The Baryonic Mass Function of Spiral Galaxies: Clues to Galaxy Formation

We compute the baryonic mass function (BMF) of disc galaxies using the best LFs and baryonic M/L ratios reliable for this goal. For baryonic masses (M_b) ranging between 10^8 and 10^{11} solar masses, the BMF is featureless, i.e. it scales as M_b^{-1/2}. Outside this mass range, the BMF is a strong inverse function of M_b. The contributions to the baryon density Omega_b from objects of different mass highlight a characteristic mass scale of spirals at about 2x10^{11} solar masses, around which >50% of the total baryonic mass is concentrated. The integral value, Omega_b= 1.4x10^{-3}, confirms, to a higher accuracy, previous evidence (Persic & Salucci 1992) that the fraction of BBN baryons locked in disc galaxies is negligible and matches that of high-z Damped Lyman Alpha systems (DLAs). We investigate the scenario where DLAs are the progenitors of present-day spirals, and find a simple relationship between their masses and HI column densities by which the DLA mass function closely matches the spiral BMF.Comment: MNRAS, in press. Replaces previous, unrefereed version. 10 pages MNRAS style LaTeX, 7 figure

The Dark Matter Distribution in Disk Galaxies

We use high-quality optical rotation curves of 9 low-luminosity disk galaxies to obtain the velocity profile of the surrounding dark matter halos. We find that they increase linearly with radius at least out to the stellar disk edge, implying that, over the entire region where the stars reside, the density of the dark halo is constant. The properties of the halo mass structure found are similar to that claimed for a number of dwarf and low surface brightness galaxies, but provide a more substantial evidence of the discrepancy between the halo mass distribution predicted in standard cold dark matter scenario and those actually detected around galaxies. We find that the density profile proposed by Burkert (1995) reproduces the halo rotation curves, with halo central densities and core radii scaling as $\rho_0 \propto r_0^{-2/3}$.Comment: 8 pages, 6 figures, MNRAS accepted. New section and figures added, concerning CDM mass models. Minor changes to the rest of the pape

Cold Dark Matter Halos Must Burn

High-quality optical rotation curves for a sample of low-luminosity spirals evidence that the dark halos around galaxies are inconsistent with the output of proper CDM simulations. In fact, dark halos enveloping stellar disks are structures with approximately a constant density out to the optical edges. This is in strong disagreement with the characteristic rho(r) ~ r^(-1.5) CDM regime and severely challenges the "standard" CDM theory, also because the halo density appears to be heated up, at gross variance with the hierarchical evolution of collision-free particles.Comment: 2 figures, definitive version to appear in the Proceedings of the MPA/ESO/MPE/USM Joint Conference: "Lighthouses of the Universe: The Most Luminous Celestial Objects and their use for Cosmology", August 2001, Garching, German

Joint formation of bright quasars and elliptical galaxies in the young Universe

We show that the mass function of black holes expected from the past quasar activity (both visible and obscured) is consistent with the number of dormant black holes found in the bulges of nearby galaxies. The joint formation of quasars and bulges is addressed by means of an analytical model for galaxy formation, based on the hierarchical clustering of cold dark matter halos. The model is able to reproduce the main statistical properties of both populations under the hypotheses that (i) star formation and quasar shining follow an anti-hierarchical order, and (ii) galaxy morphology and final black hole mass are determined by the same physical process.Comment: 5 pages, 3 postscript figures included, proceedings of the IGRAP meeting "Clustering at high redshift", Marseille, June 199

Evidence for a Massive Dark Object in NGC 4350

In this work we build a detailed dynamic model for a S0 galaxy possibly hosting a central massive dark object (MDO). We show that the photometric profiles and the kinematics along the major and minor axes, including the h3 and h4 profiles, imply the presence of a central MDO of mass M = 1.5 - 9.7 10^8 solar masses, i.e. 0.3-2.8% of the mass derived for the stellar spheroidal component. Models without MDO are unable to reproduce the kinematic properties of the inner stars and of the rapidly rotating nuclear gas. The stellar population comprise of an exponential disc (27% of the light) and a diffuse spheroidal component (73% of the light) that cannot be represented by a simple de Vaucouleurs profile at any radius. The M/L ratios we found for the stellar components (respectively 3.3 and 6.6) are typical of those of disc and elliptical galaxies.Comment: 9 pages, 4 encapsulated postscript figures. Requires mn.sty, psfig.sty. Accepted for publication in MNRA

Galactic Potentials

The information contained in galactic rotation curves is examined under a minimal set of assumptions. If emission occurs from stable circular geodesic orbits of a static spherically symmetric field, with information propagated to us along null geodesics, observed rotation curves determine galactic potentials without specific reference to any metric theory of gravity. Given the potential, the gravitational mass can be obtained by way of an anisotropy function of this field. The gravitational mass and anisotropy function can be solved for simultaneously in a Newtonian limit without specifying any specific source. This procedure, based on a minimal set of assumptions, puts very strong constraints on any model of the "dark matter".Comment: A somewhat longer form of the final version to appear in Physical Review Letters.Clarification and further reference

The mass of the dark matter particle from theory and observations

We combine observed properties of galaxies as the core density and radius with the theoretical linear evolution of density fluctuations computed from first principles since the end of inflation till today. The halo radius r_0 is computed in terms of cosmological parameters. The theoretical density profiles rho(r)/rho(0) have an universal shape as a function of r/r_0 which reproduces the observations. We show that the linear approximation to the Boltzmann-Vlasov equation is valid for very large galaxies and correctly provides universal quantities which are common to all galaxies, as the surface density and density profile. By matching the theoretically computed surface density to its observed value we obtain (i) the decreasing of the phase-space density during the MD era (ii) the mass of the dark matter particle which turns to be between 1 and 2 keV and the decoupling temperature T_d which turns to be above 100 GeV (iii) the core vs. cusp discrimination: keV dark matter particles produce cored density profiles while wimps (m \sim 100 GeV, T_d \sim 5 GeV) produce cusped profiles at scales about 0.003 pc. These results are independent of the particle model and vary very little with the statistics of the dark matter particle. Non-universal galaxy quantities (which need to include non-linear effects as mergers and baryons) are reproduced in the linear approximation up to a factor of order one for the halo radius r_0, galaxy mass M_{gal}, halo central density rho_{0} and velocity dispersion sqrt{{\bar {v^2}}_{halo}} in the limiting case of large galaxies (both r_0 and M_{gal} large). This shows the power of the linear approximation scheme: although it cannot capture the whole content of the structure formation, it correctly provides universal quantities which as well as the main non-universal galaxy properties.Comment: 17 pages, 15 figures, improved and expanded version to appear in New Astronom

Dark Matter Scaling Relations

We establish the presence of a dark matter core radius, for the first time in a very large number of spiral galaxies of all luminosities. Contrary to common opinion we find that the sizes of these cores and the " DM core problem" are bigger for more massive spirals. As a result the Burkert profile provides an excellent mass model for dark halos around disk galaxies. Moreover, we find that the spiral dark matter core densities $\rho_{0}$ and core radii $r_{0}$ lie in the same scaling relation $\rho_{0}=4.5\times 10^-2 (r_{0}/kpc)^{-2/3} M_{\odot}pc^{-3}$ of dwarf galaxies with core radii upto ten times more smaller.Comment: 4 pages, 4 figures, Accepted for Publication in Apj Let