The Sun emits light, and the Moon and the planets reflect it. A pleasant adage that we have heard a thousand times, and that perhaps we have repeated ourselves. With the exception of comets, it is fully applicable in our Solar System, and even in our cosmic neighborhood. But does it apply to all corners of the Universe? Are the stars the only possible emitters of light?
We might be tempted to assert that it is. In fact, objects like nebulae are nothing but gas and dust that re-emit radiation from a star. But if we go to larger scales, we find another type of celestial objects that challenge this idea: quasars.
The tremendous jets from the AGN in the Swan A galaxy. The size of the jet far exceeds that of the galaxy itself, which appears in the center. Credit: VLA / NRAO
Quasars receive their name from the English quasi-star (almost-star), because when observing them with the telescope only a point appears in the sky, as when we observe a star. With the knowledge available in the 60s of the last century, there was no way to understand what those bodies were that, despite their specific appearance, presented radiation with characteristics very different from those expected in a star. And this not only in the visible fringe of the electromagnetic spectrum: its radio emission was enormous.
The study of the redshift of quasars brought a new surprise: they were very distant objects, located in some cases at the ends of the observable universe. It is not that they were point objects, it is that they were too far away to be able to solve their structure. It turned out then that quasars were nothing but a particular type of nuclear-active galaxies, what we call AGNs (Active Nuclei of Galaxies), the very powerful sources of radiation present in the center of some galaxies.
What we find in an AGN is, in the first place, a supermassive black hole. Not like the relatively modest black holes resulting from a supernova explosion, but one that can be billions of times the mass of the Sun. A disk of matter forms around it, on which the central monster feeds. And it is precisely the fall of this material into the central black hole that triggers one of the most spectacular phenomena in the cosmos: the ejection of two jets or jets of material perpendicular to the accretion disk, which can be much larger than the of the galaxy itself that houses the black hole.
Artist’s impression of an AGN. The donut-shaped structure contains the accretion disk. The base of the two radiation jets can be seen. Credit: ESA / NASA, the AVO project and Paolo Padovani
The radiation from the environment of these black holes (not from the black hole itself, from which we already know that nothing can escape) is not emitted by the stars that can orbit it, but by material in the disk and by the jet itself. The disk emits radiation because it is hot, like the surface of a star or an incandescent filament. The jet, for its part, has magnetic fields of enormous intensity that capture electrons and make them describe convoluted trajectories, which in turn leads the electrons to emit so-called synchrotron radiation. So we have two non-stellar radiation sources at work, leading AGNs to be the most intense permanent sources (that is, non-explosive, like supernovae or gamma-ray bursts) in the Universe.
The shadow of the superlative black hole at the center of the galaxy M87. Credit: EHT Collaboration
Quasars would be just a particular type of AGNs, such as blazars or the so-called Seyfert Galaxies. The study of AGNs is a tremendously dynamic field of contemporary astrophysics, and advances such as the observation of the shadow of the black hole of M87 are gradually shedding light on the subject. As much as shedding light on a black hole, it never seems like a good idea a priori.
If you want to deepen your knowledge of black holes and quasars, find out about the Official Master in Astronomy and Astrophysics of the International University of Valencia.
