How to measure the star formation rate
Different stages of star formation can be measured in different ways, a number of which involve so-called emission lines – narrow regions within the electromagnetic spectrum where an object emits much more radiation than at neighbouring wavelengths.
Some specific emission lines (e.g. molecular lines associated with carbon monoxide, CO) are associated with matter collapsing to form new stars. Others, such as the O III lines (from oxygen atoms that are missing two electrons) that are important for this particular study, are formed when young, very hot stars are already present, and their intense ultraviolet radiation "blows away" electrons from whatever oxygen atoms might be present. Oxygen atoms that have just lost 2 electrons will typically not be in the state of lowest possible energy; instead, they will emit radiation at very specific wavelengths, leading to O III emission lines that glow with a very particular green colour.
In the present study, star formation rates are estimated using information about this kind of O III emission, indicating the presence of hot, young stars which, in turn, is a tell-tale sign of ongoing star formation. For galaxies as distant as those under study here, cosmic expansion has "redshifted" the green colour of the oxygen lines by a significant amount: O III emission from these galaxies reaches us as near-infrared radiation. This is the reason these objects were not discovered in earlier observations: CANDELS is the first infrared survey that is capable of detecting sources as faint as glowing oxygen atoms at the other end of the universe.
For four of the galaxies, O III emission was traced directly in spectra taken with the HST's Wide Field Camera 3. For the other galaxy, the presence of O III radiation was reconstructed from the tint it imparts to the galaxies' overall colour.
What further directions for research do these results suggest?
The CANDELS observations are an ideal candidate for follow-up observations with the Hubble Space Telescope's designated successor, the James Webb Telescope, due to be launched later this decade.
"With Webb, we'll probably see even more of these galaxies, perhaps even pristine galaxies that are experiencing their first episode of star formation," says Harry Ferguson of the Space Telescope Science Institute (STScI) in Baltimore, Md., co-leader of the CANDELS survey. "Being able to probe down to dwarf galaxies in the early universe will help us understand the formation of the first stars and galaxies."