The history of star formation in the universe has been charted by astronomers who looked at how gamma rays interact with extra-galactic background light (EBL) – which is a diffuse glow of starlight that pervades the universe. While the results are consistent with direct measurements of star formation using light from galaxies, the study boosts our understanding of the mysterious era of cosmic reionization that occurred in the early universe.
The conventional way to estimate star-formation at a particular time in history of the universe is to study ultraviolet (UV) light from galaxies. This, explains astrophysicist Marco Ajello of Clemson University in South Carolina, “is almost always emission from short-lived stars with a mass more than 10 times that of our Sun”. First, researchers estimate how much longer-wavelength light from less massive stars remains undetected, then they correct for absorption of ultraviolet light by the dust clouds that usually surround star-forming regions. “Given these two corrections, the number of stars formed per year follows rather quickly,” Ajello says.
Direct observation becomes problematic, however, in the early universe. About 300,000 years after the Big Bang, the universe had cooled to the point where protons and electrons combined to form neutral hydrogen. This produced radiation that pervades the universe today as the cosmic microwave background. Measurements from around a billion years after the Big Bang, however, indicate that this hydrogen had been ionized again.
“You need UV photons to ionize the hydrogen,” says Ajello, “The community is converging on extremely faint, star-forming galaxies as the source for these.” Detecting faint galaxies at such long distances, however, is extremely challenging. The Hubble Space Telescope has detected hints that there may have been sufficient numbers of such galaxies through gravitational lensing experiments. The four values calculated thus far, however, vary widely, so independent constraints would be valuable.
Absorbing gamma rays
Clemson and colleagues detected not the emission of radiation but its absorption. They used the Fermi Large Area Telescope (LAT), which is a satellite-based gamma-ray telescope. Gamma rays emitted by astronomical sources such as supernovae and blazars must propagate through the EBL before reaching Fermi-LAT. Gamma rays interact with the EBL and the strength of this interaction increases with the energy of the gamma rays. As a result, the universe is essentially opaque to very high energy gamma rays.
Blazars help measure extragalactic background light
The researchers looked at the change in brightness of 739 galaxies called blazars – which emit jets of intense gamma rays from central supermassive black holes – at redshifts up to 3. To allow them to see even further back, they also observed a single gamma ray burst – the brightest gamma ray emitters in the universe – at redshift 4.35. “Below 10 GeV [gamma-ray energy] there is zero absorption,” explains Ajello, “As you increase the energy you see 10% absorption…then 20%…until eventually the source has disappeared completely. For every source we want to find out how quickly this happens.” This provided the researchers with a measurement of the optical thickness of the EBL at various wavelengths.
The EBL has been evolving all the time that the light has been propagating between a source and Fermi. This presents the researchers with the data they need: “We use over 700 sources, all of them with different redshifts, and so we can reconstruct how the total spectrum of the EBL has been changing over time all the way back to redshift 6,” says Ajello. “If you’re just interested in the star-formation history, then from the UV background you can work out the star-formation history.” The results are consistent with the mid-range estimates from lensing observations.
Astrophysicist Elisa Prandini of the University of Padua in Italy, who was not involved in the research, describes the constraints on ancient galaxies as “significant”, but says it is valuable on other levels too. Notably, she says, it guarantees that direct measurements of the EBL are not missing light from some unidentified source: “With the measurement performed by Fermi, you by definition cannot miss light,” she says: “If it is there, it will interact with gamma ray photons.” She says that it is “dangerous” to place too much weight on the results at high redshift as there is a only a single data point (the gamma ray burst) available so far and says “this is something that can be improved with further observations.”
The research is described in Science.
