Astronomers using the MUSE instrument on European Southern Observatory’s Very Large Telescope at the Paranal Observatory in Chile have been able to get the best three-dimensional view so far of the deep Universe.
The background image in this composite shows the Hubble image of the region known as the Hubble Deep Field South. New observations using VLT’s MUSE instrument have detected remote galaxies that are not visible to Hubble; two examples are highlighted in this composite view; these objects are completely invisible in the Hubble picture but show up strongly in the appropriate parts of the three-dimensional MUSE data. Image credit: ESO / MUSE Consortium / R. Bacon.
By taking very long exposure pictures of regions of the sky with the Hubble Space Telescope, astronomers have created so-called ‘deep fields’ that have revealed much about the early Universe.
The first deep field, the Hubble Deep Field North, was observed over ten consecutive days during Christmas 1995.
The resulting image consisted of 342 separate exposures, with a total exposure time of more than 100 hours, compared with typical Hubble exposures of a few hours.
The observed region of sky in Ursa Major was carefully selected to be as empty as possible so that Hubble would look far beyond the stars of our own Milky Way and out past nearby galaxies.
The results were astonishing – almost 3,000 galaxies were seen in the image.
In 1996 it was decided to observe a second deep field, the Hubble Deep Field South, to assess whether the Hubble Deep Field North was indeed a special area and thus not representative of the Universe as a whole. This time the field also contained a quasar, which was used as a cosmological lighthouse and provided valuable information about the matter between the quasar and the Earth.
These images gave astronomers a peephole to the ancient Universe for the first time, and caused a real revolution in modern astronomy.
But they did not hold all the answers — to find out more about the galaxies in the images, astronomers had to carefully look at each one with other instruments, a difficult and time-consuming job.
Now, for the first time, VLT’s MUSE instrument can do both jobs at once.
One of the first observations using MUSE after it was commissioned on the VLT in 2014 was a long hard look at the Hubble Deep Field South.
For every part of the view of the Hubble Deep Field South there is not just a pixel in an image, but also a spectrum revealing the intensity of the light’s different component colors at that point – about 90 000 spectra in total. These can reveal the distance, composition and internal motions of hundreds of distant galaxies – as well as catching a small number of very faint stars in our own Milky Way Galaxy.
The MUSE data revealed more than 20 very faint objects in this small patch of the sky that Hubble did not record at all.
By looking carefully at all the spectra in the MUSE observations of the Hubble Deep Field South, the astronomers measured the distances to 189 galaxies.
They ranged from some that were relatively close, right out to some that were seen when the Universe was less than 1 billion years old.
This is more than 10 times the number of measurements of distance than had existed before for this area of sky.
For the closer galaxies, MUSE can do far more and look at the different properties of different parts of the same galaxy. This reveals how the galaxy is rotating and how other properties vary from place to place. This is a powerful way of understanding how galaxies evolve through cosmic time.
“Now that we have demonstrated MUSE’s unique capabilities for exploring the deep Universe, we are going to look at other deep fields, such as the Hubble Ultra Deep field,” said team member Dr Roland Bacon from the Centre de Recherche Astrophysique de Lyon in France.
“We will be able to study thousands of galaxies and to discover new extremely faint and distant galaxies. These small infant galaxies, seen as they were more than 10 billion years in the past, gradually grew up to become galaxies like the Milky Way that we see today.”
