Why galaxies seemingly grow in old age

(Nanowerk News) Eventually, galaxies grow old and stop forming new stars. Scientists call such galaxies “quenched”. The remarkable thing is that these quenched galaxies were three times smaller around ten million years ago, when the universe was about a quarter of the age it is now. Why this is has long puzzled astronomers as they know that quenched galaxies do not grow. The popular hypothesis until now attributed the increase in the average size of quenched galaxies to small galaxies merging with other ones once they were quenched, thereby forming larger galaxies.
A team of researchers headed by Marcella Carollo, a professor of astrophysics at ETH Zurich, has now disproved this hypothesis in an extensive study ("Newly-quenched galaxies as the cause for the apparent evolution in average size of the population"). Although it is indisputable that galaxy fusions take place in space, by evaluating a vast amount of data from the Hubble Space Telescope the researchers from ETH Zurich were able to demonstrate that these mergers only made a minor contribution to the increase in the average size of quenched galaxies.
An example of a quenched galaxy
An example of a quenched galaxy (identified by a crosshair at the centre of the photo), observed during the Cosmos project. (Photo: NASA, ESA, Marcella Carollo / ETH Zurich)
Growth before they switch off
According to the study, the main reason for this growth is far simpler: galaxies remain the same size they were when they switched off. The later they switch off, the more time they have had to form new stars and grow. The further we go from the Big Bang in the course of the Universe’s history and the closer we get to the present day, the more galaxies have become part of the zoo of quenched galaxies and the larger the new arrivals are. Consequently, the average size of quenched galaxies has increased in the course of the Universe’s history.
“It’s like with flats in a city,” explains, Carollo. “If you say that their average size has increased, it usually isn’t because old ones have been enlarged, but rather because new ones have been built that are bigger than the existing ones.”
Largest coherent picture
Glimpsing into the past is not hard for astronomers: because light from distant galaxies travels for a long time – sometimes billions of years – before being picked up by a telescope, the observations made today actually correspond to the state of the galaxies in the past. The deeper astronomers look into space, the further into the past they gaze.
For their study, the scientists from ETH Zurich used the dataset from the research project Cosmos, which was obtained with the Hubble Space Telescope. This data provides the largest coherent picture of space produced with Hubble, covering a segment of two square degrees, which is equivalent to ten times the view of the full moon from Earth and the volume behind.
Studied with a single dataset
Although comparisons of galaxy sizes from different epochs are nothing new, they involved combining different datasets, which was not always easy. In their study, however, Carollo, her doctoral student Thomas Bschorr and their colleagues have now surveyed and compared over 10,000 quenched galaxies from a single dataset for the first time. “Our approach has the advantage of enabling us to measure galaxies from all epochs with one and the same benchmark,” says Bschorr.
Carollo is delighted that such a simple answer to the riddle surrounding the size of quenched galaxies was found: “Whenever we see simplicity in nature amidst apparent complexity, it’s very satisfying.”
Source: By Fabio Bergamin, ETH Zurich
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