Title: Dark matter free dwarf galaxy formation at the tips of the tentacles of jellyfish galaxies
Authors: V. Lora, R. Smith, J. Fritz, A. Pasquali, A. C. Raga
First Author’s Institution: Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico
Status: published in The Astrophysical Journal [open access]
When Jellyfish Fly
Most galaxies are part of a galaxy cluster, which is exactly what it sounds like – a large collection of galaxies that are gravitationally bound to the larger cluster, much like how stars are gravitationally bound to a larger galaxy. In addition to the galaxies themselves, there is also gas between the galaxies in the cluster, referred to as the intracluster medium (ICM). When a disk-like galaxy moves through the ICM in a galaxy cluster, some of the gas within the galaxy (the interstellar medium, or ISM) gets stripped away from the galaxy. These create long gaseous tails (or, if you will, tentacles), giving the galaxy an uncanny resemblance to a jellyfish!
Jellyfish galaxies, and their tentacles in particular, have been studied for decades. Astronomers have investigated how much of the gas in the tentacles comes from the ICM vs. the ISM, as well as where and how star formation occurs within the tentacles. Interestingly, astronomers have found star-forming regions in the tentacles that have similar masses and sizes to ultra-compact dwarf galaxies (UCDs). Today’s authors look to reproduce those results computationally and better understand how this dwarf galaxy formation channel works.
Hanging on by a Tentacle
The authors use data from the Illustris TNG50 simulation, a cosmological simulation large enough to form dozens of galaxy clusters with enough resolution to accurately model features such as the arms of spiral galaxies. The authors identify a set of jellyfish galaxies within this simulation, then make additional cuts to:
- ensure the galaxies have obvious tentacles;
- find locations of star formation within the tentacles;
- eliminate galaxies where tentacle-like features could be due to interactions with other galaxies.
These cuts leave only one galaxy with a mass of ~400 billion solar masses; compare this to the mass of the Milky Way, which is typically reported as ~1 trillion solar masses (however, a 2023 study found that the Milky Way mass was closer to ~200 billion solar masses).
The authors identify a star-forming site within one of the tentacles of this galaxy, highlighted in Figure 1. This both supports the observational evidence and suggests that this may be a new type of dwarf galaxy (more on this in a moment). Additionally, by tracking the galaxy’s history prior to the infall, they determine that the galaxy loses gas but not stars. This means that the gas in the tentacle came from the galaxy, but the stars are forming in the tentacle rather than being relocated from the galaxy. This is a consequence of ram pressure stripping (RPS), the primary physical phenomenon that creates the tails of jellyfish galaxies. Another important finding about the dwarf galaxy candidate is that it lies well outside the dark matter (DM) halo, which has important ramifications for its status as a dwarf galaxy candidate.
Dark-Matter-Deficient Dwarfs
The authors perform additional analysis on the dwarf galaxy candidate. First, they determine that the gas and stars are gravitationally bound, meaning that they can be thought of as a single system much like how a galaxy is thought of as a single system. They also look at the DM content of the dwarf galaxy candidate and find that none of it is gravitationally bound, which would make this a DM-free dwarf galaxy. Furthermore, they estimate the mass and size of the dwarf galaxy candidate to be ~200 million solar masses and ~1-1.5 kiloparsecs. Based on these findings, the authors conclude that this system represents a new kind of dwarf galaxy which they dub an RPS dwarf galaxy; additionally, RPS dwarf galaxies are unique among dwarf galaxies because they lack a DM halo due to their creation via RPS.
The authors also analyze the star formation and metallicity of the RPS dwarf, shown in Figure 2. They find a high star formation rate (SFR) compared to other star-forming regions created via RPS. They also find that the RPS dwarf is very metal-rich compared to other dwarf galaxies of similar size and mass; this is because the jellyfish galaxy is also rich in metals, so the gas stripped into the tentacle to form stars has a higher concentration of metals.
Today’s authors have found evidence of a new type of dwarf galaxy, which they call an RPS dwarf galaxy. These dwarf galaxies form via RPS in the tentacles of jellyfish galaxies and are characterized as being gravitationally self-bound, hosting star formation, and lacking a DM halo. The authors hope to continue studies of RPS dwarf galaxies, noting that other cosmological simulations that can resolve smaller amounts of mass may lead to more discoveries of RPS dwarfs with lower masses.
Astrobite edited by Amaya Sinha
Featured image credit: adapted from Figure 2 from today’s paper, bottom panel.
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