Signatures of CDM substructure in tidal debris

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Signatures of ?CDM substructure in tidal debris

• Jennifer Siegal-Gaskins
• in collaboration with Monica Valluri

Image credit Martinez-Delgado Perez
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Observed tidal streams
Also globular cluster streams (Pal 5, NGC 5466)
More data on the way e.g., SDSS-Segue, GAIA,
RAVE, SIM-Planetquest
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Tidal streams in CDM halos Part I substructure
?CDM predicts many more satellites in a Milky
Way-sized halo than are observed. e.g., Klypin
et al. 1999, Moore et al. 1999
Are they missing? e.g., Hogan Dalcanton 2000
(WDM), Spergel Steinhardt 2000 (SIDM) Or just
dark? e.g., Bullock et al. 2001b Kravtsov et al.
2004
Recently many new MW satellites have been
discovered! e.g., SDSS
Could coherent streams survive in a halo with
substructure? e.g., Ibata et al. 2002 Johnston
et al. 2002 Mayer et al. 2002 Peñarrubia et al.
2006
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Tidal streams in CDM halos Part II
non-spherical halos

• in a spherical potential, orbits are confined to
  a plane
• ? tidal debris localized to a single plane
• in a non-spherical potential, orbits not confined
  to a plane
• tidal debris fills a 3-D volume
• precession leads to heating of streams

orbits at large radii particularly sensitive to
halo shape note CDM halos likely triaxial -
more complicated orbits!
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Constraints on substructure from tidal streams?

• Is it possible to robustly detect substructure?
• Substructure could lead to heating of the streams
  -- is this a smoking gun?
• Precession in a non-spherical halo could mimic
  heating by substructure, although effects may
  decouple in, e.g., Lz distribution (precession
  doesnt increase dispersion in Lz, substructure
  does)
• But proper motions (necessary for Lz) may be
  impossible to measure for some stars
• Dwarf galaxies may already be too dispersed in Lz
  to see effect of substructure
• Is there a unique, detectable signature of
  substructure?
• Is it possible to rule out substructure?
• Would a detection of a SINGLE COHERENT STREAM
  provide strong evidence against substructure?
• Previous studies suggest that substructure is
  incompatible with cold streams (e.g. Ibata et al.
  2002)
• Can we expect coherent streams to survive in any
  scenario with substructure?

(May also be possible to constrain halo shape
with streams, but previous studies very
inconclusive!)
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Phase space properties of orbits
a surface of section plot can be used to map
out the range of possible orbits in a potential
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Example orbits spherical halo
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Example orbits oblate halo
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Example orbits prolate halo
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Simulations

• N-body tree code GADGET-2 (V. Springel)
• Static Milky Way potential
• Halo, disk, and bulge
• Total mass 1012 Msolar
• Satellite
• NFW profile
• Initially 500k particles, 1010 Msolar
• Tidally stripped to produce remnant in
  quasi-equilibrium with host potential, 150k
  particles
• 5 Gyr integration
• star particles marked
• Dark matter substructure
• Softened point masses from cosmological N-body
  simulation

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Sky distribution
displacement of debris
Spherical halo
without substructure
with substructure
broad stream
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Oblate halo
bifurcation?
clumpier, less dispersed debris WITH substructure
highly dispersed without substructure
very similar
Prolate halo
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Phase space structure
Spherical halo
without substructure
with substructure
coherent bands
non-resonant
stars at large radii
compact, little structure
resonant
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Oblate halo
little structure
stars at large radii
Prolate halo
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Heating by substructure?
Spherical halo
Vlos,GSR
Oblate halo
Prolate halo
Galactic longitude
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Future Directions

• observations!!!
• effect of n-body substructure
• effect of microhalos
• triaxial potential

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Conclusions

• Halo shape and orbital path strongly influence
  structure of tidal streams, generally more
  important than substructure for overall stream
  formation, but not clear that resonant/non-resonan
  t is important
• Substructure seems to lead to clumping
• Substructure can shift the location of debris
  (very important for modeling!)
• Cumulative effect of substructures may be
  significant
• Unique signature of substructure stars kicked to
  large distances - likely by massive substructures
• In contrast with previous studies Cannot rule
  out substructure with a coherent stream, but can
  detect substructure with unique signature!