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Title: Dark-Matter-Free Dwarf Galaxy Formation at the Tips of the Tentacles of Jellyfish Galaxies
Authors: V. Lora et al.
First Author’s Institution: Institute of Nuclear Sciences, Mexico (UNAM)
Status: Published in ApJL
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. When a disk-like galaxy moves through the intracluster medium in a galaxy cluster, some of the gas within the galaxy (the interstellar medium) gets stripped away from the galaxy. This creates 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 intracluster medium versus the interstellar medium, 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. 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 IllustrisTNG50 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; and
- 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, 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 halo of the jellyfish galaxy, which has important ramifications for its status as a dwarf galaxy candidate.
Figure 1: Different visualizations of the selected galaxy. The top panel shows neutral gas (green), dark matter (white), and star formation (rainbow). The bottom panel shows the dark matter (white) and stellar mass (rainbow). The dwarf candidate is circled in magenta in both panels. [Lora et al. 2024]
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 dark-matter content of the dwarf galaxy candidate and find that none of it is gravitationally bound, making this a dark-matter-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 a ram-pressure-stripped dwarf galaxy; additionally, ram-pressure-stripped dwarf galaxies are unique among dwarf galaxies because they lack a dark-matter halo due to their creation via ram pressure stripping.
Figure 2: Star formation rate (top panel) and oxygen abundance (proxy for metal concentration, bottom panel) of the ram-pressure-stripped candidate (magenta). [Lora et al. 2024]
Today’s authors have found evidence of a new type of dwarf galaxy, which they call a ram-pressure-stripped dwarf galaxy. These dwarf galaxies form via ram pressure stripping in the tentacles of jellyfish galaxies and are characterized as being gravitationally self-bound, hosting star formation, and lacking a dark-matter halo. The authors hope to continue studies of ram-pressure-stripped dwarf galaxies, noting that other cosmological simulations that can resolve smaller amounts of mass may lead to more discoveries of ram-pressure-stripped dwarfs with lower masses.
Original astrobite edited by Amaya Sinha.
About the author, Brandon Pries:
I am a graduate student in physics at Georgia Institute of Technology (Georgia Tech). I do research in computational astrophysics with John Wise, using machine learning to study the formation and evolution of supermassive black holes in the early universe. I’ve also done extensive research with the IceCube Collaboration as an undergraduate at Michigan State University, studying applications of neural networks to event reconstructions and searching for signals of neutrinos from dark matter annihilation.