[Paper Review] A population of dwarf galaxies deficient in dark matter

Qi Guo, Huijie Hu+10 more|arXiv (Cornell University)|Jul 31, 2019

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This study identifies 19 dwarf galaxies, including 14 isolated ones, with minimal dark matter content—challenging the standard cosmological model that predicts dark matter dominance in low-mass systems. The findings suggest that some dwarf galaxies may consist primarily of baryons even beyond their optical half-light radii, indicating a potential discrepancy with predictions of dark matter-driven structure formation.

ABSTRACT

In the standard cosmological model, dark matter drives the structure formation and constructs potential wells within which galaxies could form. The baryon fraction in dark halos could reach the universal value in rich clusters and drops fast towards low mass systems. The formation of dwarf galaxies is sensitive both to baryonic processes and the properties of dark matter due to their shallow potential well. In dwarf galaxies in the Local Group, dark matter dominates the mass content even within their half optical light radii (r_e ~ 1 kpc). However, statistical studies beyond the Local Group were hampered by the extreme faintness of such systems in the past. Here we report 19 dwarf galaxies that could mainly consist of baryons up to radii well beyond r_e, where they are expected to be dominated by dark matter. 14 of them are isolated dwarf galaxies, free from the influence of nearby bright galaxies/AGNs and high dense environments. This result provides observational evidence that could challenge the formation theory of low-mass galaxies in the framework of standard cosmology.

Motivation & Objective

• To investigate the dark matter content in dwarf galaxies beyond the Local Group, where observational constraints have been limited by faintness.
• To determine whether dwarf galaxies in low-density environments can host minimal dark matter, challenging the expectation of dark matter dominance.
• To test the standard cosmological model's predictions on baryonic fraction and dark matter distribution in low-mass systems.
• To identify isolated dwarf galaxies free from external influences to isolate intrinsic properties of dark matter deficiency.

Proposed method

• Conducted deep photometric and spectroscopic surveys to measure stellar kinematics and luminosity profiles of faint dwarf galaxies.
• Used velocity dispersion measurements to infer total mass and compare it with baryonic mass estimates from stellar light.
• Applied dynamical modeling to estimate dark matter fractions within radii extending beyond the optical half-light radius (r_e ~ 1 kpc).
• Selected isolated systems by excluding galaxies near bright neighbors or in high-density environments to minimize tidal and environmental effects.
• Compared observed mass-to-light ratios with theoretical expectations under the standard cosmological model.
• Evaluated the significance of dark matter deficiency by assessing the consistency of observed dynamics with baryonic mass alone.

Experimental results

Research questions

• RQ1 Do dwarf galaxies in low-density environments exhibit significant dark matter deficiency?
• RQ2 To what extent can the observed mass in dwarf galaxies be explained by baryonic matter alone?
• RQ3 How do the dynamical mass profiles of isolated dwarf galaxies compare with predictions from the standard cosmological model?
• RQ4 What implications does the presence of dark matter-deficient dwarfs have for galaxy formation theories?
• RQ5 Are the observed properties of these dwarf galaxies consistent with feedback-driven or alternative dark matter models?

Key findings

• 19 dwarf galaxies were identified with mass profiles consistent with baryonic matter dominance beyond their optical half-light radii.
• 14 of these galaxies are isolated, indicating that the lack of dark matter is not due to environmental effects such as tidal stripping.
• The observed velocity dispersions imply mass-to-light ratios consistent with baryonic mass alone, suggesting minimal dark matter contribution.
• The baryonic fraction in these systems exceeds expectations in the standard cosmological model, especially at larger radii.
• The findings challenge the prediction that dark matter should dominate even in low-mass systems, particularly in isolated environments.
• The existence of such systems implies a potential inconsistency with the standard model of galaxy formation in low-mass regimes.