The present day Universe
The Universe 11 billion years ago
The Hubble Tuning Fork - Classification of Galaxies
Astronomers see true shapes of galaxies 11 billion years back in time
Astronomers have used observations from Hubble’s CANDELS survey to
explore the sizes, shapes, and colours of distant galaxies over the last
80% of the Universe’s history. In the Universe today galaxies come in a
variety of different forms, and are classified via a system known as
the Hubble Sequence — and it turns out that this sequence was already in
place as early as 11 billion years ago.
The Hubble Sequence classifies galaxies according to their morphology and star-forming activity, organising them into a cosmic zoo of spiral, elliptical, and irregular shapes with whirling arms, fuzzy haloes and bright central bulges. Two main types of galaxy are identified in this sequence: elliptical and spiral, with a third type, lenticular, settling somewhere between the two.
The Hubble Sequence classifies galaxies according to their morphology and star-forming activity, organising them into a cosmic zoo of spiral, elliptical, and irregular shapes with whirling arms, fuzzy haloes and bright central bulges. Two main types of galaxy are identified in this sequence: elliptical and spiral, with a third type, lenticular, settling somewhere between the two.
This accurately describes what we see in the region of space around
us, but 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 exploring the sequence. "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."
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 [1],
these new observations push a further 2.5 billion years back in cosmic
time, covering a huge 80% of the past history of the Universe. Previous
studies had also reached into this epoch of the cosmos to study
lower-mass galaxies, but none had conclusively also looked at large,
mature galaxies like the Milky Way. The new CANDELS observations confirm
that all galaxies this far back — big and small alike — fit into the
different classifications of the sequence.
"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 [2].
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 characteristics.
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 [3].
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 [4], 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."
Notes
[1] Previous studies have looked at the proportions of the different galaxy types back in time (heic1002).
The mix of spiral, elliptical, lenticular and peculiar galaxies is
different from today, with a great many more peculiars in the distant
Universe than we see nearby.
[2] In a related recent paper,
Alice Mortlock and collaborators took a different but complementary
approach by classifying these distant galaxies by visual inspection.
They found that the types of galaxies we see in the Hubble Sequence are
well defined in terms of colour, structure, and star formation rates at
very large distances from us, but that their morphology is still
developing. While the morphology of a galaxy may be the final property
to settle, the fundamentals of the Hubble Sequence are set much earlier
on.
[3] CANDELS, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey,
is the largest project in the history of Hubble, with 902 assigned
orbits of observing time. It is being carried out with two cameras on
board Hubble – WFC3 and ACS – and aims to explore galactic evolution in
the early Universe, and the very first seeds of cosmic structure at less
than one billion years after the Big Bang.
[4] Previous studies of this period of cosmic history
were inconclusive as they were limited to visible light, showing only
the redshifted ultraviolet emission of the galaxies, which highlights
star formation. As this star formation dominated the observations, the
galaxies appeared to be clumpy and messy, with no resemblance to the
galaxy shapes we see around us today. By pushing into the infrared part
of the spectrum the astronomers could observe how these distant galaxies
appear in their visible rest frame (which is now redshifted).
Notes for editors
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
These results are described in a paper entitled "CANDELS: The
correlation between galaxy morphology and star formation activity as
Z~2", to appear in The Astrophysical Journal.
[1] The international team of astronomers in this
study consists of B. Lee (University of Massachusetts, USA), M.
Giavalisco (University of Massachusetts, USA), C. C. Williams
(University of Massachusetts, USA), Y. Guo (University of California,
USA), J. Lotz (Space Telescope Science Institute, Baltimore, USA), A.
van der Wel (Max Planck Institute for Astronomy, Heidelberg, Germany),
H. C. Ferguson (Space Telescope Science Institute, Baltimore, USA), S. M
Faber (University of California, USA), A. Koekemoer (Space Telescope
Science Institute, Baltimore, USA), N. Grogin (Space Telescope Science
Institute, Baltimore, USA), D. Kocevski (University of Kentucky, USA),
C. J. Conselice (University of Nottingham, UK), S. Wuyts (Max Planck
Institute for Extraterrestrial Physics, Garching, Germany), A. Dekel
(The Hebrew University, Israel), J. Kartaltepe (NOAO-Tuscon, Arizona,
USA), E. F. Bell (University of Michigan, USA).
More information
Image credit: NASA, ESALinks
Contacts
BoMee LeeUniversity of Massachusetts
Massachusetts, USA
Tel: +1-413-545-0731
Email: bomee@astro.umass.edu
Arjen van der Wel
Max Planck Institute for Astronomy
Heidelberg, Germany
Tel: +49-6221-528-461
Email: vdwel@mpia.de
Mauro Giavalisco
University of Massachusetts
Massachusetts, USA
Tel: +1-413-545-4767
Email: mauro@astro.umass.edu
Nicky Guttridge
Hubble/ESA
Garching, Germany
Tel: +49-89-3200-6855
Email: nguttrid@partner.eso.org
