Milky Way s Anisotropy Profile with LAMOST/SDSS and Gaia

Transcription

1 Milky Way s Anisotropy Profile with LAMOST/SDSS and Gaia Shanghai Astronomical Observatory In collaboration with Juntai Shen, Xiang Xiang Xue, Chao Liu, Chris Flynn, Chengqun Yang

2 Contents 1 Stellar Halo 2 LAMOST + SDSS + Gaia Halo Stars 3 Results 4 Conclusion

3 Velocity anisotropy β Binney 198; Binney & Tremaine 8 β = 1 (σ 2 θ + σ2 φ )/(2σ2 r ) isotropic (β = ) radial ( < β < 1) tangential ( < β < ) estimate the mass of the Milky Way through the Jeans equation galaxy formation orbits have long dynamical time scales collisionless system orbital shapes are relatively immune to adiabatic change of the gravitational potential Bulge Globular cluster Stellar and gaseous disk Stellar halo

4 Galactic mass Figure: Bird+18 subm. Mass uncertain by 2% due to uncertainties in β alone halo K giants >41 SDSS/SEGUE >69 LAMOST DR3 M(<R gc ) [1 12 M O ] assume β =, M =.9 ± M assume β =.5, M =.7 ± M 2 1 LAMOST+SEGUE K giants total mass disk+bulge NFW fit: c=25, R = kpc 1 1 Galactocentric radius R gc [kpc]

5 Velocity anisotropy β profile β profile as seen by Simulations: slowly rising radially Solar neighborhood: β >.5 Observations past 15 kpc: variety of differing results!!! Why is β profile so difficult to measure past 15 kpc? poor statistics due to small sample sizes lack of measurements of tangential velocity dispersion What is the solution? = 1 ( )/(2 2 r ) input isothermal model input =.5 model input Sommer-Larsen et al. (1994, 1997) model input Kafle et al. (212) model r gc [kpc]

6 Contents 1 Stellar Halo 2 LAMOST + SDSS + Gaia Halo Stars 3 Results 4 Conclusion

7 Galactic halo stars from LAMOST+SDSS+Gaia Selection criteria: SDSS/SEGUE+LAMOST DR5+Gaia DR2 K giants defined by T eff and log g Liu+14 photometric distances Xue+14 Blue horizontal branch (BHB) Xue+8 Limits in color and Balmer line profile photometric distances Z > 5 kpc [Fe/H] < 1.3 dex SDSS/SEGUE >41 K giants >37 BHBs LAMOST DR5: >86 K giants Z [kpc] LAMOST DR5 + Gaia DR X [kpc]

8 Contents 1 Stellar Halo 2 LAMOST + SDSS + Gaia Halo Stars 3 Results 4 Conclusion

9 Number histogram of LAMOST DR5 halo K giants 5 halo K giants number of stars Galactocentric radius r gc [kpc]

10 Median of velocity errors vs r gc Figure: Bird+18 subm. Propagate observational errors to 3D velocities line-of-sight velocity distance proper motions in ra and dec Monte Carlo sampling to estimate 3D median velocity errors median of velocity error [km s 1 ] V r V V Galactocentric radius r gc [kpc]

11 Velocity, metallicity, r gc Figure: Bird+18 subm. evidence of velocity dependency on [Fe/H] signs of substructure vr [km s 1 ] v [km s 1 ] v [km s 1 ] [Fe/H] [dex] [Fe/H] [dex] [Fe/H] [dex] vr [km s 1 ] v [km s 1 ] v [km s 1 ] Galactocentric radius rgc [kpc] Galactocentric radius rgc [kpc] Galactocentric radius rgc [kpc]

12 Velocity dispersion vs r gc Figure: Bird+18 subm. σ r > σ θ, σ φ at all r gc 3D velocity dispersion profiles dropping for r gc < 2 kpc evidence of Sagittarius stream r gc > 2 kpc velocity dispersion [km s 1 ] Galactocentric radius r gc [kpc] r

13 Anisotropy vs r gc Figure: Bird+18 subm. highly radial within r gc < 2 kpc gently falls to lower radial values for r gc > 2 kpc = 1 ( )/(2 2 r ) halo K giants Galactocentric radius r gc [kpc]

14 Remove Sagittarius Figure: Bird+18 subm. energy E vs total angular momentum L define a Milky Way potential using the python package galpy Bovy 215 select one substructure to remove and see effect on velocity anisotropy L [kpc km s 1 ] E [(km s 1 ) 2 ]

15 Anisotropy vs r gc Figure: Bird+18 subm. After removal of Sagittarius stream: highly radial profile extends further to r gc < 3 kpc gently falling section is smoothed of jagged = 1 ( )/(2 2 r ) no subtraction E L subtraction features Galactocentric radius r gc [kpc]

16 Anisotropy and [Fe/H] dependency Figure: Bird+18 subm. the most metal poor K giants less radial orbits constant β profile extending to r gc > 25 kpc β can be used to locate substructure see = 1 ( )/(2 2 r ) e.g. Loebman [Fe/H] < [Fe/H] < < [Fe/H] < Galactocentric radius r gc [kpc]

17 Anisotropy for LAMOST K giants and SDSS BHBs β for LAMOST K giants compared with SDSS BHB s in common metallicity bins: similar β profile similar β dependency on metallicity distance and metallicity determinations are in concordance = 1 ( )/(2 2 r ) K giants: 1.5 [Fe/H] < 1.3 K giants: 2.5 < [Fe/H] < 1.8 BHBs: 1.5 [Fe/H] < 1.3 BHBs: 2.5 [Fe/H] < Galactocentric radius r gc [kpc]

18 Contents 1 Stellar Halo 2 LAMOST + SDSS + Gaia Halo Stars 3 Results 4 Conclusion

19 Key Conclusions and Discoveries LAMOST/SDSS + Gaia DR2 yield over 16 halo K-giant and BHB stars furthest stars exceed 1 kpc first presentation of 3D velocity profiles for such a large and far-reaching halo star sample! 3D velocity dispersion varies with radius β > : radially dominated halo star orbits β profile is constant up to distances exceeding r gc = 2 kpc removing Sagittarius causes β to remain flattened nearly to 3 kpc : β dependence on [Fe/H]: systematic decrease of β for [Fe/H]< 1.8 K giants and BHB s both share similar: radially dominated stellar orbits for r gc < 3 kpc β dependence on [Fe/H]

20 Next steps Compare observations to galaxies formed in cosmological simulations Check the influence of removing substructure for SDSS halo samples Integrate orbits for our sample Use Jeans equation to measure Galactic mass Thanks!!!

21 End of Presentation Backup Slides

22 Distance: Gaia vs. LAMOST K giants are good distance indicators halo K-giant photometric distances (from LAMOST/SDSS) largely have smaller errors than Gaia relative error photometric distance parallax distance parallax distances distance from Sun [kpc]

23 Anisotropy for SDSS/LAMOST K giants and BHBs β for SDSS BHB s and K Giants compared with LAMOST K Giants β profile for LAMOST and SDSS K giants is similar β profile for BHB s is lower than for K giants: Why? BHB s peak at lower metallicity β depends on metallicity β binned by [Fe/H] is similar = 1 ( )/(2 2 r ) LAMOST K giants SDSS K giants SDSS BHB's Galactocentric radius r gc [kpc]