The full COSMOS field (UltraVISTA)
The field around the COSMOS survey (ground-based image)
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Hubble's COSMOS survey solves "quenched" galaxy mystery
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. 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 [1],
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 [2]. 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.
Notes
[1] 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).
[2] 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.
[1] 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).
More information
Image credit: NASA, ESA, M. Carollo (ETH Zurich)Links
Contacts
Marcella CarolloETH Zurich
Zurich, Switzerland
Tel: +4144633 3725
Email: marcella@phys.ethz.ch
Alvio Renzini
INAF, Astronomical Observatory of Padova
Padova, Italy
Tel: 049 8293 503
Email: alvio.renzini@oapd.inaf.it
Peter Capak
California Institute of Technology
California, USA
Tel: +1-626-395-6422
Email: capak@astro.caltech.edu
Nicky Guttridge
ESA/Hubble
Garching, Germany
Tel: +49-89-3200-6855
Email: nguttrid@partner.eso.org
