A team of international astronomers, led by Newcastle University, have used new data from the Dark Energy Spectroscopic Instrument (DESI), which is conducting a five year survey of large scale structure in the universe that will include optical spectra for ~3 million quasars; extremely bright galaxies powered by supermassive black holes. They found that quasars that contained more dust, and therefore appeared redder, were more likely to have stronger radio emission compared to the quasars that had very little-to-no dust, appearing very blue.

Almost every known galaxy contains a supermassive black hole, which are black holes with a mass millions to billions that of our Sun, at its centre, including our own Milky Way. In some galaxies there is lots of material in the centre, feeding and growing this supermassive black hole, making it very energetic and “active”. The most powerful type of these active galaxies are called “quasars”, which are some of the brightest objects in the Universe. Most quasars appear very blue, due to the bright disc of matter that orbits and feeds the central supermassive black hole which is very bright in optical and ultraviolet wavelengths. However, astronomers have found that a significant fraction of these quasars appear very red, although the nature of these objects is still not well understood.

In order to understand the physics of these red quasars, “spectroscopic” measurements are required, which can be used to analyse the quasar light at different wavelengths. The “shape” of the quasar’s spectrum can indicate the amount of dust present surrounding the central region. Observing the radio emission from quasars can also tell you about the energetics of the central supermassive black hole; whether it is launching powerful “winds” or “jets” that might shape the surrounding galaxy.

This new study, led by Dr Victoria Fawcett of Newcastle University, and previously Durham University, uses spectroscopic observations from DESI to measure the amount of dust (reddening) in a sample of ~35,000 quasars and link this to the observed radio emission. They find that DESI is capable of observing much more extreme red (dusty) quasars compared to similar/previous spectroscopic surveys, such as the Sloan Digital Sky Survey (SDSS). They also find that redder quasars are much more likely to have strong radio emission compared to typical blue quasars (see link to movie at the end of the article).

This reddening-radio connection is likely due to powerful outflows of gas driven away from the supermassive black hole, which slam into the surrounding dust, causing shocks and radio emission. These outflows will eventually blow away all the dust and gas in the central region of the galaxy, revealing a blue quasar and resulting in weaker radio emission. This is consistent with the emerging picture that red quasars are a younger, “blow-out” phase in the evolution of galaxies. Red quasars may therefore be extremely important for understanding how galaxies evolve over time.

Paper: https://academic.oup.com/mnras/article/525/4/5575/7273139?login=true