The Halo of the Milky

Heidi Jo NewbergRensselaer Polytechnic Institute

In search of a halo model that fits the data

Brief background on the stellar Milky WayWe 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 triaxialChi-squared triaxial spheroid fits to the data

and the problem with lumps.

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

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The Standard Galactic ModelRadial scale length (kpc)

0.22.94-53.5-52-3

BulgeSpheroidThick DiskThin DiskDark Halo

Vertical scale height or c/a

0.40.6-11.3 kpc325 pc1?

Density near Sun (Msol/pc3)

0.000260.0260.1240.009

Metallicity [Fe/H]

0.3-1.5-0.6-0.1

Vrot at Rsol (km/s)

0-501802200?

Allen’s Astrophysical Quantities, 2000

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:

222 )/(,5.3, qzyxrr

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Log(ratio of lum. in λ range of g filter to lum. in λ range of r filter)

GalacticCenter

Yanny et al. 2000

150,000 light years100,000 light years

Size of northern lump:20 kpc by > 2 kpc by < 10 kpc

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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.

GalacticCenter

Yanny et al. 2000

Newberg et al. 2002

Newberg et al. 2002

Squashedhalo

Sphericalhalo

Exponentialdisk

Prolatehalo

Newberg et al. 2002

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Press release, November 4, 2003

Blue – model Milky WayPink – model planar stream

Current controversiesTurf 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

Gal

actic

latit

ude

1,857,142 stars

Density of spheroid stars in Galactic coordinates from DR3

Gal

actic

latit

ude

Galactic longitude

Stars typically 10 kpc from the sun

Density of spheroid stars in Galactic coordinates from DR3

Gal

actic

latit

ude

Galactic longitude

-90 900-45 45

(l,b) ~ (10,40)

Thick disk selection box

Gal

actic

latit

ude

Galactic longitude

Density of thick disk stars from DR3

623364 stars

θgLong

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 modelNo 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.

222 )/(,1

qzyxrr

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1qzpyxr

Rrr core

).,,( angles by the rotated then and center, Galactic thefrom

by shifted are axes thewhere

)/'()/'(',)(

1 222

z)(dx, dy, d')(x', y', z

qzpyxrRrr core

Minimum Chi-squared modelFull model

R0 8.0 kpcp 0.73q 0.67θ 48°Rcore 15.0 kpcdx 0.1 kpcdy 3.5 kpcdz 0.1 kpcφ -8.0°ξ 12°α 1δ 3Mf 4.2Nsolar 1081 kpc-3

χ2 1.37

123359 stars

234306 stars

190221 stars

159632 stars

123359 stars

115416 stars

165620 stars

150562 stars

Minimum Chi-squared model

Full model GC power law GC power law standard model

R0 8.0 kpc 8.5 kpc 8.0 kpc 8.5 kpc 10.7 kpcp 0.73 0.73 0.74 0.72 1.0q 0.67 0.60 0.66 0.59 0.63θ 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 -α 1 1 2.9 3.0 3.1δ 3 3 0 0 0Mf 4.2 4.2 4.2 4.2 4.2Nsolar 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

Full HernquistCentered HernquistFull Power Law

Centered Power LawTraditional Power Law

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)

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

GalacticCenter

Stars inthe smooth“spheroid”population

Pal 5 globular cluster

Sagittarius dwarf tidal stream

Sagittarius dwarfTidal stream

Sun

Newberg et al. 2002