The structure of galaxies has been a basis of their classification for a long time. In general, there are two main groups: elliptical and spiral galaxies, which are further divided into smaller sub-types according to their shape, size, brightness and some other astrophysical parameters, such as asymmetry or central bulge size. However, it was not like that all the time, as different galaxies acquired their shapes not in a single point in time.
It is an interesting fact to note that astronomers have identified more spiral galaxies than ellipticals, but that is simply because they are easier to spot. More abundant spiral galaxies are brighter than their elliptical counterparts – this is because spirals spawn more young stars, while ellipticals contain less gas and dust, building material for new stars. For the same reason elliptical galaxies typically contain older stars which produce more red light than younger stars.
The authors of the study recently published on arXiv.org note that it is still not clear when and how galaxy morphology features such as disk and bulge seen in the present-day universe emerged. In this article Japanese researchers T. M. Takeuchi, K. Ohta, S. Yuma and K. Yabe describe their research aimed to determine the time when the first round disk galaxies formed.
If you analyze specific regions of the universe, the proportion of differently shaped galaxies does not remain the same. Astronomers say that when specific regions of the sky are studied in depth, more elliptical galaxies can be found in various regions of the universe. But such laws of distribution are not constant, the authors of the current study note. And this is especially true when considering the shape of the galaxy disk.
It is well known that the farther the galaxy is located, the younger their image appears to us as the light takes billions of years to reach us while travelling across the vast distances of space. According to the researchers, most of the galaxies located at redshift Z < 1 can be classified using the Hubble’s classification system, or so-call Hubble’s tuning fork (see the image below). Meanwhile, in the universe at Z > 3, the structure of galaxies becomes more or less irregular, as many of them show the presence of clumps or other irregularities. Based on this logic the emergence of galaxies with morphology matching Hubble’s classification is considered to fall within Z ~ 1-3. Speaking in terms of the universe’s age, this interval would approximately match 2.2-5.9 billion years since the Big Bang. “Formation of disk structure would also be expected at this epoch”, the researchers suggest.
“Emergence of the round disk [galaxy] population should reflect the evolution process of the disk structure. Hence, revealing evolution of the intrinsic shape of the galaxies is expected to give us an insight to physical process of the galaxy evolution”, the team explains. In order to determine the time when the first round disk galaxies appeared, they studied the intrinsic shape of main sequence galaxies in the redshift range from 2.5 to 0.5 based on their surface brightness distribution.
In this study, galaxies were classified as having a round disk when their axial ratio was no less than 0.9 (with 1.0 corresponding to a perfect circle) and the redshift range was also constrained based on this parameter. Sample galaxies for the study were collected from the Great Observatories Origins Deep Survey-South (GOODS-S).
“The emergence of the round disk can be regarded to be Z ~ 0.9, though the precise redshift determination is difficult due to the gradual evolution and the small sample size”, the researchers commented the results of the study. Based on this value of redshift, the first round disk galaxies appeared approximately 7.5 billion years ago, when the universe was more than twice younger than it is today.
The team also notes that the bar-like structure observed at the high redshifts is not likely to be a direct progenitor of bar structures in the barred galaxies of the present day: there are fewer large barred galaxies as redhift increases, with their fraction among disk galaxies only 10-20% at Z = 0.8 – 1.0. Also, the disk-like structure can be observed among star forming galaxies at Z > 1, but their intrinsic shape is more bar-like or oval, possibly because of interactions between galaxies or their mergers.
The authors suggest that the formation of round disk could be related to the growth of a central bulge of the galaxy and/or its super-massive black hole. It is likely that the bar-like structure gradually evolves into a round disk with a timescale of approximately 2 billion years, where the super-massive black hole plays an important role for dissolving the bar structure. Alternatively, a gradual cease of galaxy interaction may also make a round disk.
The scientists note that further studies are needed to investigate relations between the disk shape and the growth of a galaxy bulge as well as galaxy interaction, as such studies could provide new insights into the evolution of the galaxies.
Written by Alius Noreika