Fundamental differences in the radio properties of red and blue quasars:
  Insight from the LOFAR Two-metre Sky Survey (LoTSS)
  Red quasi-stellar objects (QSOs) are a subset of the luminous end of the
cosmic population of active galactic nuclei (AGN), most of which are reddened
by intervening dust along the line-of-sight towards their central engines. In
recent work from our team, we developed a systematic technique to select red
QSOs from the Sloan Digital Sky Survey (SDSS), and demonstrated that they have
distinctive radio properties using the Faint Images of the Radio Sky at Twenty
centimeters (FIRST) radio survey. Here we expand our study using low-frequency
radio data from the LOFAR Two-metre Sky Survey (LoTSS). With the improvement in
depth that LoTSS offers, we confirm key results: compared to a control sample
of normal "blue" QSOs matched in redshift and accretion power, red QSOs have a
higher radio detection rate and a higher incidence of compact radio
morphologies. For the first time, we also demonstrate that these differences
arise primarily in sources of intermediate radio-loudness: radio-intermediate
red QSOs are $\times 3$ more common than typical QSOs, but the excess
diminishes among the most radio-loud and the most radio-quiet systems in our
study. We develop Monte-Carlo simulations to explore whether differences in
star formation could explain these results, and conclude that, while star
formation is an important source of low-frequency emission among radio-quiet
QSOs, a population of AGN-driven compact radio sources is the most likely cause
for the distinct low-frequency radio properties of red QSOs. Our study
substantiates the conclusion that fundamental differences must exist between
the red and normal blue QSO populations.
