A team of scientists led by the Centre for Astrobiology have discovered that the bulges we see in disc galaxies formed in two waves, and that stars formed differently during these two waves.
For the study, the researchers used the Gran Telescopio Canarias (GTC) to study a sample of both disc and spheroidal galaxies in a deep sky zone in the constellation of the Great Bear. They also used imaged data from the Hubble Space Telescope and spectroscopic data from the Survey for High-z Absorption Red and Dead Sources (SHARDS) project.
In analyzing the data, the researchers found that the bulges in disc galaxies formed in two waves. A third of the bulges in these galaxies were formed at redshift 6.2, indicative of the Universe at 5% of its present age, or around 900 million years old. These structures, say the researchers, are ‘relics of the first structure formed in the Universe’.
Meanwhile, around two-thirds of the bulges observed had a mean value of redshift of approximately 1.3, meaning they were formed when the Universe was around 35% of its current age, or 4,000 million years old. The study further shows that the spheroidal galaxies in the sample had a mean redshift of 1.1, meaning they formed a similar time to the second wave disc galaxies.
The researchers also found that the central bulges of the first wave of disc galaxies are more compact and dense than those of the second. They also found that they differ in terms of star formation rates. Whereas the stars in bulges of the first wave seem to have formed quickly, on timescales of 200 million years, those in the second bulge required five times longer to form.
"This study has allowed us to explore the morphological evolution and the history of the assembly of the structural components of the galaxies, analogous to archaeological studies, analysing the information encoded in the millions of stars of each galaxy.” says Paola Dimauro, a co-author of the article, “The interesting point was to find that not all the structures were formed at the same time, or in the same way.”