Eleven billion years ago, when the Universe was half the Earth's present age, massive galaxies had already begun to form, planting the seeds for the present-day Universe. According to current models, the dense cores of massive elliptical galaxies formed first, within which there were regions of intense star formation. By looking at distant objects, we should be able to see evidence of this process. Until now, however, we had not seen anything that looked definitively like the formation of a galaxy core. But a newly discovered object, GOODS-N-774 (nicknamed “Sparky”), may be the first glimpse of exactly that.

The researchers used data from the Hubble Space Telescope, obtained as part of the CANDELS survey, as well as the Herschel Space Observatory and the W.M. Keck Observatory, to study Sparky, which has about 150 billion (1.5 x 10^11) times the mass of the Sun (solar masses), of which 100 billion is in the form of stars.

Scientists still aren’t sure why galaxies like Sparky haven’t been observed before. While there’s a good chance that they are very rare, the authors of the study suggest that many similar star-forming cores may be obscured by gas and dust. If that’s the case, visible-light and near-infrared telescopes could be missing them.

Star formation

GOODS has an effective radius of about one kiloparsec, a size consistent with a forming galactic core. Other galaxies with similar sizes have previously been observed, but Sparky is the first to have both the stellar structure and the gas dynamics of a forming core.

Because of their violent, churning gasses, unique to the early Universe, these galactic cores would be forming stars at a respectable rate, and Sparky is no exception. From their measurements, the study’s authors calculated that Sparky was forming stars that contain somewhere between 70 and 135 solar masses each year.

At that rate, Sparky could churn out as many as 300 stars per year (the Milky Way only produces 10 annually). This is especially impressive since Sparky is only six percent the size of the Milky Way. Due to the rapid pace of formation, it contains twice as many stars despite its small size.

"It's a formation process that can't happen anymore," said Erica Nelson, the paper’s lead author. "The early Universe could make these galaxies, but the modern Universe can't. It was this hotter, more turbulent place—these were boiling cauldrons forging stars."

Descendants

Sparky represents the likely progenitor of a class of massive (but relatively small) elliptical galaxies seen in the early Universe (greater than 10 billion years ago, or at a redshift of 2). They're not present in the modern Universe, having evolved into modern galaxies through mergers.

Sparky is smaller than galaxies of the same total dynamical mass that have been seen in the Sloan Digital Sky Survey, but it is strikingly similar to compact galaxies that are quiescent (having very little star formation). These compact quiescent galaxies are seen slightly later in the Universe’s timeline by looking at objects that are closer in space, which implies that objects like Sparky will ultimately evolve into compact quiescent galaxies.

But this may not be the only mechanism for the formation of the compact quiescent galaxies. Some, for example, may form most of their stars during mergers, when more than 1,000 solar masses of stars could be formed per year (compare that with the meager 70-135 solar masses per year of Sparky). Future work will likely explore which mechanism for the formation of such galaxies is the dominant one.

"I think our discovery settles the question of whether this mode of building galaxies actually happened or not," said Pieter van Dokkum, another of the paper’s authors. "The question now is, 'How often did this occur?' We suspect there are other galaxies like this that are even fainter in near-infrared wavelengths. We had been searching for this galaxy for years, and it's very exciting that we finally found it."

Nature, 2014. DOI: 10.1038/nature13616 (About DOIs).