Archive for Sunyaev-Zel’dovich Effect

First Science from Planck

Posted in The Universe and Stuff with tags , , , , , , , , , on January 11, 2011 by telescoper

It’s been quite a long wait for results to emerge from the Planck satellite, which was launched in May 2009, but today the first science results have at last been released. These aren’t to do with the cosmological aspects of the mission – those will have to wait another two years – but things we cosmologists tend to think of as “foregrounds”, although they are of great astrophysical interest in themselves.

For an overview, with lots of pretty pictures,  see the European Space Agency’s Planck site and the UK Planck outreach site; you can also watch this morning’s press briefing in full here.

A repository of all 25 science papers can be found here and there’ll no doubt be a deluge of them on the arXiv tomorrow.

A few of my Cardiff colleagues are currently in Paris living it up at the junket working hard at the serious scientific conference at which these results are being discussed. I, on the other hand, not being one of the in-crowd, am back here in Cardiff, only have a short window in between meetings, project vivas and postgraduate lectures  to comment on the new data. I’m also sure there’ll be a huge amount of interest in the professional media and in the blogosphere for some time to come. I’ll therefore just mention a couple of things that struck me immediately as I went quickly through the papers while I was eating my sandwich; the following was cobbled together from the associated ESA press release.

The first concerns the so-called  ‘anomalous microwave emission’ (aka Foreground X) , which is a diffuse glow most strongly associated with the dense, dusty regions of our Galaxy. Its origin has been a puzzle for decades, but data collected by Planck seem to confirm the theory that it comes from rapidly spinning dust grains. Identifying the source of this emission will help Planck scientists remove foreground contamination which much greater precision, enabling them to construct much cleaner maps of the cosmic microwave background and thus, among other things, perhaps clarify the nature of the various apparent anomalies present in current cosmological data sets.

Here’s a nice composite image of a region of anomalous emission, alongside individual maps derived from low-frequency radio observations as well as two of the Planck channels (left).

Credits: ESA/Planck Collaboration

The colour composite of the Rho Ophiuchus molecular cloud highlights the correlation between the anomalous microwave emission, most likely due to miniature spinning dust grains observed at 30 GHz (shown here in red), and the thermal dust emission, observed at 857 GHz (shown here in green). The complex structure of knots and filaments, visible in this cloud of gas and dust, represents striking evidence for the ongoing processes of star formation. The composite image (right) is based on three individual maps (left) taken at 0.4 GHz from Haslam et al. (1982) and at 30 GHz and 857 GHz by Planck, respectively. The size of the image is about 5 degrees on a side, which is about 10 times the apparent diameter of the full Moon.

The second of the many other exciting results presented today that I wanted to mention is a release of new data on clusters of galaxies – the largest structures in the Universe, each containing hundreds or even thousands of galaxies. Owing to the Sunyaev-Zel’dovich Effect these show up in the Planck data as compact regions of lower temperature in the cosmic microwave background. By surveying the whole sky, Planck stands the best chance of finding the most massive examples of these clusters. They are rare and their number is a sensitive probe of the kind of Universe we live in, how fast it is expanding, and how much matter it contains.

Credits: ESA/Planck Collaboration; XMM-Newton image: ESA

This image shows one of the newly discovered superclusters of galaxies, PLCK G214.6+37.0, detected by Planck and confirmed by XMM-Newton. This is the first supercluster to be discovered through its Sunyaev-Zel’dovich effect. The effect is the name for the cluster’s silhouette against the cosmic microwave background radiation. Combined with other observations, the Sunyaev-Zel’dovich effect allows astronomers to measure properties such as the temperature and density of the cluster’s hot gas where the galaxies are embedded. The right panel shows the X-ray image of the supercluster obtained with XMM-Newton, which reveals that three galaxy clusters comprise this supercluster. The bright orange blob in the left panel shows the Sunyaev-Zel’dovich image of the supercluster, obtained by Planck. The X-ray contours are also superimposed on the Planck image.

UPDATES: For other early perspectives on the early release results, see the blogs of Andrew Jaffe and Stuart Lowe; as usual, Jonathan Amos has done a very quick and well-written news piece for the BBC.


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Hot Stuff, Looking Cool..

Posted in The Universe and Stuff with tags , , , , , on September 15, 2010 by telescoper

It’s nice for a change to have an excuse to write something about science rather than science funding, as a press release appeared today concerning the discovery of a new supercluster by Planck in collaboration with the X-ray observatory XMM-Newton.

The physics behind this new discovery concerns what happens to low-energy photons from the cosmic microwave background (CMB) when they are scattered by extremely hot plasma. Basically, incoming microwave photons collide with highly energetic electrons with the result that they gain energy and so are shifted to shorter wavelengths. The generic name given to this process is inverse Compton scattering, and it can happen in a variety of physical contexts. In cosmology, however, there is a particularly important situation where this process has observable consequences, when CMB photons travel through the extremely hot (but extremely tenuous) ionized gas in a cluster of galaxies. In this setting the process is called the Sunyaev-Zel’dovich effect.

The observational consequence is slightly paradoxical because what happens is that the microwave background can appears to have a lower temperature (at least for a certain range of wavelengths) in the direction of a galaxy cluster (in which the plasma can have a temperature of 10 million degrees or more). This is because fewer photons reach the observer in the microwave part of the spectrum that would if the cluster did not intervene; the missing ones have been kicked up to higher energies and are therefore not seen at their original wavelength, ergo the CMB looks a little cooler along the line of sight to a cluster than in other directions. To put it another way, what has actually happened is that the hot electrons have distorted the spectrum of the photons passing through it.

Here’s an example of the Sunyaev-Zel’dovich effect in action as seen by Planck in seven frequency bands:

At low frequencies (in the Rayleigh-Jeans part of the spectrum) the region where the cluster is looks cooler than average, although at high frequencies the effect is reversed.

The magnitude of the temperature distortion produced by a cluster depends on the density of electrons in the plasma pervading the cluster n, the temperature of the plasma T, and the overall size of the cluster; in fact, it’s propotional to n×T integrated along the line of sight through the cluster.

Why this new result is so interesting is that it combines very sensitive measurements of the microwave background temperature pattern  with sensitive measures of the X-ray emission over the same region of the sky. Plasma hot enough to produce a Sunyaev-Zel’dovich distortion of the CMB spectrum will also generate X-rays through a process known as thermal bremsstrahlung.  The power of the X-ray emission depends on the square of the electron density n2 multiplied by the Temperature T.

Since the Sunyaev-Zel’dovich and X-ray measurements depend on different mathematical combinations of the physical properties involved the amalgamation of these two techniques allows astronomers to probe the internal details of the cluster quite precisely.

The example shown here in the top two panels is of a familiar cluster – the Coma Cluster as mapped by Planck (in microwaves) and, by an older X-ray satellite called ROSAT, in X-rays. The two distributions have very similar morphology, strongly suggesting that they have a common origin in the cluster plasma.

The bottom panels show comparisons with the distribution of galaxies as seen in the optical part of the spectrum. You can see that the hot gas I’ve been talking about extends throughout the space between the galaxies. In fact, there is at least as much matter in the hot plasma as there is in the individual galaxies in objects like this, but it’s too hot to be seen in optical light. This could reasonably be called dark matter when it comes to its lack of optical emission, but it’s certainly not dark in X-rays!

The reason why the intracluster plasma is so hot boils down to the strength of the gravitational field in the cluster. Roughly speaking, the hot matter is in virial equilibrium within the gravitational potential generated by the mass distribution within the cluster. Since this is a very deep potential well, electrons move very quickly in response to it. In fact, the galaxies in the cluster are also roughly in virial equilibrium so they too are pulled about by the gravitational field. Galaxies don’t sit around quietly in clusters, they buzz about like bees in a bottle.

Anyway, the new data arising from the combination of Planck and XMM-Newton has revealed not just one cluster, but a cluster of clusters (i.e. a “supercluster”):

It’s early days for Planck, of course, and this is no more than a taster.
The Planck team is currently analysing the data from the first all-sky survey to identify both known and new galaxy clusters for the early Sunyaev-Zel’dovich catalogue, which will be released in January of 2011 as part of the Early Release Compact Source Catalogue. The full Sunyaev-Zel’dovich catalogue may well turn out to be the most enduring legacy of the Planck mission.


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