DMAW 2010: “Dark Matter in Galaxies” Notes for the presentation November 5, 2010 Abstract We discuss the details of the mass discrepancy phenomenon in galaxies usually ac- counted by postulating the presence of a non luminous component. In the theoretical framework of Newtonian gravity and Dark matter halos we start by recalling the proper- ties of the latter as emerging from the state-of-the-art of numerical simulations performed in the current ΛCDM scenario of cosmological structure formation. We then report the complex and much-telling phenomenology of the distribution of dark matter in Spirals, Ellipticals, and dwarf Spheroidals. A care will be given to show that such a coherent observational framework is obtained from different and large samples of galaxies and by means of very different methods of investigation and by exploiting different tracers of the gravitational field. These include RC and dispersion velocities mass fitting, X-ray gas property analysis, weak and strong lens signal mass decomposition, analysis of halo and baryonic mass functions. We will then highlight the impressive evidence that the distri- bution of dark and luminous matter are closely correlated. Hints on how the empirical scenario of the mass distribution in galaxies, including the Milky Way and the nearby ones affects the cosmological investigations are given throughout the paper. Among them, the theoretical constraints on the elusive nature of the dark matter particles and its direct and indirect searches. Finally, it is suggested that at least part of this phenomenology should enter in the standard knowledge background of cosmologists. 1 Introduction Slide 3 The best physical way to introduce phenomenon of Dark Matter is the following: let us define M(r) the mass distribution of the gravitating matter and ML(r) that of all baryonic components. We can obtain both distributions from observations; we first realize that, as we start moving from a certain radius rT , function of galaxy luminosity and Hubble Type, outwards, these distributions increasingly do not match, d log M/d log r > d log ML/d log r. We are then forced to introduce a non luminous component whose mass profile Mh(r) accounts for the disagreement: d log M(r) M (r) d log M M (r) d log M = L L + h h . d log r M(r) d log r M(r) d log r This immediately shows that the phenomenon of the mass discrepancy in galaxies emerges observationally from the radial derivative of the mass distribution, more precisely, from that of the circular velocity and/or that of the dispersion velocity, quantities that 1 measure the mass profile: M ∝ r V 2(r) ∝ σ2(r). The DM phenomenon can be investigated only if we know well the distribution of luminous matter and we can accurately measure the distribution of the gravitating matter. Slides 4-6 It is worthwhile to recall the predictions for the structure of DM halos coming from the current theory of cosmological structure formation which is in well agreement with a num- ber of cosmological observations. A fundamental one, emerging from N-body simulations performed in the (Λ)CDM scenario is that virialized dark matter halos have a universal spherically averaged density profile, ρCDM (r) (Navarro, Frenk, & White 1997): ρs ρCDM (r) = 2 , (r/rs)(1 + r/rs) where ρs and rs are strongly correlated (e.g. Wechsler et al. 2002), rs ' 0.46 Mvir 8.8 11 kpc. The concentration parameter c ≡ rs/Rvir is found to be a weak 10 M −0.09 Mvir function of mass. c(Mvir) = 9.35 12 −1 (Klypin et al. 2010). Recent simula- 10 h M tions confirm these results, in particular that, at radii about 1/200 of the virial radius, the innermost region tracked by galaxy kinematics, the ΛCDM halo profile converges to a power-law of index about -1. (Navarro et al. 2010). It is beyond the scope of this talk (in the reference mode) to discuss possible modifications of the above profile. Slide 7 Galaxies, very likely, have just one type of dark halo, their luminous components, instead, show a striking variety in morphology and in the values of their basic physical quantities. The range in magnitudes and central surface densities is 15 mag and 16 mag/arcsec2. The distribution of the luminous matter is given by a stellar disk + a spheroidal central bulge and an extended HI disk in spirals and by a stellar spheroid in ellipticals and dSphs. 2 Dark Matter in Spirals Slide 9-10 The stars are distributed in a thin disk with surface luminosity (Freeman 1970): M −R/RD D −R/RD I(R) = I0 e = 2 e 2πRD where RD is the disk scale-length, I0 is the central value and MD is the disk mass. The light profile does not depend on galaxy luminosity and the disk lenght-scale RD sets a consistent scale in all objects and the size of the distribution of the luminous component. We can take that stellar disk ends at Ropt = 3.2RD. Although the mass modelling of some individual object and certain investigations require to accurately deal with non exponential stellar disks and/or with the presence of a central bulge, the above equation well represents the typical distribution of stars in spirals for the aim of this talk. Spirals have a (HI+He) gaseous disk of (very) slowing decreasing surface density. This disk, at outer radii R > Ropt, especially in the case of low luminosity objects is the major baryonic component and therefore must be carefully considered in the mass models. An inner H2 disk is also present but it is negligible with respect to the stellar one. 2 Slides 11-14 The kinematics in spirals is measured by the Doppler effect on the emission line of a par- ticular tracers of the gravitational potential: HI, CO and Hα. We measure high quality symmetric rotation of a disk from which we obtain the sky coordinates of the galaxy kine- matical center, its systemic velocity, the inclination angle and the circular velocity V (R). Optical high quality kinematics lead to rotation curves (RCs) with a spatial resolution of ∼ 2”, i.e. ∼ (RD/30 − RD/10). Rotation curves obtained from the 21-cm HI line require radio telescopes with adequate sensitivity to detect the weak signals, and suffi- cient angular resolution to resolve the details. Early work lacked both, and only in the course of several decades the equipment improved to arrive at conclusive results. A large improvement occurred by using interferometers, based on combining the signals of two or more telescopes, and, by making use of the principle of earth rotation synthesis. This leads to maps of the integrated HI distribution and the radial velocity field at a resolution of 10”. The early evidence for a mass discrepancy in galaxies namely in spirals came in the late 70s from a combination of optical and radio RCs. Roberts & Whitehurst (1975) measured the M31 outer kinematics with the NRAO telescope and Rubin & Ford (1970) the inner optical rotation curve. These data are plotted, as well as a modern RC from Carignan et al. (2006) obtained with the Effelsberg telescope. Over plotted is a RC derived if the mass would follow the light. Clearly in this object HI data are necessary to establish convincingly the presence of a profound discrepancy between kinematics and photometry, already hinted in Freeman (1970) remarks about the NGC 300 RC. Continuing the hys- torical persepctive a topical moment was when Rubin published 20 optical RCs extended to Ropt that were still rising or flattish at the last measured point. Bosma (1981) collected data and compiled HI RCs for 25 galaxies well extended beyond the optical radius, see e.g. NGC 5055 in the slide. The red bar on each curve indicate the optical radius. It is immediate that the RCs do not decline in a Keplerian way ∝ R−1/2 beyond this point. More quantitatively Bosma & van der Kruit (1979) obtained the surface photometry for these objects and found, in their outer parts, a clear increase with dM/dR radius of the local mass-to-light ratios dL/dR . If light traces the gravitating mass, this quantity is expected to be constant with radius, equal to the average mass -to- light ratio of the stellar disk. Slide 15 Today radial Tully-Fisher relationship gives a most straightforward evidence for dark matter in spirals. At different galactocentric distances, measured in units of the optical size Ri ≡ 3.2Ropt (i = 0.2, ..., 1), there exists a family of independent Tully-Fisher-like relationships, Mband = bi + ai log V (Ri) with Mband, the magnitude in a specific band, in detail the (R, I) bands. This ensamble of relations, we call the Radial Tully-Fisher (Yegorova & Salucci 2007) contains crucial information on the mass distribution in these objects. In fact, the RTF relationships show a large systematic variation with Ri of their slopes ai: that ranges across the disk between −4 and −8 and show very small r.m.s. scatter. This variation coupled with the smallness of the scatter rules out the case in which the light follows the gravitating mass, and, in particular, all mass models with no DM or even the same dark-to-luminous-mass fraction within i Ropt in all galaxies. Slides 16-19 Looking to individual RCs we realize that their profiles are a function of the galaxy mag- nitude, a feature that points towards some universality of the spiral kinematics. Persic, 3 Salucci, & Stel (1996) built a number of coadded RCs from about 600 individual RCs. The whole I-band luminosity −16.3 < MI < −23.4 of spirals was divided in 9 luminosity intervals, each of them with ∼ 1500 velocity measurements.