The GLEAMing of the first supermassive black holes: III. Radio sources with ultra-faint host galaxies

Abstract

We present deep near-infrared $K_{\mathrm{s}}$-band imaging for 35 of the 53 sources from the high-redshift ($z>2$) radio galaxy candidate sample defined in Broderick et al. (2022). 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 $K_{\mathrm{s}}\approx 21.6$$-$$23.0$ mag (2${}^{\prime \prime }$ diameter apertures; AB). The remaining eight targets are not detected to a median $3\ddot{I}ƒ$ depth of $K_{\mathrm{s}}\approx 23.3$ mag ($2^{\prime \prime }$ 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 $L_{500\mathrm{MHz}}>10^{27}$ W Hz${}^{-1}$. By plotting 150-MHz flux density versus $K_{\mathrm{s}}$-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 $3\ddot{I}ƒ$ lower limit of $K_{\mathrm{s}}\gtrsim 23.8$ mag ($1.5^{\prime \prime }$ diameter apertures); these five targets, along with a further source from Broderick et al. (2022) with a deep non-detection ($K_{\mathrm{s}}\gtrsim 23.7$ mag; $3\ddot{I}ƒ$; $2^{\prime \prime }$ diameter aperture) in the Southern H-ATLAS Regions $K_{\mathrm{s}}$-band Survey, are considered candidates to be ultra-high-redshift ($z>5$) radio galaxies. The extreme radio to near-infrared flux density ratios ($>10^5$) 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 $z\gtrsim 4.2$ is a plausible scenario for our ultra-high-redshift candidates if the stellar mass $M_{\ast }\gtrsim 10^{10.5}{\mathrm{M}}_{\odot }$. [abridged]

24 pages, 5 figures (one of which is a multi-page figure with 30 separate panels), 2 tables, accepted for publication in PASA