Sloshing in the Universe’s Biggest Bathtub

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Title: Deep Chandra Observations of Abell 2495: A Possible Sloshing-Regulated Feedback Cycle in a Triple-Offset Galaxy Cluster
Authors: Luca Rosignoli et al.
First Author’s Institution:
University of Bologna and National Institute for Astrophysics, Italy
Status: Published in ApJ

Galaxy clusters are the biggest gravitationally bound objects in the universe. They have three main components: galaxies, their surrounding dark matter halos, and hot gas between the galaxies known as the intracluster medium. It’s the interaction between these components that makes galaxy clusters so interesting, and also (we think) significantly affects galaxy formation and evolution as a whole. As an example of this interaction, many galaxy clusters have a supermassive black hole inside their central galaxy (known as the brightest cluster galaxy) that’s acting as an active galactic nucleus — it’s being fed so much material by the galaxy and the cluster that it can’t contain it all, and it’s spitting material and energy back out into the cluster.

Thanks to gravity, each of these three components in a galaxy cluster should be centered on the same point in space — the dark matter forms a kind of gravitational “pit” that both the intracluster medium and the galaxies themselves can’t help but be pulled into. Other forms of energy (like heat keeping the intracluster medium puffed out, or leftover kinetic energy keeping the galaxies orbiting) prevent the intracluster medium and galaxies from collapsing entirely into the center of the pit, but they’re still definitely centered on where the pit is deepest. It’s very interesting, therefore, when the mass centers in galaxy clusters aren’t fully lined up. In today’s article, the authors explore one of these cases.

A Misaligned Galaxy Cluster

The cluster explored in this study, called Abell 2495, has been known about for a long time — it was discovered in 1998 through an X-ray search for clusters using the ROSAT satellite. Since then, a wide variety of data have been collected about this cluster at many different wavelengths that trace different components of the cluster. The data that are most relevant here are radio data from the Very Large Array at a frequency of 5 gigahertz (tracing activity from the active galactic nucleus inside the central brightest cluster galaxy) and optical images (showing the positions of the galaxies in the cluster) from the Hubble Space Telescope.

In addition to the ROSAT observations used to discover the galaxy cluster, there were also Chandra X-ray Observatory data of Abell 2495. However, the observations from both telescopes had very low sensitivity, making it difficult to distinguish any features in the X-ray-emitting gas. The authors present six new deep Chandra observations in this article, vastly improving the sensitivity of the X-ray observations (which trace the hot gas of the intracluster medium). Figure 1 combines several of these different types of observations, plotting them together as contours. X’s mark the various centers of the galaxy cluster components.

observations of the galaxy cluster Abell 2495

Figure 1: Multiwavelength observations of the galaxy cluster Abell 2495. The greyscale (and black contours) shows the X-ray emission from the hot gas in the galaxy cluster, the red contours show hydrogen gas (Hα), and the green contours show radio emission coming from the supermassive black hole inside the cluster’s central galaxy. The X’s show the center of the cluster determined in different ways: the black X is the center of the X-ray emission, the red X is the center of the Hα emission, the yellow X is the center of mass of the cluster (determined using the positions of the individual cluster galaxies), and the green X is the center of the brightest cluster galaxy. [Rosignoli et al. 2024]

A Gassy History

In order to understand why the galaxy cluster isn’t centered properly, we need to look back at the history of its movements. Luckily, there’s an easy way to do that — the X-ray light, which traces the intracluster medium, contains signatures of any strange happenings in the cluster’s recent history. This is because the intracluster medium is normally smooth and symmetric, but it takes a while to settle after disturbances. The authors’ new Chandra observations are thus perfect for finding out what’s going on in Abell 2495.

Some of the more interesting features the authors found in the X-ray observations are shown in Figure 2. The four cavities (the dark regions in Figure 2a) are indicators that the active galactic nucleus inside Abell 2495’s brightest cluster galaxy had periods in the past where it was extra active, emitting enough energy to blast holes in the intracluster medium. What’s particularly interesting is how some of these cavities line up with the current position of the emission from that active galactic nucleus (shown with the blue contours). The authors also found a significant density jump on one side of the intracluster medium (the light region in Figure 2b), suggesting that some force has piled up the gas on one side of the cluster in a dense cold front.

depictions of the cavities and cold front seen by the authors in this work

Figure 2: Interesting features in the X-ray observations of Abell 2495. In the left panel, the four cavities discovered by the authors are circled in green. These are areas of the otherwise smooth intracluster medium where very little material is present. In the right panel, a fitted profile of the intracluster medium has been removed, showing a large asymmetry with a significant jump — a “cold front.” [Adapted from Rosignoli et al. 2024]

temperature map of the intracluster medium of Abell 2495

Figure 3: A binned temperature map of the intracluster medium inside Abell 2495, measured using the X-ray emission. The regions outlined in black are cold enough to condense significantly in a reasonable timescale. [Adapted from Rosignoli et al. 2024]

Finally, the authors bin up their X-ray image in 2D space so they can use the spectra of the X-rays to calculate the temperature of the intracluster medium gas in each bin. Figure 3 shows the resulting temperature map. The regions of the map outlined in black have gas temperatures below a key threshold that means it can likely condense enough to fuel the active galactic nucleus in the center of the cluster. This also means that the cluster is cool-core (see an astrobite about this here). Interestingly, there’s also an extended tail of cold intracluster medium spiraling away from the center of the cluster, in a position that almost lines up with the cold front discussed above.

The Cosmic Bathtub

The authors of this article believe that all of this evidence points to one very interesting phenomenon: the galaxy cluster is sloshing. This is a phenomenon that’s appeared widely in simulations and has also been observed several times. It happens when some gravitational disturbance, typically a small “sub-cluster” of galaxies, passes by the cluster. This disturbance pulls both the dark matter and the intracluster medium in its direction. The dark matter can pass right through anything in its way, but the intracluster medium of the sub-cluster collides with the other intracluster medium of the other cluster, creating cold fronts, and so the two components get separated. This results in offsets between the centers, just like the ones observed in Abell 2495! As the disturbance passes, the dark matter and intracluster medium fall back towards the center of the cluster, setting up an oscillation just like water sloshing in a bathtub.

The Scientific Relevance of Baths

The reason why sloshing is so fascinating is because it could be regulating active galactic nucleus feedback. Normally, galaxy clusters act out the feedback cycle described in the introduction — the intracluster medium cools and collapses onto the active galactic nucleus, which in turn gets extra active and heats the intracluster medium back up again. In this case, however, the authors believe that the sloshing in this galaxy cluster is driving the whole feedback cycle. There is some quantitative evidence for this — the time the cluster takes to slosh is fairly consistent with the time between the formation of the different cavities in the intracluster medium, and the size of the X-ray cavities is consistent with the amount of energy that would be involved. If this is the case, sloshing could drive the whole life cycle of the cluster.

Sloshing Towards the Future

The authors have done a lot of analysis to come up with these findings, but there are some limitations in the data that mean significant uncertainties remain. Better X-ray data could uncover more details about the X-ray cavities, helping to narrow down when they were formed and how much energy was required to create them. A clearer picture of the cold front would allow the intracluster medium in the cluster center to be examined in more detail. It would also be helpful to get a larger sample of clusters with active galactic nucleus feedback potentially regulated by sloshing to understand if this is a common process, or if Abell 2495 is unique. Either way, this is a fascinating addition to our picture of one of the most extreme places in the universe — the center of a galaxy cluster.

Original astrobite edited by Lina Kimmig.

About the author, Delaney Dunne:

I’m a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency line intensity mapping experiment based in California’s Owens Valley.