Dec. 1-8, 2010 DARK MATTER IN GALAXIES Alessandro Romeo Onsala Space Observatory Chalmers University of Technology SE 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 exponential disc goes for at least 10 scale- lengths Bland-Hawthorn et al 2005 Ferguson et al 2003 scale radius

HI Flattish radial distribution Deficiency in the centre CO and H 2 Roughly exponential Negligible mass Wong & Blitz (2002) Gas surface densities GAS DISTRIBUTION

Early discovery from optical and HI RCs Rubin et al 1980

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 1979 Radius (kpc)

Rotation Curves Coadded from 3200 individual RCs Salucci+07 6 R D 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/R D is negligible compared to the variations that V(x,L) shows as x and L vary. The URC out to 6 R D is derived directly from observations Extrapolation of URC out to virial radius by using

A Universal Mass Distribution ΛCDM URC Observed URC NFW high low Salucci+,2007 theory obs

Rotation curve analysis From 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). V tot 2 = V DM 2 + V disk 2 + V gas 2 NFW Burkert

core radius halo central density luminosity disk halo disk MASS MODELLING RESULTS fraction of DM lowest luminosities highest luminosities All structural DM and LM parameters 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 r c Kormendy & Freeman (2004)  o ~ L B r c ~ L B 0.37  ~ L B 0.20 oo rcrc  The central surface density  ~  o r c =constant

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 R D

ELLIPTICALS

Surface brightness of ellipticals follows a Sersic (de Vaucouleurs) law R e : the effective radius By deprojecting I(R) we obtain the luminosity density j(r): The Stellar Spheroid ESO 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 σ 0 of: Dark matter fraction? Stellar population? Hyde & Bernardi 2009 Fitting gives: a=1.8, b~-0.8) then: Bernardi et al. 2003

RESULTS The spheroid determines the velocity dispersion Stars dominate inside R e More complications when: presence of anisotropies different halo profile (e.g. Burkert) Two components: NFW halo, Sersic spheroid Assumed isotropy Dark-Luminous mass decomposition of velocity dispersions Not a unique model – example: a giant elliptical with reasonable parameters Mamon & Łokas 05 Dark matter profile unresolved 10 11

Weak and strong lensing SLACS: Gavazzi et al. 2007) Inside R e, 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 SPHEROID SMALL AMOUNT OF DM INSIDE R E MASS PROFILE COMPATIBLE WITH NFW AND BURKERT DARK MATTER DIRECTLY TRACED OUT TO R VIR

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 anisotropy Cusped 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 M SUN AND ρ 0 = g/cm 3 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 ;-)