Yet another enigma has been discovered about the early Universe: galaxies that seem to come out of nowhere. Most of the galaxies that have been observed from the early days of the universe were young and actively forming stars. Now, an international team of astronomers have discovered galaxies that were already mature and massive in the early days. The finding raises new questions about how these galaxies formed so rapidly and why they stopped forming stars so early.
Today the universe is filled with galaxies that have largely stopped forming stars, a sign of galactic maturity. But in the distant past, galaxies were still actively growing by consuming gas and turning it into stars. This means that mature galaxies should have been almost non-existent when the universe was still young.
Together with lead author Caroline Straatman and principal investigator Ivo Labbe, both of Leiden University, the astronomers used deep images at near-infrared wavelengths to search for galaxies in the early universe with red colors. The characteristic red colors indicate the presence of old stars and a lack of active star formation. 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, which still forms new stars at a slow rate. The newly discovered galaxies must have formed very rapidly in roughly 1 billion years, with explosive rates of star-formation. The rate of star formation must have been several hundred times larger than observed in the Milky Way today.
The galaxies were discovered after 40 nights of observing with the FourStar camera on the Magellan Baade Telescope at Carnegie's Las Campanas Observatory 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.
The image at the top of the page shows the Hubble Ultra Deep Field (HUDF). The million-second-long exposure reveals the first galaxies to emerge from the so-called "dark ages", the time shortly after the big bang when the first stars reheated the cold, dark Universe. The new image offers insights into what types of objects reheated the Universe long ago.
This historic new view is actually two separate images taken by Hubble's Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-object Spectrometer (NICMOS). Both images reveal some galaxies that are too faint to be seen by ground-based telescopes, or even in Hubble's previous faraway looks, called the Hubble Deep Fields (HDFs), taken in 1995 and 1998.
"Hubble takes us to within a stone's throw of the big bang itself," says Massimo Stiavelli of the Space Telescope Science Institute in Baltimore, USA, and the HUDF project lead. The combination of ACS and NICMOS images is used to search for galaxies that existed between 400 and 800 million years (corresponding to a redshift range of 7 to 12) after the big bang. A key question for HUDF astronomers is whether the Universe appears to be the same at this very early time as it did when the cosmos was between 1 and 2 billion years old.
This finding was published by The Astrophysical Journal Letters.