Sudden death of very early galaxies

The first galaxies that formed in the universe might have matured sooner than expected. New results from an international team of astronomers, including researchers from Swinburne University, indicate that some of the most distance galaxies are much more ‘mature’ than expected.

When we look into the distant universe, we are also looking back in time. This is because  light takes a finite time to travel through the vast distances of the universe.  The light from galaxies that are, say, 10 billion light years away have taken – you guessed it! – ten billion years to reach the Earth. This means that the light left that distance galaxy when the universe was 10 billions years younger than it is today. From observations we know that young galaxies contain a lot of gas and hence are efficient at forming stars.  These star-forming galaxies are blue in colour, while galaxies that have exhausted their fuel supply and quenched star formation will look red in colour. So when we look back in time, we expect to find young and energetic blue galaxies that are actively forming stars.

The FourStar Galaxy Evolution (ZFOURGE) Survey team has been looking deep into the universe, searching for the universe’s earliest red quiesent galaxies at near-infrared wavelengths. The team have  discovered a new group of 15 massive, passively-evolving galaxies at a distance of 12 billion light years, when the universe was only 1.6 billion years old. This means that these galaxies have matured within the first 12% of the age of the universe. The results, which will continue to puzzle the astronomers who study how galaxies form and evolve, have been published in the Astrophysical Journal Letters.

Ultra Deep Survey (UDS) field image from the Hubble Space Telescope with 4 out of the 15 very old galaxies discovered by the ZFOURGE team. The ZFOURGE galaxies exhibit the typical red colors of mature galaxies. (Credit: Caroline Straatman)

According to the lead author, Leiden astronomer Caroline Straatman, the universe we see  around us today is full of similar massive, “red and dead” galaxies in which very few new stars are formed.  The early universe was different, with most galaxies actively forming stars and growing in mass by accreting gas.  Even the ZFOURGE team were surprised to find such massive, mature galaxies in the early universe. The galaxies must have formed fast, explosively forming stars and then rapidly quenching star formation.

This discovery was made with combination of data gathered over 40 nights from the 6.5-m Magellan Baade Telescope in Chile using the purpose-built FourStar camera, and data from legacy surveys including Chandra COSMOS and GOODS (Great Observatories Origins Deep Survey).

According to Karl Glazebrook, who leads the Swinburne team, cosmological model favoured 15 years did not even predict such mature galaxies in the early universe. In 2004 Glazebrook and co-workers discovered that such galaxies did exist 3 billion years after the Big Bang. With the advent of deep imaging near-infrared surveys in the late 1990’s and more recent improved technologies, astronomers have now been able to push back the discovery of such galaxies to just 1.6 billion years after the Big Bang, which is both surprising and truly exciting.

While it remains a mystery why different galaxies behave in different ways, ZFOURGE team leader Dr. Ivo Labbe from Leiden said that these kinds of results pose new questions, such as how these systems formed so quickly and had a massive boost of size within a short period of time. Much work is yet to be done in understanding the early universe!

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[Themiya Nanayakkara & Sarah Maddison]

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