Isotropic at the Break? 3d Kinematics of Milky Way Halo Stars in the Foreground of M31

Cunningham, Emily C.; Deason, Alis J.; Guhathakurta, Puragra; Rockosi, Constance M.; Marel, Roeland P. van der; Toloba, Elisa; Gilbert, Karoline M.; Sohn, Sangmo Tony; Dorman, Claire E.

doi:10.3847/0004-637x/820/1/18

Cited by 26 publications

(40 citation statements)

References 53 publications

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“…On the other hand, there is strong evidence to expect  b 0 for the Galactic stellar halo (Kafle et al 2014), and previous studies have shown that the velocity distribution of GCs in the MW halo is mildly radial (Deason et al 2011). Furthermore, values of b < -1 are known to be unrealistic velocity anisotropies for distant halo stars (e.g., Deason et al 2013;Cunningham et al 2016). Taking all of this information into account, we set a conservative lower limit of b = -0.5 lower .…”

Section: Priorsmentioning

confidence: 99%

Bayesian Mass Estimates of the Milky Way: The Dark and Light Sides of Parameter Assumptions

Eadie

Harris

2016

ApJ

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We present mass and mass profile estimates for the Milky Way (MW) Galaxy using the Bayesian analysis developed by Eadie et al. and using globular clusters (GCs) as tracers of the Galactic potential. The dark matter and GCs are assumed to follow different spatial distributions; we assume power-law model profiles and use the model distribution functions described in Evans et al. and Deason et al. We explore the relationships between assumptions about model parameters and how these assumptions affect mass profile estimates. We also explore how using subsamples of the GC population beyond certain radii affect mass estimates. After exploring the posterior distributions of different parameter assumption scenarios, we conclude that a conservative estimate of the Galaxy's mass within 125 kpcis5.22 10 11  M , with a 50% probability region of4. ). If we consider only the GCs beyond 10 kpc, then the virial mass iś 9.02 5.69, 10.86 10 11 ( )vir 24 19 kpc). We also arrive at an estimate of the velocity anisotropy parameter β of the GC population, which is b = 0.28 with a 50% credible region (0.21, 0.35). Interestingly, the mass estimates are sensitive to both the dark matter halo potential and visible matter tracer parameters, but are not very sensitive to the anisotropy parameter.

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Section: Priorsmentioning

confidence: 99%

Bayesian Mass Estimates of the Milky Way: The Dark and Light Sides of Parameter Assumptions

Eadie

Harris

2016

ApJ

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“…As a result, a considerable effort has been expended in measuring the stellar halo's kinematic moments (e.g., Xue et al 2008;Bond et al 2010;Cunningham et al 2016). Recently, emphasis has been placed on the measurement of the velocity anisotropy parameter (β), the ratio of tangential to radial random motion, which is expected to be positive from simple numerical experiments on halo formation (Binney & Tremaine 2008).…”

Section: Introductionmentioning

confidence: 99%

“…Recently, emphasis has been placed on the measurement of the velocity anisotropy parameter (β), the ratio of tangential to radial random motion, which is expected to be positive from simple numerical experiments on halo formation (Binney & Tremaine 2008). However, measurements of β in the MW have suggested that it is negative within 15 R/kpc25 (see Kafle et al 2012, King et al 2015, but see Deason et al 2013b, Cunningham et al 2016 for alternative values), leading to speculation that the exact merger and dissipation history of the stellar halo could strongly affect its velocity anisotropy profile (Deason et al 2013a(Deason et al , 2013b. In spite of these recent efforts to infer the MW's accretion history from measurements of the density profile and β, there have been no systematic studies of how β varies with radius in realistic cosmological hydrodynamic simulations that demonstrate that β is in fact a tracer of assembly history.…”

Section: Introductionmentioning

confidence: 99%

Beta Dips in the Gaia Era: Simulation Predictions of the Galactic Velocity Anisotropy Parameter (β) for Stellar Halos

Loebman

Valluri

Hattori

et al. 2018

ApJ

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The velocity anisotropy parameter, β, is a measure of the kinematic state of orbits in the stellar halo, which holds promise for constraining the merger history of the Milky Way (MW). We determine global trends for β as a function of radius from three suites of simulations, including accretion-only and cosmological hydrodynamic simulations. We find that the two types of simulations are consistent and predict strong radial anisotropy ( b á ñ~0.7) for Galactocentric radii greater than 10 kpc. Previous observations of β for the MW's stellar halo claim a detection of an isotropic or tangential "dip" at r∼20 kpc. Using the N-body+SPH simulations, we investigate the temporal persistence, population origin, and severity of "dips" in β. We find that dips in the in situ stellar halo are long-lived, while dips in the accreted stellar halo are short-lived and tied to the recent accretion of satellite material. We also find that a major merger as early as z∼1 can result in a present-day low (isotropic to tangential) value of β over a broad range of radii and angles. While all of these mechanisms are plausible drivers for the β dip observed in the MW, each mechanism in the simulations has a unique metallicity signature associated with it, implying that future spectroscopic surveys could distinguish between them. Since an accurate knowledge of β(r) is required for measuring the mass of the MW halo, we note that significant transient dips in β could cause an overestimate of the halo's mass when using spherical Jeans equation modeling.

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“…In Deason et al (2013b), we published the first proper motions of distant Milky Way halo stars; in Cunningham et al (2016), we present the line-of-sight (LOS) velocities for those same stars. Our 13 objects are the first stars outside of the solar neighborhood to have 3D kinematic information.…”

mentioning

confidence: 99%

“…Using the 6D phase space information from our survey, we have made the first direct measure of the velocity anisotropy outside of the solar neighborhood (Cunningham et al 2016).…”

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confidence: 99%

Measuring the Stellar Halo Velocity Anisotropy With 3D Kinematics

et al. 2015

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Abstract. We present the first measurement of the anisotropy parameter β using 3D kinematic information outside of the solar neighborhood. Our sample consists of 13 Milky Way halo stars with measured proper motions and radial velocities in the line of sight of M31. Proper motions were measured using deep, multi-epoch HST imaging, and radial velocities were measured from Keck II/DEIMOS spectra. We measure β = −0.3 + 0. 4 −0 . 9 , which is consistent with isotropy, and inconsistent with measurements in the solar neighborhood. We suggest that this may be the kinematic signature of a relatively early, massive accretion event, or perhaps several such events.Global kinematic properties, such as the relative pressure between tangential and radial velocity components, otherwise known as the velocity anisotropy, can provide important insight into the formation of the stellar halo. Using the 6D phase space information from our survey, we have made the first direct measure of the velocity anisotropy outside of the solar neighborhood (Cunningham et al. 2016).In Deason et al. (2013b), we published the first proper motions of distant Milky Way halo stars; in Cunningham et al. (2016), we present the line-of-sight (LOS) velocities for those same stars. Our 13 objects are the first stars outside of the solar neighborhood to have 3D kinematic information. The left panel of Figure 1 shows a cumulative LOS velocity histogram of the 13 stars in our sample, and the right panel shows the proper motions of the halo stars color coded by LOS velocity (velocities are shown in the Galactocentric frame). We estimated the parameters of the velocity ellipsoid ( v l , σ LOS , σ l , σ b ) using Markov Chain Monte Carlo (Figure 2), and made the first measurement of the anisotropy parameter β using 3D kinematic information outside of the solar neighborhood. We find β = −0.3−0.9 , consistent with isotropy and inconsistent with solar neighborhood measurements, which find a highly radial anisotropy (β ∼ 0.5 − 0.7). Simulations of accreted stellar haloes indicate that β should increase outward (e.g. Abadi et al. 2006), and yet we observe a "dip" in the radial anisotropy profile. This dip is coincident with the break radius in the Milky Way's stellar density profile. We argue that the presence of large, dynamically "fluffy" global substructre, such as a shell (or multiple shells), is one explanation for both the steep fall-off in stellar density beyond the break radius and the decrease in anisotropy at that radius. Deason et al. (2013a) argued that a break in the Milky Way stellar density profile could be created by the build-up of stars at apocenter from either one relatively massive accretion event or several, synchronous accretion events. In this 288 at https://www.cambridge.org/core/terms. https://doi

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Isotropic at the Break? 3d Kinematics of Milky Way Halo Stars in the Foreground of M31

Cited by 26 publications

References 53 publications

Bayesian Mass Estimates of the Milky Way: The Dark and Light Sides of Parameter Assumptions

Bayesian Mass Estimates of the Milky Way: The Dark and Light Sides of Parameter Assumptions

Beta Dips in the Gaia Era: Simulation Predictions of the Galactic Velocity Anisotropy Parameter (β) for Stellar Halos

Measuring the Stellar Halo Velocity Anisotropy With 3D Kinematics

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