The Milky Way total mass profile as inferred from Gaia DR2 (1911.04557v2)

Abstract: We determine the Milky Way (MW) mass profile inferred from fitting physically motivated models to the Gaia DR2 Galactic rotation curve and other data. Using various hydrodynamical simulations of MW-mass haloes, we show that the presence of baryons induces a contraction of the dark matter (DM) distribution in the inner regions, r<20 kpc. We provide an analytic expression that relates the baryonic distribution to the change in the DM halo profile. For our galaxy, the contraction increases the enclosed DM halo mass by factors of roughly 1.3, 2 and 4 at radial distances of 20, 8 and 1 kpc, respectively compared to an uncontracted halo. Ignoring this contraction results in systematic biases in the inferred halo mass and concentration. We provide a best-fitting contracted NFW halo model to the MW rotation curve that matches the data very well. The best-fit has a DM halo mass, $M_{200}^{\rm DM}=0.97_{-0.19}^{+0.24}\times10^{12} M_\odot$, and concentration before baryon contraction of $9.4_{-2.6}^{+1.9}$, which lie close to the median halo mass--concentration relation predicted in $\Lambda$CDM. The inferred total mass, $M_{200}^{\rm total}=1.08_{-0.14}^{+0.20} \times 10^{12} M_\odot$, is in good agreement with recent measurements. The model gives a MW stellar mass of $5.04_{-0.52}^{+0.43}\times10^{10} M_\odot$ and infers that the DM density at the Solar position is $\rho_{\odot}^{\rm DM}=8.8_{-0.5}^{+0.5}\times10^{-3} M_\odot \rm{pc}^{-3}\equiv0.33_{-0.02}^{+0.02}~\rm{GeV}~\rm{cm}^{-3}$. The rotation curve data can also be fitted with an uncontracted NFW halo model, but with very different DM and stellar parameters. The observations prefer the physically motivated contracted NFW halo, but the measurement uncertainties are too large to rule out the uncontracted NFW halo.

• The paper shows that baryonic processes contract the Milky Way's dark matter halo, significantly boosting the enclosed mass at inner radii.
• It utilizes best-fitting contracted NFW models with parameters like M200 and concentration that align with the observed rotation curve data.
• The findings stress the need to account for baryonic effects to avoid biases in halo mass and concentration estimations.

The Milky Way Total Mass Profile from Gaia DR2

The paper presents an analysis of the Milky Way (MW) mass profile derived from Gaia Data Release 2 (DR2). By employing physically motivated models and hydrodynamical simulations, the researchers investigate how baryonic processes, such as star formation and gas dynamics, influence the dark matter (DM) distribution in the MW, particularly focusing on the contraction of the halo.

Key Findings

The authors illustrate that baryonic matter significantly contracts the DM halo in the MW's inner regions, notably within 20 kpc of the Galactic center. This contraction modifies the DM distribution such that the enclosed mass can increase dramatically—by factors of approximately 1.3 at 20 kpc, 2 at 8 kpc, and 4 at 1 kpc—in comparison to the distribution expected if no contraction occurred. Such changes lead to biases in halo mass and concentration estimations if contraction effects are ignored.

The paper provides a best-fitting contracted Navarro-Frenk-White (NFW) halo model aligned closely with the observed MW rotation curve data, alongside parameters for the MW mass profile:

• The DM halo mass is approximated as $$M_{200}^{\rm DM} = 0.97_{-0.19}^{+0.24} \times 10^{12}$$ solar masses.
• The concentration parameter for an uncontracted halo is $c = 9.4_{-2.6}^{+1.9}$, consistent with median predictions for halo mass-concentration relationships in $\Lambda$CDM cosmology.

Additionally, the inferred total mass of the MW, including all baryonic components and a circumgalactic medium (CGM), reaches $$M_{200}^{\rm total} = 1.08_{-0.14}^{+0.20} \times 10^{12}$$ solar masses. This estimate corresponds well with recent observational data. The DM density at the Solar position is calculated as $$\rho_{\odot}^{\rm DM} = 8.8_{-0.5}^{+0.5} \times 10^{-3}$$ $\rm{GeV} ~ \rm{cm}^{-3}$.

Implications and Future Directions

The paper's findings underscore the importance of considering baryonic effects in estimating the MW's halo properties. Systematic biases can arise from assuming a pure NFW model without considering contraction, thus impacting interpretations of galactic dynamics and DM particle constraints.

The work supports a more nuanced understanding of MW dynamics, advocating for models that include baryonic-induced halo modifications. Future studies can enhance accuracy by reducing observational uncertainties, exploring model complexities, and incorporating additional baryonic physics.

Furthermore, these insights have implications across cosmic scales. Understanding the MW good's profile provides a benchmark for studies of other galaxies, with potential outcomes informing galaxy formation theories and cosmological models of structure evolution.

In conclusion, the integration of Gaia DR2 data with hydrodynamical simulations represents a step forward in quantifying the MW's total mass profile, setting a precedent for broader applications in galactic and extragalactic astronomy.