2017MNRAS.465.1297H

Observations using the 7-mm receiver system on the Australia Telescope Compact Array have revealed large reservoirs of molecular gas in two high-redshift radio galaxies: HATLAS J090426.9+015448 (z = 2.37) and HATLAS J140930.4+003803 (z = 2.04). Optically, the targets are very faint, and spectroscopy classifies them as narrow-line radio galaxies. In addition to harbouring an active galactic nucleus the targets share many characteristics of sub-mm galaxies. Far-infrared data from Herschel-Astrophysical Terahertz Large Area Survey suggest high levels of dust (>10⁹ M_☉) and a correspondingly large amount of obscured star formation (∼1000 M_☉ yr^–1). The molecular gas is traced via the J = 1 - 0 transition of ¹²CO, its luminosity implying total H₂ masses of (1.7 ± 0.3) x 10¹¹ and (9.5 ± 2.4) x 10¹⁰ (α_CO/0.8) M_☉ in HATLAS J090426.9+015448 and HATLAS J140930.4+003803, respectively. Both galaxies exhibit molecular line emission over a broad (∼1000 km s^–1) velocity range and feature double-peaked profiles. We interpret this as evidence of either a large rotating disc or an on-going merger. Gas depletion time-scales are ∼100 Myr. The 1.4-GHz radio luminosities of our targets place them close to the break in the luminosity function. As such they represent 'typical' z > 2 radio sources, responsible for the bulk of the energy emitted at radio wavelengths from accretion-powered sources at high redshift, and yet they rank amongst the most massive systems in terms of molecular gas and dust content. We also detect 115-GHz rest-frame continuum emission, indicating a very steep high-radio-frequency spectrum, possibly classifying the targets as compact steep spectrum objects.