How does ultraviolet light affect the formation of the first stars?
Unlike clouds in the Milky Way, the first star-forming clouds were composed entirely of hydrogen and helium. In order to form stars, these clouds needed to condense to very high densities, which required them to cool to very low temperatures. In this kind of gas, such cooling could only happen after the formation of molecular hydrogen – two hydrogen atoms latched together. However, such molecules can be destroyed by ultraviolet light – which was likely produced in copious amounts by the very massive first stars.
Image: A cluster of newborn stars in NGC 7129, an interstellar (between stars) molecular cloud, picture obtained with the NASA Spitzer Space Telescope.
Credit:NASA/JPL/Caltech/Harvard-Smithsonian CfA
How do X-rays affect the temperature of the intergalactic gas?
X-rays are photons – particles of light – with extremely high energies. Because they are so energetic, they can travel through large swathes of the Universe before being fully absorbed by matter. But, as they travel, they collide with matter particles. An X-ray transfers some of its energy to the matter during each of these collisions, which then heats the gas. Without this process, the intergalactic gas - gas between galaxies - is very cold at early times, so even a small background is enough to change the gas temperature significantly.
Image: Galaxy cluster Abell 2125 as seen by the Chandra X-Ray Observatory, with several massive multimillion degree Celsius gas clouds.
Credit: MSFC
How do "radiation backgrounds" of ultraviolet light and X-rays build up over time?
Because the first stars were so massive, they were also extremely hot, which in turn made them powerful ultraviolet sources. As these stars formed, they flooded the Universe with ultraviolet light, building up a background that subsequent generations of stars had to overcome. Meanwhile, when these stars died (and perhaps through other processes as well), black holes formed. If these black holes encountered clouds of gas, their strong gravity would suck the clouds in – but not before the clouds underwent intense heating, allowing them to produce X-rays. Because the X-rays could travel so far through the Universe, an X-ray background likely built up early in the Universe’s history.
Image: A computer simulation of the heating of the Universe by X-rays. Orange regions are near black holes emitting X-rays, where heating is strong, while blue regions remain cold.
Credit: Andrei Mesinger
How do these radiation backgrounds affect the growth of stars and galaxies?
As the first generations of stars form, ultraviolet light builds up in the Universe – making it more difficult for later stars to form! If the background were absent, star formation would proceed very rapidly, as more and more star-forming clouds formed. But the background slows this process down, because so many of the clouds are unable to cool in the presence of the ultraviolet light. Meanwhile, if black holes form and grow at this early time, an X-ray background also builds up. This heats the Universe, which makes it harder for clouds to form as well. Of course, we have not yet observed this era, so the strength of these feedback processes are currently unknown. The ultraviolet and X-ray backgrounds are in fact key targets of forthcoming observations, because they would tell us the overall rate at which stars and black holes form during the Cosmic Dawn.
Image: IRAS 20324+4057, aka "the Tadpole," is gas and dust surrounding multiple protostars, and the intense blue glow is caused by nearby stars' ultraviolet radiation, which also sculpts its tail into a long, wiggly shape.
Credit: NASA/JPL-Caltech/ESA, the Hubble Heritage Team STScI/AURA and IPHAS
