Revisiting the Dragonfly galaxy - I. High-resolution ALMA and VLA observations of the radio hotspots in a hyper-luminous infrared galaxy at z=1.92

Radio-loud active galactic nuclei (RLAGNs) are rare among AGN populations. Lacking high-resolution and high-frequency observations, their structure and evolution stages are not well understood at high redshifts. In this work, we report Atacama Large Millimeter/submillimeter Array 237 GHz continuum observation at 0.023 arcsec resolution and Very Large Array (VLA) 44 GHz continuum observation at 0.08 arcsec resolution of the radio continuum emission from a high-redshift radio and hyper-luminous infrared galaxy at z = 1.92. The new observations confirm the South-East (SE) and North-West (NW) hotspots identified by previous low-resolution VLA observations at 4.7 and 8.2 GHz and identify a radio core undetected in all previous observations. The SE hotspot has a higher flux density than the NW one does by a factor of 6, suggesting that there can be a Doppler boosting effect in the SE one. In this scenario, we estimate the advance speed of the jet head, ranging from similar to 0.1c to 0.3c, which yields a mildly relativistic case. The projected linear distance between the two hotspots is similar to 13 kpc, yielding a linear size (<= 20 kpc) of a compact-steep-spectrum (CSS) source. Combined with new high-frequency (upsilon(obs) >= 44 GHz) and archived low-frequency observations (upsilon(obs) <= 8.2 GHz), we find that injection spectra of both NW and SE hotspots can be fitted with a continuous injection (CI) model. Based on the CI model, the synchrotron ages of NW and SE hotspots have an order of 105 yr, consistent with the order of magnitude 10(3)-10(5) yr observed in CSS sources associated with radio AGNs at an early evolution stage. The CI model also favours the scenario in which the double hotspots have experienced a quiescent phase, suggesting that this RLAGN may have transient or intermittent activities.