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Signatures of ΛCDM substructure in tidal debris

Signatures of ΛCDM substructure in tidal debris . Jennifer Siegal-Gaskins in collaboration with Monica Valluri. Image credit: Martinez-Delgado & Perez. bifurcation. Monoceros stream. Sgr stream. ‘orphan’ stream. NGC 5907, Shang et al. 1998. `Field of Streams’, Belokurov et al. 2006 (SDSS).

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Signatures of ΛCDM substructure in tidal debris

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  1. Signatures of ΛCDM substructure in tidal debris Jennifer Siegal-Gaskins in collaboration with Monica Valluri Image credit: Martinez-Delgado & Perez J. Siegal-Gaskins

  2. bifurcation Monoceros stream Sgr stream ‘orphan’ stream NGC 5907, Shang et al. 1998 `Field of Streams’, Belokurov et al. 2006 (SDSS) Observed tidal streams Also: globular cluster streams (Pal 5, NGC 5466) More data on the way: e.g., SDSS-Segue, GAIA, RAVE, SIM-Planetquest J. Siegal-Gaskins

  3. Image credit: A. Kravtsov 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 J. Siegal-Gaskins

  4. Prolate Oblate q > 1 q < 1 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! J. 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!) J. Siegal-Gaskins

  6. resonances Phase space properties of orbits a ‘surface of section plot’ can be used to map out the range of possible orbits in a potential J. Siegal-Gaskins

  7. Example orbits: spherical halo J. Siegal-Gaskins

  8. Example orbits: oblate halo J. Siegal-Gaskins

  9. Example orbits: prolate halo J. Siegal-Gaskins

  10. 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 J. Siegal-Gaskins

  11. Sky distribution displacement of debris Spherical halo without substructure with substructure broad stream J. Siegal-Gaskins

  12. Oblate halo bifurcation? clumpier, less dispersed debris WITH substructure highly dispersed without substructure very similar Prolate halo J. Siegal-Gaskins

  13. Phase space structure Spherical halo without substructure with substructure coherent bands non-resonant stars at large radii compact, little structure resonant J. Siegal-Gaskins

  14. Oblate halo little structure stars at large radii Prolate halo J. Siegal-Gaskins

  15. Heating by substructure? Spherical halo Vlos,GSR Oblate halo Prolate halo Galactic longitude J. Siegal-Gaskins

  16. Future Directions • observations!!! • effect of n-body substructure • effect of microhalos • triaxial potential J. Siegal-Gaskins

  17. 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! J. Siegal-Gaskins

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