Cookies

We use cookies to ensure that we give you the best experience on our website. You can change your cookie settings at any time. Otherwise, we'll assume you're OK to continue.

Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

Klindt, L, Alexander, D M, Rosario, D J, Lusso, E & Fotopoulou, S (2019). Fundamental differences in the radio properties of red and blue quasars: evolution strongly favoured over orientation. Monthly Notices of the Royal Astronomical Society 488(3): 3109-3128.

Author(s) from Durham

Abstract

A minority of the optically selected quasar population are red at optical wavelengths due to the presence of dust along the line of sight. A key focus of many red quasar studies is to understand their relationship with the overall quasar population: are they blue quasars observed at a (slight) inclination angle or do they represent a transitional phase in the evolution of quasars? Identifying fundamental differences between red and blue quasars is key to discriminate between these two paradigms. To robustly explore this, we have uniformly selected quasars from the Sloan Digital Sky Survey with mid-infrared counterparts, carefully controlling for luminosity and redshift effects. We take a novel approach to distinguish between colour-selected quasars in the redshift range of 0.2 < z < 2.4 by constructing redshift-sensitive g* − i* colour cuts. From cross-matching this sample to the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey, we have found a factor ≈ 3 larger fraction of radio-detected red quasars with respect to that of blue quasars. Through a visual inspection of the FIRST images and an assessment of the radio luminosities (rest-frame
L
1.4GHz
L1.4GHz
and
L
1.4GHz
/
L
6μm
L1.4GHz/L6μm
⁠), we find that the radio-detection excess for red quasars is primarily due to compact and radio-faint systems (around the radio-quiet – radio-loud threshold). We show that our results rule out orientation as the origin for the differences between red and blue quasars and argue that they provide broad agreement with an evolutionary model.