    {
      "name": "Fabrizio Fiore"
    },
    {
      "name": "Huiyang Mao"
    },
    {
      "name": "Marta Galbiati"
    },
    {
      "name": "Weichen Wang"
    },
    {
      "name": "Nicolas Ledos"
    },
    {
      "name": "Bahram Mobasher"
    },
    {
      "name": "Antonio Pensabene"
    },
    {
      "name": "Giada Quadri"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/519vxj9hbcpay.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.01454",
  "fulltext_url": null,
  "id": "519vxj9hbcpay",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.01454",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.GA"
    ],
    [
      "openalex",
      "3103|Galaxies: Formation, Evolution, Phenomena"
    ]
  ],
  "openalex": "W7216077523",
  "pdf_url": "https://arxiv.org/pdf/2610.01454",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-01",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/519vxj9hbcpay.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:26:03Z",
      "native_id": "2610.01454",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.01454"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    }
  ],
  "title": "Constraining the density distribution of the cold Circumgalactic Medium of quasars at z>2 through Lya, HeII and Ha emission",
  "title_text": "Constraining the density distribution of the cold Circumgalactic Medium of quasars at z>2 through Lya, HeII and Ha emission",
  "type": "preprint",
  "updated_at": "2026-10-30T12:26:03Z",
  "url": "https://research.draconitic.com/paper/519vxj9hbcpay",
  "venue": "arXiv",
  "versions": []
}