The Local Dark Matter Density

Abstract: I review current efforts to measure the mean density of dark matter near the Sun. This encodes valuable dynamical information about our Galaxy and is also of great importance for 'direct detection' dark matter experiments. I discuss theoretical expectations in our current cosmology; the theory behind mass modelling of the Galaxy; and I show how combining local and global measures probes the shape of the Milky Way dark matter halo and the possible presence of a 'dark disc'. I stress the strengths and weaknesses of different methodologies and highlight the continuing need for detailed tests on mock data - particularly in the light of recently discovered evidence for disequilibria in the Milky Way disc. I highlight several recent measurements in order of increasing data complexity and prior, and, correspondingly, decreasing formal error bars. Comparing these measurements with spherical extrapolations from the Milky Way's rotation curve, I show that the Milky Way is consistent with having a spherical dark matter halo at the Solar position R0. The very latest measures based on ~10,000 stars from the Sloan Digital Sky Survey appear to favour little halo flattening at R0, suggesting that the Galaxy has a rather weak dark matter disc, with a correspondingly quiescent merger history [Abridged].

• The paper presents a comprehensive review of techniques to measure local dark matter density using stellar tracers and numerical simulations.
• The study employs Jeans equation modeling alongside dark-matter-only and baryonic simulations to address observational uncertainties and galactic disequilibria.
• The review highlights future prospects with Gaia data to refine dark matter estimates and clarify the Milky Way’s halo structure.

An Expert Review on "The Local Dark Matter Density" by J. I. Read

The paper by J. I. Read provides an exhaustive review of the methodologies and findings associated with measuring the local dark matter density ($dm$) near the Sun. This measurement is crucial for understanding the Milky Way's structure and aids in interpreting results from direct detection dark matter experiments. The paper also explores the theoretical framework, observational strategies, and potential challenges related to this field of research.

Theoretical Framework and Simulations

The work begins with a discussion of the $\Lambda$ Cold Dark Matter ($\Lambda$CDM) model, which is currently the most accepted cosmological model featuring dark matter. Within this framework, numerical simulations, particularly "dark-matter-only" (DMO) simulations, are instrumental in predicting parameters like the local dark matter density and velocity distribution function. The paper emphasizes the importance of baryonic physics, such as gas cooling and stellar feedback, which significantly influence the dark matter distribution and must be incorporated for accurate galaxy models.

Methodologies for Measuring $dm$

The paper outlines various approaches to measure $dm$, focusing primarily on the use of tracer stars within the Milky Way's stellar disc. These methodologies are fundamentally based on the Jeans equations derived from the collisionless Boltzmann equation under the assumption of a steady-state galaxy. Read highlights both the advantages and limitations of different techniques, including distribution function modelling and moment methods. A key insight from the paper is the significance of selecting appropriate stellar tracers and accounting for uncertainties such as observational errors and disequilibria in the Milky Way disc.

Observational Data and Recent Measurements

Recent efforts have leveraged large datasets from stellar surveys such as SDSS to determine $dm$. The paper presents a compilation of historical and modern estimates, noting the convergence of values post-Hipparcos towards a range consistent with other global constraints derived from the Milky Way's rotation curve. A notable contribution is the updated compilation of the baryonic surface density $\Sigma_b$ that remains a primary source of uncertainty in discerning $dm$.

Implications for Galactic Halo Shape

The paper explores the implications for the shape of the Milky Way's dark matter halo and the presence of an accreted dark disc. Comparisons between local measures ($dm$) and extrapolated global estimates ($dm,ext$) suggest a potential lack of halo flattening, indicative of a weak dark matter disc. This observation aligns with a historically quiescent merger history for the Milky Way. However, the conclusion acknowledges that systematic biases in the survey data could influence these findings.

The Role of Gaia and Future Prospects

Looking ahead, the Gaia satellite is expected to revolutionize measures of $dm$ by providing high-precision astrometric data for millions of stars. The paper posits that Gaia's data could facilitate moving beyond the one-dimensional approximation, global modelling of the Galactic potential, and precise determination of $dm$. Such advancements will enhance our understanding of the Milky Way's dynamics and dark matter distribution.

Conclusion

Read's review synthesizes a complex field, highlighting both historical progress and modern challenges. By integrating theoretical expectations with observational data, the paper provides comprehensive insights into the local dark matter density. With forthcoming data from Gaia, the precision of these measurements is poised to evolve, offering impactful contributions to the analysis of dark matter within our Galaxy. The paper is an essential resource for researchers aiming to understand the current status and future directions in this vital domain of astrophysics.