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

Abstract

Using the example of the tidal stream of the Milky Way globular cluster Palomar 5 (Pal 5), we demonstrate how observational data on tidal 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 of globular cluster streams and a continuous likelihood function built from these models. Inference is performed with Markov chain Monte Carlo. By generating ≈ 10 7 model streams, we show that the unique 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 readily-available 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 ± 0.25 kpc, and the transverse velocity is 253 ± 16 km s −1 . Both estimates are in excellent agreement with independent measurements of these two 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 ± 0.4) × 10 11 M . 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 -at least 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. Our inferred heliocentric distance of Pal 5 is 23.6 +0.8 −0.7 kpc, in perfect agreement with, and more precise than estimates from isochrone fitting of deep HST imaging data. We conclude that finding and modeling more globular cluster streams is an efficient way for mapping out the structure of our Galaxy to high precision. With more observational data and by using additional prior information, the precision of this mapping can be significantly increased.

The Phoenix Stream: A Cold Stream in the Southern Hemisphere

“…The fact that ODs N and S are approximately equally separated from the central OD could point to epicyclic ODs such as the ones reported in Küpper et al (2015) for Palomar 5. Table 2 summarizes the positions and angular separation of the ODs with regard to the central peaks.…”

“…There is evidence that the MW potential, at least in its inner parts, is well-approximated by an axisymmetric potential (see Küpper et al 2015). The Phoenix stream lies at 18.4 kpc from the Galactic center where the MW potential should be reasonably spherical (Bell et al 2008;Deason et al 2011;Küpper et al 2015). Under the assumption that the stream formed through the interaction with the MW potential only, we expect that it should be confined to a plane passing through the center of the Galaxy.…”

“…For more complex potentials this assumption only holds close to the progenitor itself. There is evidence that the MW potential, at least in its inner parts, is well-approximated by an axisymmetric potential (see Küpper et al 2015). The Phoenix stream lies at 18.4 kpc from the Galactic center where the MW potential should be reasonably spherical (Bell et al 2008;Deason et al 2011;Küpper et al 2015).…”

“…Perhaps the most conspicuous examples of Galactic streams are those associated with the Pal 5 GC and the Sagittarius dwarf (Odenkirchen et al 2001;Newberg et al 2002). The tidal nature of such streams makes them useful probes of the dark matter distribution across the halo (Johnston et al 2005;Küpper et al 2015). Detailed modeling of a stream's position, distance, kinematics, gaps, and overdensities (ODs) in extended cold streams also leads to constraints on the amount of dark matter fragments orbiting the halo, known as subhalos (Yoon et al 2011;Ngan et al 2015), on the progenitor's properties and on Galactic parameters (Koposov et al 2010).…”

Exploring the reality of density substructures in the Palomar 5 stellar stream

“…To detect the position and the size of the overdensities of the Pal 5 stream in the SDSS, we apply a very similar method to that used by Küpper et al (2015). First, to bring out the stars that could belong to the Pal 5 stream, we select the stars from the SDSS DR9 with dereddened g0 < 22.5 and that follow the colour criterion of Odenkirchen et al (2003).…”