The galaxies we see today are thought to be formed from smaller “building blocks”, which have attracted each other with their gravity during the process of galaxy formation and consequently merged into larger entities. This theory has quite a solid ground based on practical evidence: the very distant high-redshift galaxies, which are observed by astronomers, typically contain less matter and have smaller dimensions. So how do these colossal objects gain their “weight”? Is this process steady and continuous, or do they feed on some larger “dishes”?

A team of scientists from Japan and South Korea explored the idea that galaxies are made from larger clumps of interstellar dust and gas. The best way to prove this theory was to look for any possibly remaining relics of such building blocks throughout the visible Universe. The researchers also assumed that the modern irregular galaxies could be the most similar to those ancient building blocks that have existed in the early cosmos.

The authors claim in their study (available on arXiv.org) that, even though the irregular galaxies comprise only a few percent of all galaxies, they are still important, especially if they are not related to any large-scale interactions that could distort their original ‘free-form’ structure. Those genuine irregular galaxies (as the authors call them) have no distinct regular structures like bulge, disc, bar, spiral arms or nucleus; so it can be suggested that the irregular galaxies are still in an early evolutionary phase. Furthermore, these objects are less massive, although they contain the sufficiently large amounts of interstellar gas – a perfect combination for any relic of building blocks of galaxies.

The scientists used the data from the two recent studies on the galaxy morphology based on the Sloan Digital Sky Survey (SDSS) to select a sample of 66 irregular galaxies. Then they applied an additional classification, according to which these objects were categorized into genuine irregular galaxies, or GIGs, and non-GIGs, which basically show any signs of ‘regular’ galaxy interactions.

In total, 33 genuine irregular galaxies were found, so this new sub-sample could be subjected to the further analysis of the observational properties. The results of this analysis indicated, that GIGs have smaller physical dimensions and absolute luminosities compared to those of normal galaxies. The absence of the galactic nucleus suggests that these objects do not contain supermassive black holes. Also, a relatively lower value of metallicity – an indicator of galaxy-comprising stars age – also adds to the other evidence proving younger stages of evolution, compared to normal galaxies. Thus, GIGs can be regarded as the relics of building blocks of galaxies, according to the authors.

The team also suggests that approximately 50 percent of all irregular galaxies in the previous studies on galaxy morphology are non-GIGs, so they recommend to omit this selection bias from the future studies.

By Alius Noreika, Source: www.technology.org