Planck traces the coldest objects in the nearby Universe
11 January 2011With its power to detect cosmic material at unprecedentedly low temperatures, Planck has completed the first unbiased, all-sky survey of compact cold and dusty objects in the Milky Way and, at the same time, the first all-sky survey of cool dust in other galaxies. These extensive data sets allow astronomers to shed new light on the earliest phases of star formation. The public release of the Planck Early Cold Core Catalogue will offer the community a large number of new, cold galactic targets to be studied with other telescopes, including ESA's Herschel Space Observatory. These are amongst the highlights presented by the Planck Collaboration at a conference held from 10 to 14 January 2011 in Paris, France.
The coldest agglomerations of matter found within molecular clouds, both in the Milky Way and in other galaxies, are a key element to understanding the very early stages of stellar formation, as it is from these cold and dense clumps that stars are born. Because of their extremely low temperatures, these still poorly understood objects emit most of their radiation in the sub-millimetre region of the electromagnetic spectrum, and therefore represent an ideal target for ESA's Planck mission.
Using observations performed with the three highest frequency channels of the High Frequency Instrument (HFI) on board Planck, astronomers have obtained the first unbiased, all-sky survey of compact cold dust clumps. The full Cold Core Catalogue of Planck Objects (C3PO), assembled using data from Planck's first all-sky survey, contains over 10,000 objects, and the most reliable among those detections, amounting to 915 objects, have been compiled in the Early Cold Core Catalogue (ECC). The ECC is part of the Early Release Compact Source Catalogue (ERCSC), the first scientific product arising from this mission to be made publicly available.
"Thanks to Planck's ability to measure extremely low temperatures with very high accuracy over the entire sky, we have been able to track down the distribution of the coldest dust on very large scales throughout the Milky Way," comments Ludovic Montier from the Institut de Recherche en Astrophysique et Planétologie (IRAP) in Toulouse, France, who led the effort of compiling and analysing the C3PO sample. "Instead of the compact cores that we expected to find, we have detected mainly objects which are rather elongated and have very low temperatures, between 7 and 16 Kelvin. These clumps are not isolated but appear to be all linked to one other, forming huge filamentary structures," he adds.
Most of the cold clumps detected by Planck are located in the solar neighbourhood, but some are more distant, up to 4 kiloparsec away from us. As expected, they tend to be mostly concentrated along the Milky Way's plane, although several have been identified also at high galactic latitudes. "The majority of the cold material detected by Planck is either organised in filaments or located at the illuminated edges of pillars of dense gas," notes Mika Juvela from the University of Helsinki, in Finland, who co-led the investigation along with Ludovic Montier. "In addition, by combining the data with other observations we found that these cold clumps are aligned with hydrogen gas shells and other galactic regions of active star formation, supporting the scenario that the formation of stars might be triggered by earlier stellar populations," he adds.
The low angular resolution of Planck, however, does not allow it to peer into the small cores inside these cold clumps. "This is where Herschel comes into play," explains Isabelle Ristorcelli, also from the Institut de Recherche en Astrophysique et Planétologie (IRAP), who, along with Mika Juvela, led a first study based on follow-up observations of a handful of Planck's detections using ESA's infrared observatory, Herschel. "Thanks to Herschel's higher resolution, we have been able to peer deeply into the clumps detected by Planck and to scrutinise their detailed structure," she adds. The timely release of the Early Cold Core Catalogue provides the community with several hundred objects to follow up and investigate in greater detail, exploiting the unique opportunity of employing Planck and Herschel together in order to achieve a complete view of the distribution and properties of cold dust in the Milky Way on large and small scales alike.
Studying the emission from cold dust in our own Galaxy is also of enormous importance to understand the behaviour of dust and the overall star formation processes in other galaxies. "The data gathered within the Milky Way represent a local guidebook to interpret what we see, with much less detail, in galaxies outside our own," explains Dave Clements from Imperial College London, U.K., who guided a study based on Planck observations of almost 500 nearby galaxies, up to redshift z~0.25. "Understanding the role of cold dust in the local galaxy population is, in turn, extremely useful to calibrate observations of galaxies at much higher redshift, such as those that are currently being performed with Herschel. Together, these two ESA missions will allow us to build a ladder connecting our Milky Way to the faint and distant galaxies and uncovering the evolution of dusty, star forming galaxies throughout cosmic history," he adds.
Providing the first all-sky survey of cool dust in galaxies, Planck has enabled astronomers to see colder dust than has been detected so far, with temperatures as low as about 10 Kelvin, highlighting how this additional component has been missed until now in the observations of local galaxies. "The presence of previously undetected cold dust in these galaxies indicates that they must radiate a significantly larger amount of energy than we thought at far-infrared and sub-millimetre wavelengths," notes Ranga-Ram Chary from the U.S. Planck Data Center in Pasadena, California, U.S.A., who led an analysis of the emission, in these wavelengths, from a large sample of nearby galaxies detected by Planck. "This new evidence suggests that the extrapolations currently used for distant galaxies, which are based on local ones, are incorrect as there appears to be more dust than has been accounted for until now. This calls for a re-evaluation of these galaxies, which are still extreme objects, shining brightly and forming stars at a very high rate, but slightly less extreme than we previously believed," he adds.
Following the trail of cold material throughout the Universe, Planck is tracing cosmic dust down to some of the lowest temperatures measured thus far. "These exciting, first results show how Planck is an excellent machine to trace the densest and coldest regions of the interstellar medium in the Milky Way and, at the same time, to locate the coldest amongst outer galaxies," concludes Jan Tauber, ESA's Planck Project Scientist. "It is in these extremely cold objects that the history of star formation is encoded and, thanks to Planck, we are now starting to disclose the rich reservoir of information they conceal."
Notes for editors
ESA's Planck mission maps the sky in nine frequencies using two state-of-the-art instruments, designed to produce high-sensitivity, multi-frequency measurements of the diffuse sky radiation: the High Frequency Instrument (HFI) includes the frequency bands 100 – 857 GHz, and the Low Frequency Instrument (LFI) includes the frequency bands 30-70 GHz.
The Planck Early Release Compact Source Catalogue (ERCSC), including the Early Cold Core Catalogue, and the first scientific results to emerge from this mission are being presented this week (10-14 January 2011) at the conference "The Millimeter and Submillimeter Sky in the Planck Mission Era" held in Paris, France.
The ERCSC is based on the scans gathered during Planck's first all-sky survey, between 13 August 2009 and 6 June 2010. Planck will continue to gather data at least until the end of 2011, during which time it will have completed over four all-sky scans.
The Planck Scientific Collaboration consists of all the scientists who have contributed to the development of the Planck mission, and who participate in the scientific exploitation of the Planck data during the proprietary period, which nominally ends with the release of the scientific products to the community 3.5 yr after launch, i.e. in January 2013. 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.
A follow-up programme of Planck C3PO sources is being undertaken within the Herschel Open Time Key Programme "Galactic Cold Cores: A Herschel survey of the source populations revealed by Planck", PI: Mika Juvela. Some 100-150 separate fields will be mapped with Herschel at five far-infrared/submillimetre frequencies; to date about 15 fields have been observed with the remainder of the observations to be completed in 2011.
Planck Collaboration 2011, "Planck Early Results: The Planck view of nearby galaxies", submitted to Astronomy & Astrophysics
Planck Collaboration 2011, "Planck Early Results: The submillimetre properties of a sample of Galactic cold clumps", submitted to Astronomy & Astrophysics
Planck Collaboration 2011, "Planck Early Results: The Galactic Cold Core Population revealed by the Planck first all sky survey", submitted to Astronomy & Astrophysics
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