Signatures of ΛCDM substructure in tidal debris
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Transcript of Signatures of ΛCDM substructure in tidal debris
TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 1
Signatures of ΛCDM substructure in tidal debris
Jennifer Siegal-Gaskinsin collaboration with Monica Valluri
Image credit: Martinez-Delgado & Perez
TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 2
Observed tidal streams
NGC 5907, Shang et al. 1998
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Sgr stream bifurcation Monoceros stream
`Field of Streams’, Belokurov et al. 2006 (SDSS)
Also: globular cluster streams (Pal 5, NGC 5466)
More data on the way: e.g., SDSS-Segue, GAIA, RAVE, SIM-Planetquest
TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 3
Tidal streams in CDM halos Part I: substructure
Image credit: A. Kravtsov
Recently many new MW satellites have been discovered! e.g., SDSS
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
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
TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 4
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
Prolate Oblate
q > 1 q < 1
orbits at large radii particularly sensitive to halo shape
note: CDM halos likely triaxial - more complicated orbits!
TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 5
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 doesn’t 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!)
TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 6
Phase space properties of orbitsa ‘surface of section plot’ can be used to map out
the range of possible orbits in a potential
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resonances
TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 7
Example orbits: spherical halo
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TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 8
Example orbits: oblate halo
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Example orbits: prolate halo
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SimulationsN-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
TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 11
Sky distribution
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Spherical halodisplacement of debris
broad stream
without substructure with substructure
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Oblate halo
Prolate halo
bifurcation?
clumpier, less dispersed debris WITH substructure
highly dispersed without substructure
very similar
TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 13
Phase space structure
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Spherical halo
stars at large radiicompact, little structure
coherent bands
without substructure with substructure
non-resonant
resonant
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Oblate halo
Prolate halo
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stars at large radii
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Heating by substructure?
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Prolate halo
Spherical halo
Oblate haloV l
os,G
SR
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-resonant 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!