Billions of light-years from Earth, giant clouds of hydrogen gas produce a special type of radiation, a type of ultraviolet light known as Lyman's alpha radiation. The huge clouds emitting light are the Lyman Alpha Bubbles (LAB). LABs are several times larger than our Milky Way galaxy, but were only discovered 20 years ago. To produce this radiation, an extremely powerful source of energy is needed – energy equal to the radiation of billions of suns.
New research, published March 9 in the journal Nature Astronomy, suggests that the energy source is at the center of the star-forming galaxies around which LABs exist.
The study focuses on Lyman-alpha blob 6 (LAB-6), located over 18 billion light-years away towards the constellation Grus. The joint team discovered a unique feature of LAB-6 – its hydrogen gas seemed to fall inward. LAB-6 is the first LAB with conclusive evidence for this so-called falling gas signature. The falling gas had little abundance of metallic elements, suggesting that the falling LAB hydrogen gas originated from the intergalactic medium and not from the star-forming galaxy itself.
The amount of incident gas is too small to trigger the observed Lyman-alpha radiation. The data obtained indicate that the central star-forming galaxy is the main source of energy responsible for the emission of Lyman-alpha. They also raise new questions about the structure of the LAB.
'This is a real mystery. We expect gas to appear around star-forming galaxies – they need gas for materials, ”said Zheng Zheng, associate professor of physics and astronomy at the University of Utah and co-author of the study. 'But it appears to be the only Lyman-alpha ball with falling gas. Why are they so rare? '
The authors used the Very Large Telescope (VLT) at the European Southern Observatory (ESO) and the Atacama Large Millimeter / Submillimeter Array (ALMA) to obtain data. The lead author of the Purple Mountains Observatory Iping Ao (Chinese Academy of Sciences) first discovered the LAB-6 system over a decade ago. He knew that even then, there was something special about the system, given the extreme size of the hydrogen droplet. He jumped at the opportunity to look closely.
'Fortunately, we were able to obtain the data necessary to capture the molecular composition from ALMA, which determines the speed of the galaxy. ESO's VLT Optical Telescope has given us the important spectral light profile of Lyman-alpha 'emission.
The authors of the new study discovered the nature of the incident gas by analyzing the kinematics of the Lyman-alpha emission. After emitting Lyman's alpha photon, he collides with an environment filled with hydrogen atoms. It bumps into these atoms many times, like a ball moving in a pinball machine, before leaving the environment. This outlet causes the emission to spread outward over long distances.
Falling gas can occur in several different ways. This could be the second stage of a galactic explosion – if massive stars die, they explode and push gas outward, which then falls inward. Another option is a cold stream: filaments of hydrogen float between celestial objects, which can be pulled into the center of a potential well, creating a feature of the falling gas.
The scientists' model assumes that the falling gas in this LAB comes from the latter scenario. They analyzed the shape of the Lyman-alpha light profile, which indicates very little metal dust. In astronomy, metals are heavier than helium. The stars produce all the heavy elements in the universe – when they explode, they produce metallic elements and spread them through intergalactic space.
