What Makes Red Quasars Redder Than Blue Quasars? (Cosmology)

D.J. Rosario and colleagues in their recent paper, presented a plausible scenario that connects the windy dust to the reddening of the red quasars and their radio properties. They postulated that, dusty winds are widespread among red quasars, and the shocks that these winds drive into the interstellar medium of their host galaxies are responsible for the particular radio properties of red quasars. Their study recently appeared in Arxiv.

A quasar also known as a quasi-stellar object (QSO), is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk. As gas in the disk falls towards the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum.

Red quasars are quasars with optical colors that are redder than normal/blue quasars, thought to be the result of moderate levels of dust extinction within the quasar host galaxy. As the colours of red quasars are not as distinct as those of normal or blue quasars, they can be difficult to separate photometrically from stars and compact galaxies. Earlier strong radio emission was used to select and study red quasars. Overtime, growing evidences suggested that redder quasars were associated with a high fraction of radio sources, though, clear conclusions were elusive.

In previous studies, D.J. Rosario and colleagues demonstrated that red quasars have distinct radio properties that fundamentally distinguish them from blue quasars, specifically, a higher incidence of low-power radio emission (1.4 GHz luminosities L1.4≈ 1025 – 1027 W Hz¯1) that is physically compact when imaged by arcsecond-resolution radio surveys such as FIRST.

“We present e-MERLIN imaging of a set of intermediate-redshift, luminous red and normal quasars carefully selected to have radio properties that span the range over which red quasars show the most divergence from the general population. With an angular resolution ×25 better than FIRST survey, we resolve structures within the host galaxies of these quasars (>2 kpc).”

Now, they carried out a careful and controlled study on the radio properties of red quasars. They found a much higher detection rate of radio sources among red quasars compared to equally luminous blue quasars at the same redshifts. This result is attributable to a substantial excess of radio compact sources among red quasars, with relativity low radio powers (L1.4< 1027 W Hz¯1, where L1.4 is the integrated rest-frame 1.4 GHz radio luminosity of a radio source).

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Figure 1. Maximum radio source size vs. 1.4 GHz radio power (L1.4) for our rQSOs (red points) and cQSOs (cyan points). Filled circles and left-pointing arrows show extended sources and unresolved sources respectively. The dashed vertical lines mark the typical physical resolution of the FIRST survey and the VLBA at L-band at a characteristic redshift of z = 1.3. The dotted horizontal line marks a rough transition between low-power and high-power radio sources. The coloured bands spanning the FIRST resolution limit graphically show the relative fractions of radio rQSOs and cQSOs that are determined to be compact or resolved by FIRST. Note that compact sources are more commons among rQSOs compared to cQSOs at low radio luminosities, but they are found equally among more luminous systems. They mark using shaded bands the approximate size rangesof the CSS, GPS, and CSO categories of jetted radio sources. The largest extended radio sources, those that are well-resolved by FIRST, are the classical double-lobed FRI/II systems. © Rosario et al.

“We found that the excess radio emission in red quasars can be attributed to radio structures that are confined to galaxy scales (< 10 kpc), while we confirmed previous results that red and blue quasars have similar incidences of radio jets and lobes on circumgalactic or larger scales (> 10 kpc).”

Their results indicated that the primary mechanism that generates the enhanced radio emission in red quasars is not directly connected with the nuclear engine or accretion disc, instead it is likely to arise from extended components such as AGN-driven jets or winds.

“Since the key feature that defines radio quasars is the presence of moderate dust extinction towards their nuclei, we postulate that dusty winds are widespread among red quasars, and the shocks that these winds drive into the interstellar medium of their host galaxies are responsible for the particular radio properties of red quasars.”

— concluded authors of the study

Reference: D.J. Rosario, D.M. Alexander, J. Moldon, L. Klindt, A.P. Thomson, L. Morabito, V.A. Fawcett, C.M. Harrison, “Fundamental differences in the radio properties of red and blue quasars: kiloparsec-scale structures revealed by e-MERLIN”, Arxiv, pp. 1-21, 2021. https://arxiv.org/abs/2106.02646

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