An international team of scientists has discovered how newly formed dwarf galaxies caused a massive increase in the temperature of the universe some 13 billion years ago. The study offers new insight on what went on in the universe during the early part of its history.
The Universe After The Big Bang
During the early years following the Big Bang, the universe became so dense and hot that the matter surrounding it was ionized instead of remaining in a neutral state. It would stay heated for the next 380,000 years until the expansion of the universe allowed it to cool.
The matter in the universe took on a neutral form, allowing the first cosmic structures to take shape such as helium and hydrogen gas clouds. These gas clouds later grew in mass with the help of gravity and then they collapsed to form the first set of stars and galaxies in the universe.
However, the universe went through another massive transformation as it became heated again about a billion years after the Big Bang.
In an event referred to by astronomers as the "cosmic reionization," the hydrogen that was abundant in the universe became ionized once more, much like how it had immediately after the Big Bang. While the exact cause of the event is still debated, some scientists believe galaxies could have played a key role in the transformation.
In a study featured in the journal Nature, Trinh Thuan, an astronomer from the University of Virginia, led a team of researchers to find out the impact galaxies had on the heating of the universe.
Trinh and his colleagues made use of data collected from the Hubble Space Telescope's ultraviolet spectrometer to examine a nearby dwarf galaxy that was releasing large numbers of ionizing photons into the space between galaxies known as the intergalactic medium. It is believed that these types of photons are the likely cause of the reionization of the universe.
The researchers consider this nearby galaxy as a suitable example of the dwarf galaxies that are thought to be the cause of the early universe's reionization.
"The finding is significant because it gives us a good place to look for probing the reionization phenomenon, which took place early in the formation of the universe that became the universe we have today," Trinh said.
The normal matter found in the early universe was comprised mainly of gas. Clouds of gas helped form the first galaxies in existence by facilitating the birth of stars and star clusters.
The ultraviolet radiation that these stars produce contains much of the ionizing photons that are present in the universe. This is why many scientists suspect that galaxies likely triggered the cosmic reionization.
However, galaxies would have to release these ionizing photons into the intergalactic medium before they can cause reionization. Researchers have yet to find a galaxy capable of emitting enough ionizing radiation to trigger such an event.
To help solve this mystery, Trinh and his colleagues studied the nature of a special and rare class of cosmic structures known as "green pea" galaxies.
These galaxies appear as round and compact objects that turn green when exposed to light sensors. Scientists believe these green pea galaxies host powerful winds that are capable of ejecting ionizing photons.
By examining data from the Sloan Digital Sky Survey, the researchers were able to identify close to 5,000 different galaxies that are compact and capable of emitting high levels of UV radiation. They then narrowed down the list to five green pea galaxies that they will observe using the Hubble Space Telescope.
One of these green pea galaxies, designated as J0925+1403, matched the criteria that the researchers were looking for. Located around three billion light-years from the planet Earth, this particular green pea galaxy releases ionizing photons at an eight percent ejection rate.
The discovery of J0925+1403 suggests that this type of galaxy can be used to determine how the cosmic reionization occurred.
Trinh explained that as they conduct further observations of the J0925+1403 green pea galaxy using the Hubble Space Telescope, they expect to collect more information on how photons are released from this type of galaxy. Such observations are important in getting a better understanding of the early history of the universe.