How does galaxy morphology change as we look further back in time, to when the Universe was very young?
"This is a key question: when and over what timescale did the Hubble Sequence form?" says BoMee Lee of the
University of Massachusetts, USA, lead author of a new paper.
"To do this you need to peer at distant galaxies and compare them to their closer relatives, to see if
they too can be described in the same way."
Click on image to enlarge
This image shows "slices" of the universe at different times throughout its history (present day, and at
4 billion and 11 billion years ago). Each slice goes further back in time, showing how galaxies of each type appear.
The shape is that of the Hubble tuning-fork diagram, which describes and separates galaxies according to their
morphology into spiral (S), elliptical (E), and lenticular (S0) galaxies. On the left of this diagram are the
ellipticals, with lenticulars in the middle, and the spirals branching out on the right side. The spirals on the
bottom branch have bars cutting through their centers.
Illustration Credit: NASA, ESA, and M. Kornmesser (ESO)
The astronomers used Hubble to look 11 billion years back in time to when the Universe was very young,
exploring the anatomy of distant galaxies.
While it was known that the Hubble Sequence holds true as far back as around 8 billion years ago, these new
observations push a further 2.5 billion years back in cosmic time, covering a huge 80% of the past history of
"This is the only comprehensive study to date of the visual appearance of the large, massive galaxies that
existed so far back in time," says co-author Arjen van der Wel of the
Max Planck Institute for Astronomy in Heidelberg, Germany.
"The galaxies look remarkably mature, which is not predicted by galaxy formation models to
be the case that early on in the history of the Universe."
The galaxies at these earlier times appear to be split between blue star-forming galaxies with a complex structure --
including discs, bulges, and messy clumps -- and massive red galaxies that are no longer forming stars, as seen in
the nearby Universe.
Galaxies as massive as the Milky Way or more are rather rare in the young Universe. This scarcity has prevented
previous studies from being able to gather a large enough sample of mature galaxies to properly describe their
What was needed was a systematic set of observations such as those from
Hubble's CANDELS survey, which was large
enough to allow the astronomers to analyse a larger number of these galaxies consistently, and in detail.
With Hubble's Wide Field Camera 3
(WFC3), the astronomers were able to observe in the infrared part of the
spectrum to see how the galaxies appeared in their visible rest-frame, which is easier to compare with
galaxies in our neighbourhood.
"The huge CANDELS dataset was a great resource for us to use in order to consistently study ancient galaxies
in the early Universe," concludes Lee. "And the resolution and sensitivity of Hubble's WFC3 is second to none
in the infrared wavelengths needed to carry out this study. The Hubble Sequence underpins a lot of what we
know about how galaxies form and evolve -- finding it to be in place this far back is a significant discovery."