Tuesday, March 31, 2015

Enigmatic High-redshift Galaxies Discovered

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. Credit: ESA and the Planck Collaboration/ H. Dole, D. Guéry & G. Hurier, IAS/University Paris-Sud/CNRS/CNES

Many new and enigmatic high redshift galaxies that are intensively forming stars have been discovered using ESA’s Planck and Herschel satellites. These galaxies occur in clumps – and could be the long-sought formation phase of galaxy clusters. Some appear very bright, and have been found to be gravitationally lensed galaxies. These rapidly star-forming galaxies could help solve a central problem in cosmology: how did the large scale structure of galaxies form? “We were immediately shocked by the large fluxes or angular concentrations of these galaxies. Finding so many intensively star forming, dusty galaxies in such small groups was a huge surprise. We think this is a missing piece of cosmological structure formation: intensely star-forming groups of galaxies at high redshift, which are the precursors of today’s largest galaxy clusters” says Pr. Hervé Dole (IAS, Orsay) who leads the analysis.

From the standpoint of galaxy evolution, studying the intense star-formation epoch in massive dark-matter halos will provide a wealth of information on the kinematics and evolutionary history of galaxies in massive galaxy clusters. For cosmology, galaxy clusters tell us about the baryon content of the Universe, the role of dark matter in the assembly of large scale structure, and provides insights into possible deviations from our simple, Gaussian, model of the early universe. This means that the search for distant galaxy clusters and/or galaxies amplified by gravitational lensing is a very hot topic. ESA’s Planck satellite can find these rare objects over the entire sky, while ESA’s Herschel space observatory can scrutinize them in fine detail.

The Planck satellite has provided astronomers with the first all-sky submillimetre (sub-mm) survey sensitive enough to find the rarest, most luminous high−redshift sub-mm sources. These appear bright either as a result of strong gravitational lensing, or because they represent the combined far-infrared (FIR)/sub−mm emission from multiple intense starbursts in a young galaxy cluster – the most massive and rapidly collapsing dark−matter environments in the early Universe. When Planck scientists started looking for these poorly understood sources, they were surprised to find so many candidates. To confirm the nature of these objects, though, more detailed observations were needed. For this, they were awarded exceptional “must-do” observing time on the Herschel Space Observatory to study 200 of these sources. The results were surprising.

"Because we are looking so far back in time, and because the the universe is assumed to be homogenous in all directions, we think it's very similar to looking at the equivalent of what a baby cluster might look like," says Brenda L. Frye, an assistant astronomer at the University of Arizona's Steward Observatory who was involved in the research.

"In contrast to previous observations, for which the odd one or two baby clusters was found which one would put in a zoo, we now have found a real sample of 200 baby clusters."

This image, taken by the Hubble Space Telescope Focuses on two different galaxies, one named "10" and another named "15." Neither object looks as big and beautiful as the Milky Way but nonetheless they are still each a galaxy. Interestingly, galaxies 10 and 15 appear to have identical twins, as indicated. In actual fact there is only one real galaxy 10 and one real galaxy 15, and a natural telescope in space has "photocopied" their images on the sky so now there appear two of them. Trying to sort out optical illusions like this one is part of Brenda Frye's research in this joint Planck-Herschel Space Observatory collaboration. Credit: Nicole Nesvadba/IAS
This image, taken by the Hubble Space Telescope Focuses on two different galaxies, one named "10" and another named "15." Neither object looks as big and beautiful as the Milky Way but nonetheless they are still each a galaxy. Interestingly, galaxies 10 and 15 appear to have identical twins, as indicated. In actual fact there is only one real galaxy 10 and one real galaxy 15, and a natural telescope in space has "photocopied" their images on the sky so now there appear two of them. Trying to sort out optical illusions like this one is part of Brenda Frye's research in this joint Planck-Herschel Space Observatory collaboration. Credit: Nicole Nesvadba/IAS

“Most of the targets were found in the Planck data using a dedicated method looking at tiny fluctuations in the maps. These indicated the presence of high-redshift galaxies” says Dr Ludovic Montier (IRAP, Toulouse) who developed the approach. Dr Mattia Negrello (INAF) adds that “a few other targets came from the Planck Catalogue of Compact Sources”. “These two selection methods, using maps or catalogs, apply similar principles of colour selection favouring the detection of the most distant galaxies” concludes Dole. 

“Planck detects, then Herschel analyzes”. That’s how Gianfranco De Zotti, professor at the International School for Advanced Studies (SISSA) in Trieste and at INAF-Astronomical Observatory of Padova, summarizes the rationale of the study just published in Astronomy & Astrophysics. “As Mattia Negrello had already suggested in 2005, it is precisely Planck’s low resolution - optimized for the study of the cosmic microwave background but a major limitation for identifying extragalactic sources - which makes the satellite a powerful tool in the search for large-scale structures. And today we finally have the first experimental proof”. 

“The high sensitivity and angular resolution of Herschel allowed us to determine the nature of the Planck high-redshift candidates“ explains David Guéry (IAS) PhD student in charge of the analysis of the Herschel data; “The Herschel images show us either a single bright galaxy, suggesting that it is gravitationally lensed, or concentrations of many dusty galaxies, indicating a galaxy cluster.”

Dr Nicole Nesvadba (IAS) adds that “our targets are some of the brightest galaxies ever detected through gravitational lensing”.

“We are still a long way from fully understanding this new population,” says Clément Martinache (IAS, PhD student in charge of the near-infrared followup programme. “Our first near-infrared observations of some of the high-z candidates show unambiguous signs of galaxy clustering.”

“This program perfectly highlights the synergy between Planck and Herschel” says Pr Douglas Scott (UBC, Vancouver). “Planck selects the most interesting targets over the whole sky, andHerschel focuses on these spots for a refined analysis.” This means that the unique aspect of each space mission is used.

Gravitational lensing results in "photocopying" this single galaxy into three different images (labeled A1, A2, and B) This optical illusion is confirmed by analyzing spectra of these three images (see inset in upper left), which tells us that all three images are of the same object in the distant universe. The curve indicates the level of turbulence in the galaxy's gas stirred up as a result of the intense star formation. The colors represent different speeds and turbulence levels of the gas (left and right insets, respectively). Data taken with the IRAM radiointerferometer. Credit: Nicole Nesvadba/IAS
Gravitational lensing results in "photocopying" this single galaxy into three different images (labeled A1, A2, and B) This optical illusion is confirmed by analyzing spectra of these three images (see inset in upper left), which tells us that all three images are of the same object in the distant universe. The curve indicates the level of turbulence in the galaxy's gas stirred up as a result of the intense star formation. The colors represent different speeds and turbulence levels of the gas (left and right insets, respectively). Data taken with the IRAM radiointerferometer. Credit: Nicole Nesvadba/IAS

“Planck is a great experiment focused towards Cosmic Microwave Background cosmology, but it also allows major breakthroughs in other fields”, explains Dr Jean-Loup Puget (ESA), the Planck HFI Principal Investigator. “This has been shown recently with Galactic magnetic fields or polarized dust, and here we see it again for large-scale structure formation and high redshift galaxies.”

Dr Bruno Altieri (ESA), adds “Herschel has already discovered many high-redshift galaxies in deep surveys. These results from Planck open a new window with very bright, lensed galaxies and star-forming cluster candidates.”

“More observations are needed,” comments Dr David L. Clements (IC, UK), “and theorists have to work out the implications, but this huge new catalog of distant galaxies and clusters is the essential first step. There are going to be lots of exciting new results!”

"It was not known whether young galaxies form stars gradually, like marathon runners pacing themselves or in bursts," Frye says. "It turns out these young galaxies were not forming slowly, but in a dramatic way. Lighting up with star formation, they appear like fireworks going off in the sky. It's like sprinting the first mile of a 26-mile marathon, and then walking the rest of the way."

Pr Dole concludes “Although the data were taken a few years ago, when Planck and Herschel were operating, we’re only at the start of this project, with many exciting targets being followed up using other observatories. The impressive synergy we see here between the major European space missions Planck and Herschel, was only possible because of the many colleagues working worldwide for years on this project, and thanks to the broader Planck and Herschel science teams. This is a fantastic effort, and more impressive results are expected in the coming months. And, at the same time, we’re preparing for the future with Euclid, the next ESA mission for cosmology.”

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.

Credit: ias.frESAsissa.it

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