Dark baryons and rotation curves

The best measured rotation curve for any galaxy is that of the dwarf spiralXXXX DDO 154, which extends out to about 20 disk scale lengths. It provides an ideal laboratory for testing the universal density profile prediction from high resolution numerical simulations of hierarchical clustering in cold dark matter dominated cosmological models. We find that the observed rotation curve cannot be fit either at small radii, as previously noted, or at large radii. We advocate a resolution of this dilemma by postulating the existence of a dark spheroid of baryons amounting to several times the mass of the observed disk component and comparable to that of the cold dark matter halo component. Such an additional mass component provides an excellent fit to the rotation curve provided that the outer halo is still cold dark matter-dominated with a density profile and mass-radius scaling relation as predicted by standard CDM-dominated models. The universal existence of such dark baryonic spheroidal components provides a natural explanation of the universal rotation curves observed in spiral galaxies, may have a similar origin and composition to the local counterpart that has been detected as MACHOs in our own galactic halo via gravitational microlensing, and is consistent with, and even motivated by, primordial nucleosynthesis estimates of the baryon fraction.Comment: 16 pages LaTeX, 2 postscript figures. To be published in The Astrophysical Journal, Letter

Comment on "Scalar-tensor gravity coupled to a global monopole and flat rotation curves" by Lee and Lee

The recent paper by Lee and Lee (2004) may strongly leave the impression that astronomers have established that the rotation curves of spiral galaxies are flat. We show that the old paradigm of Flat Rotation Curves lacks, today, any observational support and following it at face value leads to intrinsically flawed alternatives to the Standard Dark Matter Scenario. On the other side, we claim that the rich systematics of spiral galaxy rotation curves, that reveals, in the standard Newtonian Gravity framework, the phenomenon of dark matter, in alternative scenarios, works as a unique benchmark.Comment: 3 pages, 2 figures, accepted in Phys. Rev.

Non-thermal emission in the lobes of radio galaxies.

Radio and gamma-ray measurements of radiogalaxy lobes are useful to determine whether emission in these widely separated spectral regions is mainly by non-thermal (NT) electrons. This is of interest as there is yet no proof for a significant emission component from pion decay following NT proton interactions in the ambient lobe gas. An assessment of the hadronic yield needs full accounting of the local (FGL) and background (EBL, CMB) radiation fields in the lobes. Assuming a truncated single-PL electron energy distribution, exact calculation of the emission by NT electrons in the magnetized plasma in the Fornax A lobes leads to the conclusion that its Fermi-LAT emission is mostly IC/GFL: this result weakens earlier conclusions on the hadronic origin of the LAT emission. Similar analyses of the lobe emissions of Cen A, Cen B, and NGC 6251 suggest their measured LAT emissions, too, to be of IC/(EBL, CFGL, CMB) nature. Measured emissions of distant radio-galaxy lobes (3C98, Pictor A, DA240, Cygnus A, 3C326, and 3C236) are currently limited to the radio and X-ray bands: they can give no information on the presence of NT protons, but do trace the properties of NT electrons, and allow calculations of the related IC gamma-ray emission to be performed. The e/B energy density ratios, U_e/U_B, turn out to be in the range ~1-100. The NT proton energy density, U_p, is spectrally constrained to be less than a few tens of eV/cm3. Despite this limit, arguably U_p >> U_e -- as suggested by arguments of lobe internal vs external pressure. Thus the lobes' NT energy budget is likely dominated by particles. Given the low thermal energy densities measured in lobes, NT energy dominance is probably a general feature of lobe energetics

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

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

High-energy emission from star-forming galaxies

Adopting the convection-diffusion model for energetic electron and proton propagation, and accounting for all the relevant hadronic and leptonic processes, the steady-state energy distributions of these particles in the starburst galaxies M82 and NGC253 can be determined with a detailed numerical treatment. The electron distribution is directly normalized by the measured synchrotron radio emission from the central starburst region; a commonly expected theoretical relation is then used to normalize the proton spectrum in this region, and a radial profile is assumed for the magnetic field. The resulting radiative yields of electrons and protons are calculated: the predicted >100MeV and >100GeV fluxes are in agreement with the corresponding quantities measured with the orbiting Fermi telescope and the ground-based VERITAS and HESS Cherenkov telescopes. The cosmic-ray energy densities in central regions of starburst galaxies, as inferred from the radio and gamma-ray measurements of (respectively) non-thermal synchrotron and neutral-pion-decay emission, are U=O(100) eV/cm3, i.e. at least an order of magnitude larger than near the Galactic center and in other non-very-actively star-forming galaxies. These very different energy density levels reflect a similar disparity in the respective supernova rates in the two environments. A L(gamma) ~ SFR^(1.4) relationship is then predicted, in agreement with preliminary observational evidence.Comment: Invited talk at SciNeGHE2010 (8th Wotkshop on Science with the New Generation of High Energy Gamma-ray Experiments): Gamma-ray Astrophysics in the Multimessenger Context (Trieste, Sept.8-10, 2010