PASADENA, CA — Astronomers are puzzled by the discovery of a mysterious, giant object that existed when the universe was only about 800 million years old. At first, they didn't believe it was even real.
Named Himiko for a legendary Japanese queen, the object is what's known as an extended Lyman-Alpha blob, a huge body of gas that may be the precursor to a galaxy. It is 55,000 light years across - comparable to the radius of the Milky Way - and may have a black hole at its heart.
Researchers are still not sure what Himiko actually is. Because it is one of the most distant objects ever found, it is very faint. It could be ionized gas powered by a super-massive black hole; a primordial galaxy with large gas accretion; a collision of two large young galaxies; super wind from intensive star formation; or a single giant galaxy with a large mass of about 40 billion suns.
“I am very surprised by this discovery," explained lead author Masami Ouchi, a fellow at the Observatories of the Carnegie Institution who led an international team of astronomers from the US, Japan, and the UK. "I have never imagined that such a large object could exist at this early stage of the universe’s history. According to the concordance model of Big Bang cosmology, small objects form first and then merge to produce larger systems. This blob had a size of typical present-day galaxies when the age of the universe was about 800 million years old - only six percent of the age of today’s universe!”
Extended blobs discovered thus far have mostly been seen from when the universe was two to three billion years old - none from when it was younger. Himiko is one of the earliest objects ever seen, dating from the period when neutral hydrogen began to form quasars, stars, and the first galaxies.
The team initially identified Himiko among 207 distant galaxy candidates seen at optical wavelengths using the Subaru telescope from the Subaru/XMM-Newton Deep Survey Field located in the constellation of Cetus. They then made spectroscopic observations to measure the distance with the Keck/DEIMOS and Carnegie’s Magellan/IMACS instrumentation.
Himiko was an extraordinarily bright and large candidate for a distant galaxy. “We hesitated to spend our precious telescope time by taking spectra of this weird candidate. We never believed that this bright and large source was a real distant object. We thought it was a foreground interloper contaminating our galaxy sample,” continued Ouchi. “But we tried anyway. Then, the spectra exhibited a characteristic hydrogen signature clearly indicating a remarkably large distance—12.9 billion light years!”
Using infrared data from NASA’s Spitzer Space Telescope and the United Kingdom Infrared Telescope, radio data from the VLA, and X-ray imaging from the XMM-Newton satellite, the team was able to estimate the star-formation rate and stellar mass of this galaxy. “We found that the stellar mass of Himiko is an order of magnitude larger than other objects known at a similar epoch, but we cannot as yet tell if the center houses an active and growing black hole,” said James Dunlop, a team member at Edinburgh.
“One of the puzzling things about Himiko is that it is so exceptional,” said Carnegie’s Alan Dressler, a member of the team. “If this was the discovery of a class of objects that are ancestors of today’s galaxies, there should be many more smaller ones already found — a continuous distribution. Because this object is, to this point, one-of-a-kind, it makes it very hard to fit it into the prevailing model of how normal galaxies were assembled. On the other hand, that’s what makes it interesting!”
The research is published in the May 10, 2009, issue of The Astrophysical Journal.