Proto-cluster candidates
The Planck all-sky map at submillimetre wavelengths (545 GHz). The
band running through the middle corresponds to dust in our Milky Way
galaxy. The black dots indicate the location of the proto-cluster
candidates identified by Planck and subsequently observed by Herschel.
The inset images showcase some of the observations made by Herschel’s
SPIRE instrument; the contours represent the density of galaxies.Copyright: ESA and the Planck Collaboration/ H. Dole, D. Guéry & G. Hurier, IAS/University Paris-Sud/CNRS/CNES. Hi-res Image
The history of the Universe
Copyright: ESA
By combining observations of the distant Universe made with ESA’s
Herschel and Planck space observatories, cosmologists have discovered
what could be the precursors of the vast clusters of galaxies that we
see today.
Galaxies like our Milky Way with its 100 billion stars are usually not
found in isolation. In the Universe today, 13.8 billion years after the
Big Bang, many are in dense clusters of tens, hundreds or even thousands
of galaxies.
However, these clusters have not always existed, and a key question in
modern cosmology is how such massive structures assembled in the early
Universe.
Pinpointing when and how they formed should provide insight into the
process of galaxy cluster evolution, including the role played by dark
matter in shaping these cosmic metropolises.
Now, using the combined strengths of Herschel and Planck, astronomers
have found objects in the distant Universe, seen at a time when it was
only three billion years old, which could be precursors of the clusters
seen around us today.
Planck’s main goal was to provide the most precise map of the relic
radiation of the Big Bang, the cosmic microwave background. To do so, it
surveyed the entire sky in nine different wavelengths from the
far-infrared to radio, in order to eliminate foreground emission from
our galaxy and others in the Universe.
But those foreground sources can be important in other fields of
astronomy, and it was in Planck’s short wavelength data that scientists
were able to identify 234 bright sources with characteristics that
suggested they were located in the distant, early Universe.
Herschel then observed these objects across the far-infrared to
submillimetre wavelength range, but with much higher sensitivity and
angular resolution.
Herschel revealed that the vast majority of the Planck-detected sources
are consistent with dense concentrations of galaxies in the early
Universe, vigorously forming new stars.
Each of these young galaxies is seen to be converting gas and dust into
stars at a rate of a few hundred to 1500 times the mass of our Sun per
year. By comparison, our own Milky Way galaxy today is producing stars
at an average rate of just one solar mass per year.
While the astronomers have not yet conclusively established the ages and
luminosities of many of these newly discovered distant galaxy
concentrations, they are the best candidates yet found for
‘proto-clusters’ – precursors of the large, mature galaxy clusters we
see in the Universe today.
“Hints of these kinds of objects had been found earlier in data from
Herschel and other telescopes, but the all-sky capability of Planck
revealed many more candidates for us to study,” says Hervé Dole of the
Institut d’Astrophysique Spatiale, Orsay, lead scientist of the analysis
published today in Astronomy & Astrophysics.
“We still have a lot to learn about this new population, requiring
further follow-up studies with other observatories. But we believe that
they are a missing piece of cosmological structure formation.”
“We are now preparing an extended catalogue of possible proto-clusters
detected by Planck, which should help us identify even more of these
objects,” adds Ludovic Montier, a CNRS researcher at the Institut de
Recherche en Astrophysique et Planétologie, Toulouse, who is the lead
scientist of the Planck catalogue of high-redshift source candidates,
which is about to be delivered to the community.
“This exciting result was possible thanks to the synergy between
Herschel and Planck: rare objects could be identified from the Planck
data covering the entire sky, and then Herschel was able to scrutinise
them in finer detail,” says ESA’s Herschel Project Scientist, Göran
Pilbratt.
“Both space observatories completed their science observations in 2013,
but their rich datasets will be exploited for plentiful new insights
about the cosmos for years to come.”
Note for Editors
“High-redshift infrared galaxy overdensity candidates and lensed sources discovered by Planck and confirmed by Herschel-SPIRE,” is authored by the Planck Collaboration.
Planck detected the sky at nine frequencies, from 30 GHz to 857 GHz. The
Planck frequencies used to detect the candidate proto-clusters in this
study were 857 GHz, 545 GHz and 353 GHz. The follow-up observations made
by Herschel’s SPIRE instrument were at 250, 350 and 500 microns. The
SPIRE 350 micron and 500 micron bands overlap with Planck’s High
Frequency Instrument (HFI) at 857 GHz and 545 GHz.
The Planck Scientific Collaboration consists of all the scientists who
have contributed to the development of the mission, and who participate
in the scientific exploitation of the data during the proprietary
period. These scientists are members of one or more of four consortia:
the LFI Consortium, the HFI Consortium, the DK-Planck Consortium and
ESA’s Planck Science Office. The two European-led Planck Data Processing
Centres are located in Paris, France and Trieste, Italy. The LFI
consortium is led by N. Mandolesi, ASI, Italy (deputy PI: M. Bersanelli,
Universita’ degli Studi di Milano, Italy), and was responsible for the
development and operation of LFI. The HFI consortium is led by J.L.
Puget, Institut d’Astrophysique Spatiale in Orsay, France (deputy PI: F.
Bouchet, Institut d’Astrophysique de Paris, France), and was
responsible for the development and operation of HFI.
For more information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799; +34 91 8131 199
Mob: +31 61 594 3954
Email: Markus.Bauer@esa.int
Hervé Dole
Institut d’Astrophysique Spatiale (CNRS & Univ. Paris-Sud) and Institut Universitaire de France Orsay, France
Tel: +33 1 69 85 85 72
Email: Herve.Dole@ias.u-psud.fr
Ludovic Montier
Institut de Recherche en Astrophysique et Planétologie (CNRS & Univ. Paul Sabatier Toulouse III), Toulouse, France
Tel: +33 5 61 55 65 51
Email: Ludovic.Montier@irap.omp.eu
Jan Tauber
ESA Planck Project Scientist
Tel: +31 71 565 5342
Email: Jan.Tauber@esa.int
Göran Pilbratt
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Email: gpilbratt@cosmos.esa.int
