{
  "abstract": "Extended Ly$α$ emission is commonly detected around luminous quasars at $z\\sim2$-3 and traces cold gas in the circumgalactic environment of the AGN. Together with the spatial distribution of galaxies, Ly$α$ probes gas and galaxy assembly in high-redshift overdensities. We use deep KCWI wide-field spectroscopy and HST/F140W imaging to investigate the assembling cluster CARLA J0800+4029 at $z=1.986\\pm0.014$. No robust extended Ly$α$ emission is detected at the radio-loud quasar position. Instead, the dominant Ly$α$ emission is offset from the AGN and coincides with a disturbed structure composed of galaxies, tidal features, and diffuse intra-halo light (IHL). The detected structure has $L_{\\rm Lyα}=1.42\\times10^{43}$ erg/s and reaches surface-brightness levels of $\\sim10^{-17}$ erg/s/cm$^2$/arcsec$^2$. At the quasar position, a matched $3σ$ limit gives $L_{\\rm Lyα}<4.20\\times10^{42}$ erg/s, a factor of $\\sim3.4$ below the detected offset emission and among the most stringent constraints at similar redshift. The Ly$α$ emission is dominated by one extended region near systemic velocity, with FWHM $\\sim300$-500 km/s. The associated tidal structure has an IHL fraction of 18.8%, and contains a spatially coherent absorption-like decrement at $\\sim+1000$ km/s. The offset and alignment between Ly$α$ emission and IHL suggest that gas and stars have been redistributed together during assembly. The lack of a comparable Ly$α$ halo around the quasar further suggests that gas in the immediate AGN environment is ionized, depleted, or suppressed, while the offset region remains favorable for Ly$α$ emission. CARLA J0800+4029 reveals baryon redistribution during cluster assembly, with the dominant observable Ly$α$ reservoir decoupled from the central AGN and linked to ongoing tidal assembly.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.37445",
  "author_total": 6,
  "authors": [
    {
      "name": "Sofia G Gallego"
    },
    {
      "name": "Simona Mei"
    },
    {
      "name": "Emanuele Daddi"
    },
    {
      "name": "Gaël Noirot"
    },
    {
      "name": "Dominika Wylezalek"
    },
    {
      "name": "R. Michael Rich"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/2zwyr2n58ygwb.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.37445",
  "fulltext_url": null,
  "id": "2zwyr2n58ygwb",
  "issns": [],
  "landing_url": "https://hal.science/hal-05780446v1",
  "language": "en",
  "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"
    ],
    [
      "hal",
      "0.phys"
    ],
    [
      "hal",
      "1.phys.astr"
    ],
    [
      "openalex",
      "3103|Galaxies: Formation, Evolution, Phenomena"
    ]
  ],
  "openalex": "W7214966069",
  "pdf_url": "https://arxiv.org/pdf/2609.37445",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-29",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/2zwyr2n58ygwb.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:17:48Z",
      "native_id": "hal-05780446",
      "source": "hal",
      "url": "https://hal.science/hal-05780446v1"
    },
    {
      "first_seen": "2026-10-30T12:25:31Z",
      "native_id": "2609.37445",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.37445"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    },
    {
      "field": "physics",
      "origin": "hal:0.phys",
      "subfield": "general"
    }
  ],
  "title": "Caught in the act: diffuse stellar and Ly$α$ emission around a halo-less quasar in the assembling cluster CARLA J0800+4029 at z=2",
  "title_text": "Caught in the act: diffuse stellar and Lyα emission around a halo-less quasar in the assembling cluster CARLA J0800+4029 at z=2",
  "type": "preprint",
  "updated_at": "2026-10-30T12:25:31Z",
  "url": "https://research.draconitic.com/paper/2zwyr2n58ygwb",
  "venue": "HAL",
  "versions": []
}