New method for galaxy growth observed
by Jim Allen
for ASTRONOMY NOW
Posted: 14 October 2010
A team of European astronomers using the Very Large Telescope (VLT) have found evidence for a new mechanism for galaxy growth.
Observations of early galaxies have found them to be smaller than their modern counterparts, suggesting that during the evolution of the Universe galaxies have increased in size. One way for this to occur is for two smaller galaxies to collide and merge. However, this does not account for all of the increase in galaxy size. Using the European Southern Observatory (ESO) VLT, the team was able to investigate the make-up of three distant galaxies, with ages of around two billion years, and have found evidence for another mechanism of galaxy growth.
An artist’s impression of a young galaxy accreting material (ESO/L. Calçada).
To ensure that the galaxies were not formed by a previous merger the astronomers selected galaxies with very regular rotation patterns. Using SINFONI (Spectrograph for INtegral Field Observations in the Near Infrared) the team was able to measure the composition of different regions in the same galaxy at these very large distances.
The centres of galaxies are mainly composed of elements heavier than hydrogen and helium (what astronomers call high metallicity materials) whilst their outer edges consist of material made from lighter, lower metallicity matter. Measurements of the three distant galaxies using the VLT gave opposite results, with the galactic centres containing lower metallic matter than the outer edges, whilst also finding large star formation rates (~120 solar masses/year) occurring in the central regions.
These results point towards an extra method of galaxy growth called the ‘cold flow’ model. Here, gas and dust from outside the galaxy is sucked into the centre fuelling star formation. Because the gas and dust from outside the galaxy is from the early Universe, it will be made of low metallicity material.
This discovery has given vital clues about the evolution of the Universe. According to the team leader from Osservatorio Astrofisico di Arcetri, Dr. Giovanni Cresci, “this is the first evidence for massive infall of pristine gas in the early Universe and it finally solves the problem of providing to the galaxies fuel to form their stars in a continuous way”.
The team now hopes to observe galaxies at higher redshifts. “We hope to understand more about the formation and early evolution of galaxies using the chemical abundances in their gas as a probe, linking the properties of high redshift sources with what is observed in local galaxies,” Cresci tells Astronomy Now.
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