On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution

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 tracer population. Also, there is a degeneracy between the parameters of the halo and that of the disk or bulge. Most previous attempts that use halo stars have made arbitrary assumptions about these. In this paper, we decompose the Galaxy into 3 major componentsa bulge, a Miyamoto-Nagai disk and an NFW dark matter halo and then model the kinematic data of the halo Blue Horizontal Branch and K-giant stars from the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Additionally, we use the gas terminal velocity curve and the Sgr A * proper motion. With the distance of the Sun from the centre of Galaxy R ⊙ = 8.5 kpc, our kinematic analysis 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 tracer velocity anisotropy is radially biased with β s = 0.4 ± 0.2 in the outer halo. We measure halo virial mass M vir to be 0.80 +0.31 −0.16 × 10 12 M ⊙ , concentration c to be 21.1 +14.8 −8.3 , disk mass to be 0.95 +0.24 −0.30 × 10 11 M ⊙ , disk scale length to be 4.9 +0.4 −0.4 kpc and bulge mass to be 0.91 +0.31 −0.38 × 10 10 M ⊙ . 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 but interestingly this suggests a truncation of the stellar halo density rather than a small overall mass of the Galaxy. Our estimates of local escape velocity v esc = 550.9 +32.4 −22.1 kms −1 and dark matter density ρ DM ⊙ = 0.0088 +0.0024 −0.0018 M ⊙ pc −3 (0.35 +0.08 −0.07 GeV cm −3 ) are in good agreement with recent estimates. Some of the above estimates, in particular M vir , are depended on the adopted value of R ⊙ and also, on the choice of the outer power-law index of the tracer number density.

The Milky Way's rotation curve out to 100 kpc and its constraint on the Galactic mass distribution

“…The results are in excellent agreement with the recent measurements of Kafle et al (2014). The concentration parameter c is 18.06 +1.26 −0.90 , which also agrees well with the recent determinations of Kafle et al (2014) and Piffl et al (2014). As expected from Eq.…”

“…From the proper motions of main-sequence stars measured by HST, Deason et al (2013) find the halo is isotropic (β = 0.0 +0.2 −0.4 ) at r = 24 ± 6 kpc. The halo beyond 50 kpc is again found to be radially biased with β = 0.4 ± 0.2, based on the most recent study of Kafle et al (2014). Table 2 summarizes these latest measurements of β.…”

What to expect from dynamical modelling of galactic haloes

“…Deg & Widrow 2014;Kafle et al 2014;Wang et al 2015). Despite the difference to other methods, we also see a strong anticorrelation between M 200 and c 200 .…”

The stellar mass, star formation rate and dark matter halo properties of LAEs at z ∼ 2

“…A red (blue) curve indicates the evolution of the mode of the M h distribution starting from the mass of our z = 2.2 LAEs shown by a red star (Guaita et al 2010, 's z = 2.1 LAEs shown by a blue circle), with a shaded region indicating the 68% confidence interval of the distribution. Black and gray rectangles represent the measured halo mass ranges of the MW and the LMC, respectively (e.g., Wilkinson & Evans 1999;Kafle et al 2014;van der Marel & Kallivayalil 2014;Eadie et al 2015;Peñarrubia et al 2016, see also Wang et al 2015). Bielby et al 2016;Diener et al 2017;Ouchi et al 2017).…”