Caught in the act: diffuse stellar and Lyα emission around a halo-less quasar in the assembling cluster CARLA J0800+4029 at z=2
Abstract
Extended Lyα emission is commonly detected around luminous quasars at z∼2-3 and traces cold gas in the circumgalactic environment of the AGN. Together with the spatial distribution of galaxies, Lyα 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±0.014. No robust extended Lyα emission is detected at the radio-loud quasar position. Instead, the dominant Lyα 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 LLyα=1.42×1043 erg/s and reaches surface-brightness levels of ∼10−17 erg/s/cm2/arcsec2. At the quasar position, a matched 3σ limit gives LLyα<4.20×1042 erg/s, a factor of ∼3.4 below the detected offset emission and among the most stringent constraints at similar redshift. The Lyα emission is dominated by one extended region near systemic velocity, with FWHM ∼300-500 km/s. The associated tidal structure has an IHL fraction of 18.8%, and contains a spatially coherent absorption-like decrement at ∼+1000 km/s. The offset and alignment between Lyα emission and IHL suggest that gas and stars have been redistributed together during assembly. The lack of a comparable Lyα 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 Lyα emission. CARLA J0800+4029 reveals baryon redistribution during cluster assembly, with the dominant observable Lyα reservoir decoupled from the central AGN and linked to ongoing tidal assembly.
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BibTeX
@misc{gallego2026caught,
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}},
author = {Sofia G Gallego and Simona Mei and Emanuele Daddi and Gaël Noirot and Dominika Wylezalek and R. Michael Rich},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.37445},
eprint = {2609.37445},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05780446v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Caught in the act: diffuse stellar and Lyα emission around a halo-less quasar in the assembling cluster CARLA J0800+4029 at z=2
AU - Sofia G Gallego
AU - Simona Mei
AU - Emanuele Daddi
AU - Gaël Noirot
AU - Dominika Wylezalek
AU - R. Michael Rich
PY - 2026
DA - 2026/09/29
PB - HAL
DO - 10.48550/arxiv.2609.37445
UR - https://hal.science/hal-05780446v1
AB - Extended Lyα emission is commonly detected around luminous quasars at z∼2-3 and traces cold gas in the circumgalactic environment of the AGN. Together with the spatial distribution of galaxies, Lyα 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±0.014. No robust extended Lyα emission is detected at the radio-loud quasar position. Instead, the dominant Lyα 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_Lyα=1.42×10^43 erg/s and reaches surface-brightness levels of ∼10^−17 erg/s/cm^2/arcsec^2. At the quasar position, a matched 3σ limit gives L_Lyα<4.20×10^42 erg/s, a factor of ∼3.4 below the detected offset emission and among the most stringent constraints at similar redshift. The Lyα emission is dominated by one extended region near systemic velocity, with FWHM ∼300-500 km/s. The associated tidal structure has an IHL fraction of 18.8%, and contains a spatially coherent absorption-like decrement at ∼+1000 km/s. The offset and alignment between Lyα emission and IHL suggest that gas and stars have been redistributed together during assembly. The lack of a comparable Lyα 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 Lyα emission. CARLA J0800+4029 reveals baryon redistribution during cluster assembly, with the dominant observable Lyα reservoir decoupled from the central AGN and linked to ongoing tidal assembly.
ER -