DARK MATTER IN GALAXIESAlessandro Romeo

Onsala Space ObservatoryChalmers University of Technology

SE-43992 Onsala, Sweden

Overview

Dark matter in SPIRALS

Dark matter in ELLIPTICALS

Dark matter in DWARF SPHEROIDALS

Detecting dark matter

Conclusions

SPIRALS

Stellar Discs

M33 very smooth structure

NGC 300 - exponential disc goes for at least 10 scale-lengths

Bland-Hawthorn et al 2005Ferguson et al 2003

scaleradius

HI

Flattish radial distribution

Deficiency in the centre

CO and H2

Roughly exponential

Negligible mass

Wong & Blitz (2002)

Gas surface densities

GAS DISTRIBUTION

Early discovery from optical and HI RCs

MASS DISCREPANCY AT LARGE RADII

disk

observed

NO RC FOLLOWS THE DISK VELOCITY PROFILE

Rubin et al 1980

disk

The mass discrepancy emerges as a disagreement between light and mass distributions

GALEX

SDSS

Extended HI kinematics traces dark matter

- -

NGC 5055 Light (SDSS) HI velocity field

Bosma, 1981

Bosma, 1981

Bosma 1979Radius (kpc)

Rotation Curves

Coadded from 3200 individual RCs

Salucci+07

6 RD

mag

TYPICAL INDIVIDUAL RCs OF INCREASING LUMINOSITY

Low lum

high lum

The Concept of Universal Rotation Curve (URC)

The Cosmic Variance of the value of V(x,L) in galaxies of the same luminosity L at the same radius x=R/RD is negligible compared to the variations that V(x,L) shows as x and L vary.

The URC out to 6 RD is derived directly from observationsExtrapolation of URC out to virial radius by using

A Universal Mass Distribution

ΛCDM URC Observed URC

NFW

high

low

Salucci+,2007

theory

obs

obs

Rotation curve analysisFrom data to mass models

➲ from I-band photometry

➲ from HI observations

➲ Dark halos with constant density cores (Burkert)

Dark halos with cusps (NFW, Einasto)

The mass model has 3 free parameters:

disk mass, halo central density and core radi radius (halo length-scale).

Vtot

2 = VDM

2 + Vdisk

2 + Vgas

2

NFW

Burkert

core radius

halo central density

luminosity

disk

halo

halo

halo

diskdisk

MASS MODELLING RESULTS

fract

ion o

f D

M

lowest luminosities highest luminosities

All structural DM and LMparameters are related to luminosity.g

Smaller galaxies are denser and have a higher proportion of dark matter.

Dark Halo Scaling Laws

There exist relationships between halo structural quantiies and luminosity. Investigated via mass modelling of individual galaxies - Assumption: Maximun Disk, 30 objects-the slope of the halo rotation curve near the center gives the halo core density - extended RCs provide an estimate of halo core radius rc

Kormendy & Freeman (2004)

o ~ LB- 0.35

rc ~ LB 0.37

~ LB 0.20

o

rc

The central surface density ~ orc =constant 3.0

2.5

2.0

1.5

1.0

SPIRALS: WHAT WE KNOW

A UNIVERSAL CURVE REPRESENTS ALL THE INDIVIDUAL RCsMORE PROPORTION OF DARK MATTER IN SMALLER SYSTEMSRADIUS AT WHICH THE DM SETS IN FUNCTION OF LUMINOSITYMASS PROFILE AT LARGER RADII COMPATIBLE WITH NFWDARK HALO DENSITY SHOWS A CENTRAL CORE OF SIZE 2 RD

ELLIPTICALS

Surface brightness of ellipticals follows a Sersic (de Vaucouleurs) law

Re : the effective radius

By deprojecting I(R) we obtain the luminosity density j(r):

The Stellar Spheroid

R Rr

drrrjdzrjRI

22

)(2)()(

ESO 540 -032

Sersic profile

SDSS early-type galaxies

The Fundamental Plane: central velocity dispersion, half-light radius and surface brightness are related

From virial theorem

FP “tilt” due to variations with σ0 of: Dark matter fraction? Stellar population?

Hyde & Bernardi 2009

Fitting

gives: a=1.8 , b~-0.8)then:

Bernardi et al. 2003

RESULTSThe spheroid determines the velocity dispersionStars dominate inside R

e

More complications when:presence of anisotropiesdifferent halo profile (e.g. Burkert)

Two components: NFW halo, Sersic spheroid Assumed isotropy

Dark-Luminous mass decomposition of velocity dispersionsNot a unique model – example: a giant elliptical with reasonable parameters

Mamon & Łokas 05

Dark matter profile unresolved

1011

Weak and strong lensing

SLACS: Gavazzi et al. 2007)

Inside Re, the total (spheroid + dark halo) mass increases proportionally to the radius

Gavazzi et al 2007

UNCERTAIN DM DENSITY PROFILEI

Mass Profiles from X-ray

Temperature

Density

Hydrostatic Equilibrium

M/L profile

NO DM

Nigishita et al 2009

CORED HALOS?

ELLIPTICALS: WHAT WE KNOW

A LINK AMONG THE STRUCTURAL PROPERTIES OF STELLAR SPHEROIDSMALL AMOUNT OF DM INSIDE RE

MASS PROFILE COMPATIBLE WITH NFW AND BURKERTDARK MATTER DIRECTLY TRACED OUT TO RVIR

dSphs

Low-luminosity, gas-free satellites of Milky Way and M31

Large mass-to-light ratios (10 to 100 ), smallest stellar systems containing dark matter

Dwarf spheroidals: basic properties

Luminosities and sizes of Globular Clusters and dSph

Gilmore et al 2009

Velocity dispersion profiles

dSph dispersion profiles generally remain flat up to large radii

Wilkinson et al 2009

STELLAR SPHEROID

Mass profiles of dSphs

Jeans equation relates kinematics, light and underlying mass distribution

Make assumptions on the velocity anisotropy and then fit the dispersion profile

Results point to cored distributions

Jeans’ models provide the most objective sample comparison

Gilmore et al 2007

DENSITY PROFILE

n(R)

PLUMMER PROFILE

Degeneracy between DM mass profile and velocity anisotropyCusped and cored mass models fit dispersion profiles equally well

However: dSphs cored model structural parameters agree with those of Spirals and Ellipticals

Halo central density vs core radius

σ(R

) km

/s

Donato et al 2009

Walker et al 2009

NFW+anisotropy = CORED

DSPH: WHAT WE KNOW

PROVE THE EXISTENCE OF DM HALOS OF 1010 MSUN AND ρ0 =10-21 g/cm3

DOMINATED BY DARK MATTER AT ANY RADIUS MASS PROFILE CONSISTENT WITH AN EXTRAPOLATION OF THE URC HINTS FOR THE PRESENCE OF A DENSITY CORE

DETECTING DARK MATTER

DM

CONCLUSIONS

The distribution of DM halos around galaxies shows a striking and complex phenomenology.

Observations and experiments, coupled with theory and simulations, will (hopefully) soon allow us to understand two fundamental issues:

The nature of dark matter itself

The process of galaxy formation

Thanks …..

That’s enough with Dark Matter!

Switch on the light ;-)

19.10.10