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The Halo of the Milky. Heidi Jo Newberg Rensselaer Polytechnic Institute. In search of a halo model that fits the data. Brief background on the stellar Milky Way We tried to measure the halo shape and found the Sagittarius dwarf tidal stream.

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The Halo of the Milky


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    1. The Halo of the Milky Heidi Jo Newberg Rensselaer Polytechnic Institute

    2. In search of a halo model that fits the data Brief background on the stellar Milky Way We tried to measure the halo shape and found the Sagittarius dwarf tidal stream. We improved our technique and found another tidal stream. Argument that the spheroid is triaxial Chi-squared triaxial spheroid fits to the data and the problem with lumps.

    3. http://www.phy.cuhk.edu.hk/people/teach/mcchu/gee240m/Chap_15/Sec15_1.htmlhttp://www.phy.cuhk.edu.hk/people/teach/mcchu/gee240m/Chap_15/Sec15_1.html

    4. Ben Moore’s N-body site

    5. The Standard Galactic Model Radial scale length (kpc) 0.2 2.9 4-5 3.5-5 2-3 Vertical scale height or c/a 0.4 0.6-1 1.3 kpc 325 pc 1? Density near Sun (Msol/pc3) 0.00026 0.026 0.124 0.009 Metallicity [Fe/H] 0.3 -1.5 -0.6 -0.1 Vrot at Rsol (km/s) 0-50 180 220 0? Bulge Spheroid Thick Disk Thin Disk Dark Halo Allen’s Astrophysical Quantities, 2000

    6. The first attempt to measure the shape of the Galactic halo using the Sloan Digital Sky Survey It was 1998. We had the first scan of data from the Sloan Digital Sky Survey. I had been writing software for 6 years. Brian Yanny and I thought we would try to measure the flattening parameter of the Galactic spheroid:

    7. Pal 5 Globular Cluster

    8. Yanny et al. 2000 Log(luminosity in wavelength range of g filter) Log(ratio of lum. in λ range of g filter to lum. in λ range of r filter)

    9. Yanny et al. 2000 Galactic Center

    10. 100,000 light years 150,000 light years Size of northern lump: 20 kpc by > 2 kpc by < 10 kpc

    11. Kathryn Johnston

    12. Newberg et al. 2003

    13. A tremendous number of papers exist studying the Sgr dwarf tidal stream Measurements of density, position, and stellar velocities of stream stars. Models of dwarf disruption, that depend on the Galactic potential (various papers claim that q=1, q>1, q<1) Models that show the Sgr dwarf interacted with the LMC several billion years ago, which threw it into this destructive orbit. Possibility that the Sgr dwarf tidal stream goes through the solar position, and that it could contribute ~1% to the dark matter density at the Earth. Theoretical limits on the lumpiness of the Galactic dark matter halo. Claim that a Galactic globular cluster was stripped from the Sgr dwarf and is currently in the tidal stream.

    14. Yanny et al. 2000 Galactic Center

    15. Newberg et al. 2002

    16. Newberg et al. 2002

    17. Squashed halo Spherical halo Prolate halo Exponential disk Newberg et al. 2002

    18. Newberg et al. 2002

    19. Blue – model Milky Way Pink – model planar stream Press release, November 4, 2003

    20. Current controversies Turf war over name: Monoceros stream, stream in the Galactic plane, Galactic Anticenter Stellar Structure (GASS), One Ring or “Ring,” Canis Major dwarf galaxy, Argo structure. Is the entire structure due to the Galactic warp? How many times does the stream wrap the Galaxy? Has the Canis Major dwarf galaxy been discovered, and is it the progenitor of the tidal stream? Did this merger puff up, or even create the Galactic thick disk? Is it related to the “metal-weak thick disk?” Is the purported Canis Major dwarf galaxy really an artifact of the Galactic warp? Is the purported Canis Major structure really an artifact of a hole in the Galactic extinction – and the real center of the structure in the Argo Navis Constellation? Is the Argo structure the Galactic warp?

    21. Newberg et al. 2002

    22. (l,b) ~ (10,40) Spheroid star selection box

    23. Density of spheroid stars in Galactic coordinates from DR3 1,857,142 stars Galactic latitude Galactic longitude

    24. Density of spheroid stars in Galactic coordinates from DR3 Galactic latitude Galactic longitude

    25. Stars typically 10 kpc from the sun

    26. Density of spheroid stars in Galactic coordinates from DR3 Galactic latitude Galactic longitude

    27. -45 0 45 90 -90

    28. (l,b) ~ (10,40) Thick disk selection box

    29. Density of thick disk stars from DR3 623364 stars Galactic latitude Galactic longitude

    30. For close stars, the maximum density is in quadrant IV and the minimum is is quadrant II. For distances larger than the Sun-GC dist., the max. is in quadrant I and the min. is in quadrant II. For large distances, the minimum is perpendicular to the major axis of the spheroid. θ gLong

    31. 155°

    32. Finding a model No model of the commonly used form will work. The triaxial power law still puts too many stars in the Galactic center. The Hernquist profile fits better, but leaves excess counts in the south.

    33. Minimum Chi-squared model Full model R08.0 kpc p 0.73 q 0.67 θ 48° Rcore 15.0 kpc dx 0.1 kpc dy 3.5 kpc dz 0.1 kpc φ -8.0° ξ 12° α1 δ3 Mf 4.2 Nsolar 1081 kpc-3 χ2 1.37

    34. 123359 stars

    35. 234306 stars

    36. 190221 stars

    37. 159632 stars

    38. 123359 stars

    39. 115416 stars

    40. 165620 stars

    41. 150562 stars

    42. Minimum Chi-squared model Full model GC power law GC power law standard model R08.0 kpc 8.5 kpc 8.0 kpc 8.5 kpc 10.7 kpc p 0.73 0.73 0.74 0.72 1.0 q 0.67 0.60 0.66 0.59 0.63 θ 48° 70 52 72 - Rcore 15.0 kpc 14.0 kpc - - - dx 0.1 kpc - 0.2 kpc - - dy 3.5 kpc - 3.0 kpc - - dz 0.1 kpc - 0.0 kpc - - φ -8.0° -4.5 -6.5 -4.0 - ξ 12° 14 16 14 - α1 1 2.9 3.0 3.1 δ3 3 0 0 0 Mf 4.2 4.2 4.2 4.2 4.2 Nsolar 1081 kpc-3 1096 kpc-3 1412 kpc-3 1341 kpc-3 1539 kpc-3 χ2 1.37 1.42 1.49 1.51 1.92

    43. Full Hernquist Centered Hernquist Full Power Law Centered Power Law Traditional Power Law

    44. Local density of stellar halo 0.00026 Msol/pc3(Allen’s Astrophysical Quantities, 2000) 1100-1500 F stars/kpc3 at the solar position (this talk) In Pal 5, the ratio Msol/F stars ~ 5, therefore, we estimate a local density of: 5.5x10-6-7.5x10-6 Msol/pc3 (30-50 times smaller than Allen)

    45. Conclusions A large part of the Galactic halo is inhabited by lumps and tidal streams with size scales of the order of 10 kpc. The smoothest component we can find is biased towards a non-axisymmetric shape and a Hernquist rather than a power-law profile. One cannot measure q from a pencil-beam survey or even a strip of the sky 100 degrees long.

    46. SEGUE as of September 30, 2004 Black= completed stripe or plate pair Imaging: 3900 sq deg, mostly low |b|; 750/3900 sq deg  19% complete Spectra: 240,000 stellar spectra; 29/400 plates  7% complete (17K stars)

    47. Sagittarius dwarf tidal stream Newberg et al. 2002 Pal 5 globular cluster Stars in the smooth “spheroid” population Ring 11 kpc Sun Galactic Center Sagittarius dwarf Tidal stream