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

Cosmic-Ray Proton to Electron Ratios

A basic quantity in the characterization of relativistic particles is the proton-to-electron (p/e) energy density ratio. We derive a simple approximate expression suitable to estimate this quantity, U_p/U_e = (m_p/m_e)^(3-q)/2, valid when a nonthermal `gas' of these particles is electrically neutral and the particles' power-law spectral indices are equal -- e.g., at injection. This relation partners the well-known p/e number density ratio at 1 GeV, i.e. N_p/N_e = (m_p/m_e)^{(q-1)/2}.Comment: 4 pages; to be published in Proc. of MGM13 (13th Marcel Grossmann Meeting -- Stockholm July 1-7, 2012

A STIS Survey for OVI Absorption Systems at 0.12 < z < 0.5 I.: The Statistical Properties of Ionized Gas

We have conducted a systematic survey for intervening OVI absorbers in available echelle spectra of 16 QSOs at z_QSO = 0.17-0.57. These spectra were obtained using HST/STIS with the E140M grating. Our search uncovered a total of 27 foreground OVI absorbers with rest-frame absorption equivalent width W_r(1031) > 25mA. Ten of these QSOs exhibit strong OVI absorbers in their vicinity. Our OVI survey does not require the known presence of Lya, and the echelle resolution allows us to identify the OVI absorption doublet based on their common line centroid and known flux ratio. We estimate the total redshift survey path, \Delta z, using a series of Monte-Carlo simulations, and find that \Delta z=1.66, 2.18, and 2.42 for absorbers of strength W_r = 30, 50 and 80mA, respectively, leading to a number density of dN(W > 50mA)/dz = 6.7 +/- 1.7 and dN(W > 30mA)/dz = 10.4 +/- 2.2. In contrast, we also measure dN/dz = 27 +/- 9 for OVI absorbers of W_r > 50mA at |\Delta v|< 5000 kms from the background QSOs. Using the random sample of OVI absorbers with well characterized survey completeness, we estimate a mean cosmological mass density of the OVI gas \Omega(OVI)h = 1.7 +/- 0.3 x 10^-7. In addition, we show that <5% of OVI absorbers originate in underdense regions that do not show a significant trace of HI. Furthermore, we show that the neutral gas column N(HI) associated with these OVI absorbers spans nearly five orders of magnitude, and show moderate correlation with N(OVI). Finally, while the number density of OVI absorbers varies substantially from one sightline to another, it also appears to be inversely correlated with the number density of HI absorbers along individual lines of sight.Comment: 12 pages. ApJ accepte

The LCO/Palomar 10,000 km/sec Cluster Survey. I. Properties of the Tully-Fisher Relation

The first results from a Tully-Fisher (TF) survey of cluster galaxies are presented. The galaxies are drawn from fifteen Abell clusters that lie in the redshift range 9000-12,000 km/sec and are distributed uniformly around the celestial sky. The data set consists of R-band CCD photometry and long- slit H-alpha spectroscopy. The rotation curves (RCs) are characterized by a turnover radius (r_t) and an asymptotic velocity v_a, while the surface brightness profiles are characterized in terms of an effective exponential surface brightness I_e and a scale length r_e. The TF scatter is minimized when the rotation velocity is measured at 2.0 +/- 0.2 r_e; a significantly larger scatter results when the rotation velocity is measured at > 3 or < 1.5 scale lengths. This effect demonstrates that RCs do not have a universal form, as has been suggested by Persic, Salucci, and Stel. In contrast to previous studies, a modest but statistically significant surface-brightness dependence of the TF relation is found, log v = const + 0.28*log L + 0.14*log I_e. This indicates a stronger parallel between the TF relation and the FP relations of elliptical galaxies than has previously been recognized. Future papers in this series will consider the implications of this cluster sample for deviations from Hubble flow on 100-200 Mpc scales.Comment: 35 pages, 8 figures, uses aaspp4.sty. Submitted to ApJ. Also available at http://astro.stanford.edu/jeff

The density profile of equilibrium and non-equilibrium dark matter halos

We study the diversity of the density profiles of dark matter halos based on a large set of high-resolution cosmological simulations of 256^3 particles. The cosmological models include four scale-free models and three representative cold dark matter models. The simulations have good force resolution, and there are about 400 massive halos with more than 10^4 particles within the virial radius in each cosmological model. Our unbiased selection of all massive halos enables to quantify how well the bulk of dark matter halos can be described by the Navarro, Frenk & White (NFW) profile which was established for equilibrium halos. We find that about seventy percent of the halos can be fitted by the NFW profile with a fitting residual dvi_{max} less than 30% in Omega_0=1 universes. This percentage is higher in lower density cosmological models. The rest of the halos exhibits larger deviations from the NFW profile for more significant internal substructures. There is a considerable amount of variation in the density profile even for the halos which can be fitted by the NFW profile (i.e. dvi_{max}<0.30). The distribution of the profile parameter, the concentration $c$, can be well described by a lognormal function with the mean value \bar c slightly smaller (15%) than the NFW result and the dispersion \sigma_c in \ln c about 0.25. The more virialized halos with dvi_{max}<0.15 have the mean value \bar c in good agreement with the NFW result and a slightly smaller dispersion \sigma_c (about 0.2). Our results can alleviate some of the conflicts found recently between the theoretical NFW profile and observational results. Implications for theoretical and observational studies of galaxy formation are discussed.Comment: The final version accepted for publication in ApJ; one figure and one paragraph added to demonstrate that all the conclusions of the first version are solid to the resoltuion effects; 19 pages with 6 figure

Measuring the Spin of Spiral Galaxies

We compute the angular momentum, the spin parameter and the related distribution function for Dark Matter halos hosting a spiral galaxy. We base on scaling laws, inferred from observations, that link the properties of the galaxy to those of the host halo; we further assume that the Dark Matter has the same total specific angular momentum of the baryons. Our main results are: (i) we find that the gas component of the disk significantly contributes to the total angular momentum of the system; (ii) by adopting for the Dark Matter the observationally supported Burkert profile, we compute the total angular momentum of the disk and its correlation with the rotation velocity; (iii) we find that the distribution function of the spin parameter $\lambda$ peaks at a value of about 0.03, consistent with a no-major-merger scenario for the late evolution of spiral galaxies.Comment: 4 pages, 2 figures. Minor changes. Accepted on ApJ