Some galaxies hit a point in their lives when their star formation is snuffed out, and they become "quenched". Quenched galaxies in the distant past appear to be much smaller than the quenched galaxies in the Universe today.
Sample of non-star-forming galaxies from the COSMOS survey
Image credit: NASA, ESA, M. Carollo (ETH Zurich)
This has always puzzled astronomers — how can these galaxies grow if they are no longer forming stars? A team of astronomers has now used a huge set of Hubble observations to give a surprisingly simple answer to this long-standing cosmic riddle.
Until now, these small, snuffed-out galaxies were thought to grow into the larger quenched galaxies we see nearby.
As these galaxies are no longer forming new stars, they were thought to grow by colliding and merging with other smaller quenched galaxies some five to ten times less massive. However, these mergers would require many such small galaxies floating around for the quenched population to snack on — which we do not see.
Until recently it had not been possible to explore a sufficient number of quenched galaxies, but now a team of astronomers has used observations from the Hubble COSMOS survey to identify and count these switched-off galaxies throughout the last eight billion years of cosmic history.
"The apparent puffing up of quenched galaxies has been one of the biggest puzzles about galaxy evolution for many years," says Marcella Carollo of ETH Zurich, Switzerland, lead author on a new paper exploring these galaxies. "No single collection of images has been large enough to enable us to study very large numbers of galaxies in exactly the same way — until Hubble's COSMOS," adds co-author Nick Scoville of Caltech, USA.
The team used the large set of COSMOS images , alongside additional observations from the Canada–France–Hawaii Telescope and the Subaru Telescope, both in Hawaii, USA, to peer back to when the Universe was less than half its present age. These observations mapped an area in the sky almost nine times that of the full Moon.
The quenched galaxies seen at these times are small and compact — and surprisingly, it seems they stay that way. Rather than puffing up and growing via mergers over time, these small galaxies mostly keep the size they had when their star formation switched off . So why do we see these galaxies apparently growing larger over time?
"We found that a large number of the bigger galaxies instead switch off at later times, joining their smaller quenched siblings and giving the mistaken impression of individual galaxy growth over time," says co-author Simon Lilly, also of ETH Zurich. "It's like saying that the increase in the average apartment size in a city is not due to the addition of new rooms to old buildings, but rather to the construction of new, larger apartments," adds co-author Alvio Renzini of INAF Padua Observatory, Italy.
This tells us a lot about how galaxies have evolved over the last eight billion years of the Universe's history. It was already known that actively star-forming galaxies were smaller in the early Universe, explaining why they were smaller when their star formation first switched off.
"COSMOS provided us with simply the best set of observations for this sort of work — it lets us study very large numbers of galaxies in exactly the same way, which hasn't been possible before," adds co-author Peter Capak, also of Caltech. "Our study offers a surprisingly simple and obvious explanation to this puzzle. Whenever we see simplicity in nature amidst apparent complexity, it's very satisfying," concludes Carollo.
 In making the COSMOS survey, Hubble photographed 575 slightly overlapping views of the Universe using the Advanced Camera for Surveys (ACS) aboard Hubble. It took nearly 1000 hours of observations and is the largest project ever conducted with Hubble. This survey has proved invaluable; it has helped to map dark matter in 3D (heic0701), to further understand the effects of gravitational lensing (heic0806), and to characterise the expansion of the Universe (heic1005).
 There is still the possibility of growth via mergers for a fraction of this quenched population, but not a majority, as previously thought.
Notes for editors
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The research is presented in a paper entitled “Newly-quenched galaxies as the cause for the apparent evolution in average size of the population”, for publication in The Astrophysical Journal.
 The international team of astronomers in this study consists of C. M. Carollo (Swiss Federal Institute of Technology [ETH Zurich], Switzerland), T. J. Bschorr (Swiss Federal Institute of Technology [ETH Zurich], Switzerland), A. Renzini (Padova Observatory, Italy), S. J. Lilly (Swiss Federal Institute of Technology [ETH Zurich], Switzerland), P. Capak (Spitzer Science Center, California Institute of Technology, USA), A. Cibinel (Swiss Federal Institute of Technology [ETH Zurich], Switzerland), O. Ilbert (Laboratoire d’Astrophysique de Marseille, France), M. Onodera (Swiss Federal Institute of Technology [ETH Zurich], Switzerland), N. Scoville (California Institute of Technology, USA), E. Cameron (Swiss Federal Institute of Technology [ETH Zurich], Switzerland), B. Mobasher (University of California, USA), D. Sanders (University of Hawaii, USA), Y. Taniguchi (Ehime University, Japan).
| ESA/Hubble Information Centre
NASA spacecraft investigate clues in radiation belts
28.03.2017 | NASA/Goddard Space Flight Center
Researchers create artificial materials atom-by-atom
28.03.2017 | Aalto University
The Institute of Semiconductor Technology and the Institute of Physical and Theoretical Chemistry, both members of the Laboratory for Emerging Nanometrology (LENA), at Technische Universität Braunschweig are partners in a new European research project entitled ChipScope, which aims to develop a completely new and extremely small optical microscope capable of observing the interior of living cells in real time. A consortium of 7 partners from 5 countries will tackle this issue with very ambitious objectives during a four-year research program.
To demonstrate the usefulness of this new scientific tool, at the end of the project the developed chip-sized microscope will be used to observe in real-time...
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
28.03.2017 | Life Sciences
28.03.2017 | Information Technology
28.03.2017 | Physics and Astronomy