Breaking tidal stream degeneracies with LAMOST

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Breaking tidal stream degeneracies with LAMOST

• Jorge Peñarrubia (IoA)

Cambridge 2nd December 08
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Local Group Cosmology

• Cosmological Paradigm predicts that galaxies
  form through mergers of smaller galaxies

This process continues nowadays in the Milky Way
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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 blur
with time
Photometric surveys can only reveal the most
recent accretions
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Full 3D Kinematics

• Unbound particles escape through leading and
  trailing tails
• Tails approx. follow the orbit of the progenitor
  system .

• strong constraints on the
• present host potential
• progenitors orbit
• progenitors mass lost fraction
• progenitors luminosity

for each accretion event
Peñarrubia et al. (2005)
cosmological merger tree
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Radial velocity surveys

• Proper motions can only be measured in a small
  volume (GAIAlt20 kpc)
• In contrast, radial velocities can be measured lt
  1 Mpc (DEIMOS _at_ Keck)
• Mapping the sky via Radial velocity surveys
• RAVE mlt16 gt Dlt 1.6 kpc (M5) 120
  objects - field
• LAMOST mlt20 gt Dlt 10 kpc (M5) 4000
  objects - field

Position radial velocity 4D info of
stellar streams
to infer the orbitmass of progenitor we need
numerical modelling
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Numerical modelling of tidal streams
Owing to the large parameter space model
degeneracies are unavoidable Free parameters

• Flattening (q) of the host potential
• Orbital apocentre
• Orbital inclination
• Orbital eccentricity
• Mass and concentration of the satellites DM
  halo
• Segregation of the satellites stellar
  component
• Satellite luminosity
• Accretion time
• (if progenitor is unknown)
• Present progenitor position velocity (6
  param.)

Constraints

• position of stream pieces from photometric
  surveys

spectroscopic surveys will break fundamental
model degeneracies
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Ideal Targets for LAMOST

• Previous photometric surveys have revealed a
  large number of stream-like structures at Dlt 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??
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Example 1 The Sgr stream and the shape of the
Milky Way potential
Belokurov et al. 2006 (SDSS2MASS)
Sgr core
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Example 1 The Sgr stream and the shape of the
Milky Way potential

• Constraints
• Sgr dwarfs position
  (D,l,b) (25 kpc, 5.60,-14.50)
• Sgr dwarfs radial velocity
  vrad 171 km/s
• Orbital plane inclination i76o
• Free parameters
• tangential vel. (vtan) (eccentricityrapo)
• halo axis-ratio (qh)

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Example 1 The Sgr stream and the shape of the
Milky Way potential
from 2-MASS (M-giants)

• Oblate halo models (0.85ltqlt0.95) match precession
  rate
• Prolate halo models (qgt1) match radial velocities
  along the stream

Johnston et al. 2005
using the same data inconsistent results !!
Law et al. 2005
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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
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Example 2 The Monoceros stream
Degenerated model Prograde vs Retrograde
orbits
Radial velocities between l gt 220o l
lt 110o will break model degeneracy
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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.
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Example 4 Halo clumpiness
According to CDM, there are 104 subhaloes in the
MW with Mgt107 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