Luminous infrared galaxies shine with the radiative output of tens of Milky Way galaxies, or even more. Their most striking feature, however, is not their tremendous energy output but the fact that nearly all of their radiation is invisible and at infrared wavelengths. The source of the energy is intense star formation that takes place within dust-filled clouds. The ultraviolet and visible light produced by hot young stars is absorbed by the dust grains and is then remitted as infrared radiation. Collisions between galaxies are expected to trigger enhanced star formation, and so luminous galaxies are thought to result from such interactions. These objects may be representative of a phase of stellar growth and enrichment that most galaxies briefly experience, especially during early times in the age of the universe when collisions were more common. The connections between galaxy interactions and star formation, however, are imperfectly understood, in part because the obscuring dust makes it difficult to probe the small nuclei of merging galaxies.
In the local universe (that is, within a billion light-years or so of us) the galaxy NGC6240 is unusual, being a luminous infrared source in the throes of a major merger of two galaxies, experiencing star formation at an estimated rate of about twenty-five solar masses per year, and radiating about twenty times as much energy as does the Milky Way. It is also known to be a powerful emitter of X-rays. The origin of these X-rays has been uncertain: they could., for example, come from intermediate-sized black holes formed in the star formation process, or perhaps from the supermassive black holes at the center of the nuclei.
CfA astronomers Junfeng Wang, Emanuele Nardini, Giuseppina Fabbiano, Margarita Karovska, Martin Elvis, Guido Risaliti, and Andreas Zezas, with three colleagues, used images and spectra from the Chandra X-ray Observatory to examine the nuclear region of NGC6240 with high spatial resolution and sensitivity. They were able to identify the two nuclei and study the diffuse very hot gas (about seventy million degrees kelvin) in the inner galaxy out to a distance of nearly twenty thousand light-years. For the first time, they were able to obtain the spatial distribution of the highly ionized, gaseous iron produced in this hot medium. They found that its morphology corresponds closely to that of the shocked molecular hydrogen gas seen in the infrared and that has been known for many years. Based on this correspondence, and incorporating an analysis of the new data, the scientists were able to conclude that there are almost a million solar-masses of hot iron in the region, and that both the quantity and energetics are consistent with a series of supernova explosions; the cataclysms produce gas moving at velocities of up to 2200 kilometers per second which shocks the material to the extreme temperatures seen. The results confirm that rapid star formation (supernovae signal the deaths of massive young stars ) is indeed at work, and responsible for both the luminous infrared and the X-ray emission.
"Fast and Furious: Shock Heated Gas as the Origin of Spatially Resolved Hard X-ray Emission in the Central 5 kpc of the Galaxy Merger NGC 6240,” Junfeng Wang, Emanuele Nardini, Giuseppina Fabbiano, Margarita Karovska, Martin Elvis, Silvia Pellegrini, Claire Max, Guido Risaliti, Vivian U, and Andreas Zezas, ApJ 781, 55 2014.