Abstract
Context. Contrary to expectations from scenarios of black hole growth driven by galaxy interactions and mergers, dual active galactic nuclei (AGN) with kiloparsec separations are rarely observed and are very difficult to identify, in particular at high redshifts (i.e. z > 2). Aims. Focussing on the recently discovered dual AGN system LBQS 0302−0019 at z = 3.29, we seek to identify further group members in its environment and to understand their formation history through deep high-angular-resolution imaging. Methods. We present deep Hubble Space Telescope (HST) Wide-field Camera 3 near-infrared imaging of LBQS 0302−0019. In combination with ground-based VLT/HAWK-I imaging, we infer accurate sizes, colours, ages, and stellar masses of companion galaxies. Results. We clearly detect four companion objects close to LBQS 0302−0019 that also have faint signatures in the ground-based images. We constrain light-weighted ages and masses for the two most prominent companions, Jil1 and Jil2, to t⋆ = 252−109+222 Myr with log(M⋆/[M⊙]) = 11.2−0.1+0.3 and t⋆ = 19−14+74 Myr with log(M⋆/[M⊙]) = 9.4−0.4+0.9, respectively. The HST data also show that the obscured AGN, previously identified by strong nebular He II emission, is associated with the young massive companion Jil2. Because very massive stars of the starburst cannot be solely responsible for the He II emission, we strengthen our initial conclusion that Jil2 has been hosting an AGN. Conclusions. If the young starburst of Jil2 had been accompanied by sustained black hole growth, Jil2 may have contributed He II-ionising flux to create the large He II Lyα proximity zone around LBQS 0302−0019. Hence, the duration of the current luminous AGN episode of LBQS 0302−0019 may have been overestimated.
Highlights
During the evolution of massive galaxies, their nuclei are expected to show recurrent phases of activity in order to grow their super-massive black holes (SMBHs) as observed from the cosmic accretion rate distribution (e.g. Soltan 1982; Marconi et al 2004; Schulze et al 2015; Georgakakis et al 2017)
While the NIR image obtained with the High Acuity Wide field K-band Imager (HAWK-I) at the Very Large Telescope (VLT) reveals signatures of distortion in the morphology, a clear interpretation is limited by the depth and resolution of the groundbased observations
In order to account for potential systematic uncertainties in the empirical point-spread function (PSF), we averaged only three of the four bright stars and five of the six faint stars; this led to 24 different PSF combinations, which we later used to determine the uncertainties of the derived parameters
Summary
If the young starburst of Jil had been accompanied by sustained black hole growth, Jil may have contributed He iiionising flux to create the large He ii Lyα proximity zone around LBQS 0302−0019. The duration of the current luminous AGN episode of LBQS 0302−0019 may have been overestimated. Galaxies: interactions – galaxies: high-redshift – galaxies: starburst – quasars: individual: LBQS 0302−0019 – galaxies: active Key words. galaxies: interactions – galaxies: high-redshift – galaxies: starburst – quasars: individual: LBQS 0302−0019 – galaxies: active
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