Globular Cluster Streams as Galactic High-Precision Scales - The Poster Child Palomar 5

TL;DR

This paper demonstrates how the Palomar 5 globular cluster stream can be modeled using Bayesian inference to precisely measure Galactic parameters, providing a new high-precision scale for the Milky Way.

Contribution

It introduces a Bayesian framework combining stream detection and modeling to constrain Galactic parameters using globular cluster streams, with no prior assumptions on model parameters.

Findings

Measured the Sun's distance from Galactic Center as 8.30±0.25 kpc

Estimated the Galactic mass within 19 kpc as (2.1±0.4)×10^{11} M⊙

Determined the dark halo's flattening parameter as q_z=0.95^{+0.16}_{-0.12}

Abstract

Using the example of the tidal stream of the Milky Way globular cluster Palomar 5 (Pal 5), we demonstrate how observational data on streams can be efficiently reduced in dimensionality and modeled in a Bayesian framework. Our approach combines detection of stream overdensities by a Difference-of-Gaussians process with fast streakline models, a continuous likelihood function built from these models, and inference with MCMC. By generating $\approx10^7$ model streams, we show that the geometry of the Pal 5 debris yields powerful constraints on the solar position and motion, the Milky Way and Pal 5 itself. All 10 model parameters were allowed to vary over large ranges without additional prior information. Using only SDSS data and a few radial velocities from the literature, we find that the distance of the Sun from the Galactic Center is $8.30\pm0.25$ kpc, and the transverse velocity is $253\pm16$ km/s. Both estimates are in excellent agreement with independent measurements of these quantities. Assuming a standard disk and bulge model, we determine the Galactic mass within Pal 5's apogalactic radius of 19 kpc to be $(2.1\pm0.4)\times10^{11}$ M$_\odot$. Moreover, we find the potential of the dark halo with a flattening of $q_z = 0.95^{+0.16}_{-0.12}$ to be essentially spherical within the radial range that is effectively probed by Pal 5. We also determine Pal 5's mass, distance and proper motion independently from other methods, which enables us to perform vital cross-checks. We conclude that with more observational data and by using additional prior information, the precision of this method can be significantly increased.