1. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins1 Signatures of ΛCDM substructure in tidal debris Jennifer Siegal-Gaskins in collaboration with Monica Valluri Image credit: Martinez-Delgado & Perez

2. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins2 Observed tidal streams NGC 5907, Shang et al. 1998 ‘orphan’ stream Sgr stream bifurcationMonoceros 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

3. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins3 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

4. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins4 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 ProlateOblate q > 1 q < 1 orbits at large radii particularly sensitive to halo shape note: CDM halos likely triaxial - more complicated orbits!

5. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins5 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!)

6. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins6 Phase space properties of orbits a ‘surface of section plot’ can be used to map out the range of possible orbits in a potential resonances

7. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins7 Example orbits: spherical halo

8. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins8 Example orbits: oblate halo

9. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins9 Example orbits: prolate halo

10. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins10 Simulations N-body tree code GADGET-2 (V. Springel) Static Milky Way potential: –Halo, disk, and bulge -Total mass ~ 10 12 M solar Satellite: -NFW profile -Initially 500k particles, 10 10 M solar -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

11. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins11 Sky distribution Spherical halo displacement of debris broad stream without substructure with substructure

12. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins12 Oblate halo Prolate halo bifurcation? clumpier, less dispersed debris WITH substructure highly dispersed without substructure very similar

13. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins13 Phase space structure Spherical halo stars at large radii compact, little structure coherent bands without substructurewith substructure non-resonant resonant

14. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins14 Oblate halo Prolate halo little structure stars at large radii

15. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins15 Heating by substructure? Prolate halo Spherical halo Oblate halo V los,GSR Galactic longitude

16. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins16 Future Directions observations!!! effect of n-body substructure effect of microhalos triaxial potential

17. TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins17 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!