The dwarf galaxy problem, also known as the missing satellites problem, arises from a mismatch between observed

dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...

numbers and collisionless numerical

cosmological Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe, the cosmos. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', with the meaning of "a speaking of the wo ...

simulation A simulation is an imitative representation of a process or system that could exist in the real world. In this broad sense, simulation can often be used interchangeably with model. Sometimes a clear distinction between the two terms is made, in ...

s that predict the evolution of the distribution of

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic pa ...

in the

universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...

. In simulations,

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

clusters hierarchically, in ever increasing numbers of halo "blobs" as halos' components' sizes become smaller-and-smaller. However, although there seem to be enough observed normal-sized galaxies to match the simulated distribution of dark matter halos of comparable mass, the number of observed dwarf galaxies is

orders of magnitude In a ratio scale based on powers of ten, the order of magnitude is a measure of the nearness of two figures. Two numbers are "within an order of magnitude" of each other if their ratio is between 1/10 and 10. In other words, the two numbers are wi ...

lower than expected from such simulation. Observations from 2024 suggest that this problem may be resolved for the Milky Way, but some simulations show that the problem of observing too few satellites still exists for other galaxies.

Context

For example, around 38 dwarf galaxies have been observed in the Local Group, and only around 11 orbiting the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

,For a detailed and up to date list see

List of Milky Way's satellite galaxies The Milky Way has several smaller galaxies gravitationally bound to it, as part of the Milky Way subgroup, which is part of the local galaxy cluster, the Local Group. There are 61 small galaxies confirmed to be within of the Milky Way, but not ...

. yet dark matter simulations predict that there should be around 500 dwarf satellites for the Milky Way alone. The dwarf galaxy problem still persists, but now there are too many observed instead of too few. Based on current simulations there should be around 220 satellite galaxies in the

, however discoveries in 2024 point the number to be around at least 500 satellites. While the problem may have been resolved for the Milky Way when taking the luminosity function (astronomy) into account, for some early-type galaxies fewer galaxies are being observed than the number predicted by simulations. The problem of finding too many galaxies may lie in the simulations used. This implies that simulations still need to be improved upon in order to solve the dwarf galaxy problem, not just for the Milky Way but for other galaxies. The Vera C. Rubin Observatory, which finished construction in 2024, will be conducting wide field surveys of the night skies, leading to discoveries which may help scientists better understand this issue.

Prospective resolution

There are two main alternatives which may resolve the dwarf galaxy problem: The smaller-sized clumps of dark matter may be unable to obtain or retain the baryonic matter needed to form stars in the first place; or, after they form, dwarf galaxies may be quickly “eaten” by the larger galaxies that they orbit.

Baryonic matter too sparse

One proposal is that the smaller halos do exist but that only a few of them end up becoming visible, because they are unable to acquire enough

baryonic matter In particle physics, a baryon is a type of composite subatomic particle that contains an odd number of valence quarks, conventionally three. Protons and neutrons are examples of baryons; because baryons are composed of quarks, they belong to ...

to form a visible dwarf galaxy. In support of this, in 2007 the Keck telescopes observed eight newly discovered ultra-faint Milky Way dwarf satellites of which six were around 99.9% dark matter (with a mass-to-light ratio of about 1,000). Density profiles from 2022 suggest that dwarf galaxies have a constant-density core, though

simulations suggest that there should be less baryon density.

Early demise of young dwarfs

The other popular proposed solution is that dwarf galaxies may tend to merge into the galaxies they orbit shortly after star-formation, or to be quickly torn apart and tidally stripped by larger galaxies, due to complicated orbital interactions. Tidal stripping may also have been part of the problem of detecting dwarf galaxies in the first place: Finding dwarf galaxies is an extremely difficult task, since they tend to have low surface brightness and are highly diffuse – so much so that they are close to blending into background and foreground stars.

Footnotes

References

External links

* {{DEFAULTSORT:Dwarf Galaxy Problem Dark matter Galaxies Large-scale structure of the cosmos Unsolved problems in physics