On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution (1408.1787v1)

Abstract: Halo stars orbit within the potential of the Milky Way and hence their kinematics can be used to understand the underlying mass distribution. However, the inferred mass distribution depends sensitively upon assumptions made on the density and the velocity anisotropy profiles of the tracers. Also, there is a degeneracy between the parameters of the halo and that of the disk or bulge. Here, we decompose the Galaxy into bulge, disk and dark matter halo and then model the kinematic data of the halo BHB and K-giants from the SEGUE. Additionally, we use the gas terminal velocity curve and the Sgr A$^*$ proper motion. With $R_\odot = 8.5$kpc, our study reveals that the density of the stellar halo has a break at $17.2^{+1.1}_{-1.0}$ kpc, and an exponential cut-off in the outer parts starting at $97.7^{+15.6}_{-15.8}$kpc. Also, we find the velocity anisotropy is radially biased with $\beta_s= 0.4\pm{0.2}$ in the outer halo. We measure halo virial mass $M_{\text{vir}} = 0.80^{+0.31}_{-0.16} \times 10^{12} M_{\odot}$, concentration $c=21.1^{+14.8}_{-8.3}$, disk mass of $0.95^{+0.24}_{-0.30}\times10^{11} M_{\odot}$, disk scale length of $4.9^{+0.4}_{-0.4}$ kpc and bulge mass of $0.91^{+0.31}_{-0.38} \times 10^{10} M_{\odot}$. The mass of halo is found to be small and this has important consequences. The giant stars reveal that the outermost halo stars have low velocity dispersion interestingly suggesting a truncation of the stellar halo density rather than a small overall mass of the Galaxy. Our estimates of local escape velocity $v_{\rm esc} = 550.9^{+32.4}_{-22.1}$ kms$^{-1}$ and dark matter density $\rho^{\rm DM}{\odot} = 0.0088^{+0.0024}{-0.0018} M_{\odot} {\rm pc^{-3}} $ ($0.35^{+0.08}_{-0.07}$ GeV cm$^{-3}$) are in good agreement with recent estimates. Some of the above estimates are depended on the adopted value of $R_\odot$ and of outer power-law index of the tracer number density.

Understanding the Stellar Halo and Mass Distribution of the Milky Way

The paper by Kafle et al., titled "On the Shoulders of Giants: Properties of The Stellar Halo And The Milky Way Mass Distribution," provides a comprehensive analysis of the properties of the Milky Way's stellar halo and its implications for the galaxy's mass distribution. The paper uses an extensive dataset from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) to model the Milky Way's components, namely the bulge, disk, and dark matter halo, employing sophisticated kinematic analyses.

Key Findings and Numerical Results

The authors decompose the Galaxy into a bulge, a Miyamoto-Nagai disk, and an NFW dark matter halo, modeling the kinematic data of the halo Blue Horizontal Branch (BHB) and K-giant stars. The kinematic analysis identifies a break in the stellar halo density at approximately 17.2 kpc and an exponential cut-off starting around 97.7 kpc. This suggests a truncation in the density profile rather than a small mass of the Galaxy, potentially indicating limitations in the observational data at such distant reaches.

The paper estimates the virial mass of the halo at $$0.80^{+0.31}_{-0.16} \times 10^{12} \, \text{M}_\odot$$ and the halo concentration at $21.1^{+14.8}_{-8.3}$. The disk mass is calculated to be $$0.95^{+0.24}_{-0.30} \times 10^{11} \, \text{M}_\odot$$, and the bulge mass is $$0.91^{+0.31}_{-0.38} \times 10^{10} \, \text{M}_\odot$$. This lightweight halo has significant implications for understanding the Milky Way's dynamics and resolving the missing satellite problem in $\Lambda$CDM cosmology, suggesting that the scarcity of massive subhalos is not necessarily a flaw of the $\Lambda$CDM model but could be a result of assuming larger Galaxy mass.

The local escape velocity, $$v_{\rm esc} = 550.9^{+32.4}_{-22.1} \, \text{km s}^{-1}$$, is consistent with recent estimates, as is the local dark matter density, $$\rho^{\rm DM}_{\odot} = 0.0088^{+0.0024}_{-0.0018} \, \text{M}_\odot \, \text{pc}^{-3}$$. These calculations are crucial for direct detection experiments targeting dark matter.

Theoretical and Practical Implications

The findings have far-reaching implications for both theoretical models and practical astrophysical applications. The detailed modeling of the Milky Way's components aids in the refinement of dynamical models that simulate galaxy formation and evolution. The ability to accurately measure the Galaxy's mass distribution influences our understanding of its formation history and its role in the broader cosmological context.

On a practical level, the meticulous characterization of the Milky Way's mass distribution aids in simulating the tidal streams and orbits of surrounding satellite galaxies, which is vital for studies of galactic dynamics and satellite interactions.

Speculation on Future Developments

Looking forward, advancements in observational technologies, such as more precise astrometric measurements and deeper surveys, will allow for even more refined models of the Galaxy's structure. Further integration of multi-wavelength data could also provide additional insights into the complex interplay between baryonic matter and dark matter within the Galactic halo. Additionally, continued developments in computational techniques will enhance the ability to simulate and analyze complex galactic interactions at unprecedented scales, offering potential breakthroughs in understanding the processes governing galaxy formation and the evolution of cosmic structures.

In conclusion, the paper by Kafle et al. offers a robust analysis of the Milky Way's mass distribution, providing important insights and constraints that will guide future research in galactic dynamics and cosmology.