“The first light of the universe might imply the existence of an important population of more exotic objects like faint quasars, X-ray binary stars, or perhaps even decaying/annihilating particles.” —Harvard-Smithsonian Center for Astrophysics
The image at the top of the page shows newly discovered quasar’s light has been traveling nearly 13 billion of the universe’s 13.7 billion years to reach us here in Earth. P352-15 is the first quasar with clear evidence of radio jets seen within the first billion years of the universe’s history. It’s highly unusual to find radio jet-emitting quasars such as this one from the period just after the universe’s lights came back on
Roughly 400,000 years after the Big Bang, the universe – bathing in the afterglow of radiation that we see today as the cosmic microwave background – began to enter the cosmic “dark ages,” so named because the luminous stars and galaxies we see today had yet to form. Most of the matter in the cosmos at this stage was dark matter with the scant remaining ordinary matter comprised largely of neutral hydrogen and helium..
Over the next few hundred million years, the universe entered a crucial turning point in its evolution, known as the Epoch of Reionization. During this period, the predominant dark matter began to collapse into halo-like structures through its own gravitational attraction. Ordinary matter was also pulled into these halos, eventually forming the first stars and galaxies, which, in turn, released large amounts of ultraviolet light. That light was energetic enough to strip the electrons out of the surrounding neutral matter, a process known as cosmic reionization.
A NASA/ESA Hubble Space Telescope image of the rapidly fading visible-light fireball from a gamma-ray burst (GRB) in a distant galaxy. A new study used the spectra of 140 GRB afterglows to estimate the amount of ionizing radiation from massive stars that escapes from galaxies to ionize the intergalactic medium, and finds the surprising result that it is very small. (Andrew Fruchter (STScI) and NASA/ESA)
Though the Epoch of Reionization took place deep in the universe’s past, it lies at the very frontier of our current cosmological observations. The sparsely distributed hot gas that exists in the space between galaxies, the intergalactic medium, is ionized. The question is, how? Astronomers know that once the early universe expanded and cooled enough, hydrogen (its main constituent) recombined into neutral atoms. Then, once newly formed massive stars began to shine in the so-called “era of reionization,” their extreme ultraviolet radiation presumably ionized the gas in processes that continue today
Ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom, causing the atom to become charged or ionized. Only the high frequency portion of the electromagnetic spectrum which includes X rays and gamma rays is ionizing
One of the key steps, however, is not well understood, namely the extent to which the stellar ionizing radiation escapes from the galaxies into the IGM. Only if the fraction escaping was high enough during the era of reionization could starlight have done the job, otherwise some other significant source of ionizing radiation is required. That might imply the existence of an important population of more exotic objects like faint quasars, X-ray binary stars, or perhaps even decaying/annihilating particles.
Direct studies of extreme ultraviolet light are difficult because the neutral gas absorbs it very strongly. Because the universe is expanding, the spectrum absorbed covers more and more of the optical range with distance until optical observations of cosmologically remote galaxies are essentially impossible.
CfA astronomer Edo Berger joined a large team of colleagues to estimate the amount of absorbing gas by looking at the spectra of gamma-ray burst (GRB) afterglows. GRBs are very bright bursts of radiation produced when the core of a massive star collapses. They are bright enough that when their radiation is absorbed in narrow spectral features by gas along the line-of sight, those features can be measured and used to calculate the amount of absorbing atomic hydrogen. That number can then be directly converted into an escape fraction for the ultraviolet light of the associated galaxy. Although a single observation of a GRB in one galaxy does not provide a robust measure, a sample of GRBs is thought to be able to provide a representative measure across all sightlines to massive stars.
The astronomers carefully measured the spectra of 140 GRB afterglows in galaxies ranging as far away as epochs slightly less than one billion years after the big bang. They find a remarkably small escape fraction – less than about 1% of the ionizing photons make it out into the intergalactic medium. The dramatic result finds that stars provide only a small contribution to the ionizing radiation budget in the universe from that early period until today, not even in galaxies actively making new stars.
The authors discuss possible reasons why GRBs might not provide an accurate measure of the absorption, although none is particularly convincing. The result needs confirmation and additional measurements, but suggests that a serious reconsideration of the ionizing budget of the intergalactic medium of the universe is needed.