Under (Ram) Pressure! Stripping Galaxies of Their Gas

Title: Ram-pressure stripped radio tails detected in the dynamically active environment of the Shapley Supercluster

Authors: Merluzzi, P., Venturi, T., Busarello, G., et al.

First Author’s Institution: INAF-Osservatorio Astronomico di Capodimonte, salita Moiariello 16, I-80131 Napoli, Italy

Status: Submitted to MNRAS [open access]

An image of space containing a prominent spiral galaxy in the top left. There is a blue-purple stripe extending from the galaxy down towards the bottom right corner.
Figure 1: Here you can clearly see the effects of ram-pressure stripping on a galaxy (ESO 137-001 in this case, not one of the galaxies included in today’s paper). Stars have remained in the galactic disc, but the ISM has been “left behind” creating the blue-purple tail you can see in the picture. NASA/ESA/Hubble Heritage Team/M. Sun.

We often think of space as being empty, but in reality, there’s a lot of low-density gas out in space, even in the seemingly empty spots. Even though this gas is difficult to directly observe, we can see its effects on a whole range of astronomical objects. For example, galaxies in a cluster are surrounded by hot, low-density gas known as the intracluster medium (ICM). The ICM can exert a drag force on galaxies moving through it, and if this force is stronger than the gravity which binds the cooler gas between the galaxy’s stars, known as the interstellar medium (ISM), to the galaxy, then the ISM can be “left behind”, stripping the galaxy of its gas in a process known as ram pressure stripping.

Ram pressure stripping can have a huge impact on the evolutionary trajectory of a galaxy; if a galaxy loses all of its gas, it can no longer form new stars. Observationally, the effects of ram pressure stripping can be seen as an offset between a galaxy’s stars and its ISM, with the ISM often forming a long tail and creating “jellyfish galaxies”.

Galaxies in clusters – gravitationally bound groups of hundreds to thousands of galaxies – are particularly susceptible to ram pressure stripping due to the presence of the ICM. The fact that cluster galaxies tend to show lower star formation rates supports this idea, since the gas needed to form new stars has been stripped away from the galaxy. So, cluster galaxies give us an excellent opportunity to observe ram pressure stripping and study its effects. It can take up to a billion years for ram pressure stripping to occur, so this isn’t a process that astronomers can watch in real-time. Instead, astronomers can search for galaxies in different stages of being stripped.

A heatmap which generally goes from dark blue at the very edges to light blue, to green, to yellow. There are small regions of red, particularly in the center and center-left. The more prominent regions are labelled. The central red region is also annotated with a blue circle and two blue crosses. In the bottom right corner another red region is annotated with a blue square.
Figure 2: This heat map shows you the Shapley supercluster, and the position of the four galaxies analysed in today’s paper. The map is colour-coded according to stellar surface density, meaning that redder areas are areas where the concentration of stars is higher. The blue circle shows the position of SOS 61086, the blue crosses show the positions of SOS 114732 and SOS 90630m and the blue square shows the position of ShaSS 421. Written labels show the Abell clusters – substructures of the larger supercluster. Figure 1 from today’s paper.

The authors of today’s paper search for evidence of ram pressure stripping in galaxies in the Shapley Supercluster – the largest group of gravitationally bound galaxies observed to date (see Figure 2 for a map). Usually, galaxies come in standard shapes, like discs and ellipses, but ram pressure stripping can disturb galaxies and create irregular features like bright knots and tails. The authors identified 13 oddly-shaped galaxies and found that 4 of them had tails of ionised gas, which was observed in optical wavelengths using the Wide-Field Spectrograph at the Australian National University 2.3m Telescope and the Multi-Unit Spectroscopic Explorer at the Very Large Telescope. These 4 galaxies were observed in radio wavelengths using the Giant Metrewave Radio Telescope, MeerKAT, and the Australian Square Kilometre Array Pathfinder, and the authors saw that 3 of the galaxies had radio tails around 40 kiloparsecs (about as large as the Milky Way, which is about 30 kiloparsecs in diameter!) long.

The background is a black and white image of galaxies, which appear as white bright spots. On the left side, there is a larger galaxy. Over top the image are red and cyan contours. They follow the shape of the large galaxy and extend further to the left side of the image.
Figure 3: Here you can see the radio (red contours) and ionised gas (cyan contours) tails for SOS 90630. The background image was taken by the VLT Survey Telescope, and the radio and optical data are superimposed. You can see that the radio and ionised gas emission extends far behind the stellar component of the galaxy, which is what’s seen in the background image. Figure 3 from today’s paper.

The radio wavelengths from the tails are emitted by highly energetic electrons, which are travelling close to the speed of light. When high speed electrons encounter magnetic fields, they move along spiral paths and emit radio wavelengths through a process called synchrotron emission. Astronomers and physicists understand the process of synchrotron emission very well, but it’s still unclear what has produced these relativistic electrons and whether the radio tail was formed at the same time as the ionised gas tail.

To determine whether the radio and ionised gas tails were created at the same time, the authors estimated the ages of each tail. For the ionised gas tail, the authors used the results of hydrodynamical simulations of ram pressure stripping to estimate how long it would take for a tail to extend to around 40 kiloparsecs. They also estimated the age of young stars in the tail to ensure that the two age estimates were in good agreement. Since synchrotron emission is well-understood, astronomers can calculate the rate at which electrons lose energy due to synchrotron radiation. Combining this with the observed current energy of the electrons gives us an estimate of how long the electrons have been relativistic. In the case of all three galaxies with observed radio tails, the authors found that the three age estimates (~120 million years for SOS 90630, ~60 million years for SOS 114372, and ~250 million years ago for SOS 61086) were consistent, suggesting that the two tails were both formed by the same ram pressure stripping event.

The authors consider three possible sources of relativistic electrons: star formation in the radio tail, relativistic electrons that were stripped from the galactic disc, and electrons that got accelerated by a quickly moving front of gas known as a shock. Each different process will generate a slightly different radio spectrum, and by inspecting the radio emission from the four galaxies, the authors conclude that the majority of the electrons likely originated in the galaxy’s disc and were dragged into the tail by ram pressure stripping. But, some of the emission can be traced to the other two processes: in the case of SOS 90630 the authors also observed some star formation near the end of the tail, suggesting that this process is also contributing to the radio emission. In the case of SOS 114372, the authors found that the “age” of the electrons was a bit younger, suggesting that they’ve been re-accelerated by shocks at some point.

All 4 of the galaxies studied in this paper were found near past or ongoing interactions between galaxy clusters. This observation supports the idea that collisions and mergers locally increase the ram pressure of the ICM on galaxies’ ISM, triggering ram pressure stripping events which can significantly alter a galaxy’s evolutionary trajectory. The results of today’s paper also show that such interactions may often create tails of both ionised gas and relativistic electrons, creating a more complete understanding of this key process.

Astrobite edited by Brandon Pries

Featured image credit: NASA/ESA/Hubble Heritage Team/M. Sun 

About Nathalie Korhonen Cuestas

Nathalie Korhonen Cuestas is a first year PhD student at Northwestern University, where her research focuses on the chemical evolution of galaxies.

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