[Paper Review] Dark matter deficient galaxies in the Illustris flat-$Λ$CDM model structure formation simulation

Hai Yu, Bharat Ratra+1 more|arXiv (Cornell University)|Sep 16, 2018

Galaxies: Formation, Evolution, Phenomena38 references5 citations

TL;DR

This study uses the Illustris flat-ΛCDM cosmological simulation to investigate the formation and prevalence of dark matter deficient galaxies—galaxies with M_dm/M_star < 1. It finds that such galaxies, including massive, compact systems like subhalo 476171 (M_star ≈ 9×10^10 M_sun, M_dm/M_star ≈ 0.1), are common in the simulation, suggesting they should exist in the real universe and could provide critical tests of ΛCDM and dark matter physics.

ABSTRACT

Surveying dark matter deficient galaxies (those with dark matter mass to stellar mass ratio $M_{ m dm}/M_{ m star}&lt;1$) in the Illustris simulation of structure formation in the flat-$Λ$CDM cosmogony, we find $M_{ m star} \approx 2 imes 10^8\, M_\sun$ galaxies that have properties similar to those ascribed by \citet{vanDokkumetal2018a} to the ultra-diffuse galaxy NGC1052-DF2. The Illustris simulation also contains more luminous dark matter deficient galaxies. Illustris galaxy subhalo 476171 is a particularly interesting outlier, a massive and very compact galaxy with $M_{ m star} \approx 9 imes 10^{10}\, M_\sun$ and $M_{ m dm}/M_{ m star} \approx 0.1$ and a half-stellar-mass radius of $\approx 2$ kpc. If the Illustris simulation and the $Λ$CDM model are accurate, there are a significant number of dark matter deficient galaxies, including massive luminous compact ones. It will be interesting to observationally discover these galaxies, and to also more clearly understand how they formed, as they are likely to provide new insight into and constraints on models of structure formation and the nature of dark matter.

Motivation & Objective

• To determine the frequency and properties of dark matter deficient galaxies in the Illustris ΛCDM simulation.
• To assess whether galaxies like NGC1052-DF2—previously reported as having low M_dm/M_star—can naturally form in the standard ΛCDM framework.
• To investigate the formation mechanisms of extreme dark matter deficient galaxies, especially massive and compact ones.
• To evaluate the observational detectability of such galaxies based on stellar mass and luminosity.
• To explore whether these galaxies could provide new constraints on structure formation and dark matter models.

Proposed method

• Analyzing the Illustris simulation, a large-scale hydrodynamical cosmological simulation of structure formation in flat-ΛCDM.
• Identifying subhalos with M_dm/M_star < 1 to classify them as dark matter deficient galaxies.
• Tracking the evolutionary history of specific subhalos, particularly subhalo 476171, using particle data from snapshots across time.
• Measuring stellar and dark matter masses, half-mass radii, and kinematic properties to characterize galaxy structure and dynamics.
• Comparing simulated galaxy statistics with observational constraints from NGC1052-DF2 and other ultra-diffuse galaxies.
• Using statistical analysis to estimate the fraction of galaxies with M_star ≈ 2×10^8 M_sun or M_star ≈ 10^9 M_sun that are dark matter deficient.

Experimental results

Research questions

• RQ1 How common are dark matter deficient galaxies (M_dm/M_star < 1) in the Illustris ΛCDM simulation?
• RQ2 Can galaxies with properties similar to NGC1052-DF2—low M_dm/M_star and moderate stellar mass—form naturally in ΛCDM?
• RQ3 What physical mechanisms lead to the formation of extremely dark matter deficient massive galaxies, such as subhalo 476171?
• RQ4 What fraction of galaxies with M_star ≈ 2×10^8 M_sun or M_star ≈ 10^9 M_sun are expected to be dark matter deficient in the ΛCDM model?
• RQ5 Are these dark matter deficient galaxies observationally detectable, and could they serve as probes of dark matter physics?

Key findings

• The Illustris simulation produces a significant number of dark matter deficient galaxies with M_star ≈ 2×10^8 M_sun and M_dm/M_star < 1, similar to NGC1052-DF2.
• More luminous and massive dark matter deficient galaxies exist in the simulation, including subhalo 476171 with M_star ≈ 9×10^10 M_sun and M_dm/M_star ≈ 0.1.
• Subhalo 476171 is a compact, massive galaxy with a half-stellar-mass radius of ≈2 kpc, indicating it is a structural outlier.
• The formation of subhalo 476171 likely involved the ejection of a tightly bound star cluster from its parent halo, resulting in the loss of most dark matter while preserving stellar structure.
• The simulation predicts that more than 0.7% of galaxies with M_star ≈ 2×10^8 M_sun and more than 0.4% with M_star ≈ 10^9 M_sun should have M_dm/M_star < 1 in the ΛCDM model.
• Given its r-band magnitude of ≈-21, subhalo 476171 is bright enough to be detectable in deep optical surveys, making it a prime observational target.