[Paper Review] Limits on Stellar Objects as the Dark Matter of Our Halo: Nonbaryonic Dark Matter Seems to be Required

Katherine Freese, Brian D. Fields+1 more|arXiv (Cornell University)|Apr 28, 1999

Dark Matter and Cosmic Phenomena1 references7 citations

TL;DR

This paper argues that stellar objects—such as faint stars, brown dwarfs, white dwarfs, and neutron stars—cannot account for the Milky Way's dark matter halo. Using observational constraints and theoretical limits, the authors show that such baryonic objects contribute at most a few percent of the galaxy's mass, with white dwarfs alone constrained to Ω_WD ≤ 3 × 10⁻³ h⁻¹; this strongly implies the need for nonbaryonic dark matter to explain the halo's mass budget.

ABSTRACT

The nature of the dark matter in the Halo of our Galaxy remains a mystery. Arguments are presented that the dark matter does not consist of ordinary stellar or substellar objects, i.e., the dark matter is not made of faint stars, brown dwarfs, white dwarfs, or neutron stars. In fact, faint stars and brown dwarfs constitute no more than a few percent of the mass of our Galaxy, and stellar remnants must satisfy $Ω_{WD} \leq 3 imes 10^{-3} h^{-1}$, where $h$ is the Hubble constant in units of 100 km/s Mpc^{-1}. On theoretical grounds one is then pushed to more exotic explanations. Indeed a nonbaryonic component in the Halo seems to be required.

Motivation & Objective

• To assess whether stellar or substellar objects could constitute the dark matter in the Milky Way's halo.
• To constrain the maximum possible contribution of baryonic objects—like brown dwarfs, white dwarfs, and neutron stars—to the halo's mass density.
• To evaluate observational and theoretical limits on the abundance of faint stars and stellar remnants in the galactic halo.
• To determine whether the observed dark matter density can be explained by baryonic matter alone or if nonbaryonic components are required.

Proposed method

• Analysis of microlensing survey data to estimate the number density of faint stars and brown dwarfs in the galactic halo.
• Application of theoretical constraints on white dwarf formation and cooling to limit their total mass contribution.
• Use of the Hubble constant h to normalize the mass density parameter Ω_WD, setting an upper bound on white dwarf mass fraction.
• Comparison of observed halo mass-to-light ratios with predictions from baryonic stellar populations to rule out significant contributions from dim stars.
• Incorporation of Big Bang nucleosynthesis (BBN) constraints to limit the total baryonic mass density in the universe.
• Combining all constraints to derive a tight upper limit on the total baryonic contribution to the halo mass budget.

Experimental results

Research questions

• RQ1 Can faint stars and brown dwarfs account for a significant fraction of the Milky Way's dark matter halo mass?
• RQ2 What is the maximum possible mass contribution of white dwarfs to the halo's dark matter budget?
• RQ3 Do observational microlensing surveys and theoretical models allow for a baryonic explanation of the halo's missing mass?
• RQ4 Is the observed dark matter density consistent with a baryonic origin, given constraints from Big Bang nucleosynthesis?
• RQ5 Does the combination of all baryonic object constraints necessitate a nonbaryonic dark matter component?

Key findings

• Faint stars and brown dwarfs contribute no more than a few percent of the total mass of the Milky Way.
• The mass density of white dwarfs in the halo is constrained to Ω_WD ≤ 3 × 10⁻³ h⁻¹, where h is the Hubble constant in units of 100 km/s/Mpc.
• The combined contribution of all baryonic stellar remnants (white dwarfs, neutron stars, brown dwarfs, etc.) is insufficient to explain the observed dark matter density.
• Theoretical and observational constraints from microlensing, BBN, and halo mass-to-light ratios collectively rule out baryonic objects as the dominant dark matter component.
• The results strongly suggest that nonbaryonic dark matter is required to account for the mass budget of the galactic halo.