Birth of a quasar observed

Observation shows how black holes and quasars affect their home galaxies

Quasar (Illustration) © ESO / M. Cornmesser
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For the first time, astronomers have directly observed the birth of a quasar with the space telescopes Hubble and Spitzer. Quasars are extremely high-energy and therefore very bright regions around black holes in centers of active galaxies. The researchers found that the star formation rate in galaxies is much higher until after the ignition of a quasar. This is the first direct evidence of how black holes affect the evolution of their mother galaxies, the researchers report in their forthcoming in the "Astrophysical Journal" published paper.

"We have been able to observe the interaction between the black hole in the center and the mother galaxy at the crucial moment - the birth of a quasar." Says Tanya Urrutia, scientist at the Leibniz Institute for Astrophysics Potsdam. "Quasars play the crucial role in galaxy evolution and determine the properties of the massive galaxies in our local universe, because they regulate the ratio of parent galaxy and star formation rate." Overall, the researchers studied 13 luminous quasars at a distance of about six billion light years. They come from a time when the universe was about half the age of the current one.

By merging two galaxies into one larger one, the active black hole radiates energy and winds into the mother galaxy and the cosmos. Previous attempts to observe quasars at the crucial stage of their birth often failed because the light from the quasars was darkened and reddened by significant amounts of dust. Dust in turn absorbs the ultraviolet and optical light and emits it back in infrared wavelengths. Only the now elaborate observation campaigns with the largest telescope in the world ("Keck") enabled the scientists to observe young quasars in a statistically relevant number.

First star formation, then quasar birth

The new work shows how this energy influences the star formation rates in the mother galaxy and that the quota significantly decreases after the ignition of a quasar. In the temporal sequence of general galaxy evolution it seems that the most active star formation rate always occurs before the growth of the black hole. "The beginning of star formation precedes the ignition of the quasar." Confirms Urrutia. The astronomers also studied how gluttonous black holes are, that is, with what rate the matter falls into the black hole. With the Spitzer Space Telescope, the researchers saw that the redder a quasar is, the more it absorbs matter. They thus witnessed a phase in which galaxies and quasars develop together and in which the physical properties of both are interconnected.

Computer generated graphic of a black hole. Black is the event horizon represented, surrounded by a circular disk of matter of gas. NASA

Astronomers believe that the transition from young, star-forming galaxies to ancient, elliptical galaxies, as we see them today in the Milky Way's environment, depends heavily on the black holes in their centers, Stars are created in areas with a lot of cold gas and dust. It is well known that galaxy mergers and collisions stimulate star formation. Already in the run-up to the now published paper astronomers had established a relationship between the mass of the central black hole and the luminosity of the mother galaxy ("Magorrian Relation"). It seems as if the black holes are developing more slowly in relation to the other processes in their parent galaxies, which were set in motion by the galaxy collision. In young quasars, this relationship was not observed. display

Although quasars release extreme energies and can be seen over billions of light years, their physical size is quite small - at least in comparison to their parent galaxy. The radiation region is roughly the size of our solar system, while the galaxy can have tens of thousands of light years. "It's amazing that something that happens on a very small scale can affect the Great Mother Galaxy so much, " says Urrutia. "It's as if someone playing with a stick on the beach would influence and determine the behavior of all the world's oceans."

(Leibniz Institute for Astrophysics Potsdam, 28.09.2012 - NPO)