## High-resolution Observation of the Sunyaev-Zel’dovich Effect With ALMA

I just saw a very interesting paper (by Kitayama et al.) on the arXiv, which I’m pretty sure presents the highest-ever resolution observations of the (Thermal) Sunyaev-Zel’dovich Effect in a galaxy cluster taken with the Atacama Large Millimetre Array (ALMA). This is basically a distortion of the spectrum of the cosmic microwave background seen in the direction of the cluster caused by scattering of CMB photons off electrons in the extremely hot plasma that pervades such an object. The key parameter to be measured along each line of sight is the Compton y-parameter, which is defined as

$y = \tau \frac{kT}{m_ec^2},$

where $\tau$ the optical depth of the cluster (which in this case is essentially the fraction of CMB photons that get scattered) and $T$ is the plasma temperature; for a more technical discussion of the process see here.

Here is the abstract of the paper:

We present the first image of the thermal Sunyaev-Zel’dovich effect (SZE) obtained by the Atacama Large Millimeter/submillimeter Array (ALMA). Combining 7-m and 12-m arrays in Band 3, we create an SZE map toward a galaxy cluster RXJ1347.5-1145 with 5 arc-second resolution (corresponding to the physical size of 20 kpc/h), the highest angular and physical spatial resolutions achieved to date for imaging the SZE, while retaining extended signals out to 40 arc-seconds. The 1-sigma statistical sensitivity of the image is 0.017 mJy/beam or 0.12 mK_CMB at the 5 arc-second full width at half maximum. The SZE image shows a good agreement with an electron pressure map reconstructed independently from the X-ray data and offers a new probe of the small-scale structure of the intracluster medium. Our results demonstrate that ALMA is a powerful instrument for imaging the SZE in compact galaxy clusters with unprecedented angular resolution and sensitivity. As the first report on the detection of the SZE by ALMA, we present detailed analysis procedures including corrections for the missing flux, to provide guiding methods for analyzing and interpreting future SZE images by ALMA.

And here is the key image, a map of the variation of the Compton y-parameter across the cluster:

It’s not at all easy to isolate the Sunyaev-Zeld’dovich effect, so this is an impressive result and the paper is well-worth reading. Observations at such high resolution will help greatly to understand the behaviour of hot gas in rich clusters, especially when combined with observations of the emission from the cluster plasma itself, which is hot enough to radiate in the X-ray part of the spectrum.