Washington: Astronomers have detected ultraviolet (UV) emissions of neutral hydrogen in Earth’s own galaxy for the first time, giving a cue about the rate at which Milky Way is making stars.
Although hydrogen is the most abundant element in the universe, the so-called Lyman alpha emission of hydrogen, lying at far UV wavelengths, had up to now escaped detection in the vicinity of the Solar System.
As a result of these findings detected by NASA’s Voyager 1 and Voyager 2 space probes, it now will be possible to test locally (within the Milky Way galaxy) models designed to measure of the same types of emissions that are observed in distant galaxies.
Lyman alpha emissions, with a wavelength of 121.6 nanometers, are the principal signature of hydrogen atoms in the universe and are used as indicators of the formation of stars in galaxies shortly after the Big Bang (the primordial universe).
However, because these emissions are at the ultraviolet end of the spectrum, they are completely obscured by the earth’s atmosphere and the proximity of the Sun, which is composed primarily of hydrogen and produces an intense flux of ultraviolet photons.
As the two Voyager probes moved farther and farther from the Sun—from 1993 to 2003, they travelled between six and 13 billion kilometers (40 to 90 Astronomical Units) from the Sun—the glow of local Lyman alpha emissions in the vicinity of the probes has become easier to detect, growing 20 times more intense than if observed from Earth orbit.
Rosine Lallement’s team of researchers used the residual ultraviolet light detected by the Voyager probes to develop a theoretical model of the interplanetary glow observable in the sky.
The model makes it possible to measure the slight excess radiance in the direction of the Milky Way, which correlates with the “red” that is characteristic of the regions that surround young, hot stars.
As a result, astronomers on Earth will be able to test models that have been developed to interpret observations of the Lyman alpha emission from very distant galaxies, where the emission is associated with the first bursts of forming stars.
The study has been recently published in the journal Science.