Himiko, a "space blob" named after a legendary queen from ancient Japan, is an enormous galaxy, 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.
The Hubble images, receiving optical and ultraviolet light, reveal three stellar clumps covering a space of 20,000 light-years. Each clump is the size of a typical luminous galaxy dating to the epoch of Himiko. Together, the clumps achieve a prodigious rate of star formation, equivalent to about one hundred solar masses per year. This is more than sufficient to explain the existence of Himiko and its gaseous halo. The observation of the three stellar clumps is exciting in itself, as it means that Himiko is a "triple merger," which, according to Ellis, is "a remarkably rare event."
But a surprising anomaly emerged when Himiko was observed by ALMA. Although the giant gas cloud was bustling with energy at ultraviolet and optical frequencies, it was comparatively sleepy in the submillimeter and radio ranges that ALMA detects. 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. When these dust clouds are heated up by the ultraviolet light emitted by the developing stars, the dust reradiates the ultraviolet light out into the universe at radio wavelengths. But ALMA did not receive significant radio signals from Himiko, suggesting that heavier elements are not present. Also missing was the spectral signature associated with the emission of gaseous carbon, something also common in galaxies with intense star formation.
Both of these nondetections—of substantial radio waves and of gaseous carbon—are perplexing since carbon is ordinarily rapidly synthesized in young stars. Indeed, carbon emission has heretofore been recommended as a tracer of star formation in distant galaxies. But, as Ellis and his fellow astronomers found, Himiko does not contain the dust clouds of heavier elements that astronomers find in typical energetic galaxies. Instead its interstellar gas is composed of hydrogen and helium—primitive materials formed in the Big Bang itself.
Ellis and his fellow astronomers did not come to this conclusion quickly. They first carefully ruled out several other possible explanations for Himiko, including that the giant blob is being created by the magnification of a foreground object by a phenomenon known as gravitational lensing, or is being powered by a massive black hole at its center. Ultimately, the team concluded that Himiko is most likely a primordial galaxy caught in the moment of its formation between 400 million to 1 billion years after the Big Bang, a period astronomers term the cosmic dawn.
"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!"
Artist's rendition of Himiko at the top of the page is based on the results from the observations of ALMA and Hubble Space Telescope. Himiko is mainly composed of clean primordial gas with a little amount of heavy elements.
The Hubble images above (Figure 1) reveal three stellar clumps aligned over 20 thousand light years. Each clump has brightness comparable to a typical luminous galaxy at the epoch of Himiko, when the Universe was only 800 million years old. The gigantic hydrogen cloud (or space blob) engulfs the three clumps. No single bright core is found, ruling out the possibility that Himiko is powered by a supermassive black hole. By combining the Hubble and Spitzer Space Telescope data, the astronomers reveal intense star formation in Himiko. Matthew Ashby, a team member at Harvard-Smithsonian Center for Astrophysics, said "We find Himiko is converting gas into stars at a rate of about a hundred solar masses per year, several times more intensely than any known object at this epoch. This intense star production rate is probably sufficient to heat the vast space blob."
The most astonishing find, however, is that the ALMA data show no signal of carbon gas which is used as the index of star formation nor radiation from dust clouds within the system heated by young stars. The radio intensity of carbon gas emission is more than 30 times weaker than present-day galaxies with comparable star formation activities. Given the sensitivity of ALMA, this is truly remarkable.
In the first few minutes after the Big Bang, light elements, e.g. hydrogen and helium, are created. On the other hand, "heavy" elements such as carbon and oxygen are synthesized by nuclear fusion reactions in stars. "Himiko reveals no radio emission from either the solid or gaseous state of heavy elements." remarked Kotaro Kohno, a member of the team. "Star formation and associated supernova explosions normally create dust clouds composed of grains of carbon, oxygen, silicon and other elements. This dust is heated by ultra-violet radiation from massive newborn stars and the warm dust then re-radiates at radio wavelengths. Such radiation is not detected in Himiko. Even more surprising, we detect no emission from gaseous carbon".
As a result, the astronomers speculate that Himiko could be composed of primordial gas, a mixture of the light elements of hydrogen and helium created in the Big Bang. If correct, this would be a landmark discovery signaling the detection of a primordial galaxy seen during its formation.
The paper reporting the results of this research, titled "An Intensely Star-Forming Galaxy at Z ~ 7 with Low Dust and Metal Content Revealed by Deep ALMA and HST Observations," will be published in the December 1, 2013, issue of the Astrophysical Journal. The work was funded by NASA through a grant from the Space Telescope Science Institute, the World Premier International Research Center Initiative (WPI Initiative), and the Japan Society for the Promotion of Science (JSPS).
The Daily Galaxy via CalTech