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Breaking tidal stream degeneracies with LAMOST

Breaking tidal stream degeneracies with LAMOST. Jorge Peñarrubia (IoA). Cambridge 2nd December 08. Local Group Cosmology. Cosmological Paradigm predicts that galaxies form through mergers of smaller galaxies. This process continues nowadays in the Milky Way…. Local Group Cosmology.

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Breaking tidal stream degeneracies with LAMOST

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  1. Breaking tidal stream degeneracies with LAMOST Jorge Peñarrubia (IoA) Cambridge 2nd December 08

  2. Local Group Cosmology Cosmological Paradigm predicts that galaxies form through mergers of smaller galaxies This process continues nowadays in the Milky Way…

  3. Local Group Cosmology The Local Group is the only system where the kinematics of individual stars can be measured with high precision why is this important? We want to decompose the hierarchical formation of our galaxy = set of individual mergers However, the remnants of accreted satellites blurwith time Photometric surveys can only reveal the most recent accretions

  4. Full 3D Kinematics • Unbound particles escape through leadingand trailing tails • Tails approx. follow the orbit of the progenitor system …. • strong constraints on the • presenthost potential • progenitor’sorbit • progenitor’smass lost fraction • progenitor’s luminosity for each accretion event Peñarrubia et al. (2005) cosmological merger tree

  5. Radial velocity surveys Proper motions can only be measured in a small volume (GAIA<20 kpc) In contrast, radial velocities can be measured < 1 Mpc (DEIMOS @ Keck) Mapping the sky via Radial velocity surveys: • RAVE: m<16 => D< 1.6 kpc (M=5) 120 objects - field • LAMOST: m<20 => D< 10 kpc (M=5) 4000objects - field Position + radial velocity = 4D info of stellar streams to infer the orbit+mass of progenitor we need numerical modelling

  6. Numerical modelling of tidal streams Owing to the large parameter spacemodel degeneracies are unavoidable Free parameters • Flattening (q) of the host potential • Orbital apocentre • Orbital inclination • Orbital eccentricity • Mass and concentration of the satellite’s DM halo • Segregation of the satellite’s stellar component • Satellite luminosity • Accretion time (ifprogenitor is unknown) • Present progenitor position + velocity (6 param.) Constraints: • position of stream pieces from photometric surveys spectroscopic surveys will break fundamental model degeneracies

  7. Ideal Targets for LAMOST Previous photometric surveys have revealed a large number of stream-like structures at D< 50 kpc Potential targets for LAMOST are: • Sagittarius stream • Monoceros stream • Virgo over-density • Hercules-Aquila over-density • Palomar 5 stream • …….. etc All located in the Northern Galactic Hemisphere !! (the South remains terra incognita) What could we learn if we had LAMOST data now??

  8. Example 1: The Sgr stream and the shape of the Milky Way potential Belokurov et al. 2006 (SDSS+2MASS) Sgr core

  9. Example 1: The Sgr stream and the shape of the Milky Way potential • Constraints: • Sgr dwarf’s position: (D,l,b) = (25 kpc, 5.60,-14.50) • Sgr dwarf’s radial velocity: vrad = 171 km/s • Orbital plane inclination: i=76o • Free parameters • tangential vel. (vtan) (eccentricity==rapo) • halo axis-ratio (qh)

  10. Example 1: The Sgr stream and the shape of the Milky Way potential from 2-MASS (M-giants) : • Oblatehalo models (0.85<q<0.95) matchprecession rate • Prolate halo models (q>1) matchradial velocitiesalong the stream Johnston et al. 2005 using the same data inconsistent results !! Law et al. 2005

  11. Example 2: The Monoceros stream Again, M-Giant over-densities • M-giants show a large dispersion on the sky • They move on nearly circular orbits Do all over-densities belong to the Mon stream? Penarrubia et al. 2005

  12. Example 2: The Monoceros stream Degenerated model : Prograde vs Retrograde orbits Radial velocities between l > 220o l < 110o will break model degeneracy

  13. Example 3: Field(s) of streams In the next few years, the number of streams detected via photometric surveys (SDSS I,II,III; Pan-STARRS) will dramatically increase. Monoceros Orphan Sgr Pal 5 Virgo Kinematics will be crucial for their modelling….

  14. Example 4: Halo clumpiness According to CDM, there are ~104 subhaloes in the MW with M>107 Msol Cold tidal streams (e.g from GCs with ~ 1 km/s) may be heated by encounters with DM subhaloes kinematical surveys of GC streams could potentially constrain the number density of DM clumps Grillmair & Dionatos 2006

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