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Investigating the Nature of Tidal Streams: Smooth vs. Lumpy Features in Cosmology

This study explores the characteristics of tidal streams in the context of Cold Dark Matter (CDM) simulations, particularly within the Andromeda galaxy. We analyze the smoothness or lumpiness of observed tidal streams, using extensive data from the Pan Andromeda Archaeological Survey. The research reveals statistical significance in density fluctuations, adding evidence against the notion of smooth dark matter halos. Through advanced simulations and photometric methods, we uncover the complexities of tidal stream dynamics and their implications for our understanding of the universe.

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Investigating the Nature of Tidal Streams: Smooth vs. Lumpy Features in Cosmology

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  1. Star Streams: Smooth or Lumpy? Ray Carlberg & PAndAS (Wayne Ngan simulations)

  2. Cosmology focus changed • Many parameters of the universe known to 2-3 digits • “just six numbers” • Two Dark numbers • What is the physics of this stuff? • Why?

  3. N-body CDM dark matter simulation: 10’s of 1000’s of halos

  4. CDM Halos ~10% mass in sub-halos • Tidal streams cannot remain smooth in CDM • Folded, • chopped, • heated • Are observed streams smooth or lumpy? • Complications: • foregrounds, • small N stats • Stream Age • Other dynamics

  5. smooth

  6. 5 sub-halos

  7. 100 sub-halos

  8. One thousand sub-halos

  9. Pan Andromeda Archaeological Survey • 400 square degrees of M31-M33 • Extends out 150 kpc from M31 • CFHT Megaprime g and i band images • i~24.5mag limit • Color-mag matched to old RGB isochrones • Gives [Fe/H] • For roughly 4 million stars • Limit ~34 mag/sq arcsec

  10. AndXXVII

  11. NW Stream

  12. Non-uniform photometric coverage

  13. Stream width 2.5kpc

  14. Density in Stream

  15. No stream at high metal abundance

  16. Statistical Significance of Lumps • Stream is a 15% overdensity • Stats dominated by background fluctuations • χ2/ νindicates >99.9999% confidence that fluctuations are in excess of noise for 1, 2, 4 and 8 degree binning (scales 3-20 kpc).

  17. Other lumpy/non-lumpy streams • Pal 5: Oddenkirchen et al. one lump in 4kpc long segment • Sgr: Majewski et al. find such small velocity deviations that assert that the MW halo must be smooth. Possibly, but velocities are culled to remove outliers that may be part of stream.

  18. Discussion • Stream is lumpy => halo cannot be smooth • Density 0-3x mean • Scales 3-20 kpc • Statistical significance >99.999% confidence • Dark matter cannot be smooth • What causes the lumpiness? • Jeans instabilities [wavelength too long] • Resonant ejection ? Not in n-body • Epicyclic orbit pileup ? [wavelengths too long] • Dark matter sub-halos: seems likely • LUMPY CDM halo consistent

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