An ancient galaxy with a frantic star formation

An article published in the journal “Nature” describes the mapping of a galaxy known as COSMOS-AzTEC-1 that showed peculiar characteristics. A team of astronomers used the ALMA radio telescope to study this starburst galaxy, a class in which there’s considerable star formation. COSMOS-AzTEC-1 is very far away so we see it as it was 12.4 million years ago and very massive and could be the progenitor of today’s large elliptical galaxies so this type of research could provide new information on their evolution.

The galaxy COSMOS-AzTEC-1 was already known thanks to previous observations that offered some information about its characteristics. The James Clerk Maxwell Telescope in Hawaii allowed to discover it while the Large Millimeter Telescope (LMT) in Mexico allowed to conduct a first study that led to the identification of huge amounts of carbon monoxide inside it with the very high rate of star formation.

The ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope, inaugurated in March 2013, made it possible to make a leap forward in some types of research. Its high resolution and sensitivity can provide much higher details than other radio telescopes with much better detection than it was possible only a few years ago.

To study the galaxy COSMOS-AzTEC-1 a team led by Ken-ichi Tadaki, a postdoctoral researcher at the Japan Society for the Promotion of Science and the National Astronomical Observatory of Japan, used the most extensive configuration of 16 kilometers, which allowed to obtain a map with the highest resolution in the details of the distribution and the movement of the gas inside it. This is the best result ever obtained for a galaxy at that distance.

The result of the mapping was surprising because generally in distant starburst galaxies star formation is active at their center, instead in COSMOS-AzTEC-1 there are two distinct large clouds at several thousand light years from the galactic center. A further anomaly is given by the fact that the clouds in that galaxy are unstable, which is the reason why the star formation reached such an intense pace, about a thousand times higher than that existing in the Milky Way. Even for a starburst galaxy, it’s a really fast star formation rate!

The top image (ALMA (ESO/NAOJ/NRAO), Tadaki et al.) shows the galaxy COSMOS-AzTEC-1 observed with the ALMA radio telescope. On the left side the molecular gas distribution is shown, on the right side the distribution of the dust particles is shown. The two large clouds far from the galactic center are indicated by arrows.

Generally, in the clouds there’s a balance between inward gravity and outward pressure. When gravity becomes dominant a cloud collapses and its gas forms stars at a quick rate. When massive stars end their lives, they explode into supernovae that eject gas, increasing outward pressure. This balance leads to a moderate star formation rate but in COSMOS-AzTEC-1 the outward pressure is much weaker than gravity and this causes a star formation rate that, according to estimates, will consume the gas in about 100 millions of years, about 10 times faster than in other comparable galaxies.

There’s no certain explanation for this anomalous star formation in COSMOS-AzTEC-1. A hypothesis is that it’s the result of a galactic merger, an event that generates instability and can compress the new galaxy’s gas clouds in an abnormal way causing an exceptional star formation. However, astronomers found no evidence of such an event, but the merger of two galaxies is only one hypothesis and it’s also possible that a galaxy absorbed some gas-rich dwarf galaxies. Ken-ichi Tadaki stated that he wants to observe other similar galaxies with the ALMA radio telescope to verify the possible relationship between stellar mergers and monster galaxies like this one.

The problem is interesting because it offers new information on the possible evolution of a massive galaxy when the universe was young. In recent years instruments such as the ALMA radio telescope are allowing a much more detailed investigation into that kind of processes and we can expect new discoveries to understand whether the galaxy COSMOS-AzTEC-1 is truly exceptional or if it’s only the first example discovered.

Artist's impression of COSMOS-AzTEC-1 (Image courtesy National Astronomical Observatory of Japan. All rights reserved)
Artist’s impression of COSMOS-AzTEC-1 (Image courtesy National Astronomical Observatory of Japan. All rights reserved)

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