J. W. Broderick, N. Seymour, G. Drouart, D. Knight, J. Afonso, C. de Breuck, T. J. Galvin, A. J. Hedge, M. D. Lehnert, G. Noirot, S. S. Shabala, R. J. Turner, J. Vernet
Abstract
We present deep near-infrared Ks -band imaging for 35 of the 53 sources from the high-redshift ( z>2 ) radio galaxy candidate sample defined in Broderick et al. (2022, PASA, 39, e061). These images were obtained using the High-Acuity Widefield K-band Imager (HAWK-I) on the Very Large Telescope. Host galaxies are detected for 27 of the sources, with Ks≈21.6 -23.0 mag (2 ^'' diameter apertures; AB). The remaining eight targets are not detected to a median 3σ depth of Ks≈23.3 mag (2'' diameter apertures). We examine the radio and near-infrared flux densities of the 35 sources, comparing them to the known z>3 powerful radio galaxies with 500-MHz radio luminosities L500MHz>1027 W Hz-1 . By plotting 150-MHz flux density versus Ks -band flux density, we find that, similar to the sources from the literature, these new targets have large radio to near-infrared flux density ratios, but extending the distribution to fainter flux densities. Five of the eight HAWK-I deep non-detections have a median 3σ lower limit of Ks≳23.8 mag (1.''5 diameter apertures); these five targets, along with a further source from Broderick et al. (2022, PASA, 39, e061) with a deep non-detection ( Ks≳23.7 mag; 3σ ; 2'' diameter aperture) in the Southern H-ATLAS Regions Ks -band Survey, are considered candidates to be ultra-high-redshift ( z>5 ) radio galaxies. The extreme radio to near-infrared flux density ratios ( >105 ) for these six sources are comparable to TN J0924 − 2201, GLEAM J0856 + 0223 and TGSS J1530 + 1049, the three known powerful radio galaxies at z>5 . For a selection of galaxy templates with different stellar masses, we show that z≳4.2 is a plausible scenario for our ultra-high-redshift candidates if the stellar mass M*≳1010.5 M⊙ . In general, the 35 targets studied have properties consistent with the previously known class of infrared-faint radio sources. We also discuss the prospects for finding more UHzRG candidates from wide and deep near-infrared surveys.
Keywords
galaxies: high-redshift; galaxies: active; infrared: galaxies; radio continuum: galaxies;
Publications of the Astronomical Society of Australia
Volume 41, Number e071
2024 October
