content 17-October-2017 09:54:09

Delving through the Milky Way with Planck

The main goal of ESA's Planck satellite was to map the Cosmic Microwave Background (CMB), the most ancient light that has travelled across the Universe. But before scientists in the Planck Collaboration could start exploiting this pool of cosmological information long and meticulous work was required to remove several foreground signals hiding the CMB. The strongest source of foreground is our Galaxy, and the information contained in these layers is helping astronomers to paint a new view of the Milky Way.

The contaminants include emissions from galaxies, galaxy clusters and, most crucially, from diffuse material within our own Galaxy, the Milky Way. The careful removal of foreground emissions that is necessary to uncover the cosmological potential of Planck's data had a very valuable by-product: the delivery of large data sets to many other fields in astrophysics.

Left: All-sky image showing density of cold cores present in the Cold Core Catalogue of Planck Objects (C3PO). Right: All-sky distribution of molecular gas seen by Planck.
Credit: ESA/Planck Collaboration

The interstellar medium (ISM) that permeates the Milky Way shines brightly in all of Planck's wavelength channels via a variety of radiation processes. The main causes of this diffuse radiation are free electrons at the longest wavelengths, and dust grains at the shortest wavelengths probed by Planck.

Data from Planck were used to compile maps of the various components of the ISM, revealing in the process some new players contributing to our Galaxy's emission.

Three-colour composite of the Perseus molecular cloud, combining 0.4 GHz, 30 GHz and 857 GHz observations, highlighting (in red) the anomalous dust emission arising from nano-sized spinning dust grains.
Credit: ESA/Planck Collaboration

With the new data, astronomers studied a mysterious emission that was detected several years ago at microwave wavelengths, confirming that its origin can be explained by spinning dust grains and that it is widespread in the ISM. Another study of Planck's maps enabled the astronomers to better quantify the amount of gas in molecular clouds where stars are being born. Locating and quantifying gas in molecular clouds is a vexed issue in the study of star formation since molecular hydrogen, the gas from which stars take shape, does not emit and must be traced indirectly. To this aim, astronomers have exploited the data from Planck to obtain the first all-sky map of the emission from carbon monoxide, a molecule that traces the densest pockets of gas where star formation is taking place across the Galaxy.

A study using some of the early data collected by Planck produced a chart of the densest and coldest cores across the Galaxy, where the earliest steps of star formation take place. This catalogue proved a very helpful resource for follow-up studies of these stellar cradles with higher resolution observations, including some performed with ESA's Herschel Space Observatory.

Last Update: 18 October 2013

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