Dwarf galaxies

The Milky Way, Andromeda and Centaurus A host flattened arrangements of satellite dwarf galaxies with correlated kinematics. The rarity of similar structures in cosmological simulations constitutes a major problem for the ΛCDM model, with no obvious solution in sight.

The existence of planes of satellites has been a cosmological surprise. However, evidence for stable, long-lived satellite planes is generally tenuous, and they are unexpected in all current models of galaxy formation.

This Review examines gas dynamics in dwarf galaxies, such as rotation curves and mass models. Star-forming dwarfs extend the dynamical laws of spiral galaxies and show small scatter around them, implying a tight coupling between baryons and dark matter.

This article reviews the properties of the dwarf galaxies of the Local Group from the point of view of their stellar dynamics and dark matter content and distribution, as inferred from the combination of observed data and dynamical models.

Galaxies and their dark-matter haloes have posed several challenges to the dark energy plus cold dark matter (ΛCDM) cosmological model. This Review discusses the most notable challenges to ΛCDM regarding dwarf galaxies and the insights provided by recent cosmological numerical simulations.

A cosmological simulation shows that low-mass galaxies can form with far less dark matter than expected, with results matching some observed characteristics. Roughly one-third of massive central galaxies may host at least one such dark-matter-deficient satellite.

This review summarizes our understanding of early-type dwarfs, which are the end-points of the evolution of low-mass galaxies. Their primeval stellar populations provide a unique laboratory for studying the physical conditions on small scales at epochs beyond z = 2.

This Review summarizes what is known of the stellar and chemical properties of nearby (<20 Mpc) star-forming dwarf galaxies. These objects resemble the earliest formed galaxies and may thus represent a window on the distant, early Universe.

This Perspective discusses massive black holes in dwarf galaxies and presents new insights on the demographics of nearby dwarf galaxies to help constrain the black hole occupation/active fraction as a function of mass and dwarf galaxy type.

This Perspective summarizes the latest observational evidence for star formation feedback and the important role of external ionizing radiation for the smallest galaxies, showing how this feedback directly impacts their properties, including their dark matter distribution.

There is an ongoing debate as to whether ultra-diffuse galaxies are regular dwarf galaxies with low star formation or massive ‘failed’ objects that quenched their star formation in the early Universe. Current evidence seems to support the former scenario.

Stars in the Tucana II ultrafaint dwarf galaxy observed out to nine half-light radii reveal the presence of an extended dark matter halo with a total mass of >10⁷ solar masses, consistent with a generalized Navarro–Frenk–White density profile and suggestive of past strong bursty feedback or an early galactic merger.

Cosmological hydrodynamical simulations of the ΛCDM Universe show that isolated quenched ultra-diffuse galaxies are formed as backsplash galaxies that were once satellites of another galactic group or cluster halo but are today a few megaparsecs away from them.

Cosmological simulation TNG50 reveals that a recently discovered population of isolated but non-star-forming ultra-diffuse galaxies may have been gas-rich satellites of much more massive galaxies in the distant past.

Up to 40% of ultra-diffuse galaxies could have formed via stripping of material by ram pressure. A study on 11 low-mass post-starburst galaxies in the Coma and Abell 2147 clusters shows ubiquitous marks of ram-pressure stripping and recent star formation, and most of them will evolve into ultra-diffuse galaxies in the next 10 Gyr.

Globular cluster NGC 2005 in the Large Magellanic Cloud (LMC) bears the elemental hallmarks of being an accreted object: a surviving fragment of a galaxy that fell into the LMC long enough ago to have erased any dynamical signature of accretion.

An exceptionally low delay of 83 minutes between variability in the accretion disk and Hα emission is reported from the nucleus of the dwarf galaxy NGC 4395. The implied black hole mass of about 10,000 solar masses is consistent with the mass–velocity dispersion relation.

Nineteen dwarf galaxies from the ALFALFA catalogue support previous observations of dwarf galaxies that suggested a deficiency in dark matter, challenging the formation theory of low-mass galaxies within the standard cold dark matter model.

Intermediate-mass black holes (BHs) in local dwarf galaxies are considered the relics of the early seed BHs. However, their growth might have been impacted by galaxy mergers and BH feedback so that they cannot be treated as tracers of the early seed BH population.

Small galaxies seem to be forming stars the old-fashioned way: in splendid isolation.

Embedding the Magellanic Clouds in a corona of ionized gas allows the gaseous Magellanic Stream to be modelled accurately.