In the early 1980’s, NASA’s Infrared Astronomical Satellite (IRAS) detected a number of unknown objects lurking in the depths of the cosmos.
At the time, these IRAS objects stirred speculation in the press. Were the infrared signals being emitted by comets inside the Solar System? Or were they failed stars (brown dwarfs) lurking beyond the orbit of Pluto? The latter theory spawned the idea that the hunt for Planet X was back on (stoking the smoldering conspiracy embers of the flawed doomsday theory that Nibiru is coming to get us). Alas, it was neither, these intense infrared signals were coming from much, much further away.
It turned out that the infrared emissions were being generated by galaxies that, bizarrely, had little optical signal. Although a high proportion of them were known to be interacting galaxies (i.e. they were colliding with other galaxies), the exact energy mechanism driving their emissions was not known.
Ultra-luminous galaxies have the luminocity of a trillion Suns, whereas our galaxy has the luminosity of a pedestrian ten billion Suns. Obviously, ultra-luminous galaxies are different animals to the Milky Way, but a galaxy is a galaxy and the energy sources are similar whether they are ultra-luminous or not. It would appear that the only difference is how active the galaxy is.
The first obvious energy source in a galaxy is star formation; the more stars that are forming, the brighter the galaxy. Secondly — as with our galaxy — the central supermassive black hole’s accretion rate contributes to the galaxy’s energy budget; the more matter being accreted by the black hole, the more energy is being generated (and therefore the brighter the galaxy).
So, when observing these ultra-luminous galaxies, surely it should be an easy task to work out where all this energy is coming from? Actually, this isn’t the case, astronomers are having a difficult job in understanding the nature of IRAS galaxies and the reason for this comes from the source of the infrared emissions. Galactic dust is being heated by the energy source, but this dust obscures the source of this heating (it is opaque to optical wavelengths).
Smithsonian Astrophysical Observatory (SAO) researcher Guido Risaliti and his team have been analyzing Spitzer data to try to characterize the infrared emissions from 71 ultra-luminous galaxies. Using a “dust emission diagnostic technique,” the team have deduced that approximately 70% of the galaxies have active nuclei (i.e. their supermassive black holes have high accretion rates). Although most of the galactic nuclei are active, it is star formation that dominates the energy production in two-thirds of the galaxies. Also, these account for the highest fraction of the brightest galaxies.
This is a significant finding as it demonstrates how a galaxy reacts when it interacts with another galaxy. It would appear that the black hole in the core of the galactic bulge is kick-started during the massive gravitational interaction, boosting energy output as it eats more matter. The interaction also boosts star birth and this energy source becomes a dominant factor. Both energy sources heat up interstellar dust, making the galaxy glow in infrared wavelengths while optical light is masked.
Source: SAO (Harvard)