Fundamental differences in the radio properties of red and blue quasars: insight from the LOFAR Two-metre Sky Survey (LoTSS)

Abstract

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 ×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.

Evidence for Radio Loud to Radio Quiet Evolution from Red and Blue Quasars

“…In this work we show how the results of Rosario et al (2020) and Fawcett et al (2020) support a picture in which both the most radio loud quasars or QSOs and the most radio quiet quasars or QSOs result from accretion onto rapidly rotating black holes in retrograde and prograde configurations, respectively. Intrinsic to this picture is the fact that retrograde accretion spins black holes toward prograde accretion, implying that radio loud quasars, if formed, constitute initial states, but never final ones.…”

“…We will show how to understand that feature as well as the tendency of the rQSOs to cQSOs ratio to peak in the radio loud range as shown in Figure 1. Rosario et al (2020). While the numbers approach unity at the extremes of radio loudness, they differ from that significantly in the middle.…”

“…In Figure 1 we reproduce the ratio of the numbers of rQSOs and cQSOs of Rosario et al (2020) as a function of radio loudness. As pointed out in their work, the ratio is significantly removed from value unity toward the radio loud/radio quiet boundary or midpoint of Figure 1 shown with a dashed line.…”

“…As pointed out in their work, the ratio is significantly removed from value unity toward the radio loud/radio quiet boundary or midpoint of Figure 1 shown with a dashed line. The values of Figure 1 come from the numbers of rQSOs and cQSOs from Rosario et al (2020) and which we separately show in Figure 2. Because the numbers reflect 6 micron, redshift matched, LOFAR detections, the actual distributions are enhanced with respect to Figure 2.…”

“…While the numbers approach unity at the extremes of radio loudness, they differ from that significantly in the middle. Rosario et al (2020), the ratio of which is plotted in Figure 1. Data points are smoothly connected for ease of comparison.…”