Sloshed and shocked: diagnosing gas motion in galaxy clusters

April 01, 2019

Recent high-resolution microwave and X-ray observations of the galaxy cluster RX J1347-1145 offer a new diagnosis tool of gas motion. Probing different parameters of the hot gas in galaxy clusters, these observations allow the MPA scientists to distinguish between gentle and violent motion of gas stirred by encounters with smaller sub-clusters.

Galaxy clusters are the largest gravitationally bound objects in the Universe. About 15% of their mass is in the form of hot (100 million Kelvins) ionised gas, whose pressure balances their own enormous gravity. The rest is in the form of dark matter, with only minor contribution from stars in the member galaxies of clusters.

Hot gas in galaxy clusters can be seen in two ways: First, it emits X-rays via thermal bremsstrahlung from scattering of electrons and protons. Second, scattering of electrons off photons of the cosmic microwave background (CMB) transfers the electron's kinetic energy to photons, distorting the black-body spectrum of the CMB; thus, galaxy clusters are visible over the uniform background of CMB in microwave bands, the effect known as the thermal Sunyaev-Zeldovich (SZ) effect. Now, here is the key: X-ray emission probes gas density because its intensity is proportional to the density squared, whereas the SZ effect probes thermal gas pressure, i.e. the product of density and temperature. Therefore, by combining X-ray and SZ images, we can directly measure the so-called "equation of state" of the gas, i.e., how gas pressure and density are related to each other.

Galaxy clusters do not stand still. They are continuously forming and growing by accreting mass from the surrounding cosmic web. As a result, hot gas in clusters is stirred and disturbed continuously. X-ray images show such disturbances, which are sensitive to irregularities in the density. But how about disturbances in gas pressure? Gentle motion of the gas at velocities much smaller than the sound speed does not alter pressure, whereas motion that is more violent generates shock waves and increases pressure. Thus, the equation of state of perturbed gas measured by a combination of X-ray and SZ data allows us to infer gas motion directly. However, spatial resolution of the SZ data has been limited because of the lack of appropriate telescopes. Now, with ALMA, a team of scientists led by Eiichiro Komatsu at MPA have succeeded in obtaining an image of the SZ effect in the distant galaxy cluster RX J1347-1145 at 5-arcsecond resolution, matching the resolution of X-ray data for the first time (Figure 1; see also the MPA press release on March 17, 2017).

Removing the smooth component, this X-ray image shows two disturbances (the left panel of Figure 2): a dipolar pattern at the centre, which is characteristic of "sloshing" of gas due to a perturbation from infall of smaller clumps of matter, and an excess X-ray emission in the south-east region. Looking at the corresponding SZ image (the right panel of Figure 2), we find nothing at the centre, indicating that the sloshing motion is gentle and does not alter pressure. On the other hand, we find a large excess SZ signal in the south-east region, which is consistent with shock-heated gas, probably due to a major merger with an infalling sub-cluster. This discovery was made possible by the high spatial resolution and high sensitivity of ALMA, both of which are unprecedented in measuring the SZ effect.

Another method to characterise the effective equation of state for the gas using the X-ray data alone was proposed by Eugene Churazov and his colleagues. It relies on comparing the amplitudes of gas perturbations seen in different energy bands (Figure 3). Given the complementarity of X-ray and SZ observations, adding SZ data enhances this approach in a major way. Recently, scientists at MPA and ESO performed a detailed joint analysis of X-ray data and SZ images as well as interferometric data of this cluster. This study, led by Luca Di Mascolo (PhD student at MPA), broadly agrees with the results of Komatsu's group. In addition, it discusses a "milder" form of the merger that leads to the perturbations observed in the X-ray and SZ signals. In this scenario, part of the south-east excess observed in the X-ray images may be due to colder gas infalling into the main cluster.

More ALMA data on other clusters have been obtained by Komatsu's team, waiting to be published. These data sets, together with new analysis methodology developed by Komatsu and Churazov's groups, open up a new window to study gas motion and dynamical states of galaxy clusters.

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