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DARK MATTER IN GALAXIES. Alessandro Romeo. Onsala Space Observatory Chalmers University of Technology SE-43992 Onsala, Sweden. Dec. 1-8, 2010. Overview Dark m atter in SPIRALS Dark matter in ELLIPTICALS Dark matter in DWARF SPHEROIDALS Detecting dark matter Conclusions.

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slide1

DARK MATTER IN GALAXIES

Alessandro Romeo

Onsala Space Observatory

Chalmers University of Technology

SE-43992 Onsala, Sweden

Dec. 1-8, 2010

slide2

Overview

Dark matter in SPIRALS

Dark matter in ELLIPTICALS

Dark matter in DWARF SPHEROIDALS

Detecting dark matter

Conclusions

slide4

Stellar Discs

M33verysmoothstructure

NGC 300 - exponential disc

goes for at least 10 scale-

lengths

scale

radius

Bland-Hawthornet al 2005

Ferguson et al 2003

slide5

Wong & Blitz (2002)

Gas surfacedensities

GAS DISTRIBUTION

HI

Flattishradialdistribution

Deficiency in the centre

CO and H2

Roughlyexponential

Negligible mass

slide6

Earlydiscoveryfromoptical and HI RCs

observed

disk

no RC followsthe disk velocityprofile

disk

Rubinet al 1980

Mass discrepancy AT LARGE RADII

slide7

Extended HI kinematicstraces dark matter

-

-

Light (SDSS)

HI velocityfield

  • NGC 5055

SDSS

Bosma, 1981

GALEX

Bosma, 1981

Radius (kpc)

Bosma 1979

The mass discrepancyemergesas a disagreementbetween light and mass distributions

slide8

mag

Salucci+07

6 RD

Rotation Curves

Coaddedfrom 3200 individualRCs

TYPICAL INDIVIDUAL RCs OF INCREASING

LUMINOSITY

Low lum

high lum

slide9

The Concept of Universal Rotation Curve (URC)

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

The URC out to 6 RD isderiveddirectlyfromobservations

Extrapolationof URC out tovirialradiusbyusing

slide10

A Universal Mass Distribution

ΛCDM URC Observed URC

NFW

theory

low

obs

high

obs

Salucci+,2007

slide11

Rotation curve analysis

From data to mass models

Vtot2 = VDM2 + Vdisk2 + Vgas2

  • fromI-bandphotometry
  • from HI observations

Dark haloswithconstant density cores (Burkert)

Dark haloswithcusps(NFW, Einasto)

The mass modelhas 3 free parameters: disk mass, halocentral density and core radi radius (halolength-scale).

NFW

Burkert

slide12

halocentral density

coreradius

luminosity

MASS MODELLING RESULTS

highestluminosities

lowestluminosities

halo

disk

disk

halo

halo

disk

All structural DM and LM

parameters are related

to luminosity.g

Smallergalaxies are denser and have a higherproportionof dark matter.

fractionof DM

slide13

Dark HaloScalingLaws

Thereexistrelationshipsbetweenhalostructuralquantiies and luminosity.

Investigated via mass modellingofindividualgalaxies

- Assumption:MaximunDisk, 30 objects

-the slopeof the halo rotation curve near the center givesthe halocore density

- extendedRCsprovidean estimate ofhalocoreradiusrc

  • Kormendy & Freeman (2004)

o

o ~ LB- 0.35

rc ~ LB0.37

 ~LB0.20

rc

The centralsurfacedensity  ~ orc=constant

3.0

2.5

2.0

1.5

1.0

slide14

SPIRALS: WHAT WE KNOW

A UNIVERSAL CURVE REPRESENTS ALL THE INDIVIDUAL RCs

MORE PROPORTION OF DARK MATTER IN SMALLER SYSTEMS

RADIUS AT WHICH THE DM SETS IN FUNCTION OF LUMINOSITY

MASS PROFILE AT LARGER RADII COMPATIBLE WITH NFW

DARK HALO DENSITY SHOWS A CENTRAL CORE OF SIZE 2 RD

slide16

The Stellar Spheroid

Surfacebrightnessofellipticalsfollows a Sersic (de Vaucouleurs) law

Re : the effectiveradius

  • Bydeprojecting I(R) weobtain the luminosity density j(r):

ESO 540 -032

Sersicprofile

slide17

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

SDSS early-typegalaxies

Bernardi et al. 2003

Fromvirialtheorem

Hyde & Bernardi 2009

Fitting

gives: a=1.8 , b~-0.8)

then:

FP “tilt” due tovariationswithσ0of:

Dark matterfraction?

Stellar population?

slide18

Dark-Luminous mass decomposition of velocity dispersions

  • Not a unique model – example: a giant elliptical with reasonable parameters

RESULTSThe spheroid determines the velocity dispersionStars dominate inside ReMore complications when:presence of anisotropiesdifferent halo profile (e.g. Burkert)

1011

Two components: NFW halo, Sersic spheroid Assumed isotropy

Mamon& Łokas05

Dark matterprofileunresolved

weak and strong lensing
Weakand strong lensing

SLACS: Gavazzi et al. 2007)

Gavazzi et al 2007

Inside Re, the total (spheroid + dark halo) mass increasesproportionallyto the radius

UNCERTAIN DM DENSITY PROFILEI

slide20

Mass ProfilesfromX-ray

Nigishitaet al 2009

Temperature

Density

M/L profile

NO DM

HydrostaticEquilibrium

  • CORED HALOS?
slide21

ELLIPTICALS: WHAT WE KNOW

A LINK AMONG THE STRUCTURAL PROPERTIES OF STELLAR SPHEROID

SMALL AMOUNT OF DM INSIDE RE

MASS PROFILE COMPATIBLE WITH NFW AND BURKERT

DARK MATTER DIRECTLY TRACED OUT TO RVIR

dwarf spheroidals basic properties
Dwarf spheroidals: basic properties
  • Low-luminosity, gas-free satellites of Milky Way and M31
  • Large mass-to-light ratios (10 to 100 ), smallest stellar systems containing dark matter

Luminosities and sizes of

Globular Clusters and dSph

Gilmoreet al 2009

velocity dispersion profiles
Velocity dispersion profiles

STELLAR SPHEROID

Wilkinson et al 2009

dSph dispersion profiles generally remain flat up to large radii

mass profiles of dsphs
Mass profiles of dSphs

Jeans’ models provide the most objective sample comparison

  • Jeans equation relates kinematics, light and underlying mass distribution
  • Make assumptions on the velocity anisotropy and then fit the dispersion profile

n(R)

PLUMMER PROFILE

DENSITY PROFILE

Results point to cored distributions

Gilmoreet al 2007

slide26

Degeneracy between DM mass profile and velocity anisotropy

Cusped and cored mass models fit dispersion profiles equally well

Walkeret al 2009

However:

dSphscoredmodelstructuralparametersagreewiththoseofSpirals and Ellipticals

σ(R) km/s

Halocentral density vs coreradius

NFW+anisotropy = CORED

Donato et al 2009

slide27

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

conclusions

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
Thanks …..
  • That’s enough with Dark Matter!
  • Switch on the light ;-)
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