AUTHORS: Rafferty, D. A.; Bîrzan, L.; Nulsen, P. E. J.; McNamara, B. R.; Brandt, W. N.; Wise, M. W.; Röttgering, H. J. A.
FIRST AUTHOR’S INSTITUTION: Leiden Observatory, Leiden University
Compact Galaxy Groups
Galaxies are not found in isolation. Instead, galaxies are much closer than chance alignments would suggest, and are typically part of large collections of galaxies, called galaxy clusters. The morphologies of these clusters range from loose associations of galaxies to compact groups, where the distance between galaxies is comparable to the size of the galaxies themselves. In 1982, Paul Hickson produced the first catalogs of compact groups. These so called Hickson Compact Groups include the famous Stephen’s Quintet and the compact group discussed in today’s paper: Hickson Compact Group 62 (HCG 62), a nearby compact group of about 60 galaxies.
However, galaxy clusters are not only comprised of galaxies. They also contain dark matter (which was probably instrumental in the cluster formation: see Ian’s astrobite) and a massive gaseous atmosphere which makes up the intracluster medium (ICM; sometimes called intragroup medium). The ICM is the subject of today’s post. The gas that makes up the ICM was heated to very high temperatures (107-108 K) by the initial gravitational collapse of galaxy groups. As a result of its high temperature, this gas is fully ionized and emits X-rays due to braking radiation (bremsstrahlung emission) and thus compact galaxy groups are very bright in X-ray as shown below.
This picture of galaxy clusters is incomplete, in part because I’ve left out a very important consideration in the ICM. As the hot gas radiates X-rays, it is losing energy, and cooling as a result. The important thing to note here is that the more dense the gas is, the faster it cools. This means the gas at the very center of the galaxy clusters cools more quickly than gas in the outskirts. The gas at the center of the cluster exerts an outward pressure that prevents the gas in the outskirts of the cluster from collapsing inwards and in order to maintain this equilibrium, the central gas must condense as it cools. Thus, the central gas flows inward — this is called a cooling flow (for a review of cooling flows, see Fabian, 1994).
The concept of a cooling flow is theoretically sound; however, there is a problem: we don’t observe intracluster gas below certain temperatures, meaning it isn’t cooling all the way. It is difficult to explain why intracluster gas would suddenly stop cooling. Many scenarios have been suggested (e.g. Fabian et al., 2001), but one of the most likely explanations is that some other source injects energy back into cooling intracluster gas, so the gas never cools off fully. That’s a lot of energy, so the favored candidates to inject this energy back into the ICM are some of the most energetic sources in the observable Universe: active galactic nuclei (AGN) which are caused by supermassive black holes at the centers of galaxies accreting material at high rates. However, it is unclear if even AGN can inject enough energy in the ICM to prevent cooling of the gas. One limiting factor is their mass, which Allison discussed in a recent post. AGNs are no stranger to Astrobites, so if you are interested in them check out these previous posts.
The energy in AGN jets
Sometimes AGNs are associated with radio jets. These radio lobes are likely due to synchrotron radiation caused by the acceleration of charged particles (e.g. electrons) in a strong magnetic field. However, it is difficult to constrain the energy in the jet since the emission efficiency, or how much of the energy in the lobe is used to produce the radio waves we are seeing, is related to electron fraction. The electron fraction is difficult to constrain, and may be different for different systems. Interestingly enough, bright radio lobes in galaxy clusters are often strongly associated with voids in X-ray emission from intracluster gas (e.g. McNamara et al., 2000; Fabian et al., 2000). In other words, the power in the radio jets actually displaces the hot, ionized gas in the ICM. Since the radio jet uses energy to push the intracluster gas away to create the void, observing the X-ray void can actually place a constraint on the amount of energy in the radio jet. This is done by calculating the total enthalpy in the cavities and converting that value to the total mechanical energy needed to displace the intracluster gas (see this paper or Bîrzan et al., 2004 for details).
Observations and Results
Rafferty et al. present new observations taken with the Chandra X-ray Observatory of HCG 62. This new data set is deeper than previous data sets; therefore it provides the best constraints to date on the energetics of the X-ray cavities in HCG 62. Rafferty et al. confirm the findings of Bîrzan et al. (2004) in that the total mechanical power needed to create the cavities is more than enough to offset the cooling losses of the ICM at the center of HCG 62. Additionally, they confirm the mechanical energy far exceeds the observed radio luminosity of the lobes. This means the majority of the particle pressure in this system is not due to electrons (which cause radio emission), and is instead the result of heavier particles (like cosmic rays) or a thermal gas. Furthermore, for the first time, the authors detect an AGN in X-ray at the center of HCG 62 and find evidence for a recent galaxy merger in the cluster.