dec. 1-8, 2010 dark matter in galaxies alessandro romeo onsala space observatory chalmers university...
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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