Unraveling Mysteries: Dwarf Galaxies & Dark Matter Debate

The dwarf galaxies neighboring our Milky Way have been perceived as ancient satellites, circling our galaxy for approximately 10 billion years. This notion suggested the presence of massive dark matter reserves safeguarding them from the immense tidal impacts triggered by our galaxy's gravitational tug. Such an assumption led to the belief that dark matter was the culprit behind the significant variances in the star velocities within these dwarf galaxies. However, the latest data from Gaia offers a radical reevaluation of dwarf galaxies' characteristics, indicating that most might have been annihilated shortly after entering the galactic halo.

image Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

Esteemed astronomers from the Paris-PSL Observatory, the Centre National de la Recherche Scientifique (CNRS), and the Leibniz Institute for Astrophysics Potsdam (AIP) have successfully traced the Milky Way's timeline through the relationship linking its orbital energy to an object upon its halo entry. 

Objects that were captured by the Milky Way's gravitational field during its early formation phase, when it was less massive, exhibit lower orbital energies compared to recent arrivals. Intriguingly, these early arrivals were found to be substantially larger than the Sagittarius dwarf galaxy that entered the halo 5-6 billion years ago. This implies a more recent arrival of most dwarf galaxies, possibly less than three billion years ago.

Shown here is an analogue of the Scultor dwarf galaxy. . Credits: Jianling Wang, François Hammer

Gas-rich Dwarf Galaxies: Journey from the Outside

This recent arrival suggests that neighboring dwarf galaxies originated from outside the halo, where nearly all dwarf galaxies are found to house enormous neutral gas reserves. Upon colliding with the hot gas in the galactic halo, these gas-rich galaxies lost their gas. The collision's violent shocks and turbulence significantly altered them.

Formerly gas-rich dwarf galaxies, governed by gas and star rotation, transform into gas-free systems upon losing their gas. Their gravitational equilibrium is maintained by the remaining stars' random motions. This violent gas loss process disrupts the dwarf galaxies, causing a mismatch between the star movement speed and their gravitational acceleration. The joint effects of gas loss and gravitational shocks due to immersion in the galaxy aptly explain the diffuse star velocities within the dwarf galaxy's remnants[1].

The Dark Matter Enigma

This study also throws light on the intriguing role of dark matter. Firstly, the lack of equilibrium hinders any dynamical mass estimation of the Milky Way dwarfs and their dark matter content. Secondly, while dark matter was previously thought to safeguard dwarf galaxies' stability, its role seems questionable for out-of-equilibrium objects.

If a dwarf galaxy already contained a substantial amount of dark matter, it would have stabilized its initial star-rotating disk, preventing the dwarf's transformation into a galaxy with random star motions. The recently described arrival of dwarf galaxies and their transformation in the halo could potentially elucidate many observed properties of these objects, particularly their star's vast distances from their centers.

Contrary to the earlier understanding of dwarf galaxies being dominated by dark matter, their properties appear to align with the absence of dark matter. This revelation prompts several questions: Where are the numerous dark matter-dominated dwarf galaxies predicted by the standard cosmological model around the Milky Way? How can we deduce the dark matter content of a dwarf galaxy if we cannot assume equilibrium? What other observations could distinguish between the proposed out-of-equilibrium dwarf galaxies and the conventional model with dark matter dominated dwarfs? Future data from Gaia may hold the key to these queries.

Sources: Phys, Royal Astronomical Society

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