Astronomers have analyzed the formation of ultradiffuse galaxies using cosmological simulations. It turned out that some of the quiescent representatives of this class – that is, galaxies in which active star formation has ended – are far away from galactic clusters. This happens due to interaction with a dense galaxy cluster, as a result of which the ultradiffuse galaxy first loses matter and stops forming new stars, and then flies away from the cluster by several megaparsecs.
Everything you need to know about ultradiffuse galaxies
What are ultradiffuse galaxies?
Ultradiffuse galaxies are called galaxies that contain hundreds to thousands of times fewer stars in sizes comparable to the Milky Way – that is, they are much lighter and fainter than a typical large galaxy, which makes them difficult to observe. The origin of objects with such properties has not yet been finally established.
According to observations, ultradiffuse galaxies can be divided into two groups: in some, relatively active star formation occurs (with a typical rate in the range of 0.001–1 solar masses per year), while others have already died out and are not forming new stars. In this case, the characteristic arrangement of quiescent galaxies is dense clusters of galaxies, and star formation is usually observed in ultradiffuse galaxies outside such clusters.
The physics of galaxies
These data are consistent with the qualitative understanding of the physics of galaxies. Being in a cluster, an ultradiffuse galaxy loses its own matter due to interaction with a dense environment, and it has nothing to make new stars out of, while outside the cluster the loss of matter is not so intense and there is enough material for star formation.
Quiescent ultradiffuse galaxies
However, some quiescent ultradiffuse galaxies are located far from dense galaxy clusters, where the density of the environment is not enough to cause the galaxy to lose a lot of matter. Qualitative reasoning is no longer sufficient to explain such observations theoretically, and detailed quantitative models and computer simulations are required.
Hydrodynamic cosmological simulations
Astronomers have followed the evolution of ultradiffuse galaxies in hydrodynamic cosmological simulations. Scientists picked an area with a linear size of 50.7 megaparsecs. Inside it, they placed about ten billion particles with masses of about 10 5 solar masses, which were simulated ordinary matter and dark matter. In addition to gravity, the simulation took into account star formation, gas cooling and heating, the evolution of the chemical composition of the universe, feedback mechanisms for stars and black holes, and magnetic fields.
The authors identified the objects by subhalos (compaction of model particles), which contained a stellar-mass component. For the identified galaxies, the mass (stellar and total), the morphological parameter (describing the shape of the galaxy), and the characteristic radius were calculated – the distance within which half of the entire stellar mass of the galaxy is contained.
Researchers attributed an individual galaxy to ultradiffuse if its mass was in the range of 10 7.5 -10 9 solar masses (corresponds to observations) and among galaxies of close masses it belonged to the five percent of the largest in radius. Thanks to these criteria, the masses and sizes of the simulated ultradiffuse galaxies practically coincided with those actually observed.
14 billion years of evolution
Starting the simulation with redshift z = 127, the scientists simulated almost 14 billion years of evolution of the allocated volume and traced the final distribution of ultradiffuse galaxies in the overall spatial structure.
It turned out that during the simulation, about a quarter of ultradiffuse galaxies were extinguished. At the same time, the global picture coincided with qualitative expectations: quiescent galaxies are mainly concentrated in clusters, and forming stars are more evenly scattered over rarefied regions.
Nevertheless, astronomers also found isolated quiescent ultradiffuse galaxies in the final distribution at distances of several megaparsecs to the nearest clusters. Having traced in more detail their movement and the rate of star formation in time, the researchers found that such objects are formed as a result of interaction with the cluster at an earlier stage of evolution (on average, 5.5 ± 2.5 billion years ago with the interaction duration of about one and a half billion years).
Galaxies flying away
Approaching the cluster, the galaxy actively forming stars enters a dense medium and loses material, as a result of which it dies out. In this case, with a successful outgoing trajectory, an ultradiffuse galaxy can leave the cluster after interaction and, on a scale of billions of years, fly away at a distance of several megaparsecs from it.
The authors noted other properties that distinguish quiescent ultradiffuse galaxies from star-forming ones and will allow the simulation results to be checked for adequacy using observations. In particular, the shape of the former is always closer to spheroidal, while among the latter there were both disc-shaped and spherical.
Quiescent galaxies are lighter
In addition, quiescent galaxies ejected from clusters turned out to be several times lighter compared to star-forming ones of the same stellar mass – that is, they lost not only ordinary matter but also a noticeable fraction of dark matter (this is also evidenced by the density profiles of dark matter, which at the edges ejected from clusters of ultradiffuse galaxies are sharper than those of similar star-forming galaxies).
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• Connellan, I. (2021, September 7). The mysteries of ultradiffuse galaxies. Cosmos Magazine.
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• ScienceDaily. (2021, September 6). Astronomers explain origin of elusive ultradiffuse galaxies.