Authors: Ákos Bogdán, Lorenzo Lovisari, Patrick Ogle, Orsolya E. Kovács, Thomas Jarrett, Christine Jones, William R. Forman, Lauranne Lanz
First Author’s Institution: Harvard & Smithsonian, Cambridge, MA, USA
Status: Submitted to the Astrophysical Journal, available on arXiv
Galaxy clusters contain hundreds of galaxies in a huge variety of shapes and sizes, ranging from irregular dwarf galaxies to giant ellipticals. The most luminous member of a cluster is known as the Brightest Cluster Galaxy, or BCG. Each BCG is different, but there are some properties that they tend to have in common – most BCGs are found at the very centre of their host cluster, and are large, elliptical galaxies, containing little gas and forming very few new stars.
The reason why most BCGs look so similar is well-understood, as it is thought that these large galaxies form via a series of galaxy mergers. These are violent cosmic events that slowly increase the size of the galaxy, whilst also destroying any delicate disk or spiral arms that the galaxy may have (click here to see a simulation of two merging spiral galaxies). Additionally, mergers can lead to gas being expelled from a galaxy, resulting in the gas-poor, quenched BCGs that we see today.
However, today’s paper presents an exciting twist to this story, by presenting data from three galaxy clusters that do not appear to follow this trend, including one BCG that doesn’t fit with our current theories at all.
The authors begin by introducing seven superluminous spiral galaxies, a recently-discovered class of huge galaxies with spiral or lenticular shapes. The great size of these galaxies is what motivates the main question of today’s paper: despite not looking like typical Brightest Cluster Galaxies, could these superluminous spiral galaxies actually be BCGs?
To answer this, we can look at the amount of X-ray radiation surrounding these galaxies. X-rays are emitted by the intracluster medium (ICM), a vast cloud of incredibly hot gas that fills a cluster, occupying the space between galaxies: if a cluster was a tasty chocolate chip muffin, the ICM would be the cake, filled with chocolate chip galaxies. Using the X-ray telescope XMM-Newton, the authors found no X-ray emission surrounding two of their galaxies. However, as Figure 1 shows, the remaining five have large amounts of X-rays being produced nearby. This indicates the presence of the ICM, meaning that these galaxies are nearby to a galaxy cluster.
It’s unusual to find spiral galaxies inside of clusters, but not unheard of. However, what makes this work so exciting is that in three of these clusters, there is not a single other galaxy that is brighter than the superluminous spiral – in other words, they are the Brightest Cluster Galaxy. Finally, one of these galaxies (J16273 in Figure 1) is not only the BCG, but is found directly in the cluster centre, in exactly the position that we would usually expect to find a BCG!
Galaxy mergers, but not as you know them
The fact that J16273 is the brightest galaxy in a cluster, and lives right in the cluster centre, makes it appear to be a fairly typical BCG. However, BCGs are elliptical galaxies, because of the large numbers of galaxy mergers that they experience. How can we explain why this one is so different from all of those that we’ve seen before?
Surprisingly, one explanation is mergers themselves. The authors suggest that J16273 was previously a regular, elliptical BCG, that recently merged with a smaller gas-rich galaxy. Under the right conditions, this merger could spin up the elliptical galaxy, with the remnants of the gas-rich galaxy forming a brand new spinning disk.
In order to really understand these giant spiral galaxies, future work will need to look at many more than just seven of them. The authors acknowledge this, and suggest that eROSITA, an ongoing X-ray survey of the sky, will be able to look at many more of these galaxies and determine whether they live in clusters, groups, or alone. eROSITA is due to release its first data at the end of 2022, and should help us to solve the mystery of how these huge spirals ended up in places we never expected to find them.
Astrobite edited by Katy Proctor
Featured image credit: NASA/ESA/Hubble/B. Holwerda