Signatures of ΛCDM substructure in tidal debris

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

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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). - PowerPoint PPT Presentation

Transcript of Signatures of ΛCDM substructure in tidal debris

Page 1: 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

Page 2: Signatures of ΛCDM substructure in tidal debris

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

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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

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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!

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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!)

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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

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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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TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 9

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

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Sky distribution

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Spherical halodisplacement of debris

broad stream

without substructure with substructure

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TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 12

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Oblate halo

Prolate halo

bifurcation?

clumpier, less dispersed debris WITH substructure

highly dispersed without substructure

very similar

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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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TeV Particle Astrophysics, Venice, August 29, 2007J. Siegal-Gaskins 14

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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TeV Particle Astrophysics, Venice, August 29, 2007J. 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!