Letter to the Editor

J-PAS: Discovery of RaJav, a bright extended Lyα Nebula at z = 2.25

¹ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, 44001 Teruel, Spain
² Unidad Asociada CEFCA-IAA, CEFCA, Unidad Asociada al CSIC por el IAA y el IFCA, Plaza San Juan 1, 44001 Teruel, Spain
³ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, CAS, Shanghai 200030, PR China
⁴ Instituto de Astrofísica de Andalucía (CSIC), P.O. Box 3004 E-18080 Granada, Spain
⁵ Instituto de Física de Cantabria, Edificio Juan Jordá, Avenida de los Castros, 39005 Santander, Spain
⁶ Observatori Astronòmic de la Universitat de València, 46980 Paterna, Spain
⁷ Instituto de Física, Universidade de São Paulo, Rua do Matão 1371, CEP 05508-090 São Paulo, Brazil
⁸ Observatório Nacional – MCTI (ON), Rua General José Cristino, 77, São Cristóvão, 20921-400, Rio de Janeiro, Brazil
⁹ Donostia International Physics Centre, Paseo Manuel de Lardizabal 4, E-20018 Donostia-San Sebastián, Spain
¹⁰ Ikerbasque, Basque Foundation for Science, E-48013 Bilbao, Spain
¹¹ Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Spain
¹² Instituto de Astrofísica de Canarias, E-38200 La Laguna, Spain
¹³ Departamento de Astronomia, Instituto de Astronomia, Geofísica e Ciências Atmosféricas da USP, Cidade Universitária, 05508-900 São Paulo, SP, Brazil
¹⁴ Instruments4, 4121 Pembury Place, La Cañada-Flintridge, CA 91011, USA

^⋆ Corresponding author: rahna.payyasseri@gmail.com

Received: 30 July 2025
Accepted: 20 October 2025

Abstract

We report the discovery of a massive Lyα nebula (potentially largest ever discovered), RaJav, at z = 2.25, associated with a quasar pair: the bright SDSS J162029.07+433451.1 (hereafter J1620+4334) and the faint newly discovered quasar JPAS-9600-10844, at 2.265 ± 0.021 using the early data release (17 deg²) of the Javalambre Physics of the Accelerating Universe Astrophysical Survey (J-PAS). The quasar JPAS-9600-10844 embedded in the nebula is located at ∼60.2 kpc (7.3″) from J1620+4334, and shows a compact structure with broad emission lines (> 3000 km/s), typical of active galactic nuclei (i.e., Lyαλ1216 and CIV λ1548). At a 2σ surface brightness (SB) contour of ∼1.86 × 10⁻¹⁶ erg s⁻¹ cm⁻² arcsec⁻², the nebula extends beyond 100 kpc and has a total Lyα luminosity of ∼5.8 ± 0.7 × 10⁴⁴ erg s⁻¹ which signifies the presence of an enormous Lyα nebula (ELAN). The nebula traces an overdensity of quasars at a redshift of 2.2–2.3 consistent with the progenitor of a massive galaxy cluster. The extended CIV emission with luminosity of ∼3.7 ± 0.5 × 10⁴⁴ erg s⁻¹ indicates that the circumgalactic medium (CGM) is metal-enriched and not primordial. The current J-PAS observations suggest photoionization and shocks due to outflows as possible ionization mechanisms. The faint extended far-UV and near-UV continuum emission likely points to ongoing star formation around the two quasars, suggesting a complex interaction in their environments. These findings provide new insights into the environment of quasars and their role in shaping the dynamics and evolution of the CGM at cosmic noon. Further spectroscopic observations will be required to fully characterize the object’s nature and its kinematic properties. This study demonstrates the unique capability of J-PAS to detect massive and rare Lyα nebulae, providing new insights into their properties, environments, and connections to large-scale structures in the cosmic web such as filaments and overdensities in a large cosmological volume.