First author’s institution: Institute of Astronomy, Cambridge, UK.
Status: Accepted to MNRAS.
Groups and clusters of galaxies, the regions of largest concentration of matter in the Universe, are also reservoirs of hot ionized gas. The intracluster gas emits in the high energy region of the electromagnetic spectrum, X-rays, due to the deceleration of charged particles when they pass each other, a process known as thermal bremsstrahlung. For some reason, this gas is observed to be very hot (~ 107-108 K) and does not easily cool down by radiating energy away, defying theoretical predictions. The solution seems to be to add a bit of heating to the mix, but what causes it?
Several heating mechanisms have been proposed. The dominant one is believed to be radiation emitted by Active Galactic Nuclei (AGN) in the group or cluster. AGN are powerhouses in the central regions of certain galaxies as a result of emission associated with a supermassive black hole. The energy from the AGN creates cavities in the gas around them, like bubbles that rise from each active galaxy. The authors of this paper identify a sample of 49 groups and clusters for which they study the properties of their X-ray cavities to understand how this might prevent the intracluster gas from cooling.
The groups and clusters studied in this paper have timescales for cooling of the gas of less than 3 Gyr. If no heating were present, the gas should be cooling efficiently, but it is not. To detect X-ray cavities, the authors use images taken by the Chandra observatory or the XMM-Newton observatory of each group or cluster in the X-rays. Since they are looking for inhomogeneities in the images, they take the image, smooth it, and subtract a “more smoothed” version from a “less smoothed version” of the same object. This allows them to identify inhomogeneities in the X-ray emission, as in Figure 1.
The authors find that 61% of the 49 groups and clusters present cavities. This percentage represents the fraction of time when the AGN is “on” and heating the gas. For clusters with small cooling time, the detection of cavities is limited by the resolution of the data, and hence this is only a lower limit to the AGN duty cycle.
Is AGN heating enough to prevent the gas from cooling? The authors address this question in Figure 2, where they compare the heating power of each observed cavity to the “cooling luminosity”, the luminosity of the gas within the radius where it would take 3 Gyr to cool. In other words, the authors use the size of the cavity and the inferred properties of the intracluster gas to obtain an estimate of the AGN heating and compare it to the amount of energy that the gas is radiating away. The points correspond to the different observed cavities, and the lines represent equality between heating and cooling (under different assumptions). In general, cavities above the black line would have sufficient energy to prevent gas from cooling. Cavities below it can have some trouble balancing the cooling and some other heating mechanism might be required. The fact that the points lie mostly within the range defined by the dashed lines, is interpreted as evidence for continuous (rather than intermittent) bubble activity from the AGN.
Overall, the question of what mechanisms make up for all the heating remains open. Even though AGNs seem to be continuously pushing gas away through the creation of bubbles, in some groups and clusters, the authors find that the X-ray cavities are not efficient enough to prevent cooling of the gas.