Jellyfish galaxies and where to find them

Daniel Cornwell at his Master’s graduation ceremony
An image of me at my Master’s graduation ceremony. Image Credit: University of Leicester.

I’m Dan Cornwell, a third year PhD student at the University of Nottingham, U.K. I work on using simulations of galaxy clusters to compare to observations of the cosmic web and its influence on galaxy evolution. Outside of research, I love being involved in outreach, going to the gym, playing tennis and baking.

Title: GASP XXXIX: MeerKAT hunts Jellyfish in A2626

Authors: Tirna Deb, Marc A.W. Verheijen, Bianca M. Poggianti et al

First Author’s institution: Kapteyn Astronomical institute, University of Groningen, The Netherlands 

Status: Published in MNRAS 

Galaxies spend their lifetimes in a range of different environments. Some galaxies evolve in isolated systems (known as field galaxies), whereas others live in denser environments, such as galaxy clusters and groups. Galaxy clusters are the largest gravitationally-bound objects in the Universe, containing thousands of galaxies across millions of light-years.  

Because of this difference in environment, galaxies in clusters tend to evolve very differently to galaxies in the field. For example, galaxies in clusters tend to be redder and more elliptical to those that are in the field. This is due to the ferocity of galaxy cluster environments—they contain extremely hot plasma known as the Intracluster medium (ICM) that can force out gas from galaxies, quenching star formation. The ICM is detectable in X-rays and is extremely hot, reaching tens of millions of Kelvin (equivalent to the core temperature of stellar red giants). Clusters also often host giant galaxies that can rip apart smaller, fainter galaxies due to the monstrous gravitational tidal forces provided by the galaxies. 

a jellyfish galaxy
Figure 1: NASA/ESA Hubble Space Telescope image of a galaxy in ESO 137-001, combined with Chandra X-ray data. The X-ray information is shown in blue and the rest of the color is from HST. Image credit: ESA, Hubble, Chandra
a jellyfish for comparison
Figure 1: BBC image of an actual jellyfish for comparison. This illustrates how Jellyfish galaxies are stripped of their gas and end up with long tails that are visually similar to the tentacles of a jellyfish. Image credit: BBC, Tim Gray.

Galaxies often contain lots of gas in the form of neutral atomic hydrogen (H I), some of which is eventually used to form stars. However, in the savage cluster environment, the ICM can strip and deplete a galaxy’s H I reservoir, causing a long tail to form in the opposite direction to which that galaxy is moving. This process is known as ram-pressure stripping. You can think of it as if your family’s dog is sticking its head out of the window of a fast-moving car. Its ears and hair are blown backwards due to the dog’s relative motion through a dense medium, the air.

Figure 1 shows an example Jellyfish galaxy (and jellyfish) which illustrates this effect. As this galaxy moves from bottom right to top left and falls towards the cluster, the ICM is stripping the galaxy of its gas, forming the bright blue tail, which is observed in X-rays.  

MeerKAT meet Jellyfish 

The authors of today’s paper use MeerKAT, a radio telescope in South Africa, to confirm the presence of six Jellyfish galaxy candidates, previously detected in visible light, in the massive galaxy cluster A2626. The authors define the conditions for a galaxy to be deemed a jellyfish as follows:the Hydrogen tail of the galaxy must be at least as long as the diameter of the stellar disc, the region of the galaxy containing nearly all the stars we can see. The researchers do this by cross-checking optical and radio observations of the six galaxies to determine the presence of a stripped hydrogen tail.  

RA/DEC plot of the galaxy cluster A2626. Included are images of several jellyfish candidate galaxies that are located within the cluster.
Figure 2: An illustration of cluster A2626 where each point in the central panel is a galaxy and the stars are Jellyfish galaxy candidates. The outer panels display optical images of these candidates. JW100 and JW103 are the confirmed Jellyfish.  

Of the six Jellyfish candidates, the authors confirmed only two of them as Jellyfish. Figure 2 shows the six galaxies and their positions in the galaxy cluster. JW100, which appears to have the longest tentacles, is located closest to the cluster core, where the ICM is dense and ripe for stripping away the gas in galaxies.  

To form stars or not to form stars 

Galaxies generally contain many millions of stars. However, to be regularly forming new stars, these galaxies need cool gas, otherwise the galaxies will quench their star formation. Therefore, one would expect the star formation rate (SFR) (the amount of stellar material produced per year) to depend on the presence of H I gas—which we already know is key to determining the presence of a Jellyfish galaxy. 

distribution of galaxy’s H I mass as a function of their star formation rate
Figure 3: Star formation rate (SFR) versus the H I mass of a galaxy. The grey hexagons are part of a wider, more general distribution of what we expect to find for most galaxies. The stars are the Jellyfish galaxies that are investigated in this work and the coloured hexagons are the Jellyfish galaxy candidates.  For reference, galaxies in other clusters are plotted in the coloured dots. 

The authors compare the SFR of the galaxies in their work to a wider, more general sample and find that the Jellyfish candidates have a higher-than-expected SFR given their H I mass. This can be interpreted as the galaxies having their H I gas stripped and becoming H I deficient, but the galaxies are not yet quenched. This implies that these galaxies are more efficiently converting H I into H II gas and that they must have evolved from right to left in Figure 3. These Jellyfish have been caught in a vital stage of their swim through the cosmos, and, as a result, form a unique population of galaxies who are likely undergoing an extreme transformation. Knowing the characteristics of these galaxies and their location will allow future studies to better find these Jellyfish. 

Astrobite edited by: Briley Lewis

Featured image credit: HST, ESA, Hubble, Chandra

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