Discovery of a giant and luminous Lyα+C IV+He II nebula at z = 3.326 with extreme emission line ratios^⋆

¹ Instituto de Astrofísica de Canarias, C/Vía Láctea, s/n, 38205 San Cristóbal de La Laguna, Tenerife, Spain
e-mail: rmarques@cab.inta-csic.es
² Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir, 28850 Torrejón de Ardoz, Madrid, Spain
³ Universidad de La Laguna, Dpto. Astrofísica, 38206 San Cristóbal de La Laguna, Tenerife, Spain
⁴ Institut d’Astrophysique de Paris, UMR7095 CNRS & Sorbonne Université (UPMC), 75014 Paris, France
⁵ Astronomy Department, Cornell University, Ithaca, NY 14853, USA
⁶ Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁷ Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
⁸ GRANTECAN, Cuesta de San José s/n, 38712 Breña Baja, La Palma, Spain
⁹ Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
¹⁰ Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA
¹¹ Department of Physics and Astronomy, University of Hawaii, 2505 Correa Road, Honolulu, HI 96822, USA
¹² Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁵ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, UK
¹⁶ Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
¹⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Received: 4 June 2019
Accepted: 18 July 2019

Abstract

We present the discovery of HLock01-LAB, a luminous and large Lyα nebula at z = 3.326. Medium-band imaging and long-slit spectroscopic observations with the Gran Telescopio Canarias reveal extended emission in the Lyα 1215 Å, C IV 1550 Å, and He II 1640 Å lines over ∼100 kpc, and a total luminosity L_Lyα = (6.4 ± 0.1)×10⁴⁴ erg s⁻¹. HLock01-LAB presents an elongated morphology aligned with two faint radio sources contained within the central ∼8 kpc of the nebula. The radio structures are consistent with faint radio jets or lobes of a central galaxy, whose spectrum shows nebular emission characteristic of a type-II active galactic nucleus (AGN). The continuum emission of the AGN at short wavelengths is however likely dominated by stellar emission of the host galaxy, for which we derive a stellar mass M_* ≃ 2.3 × 10¹¹ M_⊙. Our kinematic analysis shows that the ionized gas is perturbed almost exclusively in the inner region between the radio structures, probably as a consequence of jet–gas interactions, whereas in the outer regions the ionized gas appears more quiescent. The detection of extended emission in C IV and C III] indicates that the gas within the nebula is not primordial. Feedback may have enriched the halo at at least 50 kpc from the nuclear region. Using rest-frame UV emission-line diagnostics, we find that the gas in the nebula is likely heated by the AGN. Nevertheless, at the center of the nebula we find extreme emission line ratios of Lyα/C IV ∼60 and Lyα/He II ∼80, one of the highest values measured to date, and well above the standard values of photoionization models (Lyα/He II ∼30 for case B photoionization). Our data suggest that jet-induced shocks are likely responsible for the increase of the electron temperature and, thus, the observed Lyα enhancement in the center of the nebula. This scenario is further supported by the presence of radio structures and perturbed kinematics in this region. The large Lyα luminosity in HLock01-LAB is likely due to a combination of AGN photoionization and jet-induced shocks, highlighting the diversity of sources of energy powering Lyα nebulae. Future follow-up observations of HLock01-LAB will help to reveal the finer details of the excitation conditions of the gas induced by jets and to investigate the underlying cooling and feedback processes in this unique object.