Title

Galaxy Structure and Dark Matter

Michael MerrifieldUniversity of Nottingham

UDF #3492

Coma Cluster

2

2

212

dtIdPEKE

(Zwicky 1937)

Ngc 3198

(Begeman 1989)

NGC 3198

rrM )(

2)( rr

(Kent 1987)

rv

rrGMan

2

2

)(

)( 0aaaan

111

)(xxx

xMOdifiedNewtonianDynamics (Milgrom 1983)

Tully-Fisher

The Tully Fisher Relation

In “deep MOND,”

0aaa n

02

2

ar

GMrv

4vML

(Kent 1987)

rv

rrGMan

2

2

)(

rrM )(

2)( rr

1.1 70 Ma pc-2 Total mass within 1.1 kpc of the Galactic plane (Kuijken & Gilmore 1991)

Census of mass in baryons within 1.1 kpc of the Galactic plane (Olling & Merrifield 2001)

1.1 50 Ma pc-2bar

In the Solar neighbourhood, we have:In Situ Measurements

whereas more globally in the Milky Way:

1.1 20 Ma pc-2DM

0.4 1.1 bar

M(R0) 9.5 × 1010 Ma Total dynamical mass within the Solar circle

Mbar(R0) 5.5 × 1010 MaCensus of mass in baryons within the Solar circle (Olling & Merrifield 2001)

MDM(R0) 4.0 × 1010 Ma

0.7 Mbar(R0)

Dark matter more spherically distributed than baryons

One measure of the shape of the halo can be made using the thickness of a galaxy’s gas layer, assuming that it is in hydrostatic equilibrium:

Flatter

halo

dark matter concentrated closer to plane

Vertical gravitational pull reduces scaleheight of gas layer

Measuring Halo Shape

1.00.80.6

0.4

0.2

q = c/a

(Olling & Merrifield 2000)

(Buote & Canizares 1996)

NGC 720

Milky Way

Moore et al. (2003)Moore et al. (2003)

Milky Way

(Dubinski 1994)

Title

Where Have All the Satellites Gone?

Invisible Galaxies?

(Minchin et al. 2005)

• Never formed stars

• First star blew out gas, stopping star formation

MOND?

MOND

DMDM

MOND

1

2

MM

(Ciotti & Binney 2004)

• Destroyed by dynamical friction

05.025DM

MOND

MOND

DM

MM

For a typical galaxy-scale halo,

Navarro et al. (2004)

21)(

ss

sNFW rrrr

r

5.15.1 1

)(MM

MM rrrr

r

...all of which gives plenty of “wiggle room” to make the CDM models fit!

(Klypin, Zhao & Somerville 2002)

The Milky Way Rotation Curve in CDM

NGC 6822

N6822 rotation curve:

the prediction (de Blok 2004)

(Sanders & McGaugh 2002)

MOND Again

NGC 4472

Elliptical Galaxies

PNe Romanowsky et al. (2003)

Combined dispersion profiles

Isotropic no dark mattermodel

Isothermal halo

MOND(Sanders & McGaugh 2002)

NGC 821 (Romanowsky et al 2003)NGC 3379 (Romanowsky et al 2003)NGC 4494 (Romanowsky et al 2003)

NGC 4697 (Méndez et al. 2001)

(Romanowsky et al. 2003)

CDM prediction(Bullock et al. 2001)

Explanation for “Naked” Ellipticals

We have been able to rule out:• Systematic problems with data• Face-on highly-flattened systems• Strong radial bias in orbits

Which leaves: • Lost dark matter halos• MOND• Low concentration halos

More data on the way…

Summary

Conclusions

There is overwhelming evidence that, on the scale of galaxies, dynamically-inferred masses exceed the observed baryonic masses

For the last twenty years, the paradigm has been thatthe deficit is made up by non-baryonic dark matter

It is by no means clear at this point that Occam’s Razor supports this interpretation (c.f. MOND)

Save the paradigm detect the dark matter!

Clusters of Galaxies