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. 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.

chu/gee240m/Chap_15/Sec15_1.html

Ben Moore’s N-body site

The Standard Galactic Model Radial scale length (kpc) Bulge Spheroid Thick Disk Thin Disk Dark Halo Vertical scale height or c/a kpc 325 pc 1? Density near Sun (M sol /pc 3 ) Metallicity [Fe/H] V rot at R sol (km/s) ? Allen’s Astrophysical Quantities, 2000

The first attempt to measure the shape of the Galactic halo using the Sloan Digital Sky Survey It was 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:

Pal 5 Globular Cluster

Yanny et al 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)

Galactic Center Yanny et al. 2000

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

Kathryn Johnston

Newberg et al. 2003

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.

Galactic Center Yanny et al. 2000

Newberg et al. 2002

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

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

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?

Newberg et al. 2002

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

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

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

Stars typically 10 kpc from the sun

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

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

Galactic latitude Galactic longitude Density of thick disk stars from DR stars

θ gLong 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.

155°

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.

Minimum Chi-squared model Full model R kpc p0.73 q0.67 θ48° R core 15.0 kpc dx0.1 kpc dy3.5 kpc dz0.1 kpc φ-8.0° ξ12° α1 δ3 M f 4.2 N solar 1081 kpc -3 χ

stars

stars

stars

stars

stars

stars

stars

stars

Minimum Chi-squared model Full model GC power law GC power law standard model R kpc 8.5 kpc 8.0 kpc 8.5 kpc 10.7 kpc p q θ48° R core 15.0 kpc 14.0 kpc dx0.1 kpc kpc - - dy3.5 kpc kpc - - dz0.1 kpc kpc - - φ-8.0° ξ12° α δ M f N solar 1081 kpc kpc kpc kpc kpc -3 χ

Full Hernquist Centered Hernquist Full Power Law Centered Power Law Traditional Power Law

Local density of stellar halo M sol /pc 3 (Allen’s Astrophysical Quantities, 2000) F stars/kpc 3 at the solar position (this talk) In Pal 5, the ratio M sol /F stars ~ 5, therefore, we estimate a local density of: 5.5x x10 -6 M sol /pc 3 (30-50 times smaller than Allen)

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.

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)

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