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Dark Galaxies: Fossils of the Early Universe

Dark Galaxies: Fossils of the Early Universe

NGC 6822, the first irregular galaxy, born in the early universe, observed by the Euclid telescope. Source: ESA/Euclid/Euclid Consortium/NASA.

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Dark galaxies, so named because of their small star content, are ancient objects, fossils of the primordial universe. According to the standard model Clean development mechanism In cosmology (where Λ is the constant indicating the accelerating expansion of the universe and Clean development mechanism represent “Cold dark matter“, any., Cold dark matter), galaxies represent the result of hierarchical clustering of rather large dark matter halos. The properties of dark matter halos determine the formation time of galaxies, their gas content, and their resistance to non-gravitational processes that can significantly alter their distribution. These processes go under the general name “baryonic feedback” because they are rooted in baryonic matter, ordinary matter, which does not interact only with gravity like dark matter. These are, for example, supernova explosions, stellar winds resulting from the development of massive stars and ionizing radiation emanating from third-group stars, which sweep through the surrounding medium, removing gas necessary for the formation of new stars. If the primordial gas reserves of dark matter halos are severely damaged before reionization, the cosmic epoch in which neutral hydrogen gas ionizes after the first stars are born, star formation is interrupted or never begins: they are therefore destined to remain a mystery. Thus, dark galaxies are born in the center of dark halos, that is, dark matter halos that failed to form a noticeable amount of stars at the dawn of reionization.

Part of the gas that leads to formation
Stellar in bright galaxies (black line), in
Dark ones are poor in stars (blue line) and in
Dark without stars (red line) in it
A function of time. Galactic gas
Darkness decreases over time, instead
Of the luminous galaxies remain stationary.
Credit: arXiv.

Since dark galaxies are by definition difficult to observe, theoretical modeling is essential to study them. at recent days, Illustris TNG project It made a notable contribution in this sense: thanks to innovative cosmological hydrodynamic simulations, it actually made it possible to study the physical properties of dark galaxies and compare them with those of bright galaxies. Specifically, during the simulations, dark galaxies were defined as having a stellar mass less than one ten-thousandth of their total mass, while bright galaxies were defined as having a stellar mass greater than or equal to this value. A second subdivision was then made between dark, star-poor galaxies, thus obtaining a final sample consisting of: 14,206 bright galaxies, 19,245 dark, star-poor galaxies and 14,318 dark starless galaxies. The selected galaxies have evolved from the moment they formed within dark matter halos to the present day in order to evaluate the factors influencing their history; In particular, the researchers identified three: the original environment, merger events with other galaxies, and reionization effects.

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Galaxies born in less dense environments with little gas available for star formation have a greater probability of becoming dark at the end of reionization, which also contributes to them being more clearly distinguished from bright galaxies by a further reduction in their gas reserves. Therefore, the lower the initial gas content of a galaxy, the fewer stars it can produce. Moreover, according to the results of the simulations, the separation between the two types of galaxies will increase over time, since dark galaxies will be less likely to merge with nearby star systems than their bright counterparts. This would lead to failure to obtain new gas to reshape stars.

The conclusion seems to be the following: only galaxies that pass the epoch of reionization without fundamental changes in the original gas reserve are able to continue star formation activity and evolve as bright galaxies, while dark galaxies are unable to recover the lost gas, they fossilize in this state.

For this reason, the current picture of dark galaxies corresponds to the picture of the end of reionization. Therefore we are waiting for observational data from surveys such as Clover, A few things H Euclid To specify more stringent constraints on the predictions of this theory.

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