Astronomers at Swinburne University of Technology have discovered a rare galaxy with an atomic hydrogen gas reservoir almost eight billion times the mass of our Sun, but intriguingly, the galaxy is forming very few stars.
In most of the galaxies in the nearby Universe, the rate at which stars are born is strongly dependent on the availability of gas: the higher the gas content, the more stars are formed.
Using large galaxy surveys carried out in recent years by the Arecibo telescope in Puerto Rico, the Sloan Digital Sky Survey, and the GALEX space telescope, researchers were able to study the relationship between gas content and star formation in a sample of 800 galaxies.
They discovered galaxy GASS 3505 had a huge gas reservoir, yet it had a very low level of star formation. This makes GASS 3505 an extremely rare galaxy. The rate of star formation is so low that it would take more than the current lifetime of the Universe for the entire gas reservoir to be converted into stars.
“In order to understand why this galaxy is an oddball, we needed an instrument with high resolution to image the distribution and motion of the gas – like zooming in with a camera,” says lead researcher Dr Katinka Gereb from Swinburne’s Centre for Astrophysics and Supercomputing.
Observations with the Very Large Array show that the gas in GASS 3505 is distributed in a giant ring 300,000 lights year in diameter, rotating around the galaxy.
The researchers also carried out observations of the galaxy’s stars at high sensitivity, which revealed the presence of a faint, young population of stars spiralling around the galaxy, embedded in the gas.
“We know that in other galaxies like our Milky Way, regions of dense gas are the birthplaces of stars. Even though GASS 3505 has a lot of gas, we find that its gas density is too low to maintain a ‘healthy’ level of star formation,“ says study co-author Dr Barbara Catinella from the International Centre for Radio Astronomy Research (ICRAR) at the University of Western Australia.
Where did the gas come from?
Astronomers believe that galaxy-to-galaxy interaction is one of the main ways galaxies build up gas in the nearby Universe.
The observations of the stars in GASS 3505 reveal a long stellar stream, extending far beyond the central body of the galaxy. This stream is clear evidence that GASS 3505 swallowed a smaller galaxy in the past.
The researchers were interested to know whether the large gas reservoir of GASS 3505 could have once belonged to the smaller galaxy that was absorbed.
They carried out computer simulations of galaxy-galaxy mergers based on the laws of gravity and hydrodynamics to examine this possibility.
“Our results suggest that such a merger is a possible way to create a GASS 3505-type system,” Dr Gereb says. “However, small discrepancies in the observed and modelled properties could suggest that some extra source of gas is needed for GASS 3505 to become so gas-rich.”
Importance of radio astronomy
Thanks to large collaborative efforts, many radio telescopes are currently being built and put into operation. This new generation of telescopes will survey the atomic hydrogen content of hundreds of thousands of galaxies at high resolution, and will provide unparalleled samples to study the accretion history of galaxies from the nearby to the distant Universe.
The results of this work have been published by the Oxford University Press in the Monthly Notices of the Royal Astronomical Society.