Green Pea Galaxies And Cosmic Reionization

 According to Big Bang cosmology, the cosmic Dark Ages occurred early in the history of the Universe, that is thought to have been born approximately 13.8 billion years ago in the wild and explosive exponential inflation of Spacetime. During this very ancient era, the newborn Universe gradually cooled down from its original extremely hot and dense state, and this cooling-off period allowed electrons and protons to meet up and dance together to create hydrogen atoms. This heralded the beginning of the Dark Ages, an era enshrouded in impenetrable blackness and alluring mystery. Reionization refers to the process that caused the matter in the Universe to reionize after the Dark Ages had finally ended, when the first luminous objects formed during the Cosmic Dawn. In August 2019, an international team of cosmologists announced that they had been able to demonstrate that primordial galaxies triggered the era of reionization.

The cosmologists arrived at their conclusion by studying green pea galaxies. The Geneva, Switzerland-based astronomer, Dr. Anne Verhamme, led the team that made these important new observations. Dr. Verhamme was awarded the 2019 Marie Heim-Vogtlin prize by the Swiss National Science Foundation (SNSF).

Green pea galaxies are considered to be a type of Luminous Blue Compact Galaxy experiencing very high rates of star birth. Pea galaxies are so-named because of their relatively small size and greenish hue as seen in images obtained by the Sloan Digital Sky Survey (SDSS). The SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide angle optical telescope at Apache Point, New Mexico, in the U.S. The term redshift refers to the displacement of spectral lines toward longer wavelengths--the red end of the spectrum.

Pea galaxies were discovered by volunteer citizen scientists back in 2007. The volunteers were working within the forum section of the online astronomy project called Galaxy Zoo (GZ), part of the Zooniverse web portal.

Green Peas

Pea galaxies are oxygen-rich emission line compact galaxies, that sport a low-mass and have an upper size limit that renders them generally no larger than about 16,300 light-years across. These small galaxies usually dwell in environments that are less than two-thirds the density of normal, garden-variety galaxy environments. An average pea galaxy displays a redshift of z = 0.258, a mass of about 3,200 million times that of our Sun, a star formation rate of approximately 10 solar masses a year, and a low metallicity. Low metallicity means that the constituent stars within these small galaxies have not produced much in the way of atomic elements heavier than hydrogen. Stars are responsible for manufacturing all of the atomic elements heavier than hydrogen and helium in the Universe by way of the process of nuclear fusion. Atomic elements heavier than helium are called metals by astronomers.

These small galaxies are notable for their high star formation rates, rather than for having a brilliant active galactic nucleus (AGN). AGN are glaring accretion disks that surround supermassive black holes that are thought to exist in perhaps every large galaxy in the Cosmos--including our own large spiral Milky Way Galaxy. Pea galaxies also show a strong emission line for doubly ionized oxygen--which is a forbidden mechanism of the visible spectrum, and is only possible at very low density. When the entire SDSS catalogue was studied, only 40,222 of these small objects were detected. This suggests that green pea galaxies are rare in the Universe.

Pea galaxies are both the least massive and most active star-birthing galaxies in the Cosmos. This indicates that these small, dense galaxies would probably have been normal denizens of the ancient Universe. However, we do not see similarly active small galaxies in the modern Universe. Therefore, green pea galaxies may play an important role in telling astronomers something about how stars were born in the ancient Universe, as well as how galaxies evolved.

The green pea galaxies come from a time when the Cosmos was about three-quarters of its current age. This means that they provide valuable clues about how primordial galaxies formed, as well as the evolutionary track that galaxies eventually took in the early Universe. Some cosmologists think that pea galaxies are really old galaxies that formed most of their stellar mass several billion years ago. This is because old stars have been spectroscopically confirmed in one of a trio of galaxies studied by a team of researchers. The presence of Magnesium was detected in that one galaxy, indicating that it is very old.