DARK MATTER IN GALAXIES
Alessandro Romeo
Onsala Space Observatory
Chalmers 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 scalelengths
scale
radius
Bland-Hawthorn et al 2005
Ferguson et al 2003
Gas surface densities
GAS DISTRIBUTION
HI
Flattish radial distribution
Deficiency in the centre
CO and H2
Roughly exponential
Negligible mass
Wong & Blitz (2002)
Early discovery from optical and HI RCs
Rubin et al 1980
Extended HI kinematics traces dark matter
Light (SDSS)
HI velocity field
NGC 5055
-
SDSS
Bosma, 1981
GALEX
Bosma,
1981
Radius (kpc)
Bosma 1979
The mass discrepancy emerges as a disagreement between light and mass distributions
Rotation Curves
TYPICAL INDIVIDUAL RCs OF INCREASING
LUMINOSITY
Coadded from 3200 individual RCs
Salucci+07
Low lum
mag
high lum
6 RD
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 observations
Extrapolation of URC out to virial radius by using
A Universal Mass Distribution
ΛCDM URC
Observed URC
NFW
theory
low
obs
high
obs
Salucci+,2007
Rotation curve analysis
From data to mass models
Vtot2 = VDM2 + Vdisk2 + Vgas2
➲
from I-band photometry
➲
from HI observations
➲
Dark halos with constant density cores (Burkert)
Dark halos with cusps (NFW, Einasto)
NFW
Burkert
radi
The mass model has 3 free parameters:
disk mass, halo central density and core
radius (halo length-scale).
MASS MODELLING RESULTS
highest luminosities
lowest luminosities
halo
disk
halo
halo
disk
fraction of DM
All structural DM and LM
parameters are related
to luminosity.g
Smaller galaxies are
denser and have a higher
proportion of dark matter.
disk
luminosity
Dark Halo Scaling Laws
There exist relationships between halo structural quantiies
Investigated via mass modelling of individual galaxies
and luminosity.
- Assumption: Maximun Disk, 30 objects Kormendy & Freeman (2004)
-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
o
o ~ LB- 0.35
rc ~ LB 0.37
 ~ LB 0.20
The central surface density 
3.0
~ orc
rc
=constant
2.5
2.0
1.5
1.0

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
ELLIPTICALS
The Stellar Spheroid
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):
I ( R) 



R
 j (r ) dz  2 
j ( r ) r dr
r 2  R2
ESO 540 -032
Sersic profile
The Fundamental Plane: central velocity dispersion, half-light radius and
surface brightness are related
SDSS early-type galaxies
Bernardi et al. 2003
From virial theorem
Hyde & Bernardi 2009
Fitting
gives: a=1.8 , b~-0.8)
then:
FP “tilt” due to variations with σ0 of:
Dark matter fraction?
Stellar population?
Dark-Luminous mass decomposition of velocity dispersions
Not a unique model – example: a giant elliptical with reasonable parameters
1011
Two components: NFW halo, Sersic spheroid
Assumed isotropy
Mamon & Łokas 05
RESULTS
The spheroid determines the velocity
dispersion
Stars dominate inside Re
More complications when:
presence of anisotropies
different halo profile (e.g. Burkert)
Dark matter profile unresolved
Weak and strong lensing
SLACS: Gavazzi et al. 2007)
Gavazzi et al 2007
Inside Re, the total (spheroid + dark halo) mass increases proportionally to the
radius
UNCERTAIN DM DENSITY PROFILEI
Mass Profiles from X-ray
Nigishita et al 2009
Temperature
Density
M/L profile
NO DM
Hydrostatic Equilibrium
CORED HALOS?
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
dSphs
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
Gilmore et al 2009
Velocity dispersion profiles
STELLAR SPHEROID
Wilkinson et al 2009
dSph dispersion profiles generally remain flat up to large radii
Mass profiles of dSphs
Jeans’ models provide the most
objective sample comparison
Jeans equation relates kinematics, light and underlying mass distribution
n(R)
Make assumptions on the velocity anisotropy and then fit the dispersion profile
PLUMMER PROFILE
DENSITY PROFILE
Results point to cored distributions
Gilmore et al 2007
Degeneracy between DM mass profile and velocity anisotropy
Cusped and cored mass models fit dispersion profiles equally well
Walker et al 2009
σ(R) km/s
However:
dSphs cored model structural parameters
agree with those of Spirals and Ellipticals
Halo central density vs core radius
NFW+anisotropy = CORED
Donato et al 2009
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
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