Hubble, ALMA and Spitzer discover a trio of galaxies from the cosmic dawn
Sen—Using the combined forces of the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile and the Hubble and Spitzer space telescopes, astronomers have discovered a trio of primitive galaxies nestled inside an enormous blob of primordial gas nearly 13 billion light-years from Earth.
"This exceedingly rare triple system, seen when the universe was only 800 million years old, provides important insights into the earliest stages of galaxy formation during a period known as 'cosmic dawn,' when the universe was first bathed in starlight," said Richard Ellis, a member of the research team at the California Institute of Technology.
With a hot glowing gaseous halo extending over 55,000 light-years, not only is Himiko very large, it is extraordinarily distant, seen at a time approximately 800 million years after the Big Bang, when the universe was only 6 percent of its present size and stars and galaxies were just beginning to form.
Discovered in 2009, Himiko (after a legendary queen of ancient Japan) appeared to be a giant bubble of hot, ionized gas, and was one of the most fascinating objects to emerge from the Subaru Telescope's wide-field survey.
Composite image of Himiko. Left panel shows the section of the sky containing Himiko (identified in box) as imaged by Hubble. Upper right is a close-up of Himiko with Hubble. Lower right is the same object with additional data from Spitzer and Subaru. Image credit: NASA/Hubble; NASA/Spitzer; NAOJ/Subaru
The Hubble images, receiving optical and ultraviolet light, reveal three stellar clumps covering a space of 20,000 light-years. It is nearly 10 times larger than typical galaxies of that era and comparable in size to our own galaxy. It's possible the trio will eventually merge into a single galaxy similar to the Milky Way.
Infrared observations with Spitzer provided more clues about the object's mass, suggesting Himiko might represent a single galaxy, which would make it uncharacteristically massive for that period of the early universe. The objects are extremely energetic, suggesting they are undergoing a period of intense star formation, equivalent to about one hundred solar masses per year.
"The new observations revealed that, rather than a single galaxy, Himiko harbours three distinct, bright sources, whose intense star formation is heating and ionizing this giant cloud of gas," said Masami Ouchi, an associate professor at the University of Tokyo who led the international team of astronomers from Japan and the United States.
New data from ALMA, Hubble and Spitzer also led astronomers to speculate that Himiko could be made up almost entirely of primordial gas, a mixture of the light elements hydrogen and helium, which were created in the Big Bang. Ordinarily, intense star formation creates dust clouds that are composed of elements such as carbon, oxygen, and silicon, which are heavy in comparison to the hydrogen and helium of the early universe.
Observations with ALMA did not detect any telltale signature from carbon, suggesting that these three objects may be very primitive and have not had enough time to seed the intergalactic medium with heavy elements. If correct, this would be a landmark discovery signaling the detection of a primordial galaxy seen during its formation.
"Astronomers are usually excited when a signal from an object is detected," Ellis says, "but in this case it's the absence of a signal from heavy elements that is the most exciting result!"