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Astronomy, Astrophysics, Space, Stars

Granny galaxies discovered in the early universe

A Hubble Space Telescope color composite image, with two of the 15 mature galaxies. They exhibit the typical red colors of mature galaxies. Most of the other galaxies in the image are much closer. Credit: Caroline Straatman

A Hubble Space Telescope color composite image, with two of the 15 mature galaxies. They exhibit the typical red colors of mature galaxies. Most of the other galaxies in the image are much closer.
Credit: Caroline Straatman

An international team of researchers has discovered the most distant examples of galaxies in the early universe that were already mature and massive. The mature galaxies were found at a record-breaking distance of 12 billion light years, seen when the universe was just 1.6 billion years old. Their existence at such an early time raises new questions about what forced them to grow up so quickly.

 

“These distant and early massive galaxies are one of the Holy Grails of astronomy,” Director of the Centre for Astrophysics and Supercomputing at Swinburne University of Technology, Professor Karl Glazebrook, who was involved in the discovery, said.

“Fifteen years ago they were predicted not to even exist within the cosmological model favoured at the time. In 2004 I wrote a paper on the discovery of such galaxies existing only three billion years after the Big Bang. Now, with improved technology we are pushing back to only 1.6 billion years, which is truly exciting.”

Astronomers used deep images at near-infrared wavelengths to search for galaxies in the early Universe with red colours. These red colours indicate the presence of old stars and a lack of active star formation. Surprisingly, they located 15 galaxies at an average distance of 12 billion light years — only 1.6 billion years after the Big Bang.

The galaxies are barely detectable at visual wavelengths and are easily overlooked. But in the new near-infrared light images they are easily measured, from which it can be inferred that they already contained as many as 100 billion stars on average per galaxy.

The mature galaxies have masses similar to that of the Milky Way, but were already retired from star-formation when the universe was only 12 per cent of its current age.

“While the Milky Way still forms new stars at a slow rate today, the galaxies we discovered must have formed very rapidly in a relatively ‘short’ time — roughly one billion years — with explosive rates of star-formation. These must have been several hundred times higher than in the Milky Way today,” Macquarie University’s Dr Lee Spitler said.

“This is the best evidence to date that these galaxies grew up in a hurry. People have reported ‘old’ galaxies before, but it was never clear until our data that they were actually ‘old’. The excellent imaging products from the Magellan telescope allowed us to prove they are indeed ‘old’.”

The finding raises new questions about how these galaxies formed so rapidly and why they stopped forming stars so early.

The galaxies were discovered after 40 nights of observing with the FourStar camera on the Magellan Baade Telescope in Chile and combined with data from Hubble’s Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and the Great Observatories Origins Deep Survey. Using special filters to produce images that are sensitive to narrow slices of the near-infrared spectrum, the team was able to measure accurate distances to thousands of distant galaxies at a time, providing a 3-D map of the early universe.

Reference: Caroline M. S. Straatman, Ivo Labbé, Lee R. Spitler, Rebecca Allen, Bruno Altieri, Gabriel B. Brammer, Mark Dickinson, Pieter van Dokkum, Hanae Inami, Karl Glazebrook, Glenn G. Kacprzak, Lalit Kawinwanichakij, Daniel D. Kelson, Patrick J. McCarthy, Nicola Mehrtens, Andy Monson, David Murphy, Casey Papovich, S. Eric Persson, Ryan Quadri, Glen Rees, Adam Tomczak, Kim-Vy H. Tran, Vithal Tilvi. A Substantial Population of Massive Quiescent Galaxies at z ~ 4 from ZFOURGE. The Astrophysical Journal, 2014; 783 (1): L14 DOI: 10.1088/2041-8205/783/1/L14

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