2021MNRAS.501.5362S

We report the detection of a large (∼90 kpc) and luminous Lyα nebula [L_Lyα = (6.80 ± 0.08) x 10⁴⁴ erg s^–1] around an optically faint (r > 23 mag) radio galaxy M1513-2524 at z_em = 3.132. The double-lobed radio emission has an extent of 184 kpc, but the radio core, i.e. emission associated with the active galactic nucleus (AGN) itself, is barely detected. This object was found as part of our survey to identify high-z quasars based on Wide-field Infrared Survey Explorer (WISE) colours. The optical spectrum has revealed Lyα, N V, C IV, and He II emission lines with a very weak continuum. Based on long-slit spectroscopy and narrow band imaging centred on the Lyα emission, we identify two spatial components: a 'compact component' with high velocity dispersion (∼1500 km s^–1) seen in all three lines, and an 'extended component', having low velocity dispersion (i.e. 700-1000 km s^–1). The emission line ratios are consistent with the compact component being in photoionization equilibrium with an AGN. We also detect spatially extended associated Lyα absorption, which is blue-shifted within 250-400 km s^–1 of the Lyα peak. The probability of Lyα absorption detection in such large radio sources is found to be low (∼10 per cent) in the literature. M1513-2524 belongs to the top few percent of the population in terms of Lyα and radio luminosities. Deep integral field spectroscopy is essential for probing this interesting source and its surroundings in more detail.