A too-many dwarf galaxy satellites problem in the M83 group (2403.08717v1)

Abstract: Dwarf galaxies in groups of galaxies provide excellent test cases for models of structure formation. This led to a so-called small-scale crisis, including the famous missing satellite and too-big-to-fail problems. It was suggested that these two problems are solved by the introduction of baryonic physics in cosmological simulations. We test for the nearby grand spiral M83 - a Milky Way sibling - whether its number of dwarf galaxy companions is compatible with today's $\Lambda$ + Cold Dark Matter model using two methods: with cosmological simulations that include baryons, as well as with theoretical predictions from the sub-halo mass function. By employing distance measurements we recover a list of confirmed dwarf galaxies within 330 kpc around M83 down to a magnitude of $M_V =-10$. We found that both the state-of-the-art hydrodynamical cosmological simulation Illustris-TNG50 and theoretical predictions agree with the number of confirmed satellites around M83 at the bright end of the luminosity function (>10$^8$ solar masses) but underestimate it at the faint end (down to 10$^6$ solar masses) at more than 3$\sigma$ and 5$\sigma$ levels, respectively. This indicates a too-many satellites problem in $\Lambda$CDM for M83. The actual degree of tension to cosmological models is underestimated, because the number of observed satellites is incomplete due to the high contamination of spurious stars and galactic cirrus.

• The paper reveals a significant excess of faint dwarf satellites in the M83 group compared to ΛCDM predictions, with discrepancies exceeding 3σ and 5σ levels.
• It employs precise distance measurements of dwarf galaxies within 330 kpc and compares observed data with Illustris-TNG50 and sub-halo mass function models.
• The findings underscore the need to refine galaxy formation models and expand observational campaigns to resolve challenges in small-scale cosmology.

An Analysis of the Dwarf Galaxy Satellites Challenge in the M83 Group

The paper "A too-many dwarf galaxy satellites problem in the M83 group" presents an intriguing examination of the satellite population surrounding the M83 galaxy, testing the congruence of this population with predictions from $\Lambda$CDM cosmological models. The paper contributes significantly to the discussion around the longstanding small-scale crisis in cosmology, which includes perplexing phenomena such as the missing satellite problem and the too-big-to-fail problem. The analysis centers around M83, a spiral galaxy comparable to the Milky Way, and critically assesses the consistency of observed dwarf galaxy counts with two central $\Lambda$CDM frameworks: state-of-the-art hydrodynamical simulations, and sub-halo mass function predictions.

Methodology and Findings

The research utilizes distance measurements from known dwarf galaxies located within 330 kpc of M83, identifying those with magnitudes as faint as $M_V =-10$. Crucially, the paper highlights that neither the sophisticated hydrodynamical simulation Illustris-TNG50 nor theoretical sub-halo mass function predictions align well with the observed data at the faint end of the luminosity function. Both methodologies predict a lesser number of faint satellites than are actually observed, with discrepancies at significance levels exceeding 3$\sigma$ and 5$\sigma$ respectively.

Illustris-TNG50, a cosmological simulation with hydrodynamics, serves as a pivotal comparison against the M83 system. While it accurately predicts satellite numbers at higher luminosities (>$10^8$ solar masses), it severely underestimates them below 10$^6$ solar masses. This suggests that our current understanding and modeling of galaxy formation at small scales might be deficient, especially considering the incompleteness due to observational challenges such as stellar contamination and galactic cirrus.

Implications and Future Directions

The paper reinforces the notion that $\Lambda$CDM models, despite their progressive refinements, remain incomplete in replicating small-scale astronomical structures observed outside the Local Group. These findings prompt a reevaluation of the assumptions underlying galaxy formation and evolution models, particularly at the dwarf galaxy scale, which involve considerable uncertainties in their baryonic processes.

Practically, the existence of a "too-many satellites" problem in the M83 group suggests that deeper and more expansive observational campaigns are necessary to discover hidden dwarf galaxies. Additionally, the possibility of similar discrepancies in other galactic environments warrants further investigation and offers a substantial opportunity for advancing our comprehension of cosmic structure formation.

Theoretically, this discrepancy opens avenues for refining existing cosmological models, introducing enhanced baryonic physics, and considering modifications to dark matter distributions or properties. Future developments in this area could also explore alternative dark matter hypotheses or new cosmological parameters that might account for these observations.

In conclusion, the paper provides substantial evidence challenging the predictive capabilities of existing cosmological models regarding dwarf galaxies, thereby contributing to an ongoing discourse in astrophysics. The paper ultimately urges the astrophysics community to refine their analytical tools and models to better reflect the complexities observed in the universe, particularly at smaller scales.