A Hubble Space Telescope study of nearby galaxies is challenging a familiar explanation for how ultraviolet light escaped the first galaxies and helped transform the early universe, particularly in the context of the Hubble reionization study.
The strongest galactic winds did not accompany the greatest escape of that light. Instead, faster winds were linked to less escaping radiation, according to an Oct. 22 announcement from Johns Hopkins University. The finding suggests young stars may open an earlier escape window, before powerful winds driven by exploding stars develop.
The research, published Oct. 15 in Nature Astronomy, examined six local, compact star-forming galaxies. These are present-day comparisons for ancient galaxies, not direct observations of the universe’s first stars.
This Hubble reionization study offers new insights into the processes that governed the universe’s evolution during its earliest phases.
Why escaping light matters
The story begins with hydrogen, the simplest element. A neutral hydrogen atom has a proton and an electron. Ultraviolet light with enough energy can strip away the electron, leaving an electrically charged atom, or ion.
Long before the Sun formed, the first stars began producing this energetic light. Over time, radiation changed much of the hydrogen between galaxies from neutral gas into ionized gas. Astronomers call that process reionization.
NASA’s guide to the early universe places this era between the emergence of the first stars and roughly a billion years after the Big Bang. Its exact beginning remains uncertain.
The often-used image of a cosmic fog describes gas that blocked energetic ultraviolet light. It does not mean the entire universe had the same opacity at every wavelength. Nor was reionization a single explosion that suddenly switched on the cosmos.
Starlight must leave its home galaxy to change the gas beyond it. The new study addresses that first step.
Reading the light, not just taking pictures
The researchers used observations from Hubble’s Cosmic Origins Spectrograph, or COS. Unlike a camera, the instrument separates light into its component wavelengths. That lets scientists study what the light encountered along its path.
NASA describes spectroscopy as a way to measure properties such as a material’s composition, density, temperature and velocity. COS, installed in 2009, works in ultraviolet light and was designed to preserve as much of that faint incoming signal as possible.
In this study, absorption features provided information about moving gas. Gas removes light at particular wavelengths, leaving dips in a spectrum. The location and width of those dips help reveal motion. The team compared that evidence with measurements of how much hydrogen-ionizing radiation escaped.
In the Johns Hopkins account, senior author Stephan McCandliss described the results as consistent with supernova energy increasing the coverage of gas and dust that blocks radiation, rather than clearing an escape route.
An early window, with important limits
The six galaxies span different starburst ages. In the published study, radiation escape peaked in the youngest systems, under about 3 million years. Those systems had intense radiation and stellar winds, but lacked a fast galactic wind. Older systems had faster outflows and more dust.
Winds from young stars differ from galaxy-wide outflows. The findings favor early stellar activity helping light escape, followed by supernova-driven winds gathering material that suppresses it during roughly the first 10 million years of star formation.
That is an interpretation of a small comparison sample, not a time-lapse record of one galaxy. The observations do not establish that supernovae always block radiation, or that every ancient galaxy followed this sequence. They also do not settle which sources supplied all the radiation needed for reionization.
Johns Hopkins said the team is seeking more Hubble observing time to test these ideas and help improve models of reionization.
