Galactic energy-saving lamps

The active galactic nuclei (AGN) are not ‘shining’ perpetually. And the result of a study conducted by researchers in Zurich that the average life of an AGN lasts 200,000 years. The results, published in Monthly Notices of the Royal Astronomical Society, provide new clues about how these objects can influence the formation and evolution of the environment that hosts them.
The active galactic nuclei are the most luminous objects astrophysical universe. They do not shine permanently rather their brightness undergoes a sort of “flicker” extremely slow. And ‘what emerges from a study conducted by researchers at the institute Eidgenössische Technische Hochschule (ETH) in Zurich that has allowed us to study how these blacks holes and galactic nuclei affect the formation and evolution of the environment that hosts them. The results of this study are published in Monthly Notices of the Royal Astronomical Society.


Credit: NASA, ESA, W. Keel, Galaxy Zoo Team

We know that supermassive black holes attract the surrounding gas due to their intense gravity. As the gas orbiting the black hole, forming an accretion disk, it gets progressively hotter and hotter and starts to radiate. This phenomenon results in an active galactic nucleus, or AGN (Active Galactic Nucleus). These objects can often achieve the brightness of hundreds of billions of stars. For example, in the heart of the Milky Way lies a black hole (Sagittarius A *) that, according to some studies, he acted like a real AGN about a few million years ago.
The researchers of ETH Zurich, led by Kevin Schawinski who is also the lead author of the study, reported in their paper an important fact: the AGN does not shine permanently, rather they are reminiscent of the “flicker” of a lamp. In other words, according to the authors, an AGN “turns on and off” about every 200,000 years, a result that is based on their observations. AGNs emit radiation across the electromagnetic spectrum, from X-rays to radio waves, so telescopes can capture the X-rays that originate near the active core and can also record the visible light, although with some delay. It’s a little ‘how to deal with a gas lamp that does not shine immediately after it is turned on. Now, the visible light does not come from the galactic core, but is due to the gas distributed in interstellar space of the galaxy. The gas atoms are stimulated by the radiation of the active galactic nucleus to emit light in a manner well brand. The delay is due to the time required for the light to reach the ends of the galaxy and turn on “the gas lamp galactic”. Before this can happen, the galactic nucleus is in an off state apparent, at least if we speak in terms of visible light, and during this phase emits X-rays.


Credit: Schawinski K. et al. 2015

Analysis of the data suggests that about 5% of active galactic nuclei observed by the authors of this study seem to be in the “off” state. This means that despite such objects were revealed by telescopes that by operating in the X band, they do not radiate visible light typical of a “gas lamp galactic”. Thus, the scientists concluded that if indeed the 5% of all the observed AGN does not emit visible light, the apparent off state represents 5%, or 1/20, the total time on the “stage light-darkness” of an AGN. Put another way, it’s a little ‘how to take a photo of a person every day of his life. In the end, we are more photos of the years as an adult rather than those relating to adolescence, which lasts less over a lifetime. In fact, researchers were already familiar from previous studies that the state “off” of an AGN can be compared to teens of a person, in this case, is approximately of the order of 10 thousand years. This is the amount of time it takes the light to cross a normal galaxy.

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