Substructure and halo density profiles in a Warm Dark Matter Cosmology

We performed a series of high-resolution simulations designed to study the substructure of Milky Way-size galactic halos (host halos) and the density profiles of halos in a warm dark matter (WDM) scenario with a non-vanishing cosmological constant. The virial masses of the host halos range from 3.5 x 10^12 to 1.7 x 10^12 solar masses and they have more than 10^5 particles each. A key feature of the WDM power spectrum is the free-streaming length R_f which fixes an additional parameter for the model of structure formation. We analyze the substructure of host halos using three R_f values: 0.2, 0.1, and 0.05 Mpc and compare results to the predictions of the cold dark matter (CDM) model. We find that guest halos (satellites) do form in the WDM scenario but are more easily destroyed by dynamical friction and tidal disruption than their counterparts in a CDM model. The small number of guest halos that we find within the virial radii of host halos at z = 0 in the WDM models is the result of a less efficient halo accretion and a higher satellite destruction rate. Under the assumption that each guest halo hosts a luminous galaxy, we find that the observed circular velocity function of satellites around the Milky Way and Andromeda is well described by the R_f = 0.1 Mpc WDM model. In the R_f = 0.1-0.2 Mpc models, the surviving subhalos at z=0 have an average concentration parameter c_1/5 which is approximately twice smaller than that of the corresponding CDM subhalos. This difference, very likely, produces the higher satellite destruction rate found in the WDM models. The density profile of host halos is well described by the NFW fit whereas guest halos show a wide variety of density profiles (abridged).Comment: Uses emulateapj.sty: 10 pages, 4 figures, ApJ accepted. Some changes have been introduced as suggested by the referee: (1) the description of the numerical simulations was sligthly modified to make it clearer, (2) the ellipticities of the host halos are now measured, and (3) the discussion section was divided in two subsections and enlarge

Nonconventional short-time dc magnetometer for superconducting films

Includes bibliographical references.A nonconventional technique for the measurement of magnetic relaxation in superconducting films at short times (~ 10-5 s) is described. This technique combines the application of a pulsed magnetic field and a synchronized high-energy pulsed laser. Remanent magnetic relaxation in (Y;Gd)Ba2Cu3O7-δ thin films is reported over five decades time at reduced temperatures above 0.8.This work was partially supported by Fundació Sauberán, EX018-UBA and EX285-UBA grants

Elastic-to-plastic crossover below the peak effect in the vortex solid of YBa2Cu3O7 single crystals

We report on transport and ac susceptibility studies below the peak effect in twinned YBa2Cu3O7 single crystals. We find that disorder generated at the peak effect can be partially inhibited by forcing vortices to move with an ac driving current. The vortex system can be additionally ordered below a well-defined temperature where elastic interactions between vortices overcome pinning-generated stress and a plastic to elastic crossover seems to occur. The combined effect of these two processes results in vortex structures with different mobilities that give place to history effects.Comment: 4 pages, 4 figures. Published in PRB Rapid Comm., February 1, 200

Amplitude Zeros in Radiative Decays of Scalar Particles

We study amplitude zeros in radiative decay processes with a photon or a gluon emission of all possible scalar particles(e.g. scalar leptoquarks) which may interact with the usual fermions in models beyond the standard model. For the decays with a photon emission, the amplitudes clearly exhibit the factorization property and the differential decay rates vanish at specific values of a certain variable which are determined only by the electric charges of the particles involved and independent of the particle masses and the various couplings. For the decays with a gluon emission, even though the zeros are washed away, the differential decay rates still have distinct minima. The branching ratios as a function of leptoquark masses are presented for the scalar leptoquark decays. We also comment on the decays of vector particles into two fermions and a photon.Comment: Revtex, 17 pages + 6 figures (available upon request), Preprint, OITS559. Several typos with tex file were correcte

Amplitude Zeros in W±ZW^\pm Z Production

We demonstrate that the Standard Model amplitude for $f_1 \bar f_2 \rightarrow W^\pm Z$ at the Born-level exhibits an approximate zero located at $\cos\theta = (g^{f_1}_{-} + g^{f_2}_{-}) / (g^{f_1}_{-} - g^{f_2}_{-})$ at high energies, where the $g^{f_i}_{-}$ ($i=1,2$) are the left-handed couplings of the $Z$-boson to fermions and $\theta$ is the center of mass scattering angle of the $W$-boson. The approximate zero is the combined result of an exact zero in the dominant helicity amplitudes ${\cal M}(\pm,\mp)$ and strong gauge cancelations in the remaining amplitudes. For non-standard $WWZ$ couplings these cancelations no longer occur and the approximate amplitude zero is eliminated.Comment: 11 pages, 4 figures submitted separately as uuencoded tar-ed postscript files, FSU-HEP-940307, UCD-94-

Are Dwarf Galaxies Dominated by Dark Matter?

Mass models for a sample of 18 late-type dwarf and low surface brightness galaxies show that in almost all cases the contribution of the stellar disks to the rotation curves can be scaled to explain most of the observed rotation curves out to two or three disk scale lengths. The concept of a maximum disk, therefore, appears to work as well for these late-type dwarf galaxies as it does for spiral galaxies. Some of the mass-to-light ratios required in our maximum disk fits are high, however, up to about 15 in the R-band, with the highest values occurring in galaxies with the lowest surface brightnesses. Equally well-fitting mass models can be obtained with much lower mass-to-light ratios. Regardless of the actual contribution of the stellar disk, the fact that the maximum disk can explain the inner parts of the observed rotation curves highlights the similarity in shapes of the rotation curve of the stellar disk and the observed rotation curve. This similarity implies that the distribution of the total mass density is closely coupled to that of the luminous mass density in the inner parts of late-type dwarf galaxies.Comment: Accepted for publication in the Astrophysical Journa

Is there Evidence for Flat Cores in the Halos of Dwarf Galaxies?: The Case of NGC 3109 and NGC 6822

Two well studied dwarf galaxies -- NGC 3109 and NGC 6822 -- present some of the strongest observational support for a flat core at the center of galactic dark matter (DM) halos. We use detailed cosmologically motivated numerical models to investigate the systematics and the accuracy of recovering parameters of the galaxies. Some of our models match the observed structure of the two galaxies remarkably well. Our analysis shows that the rotation curves of these two galaxies are instead quite compatible with their DM halos having steep cuspy density profiles. The rotation curves in our models are measured using standard observational techniques. The models reproduce the rotation curves of both galaxies, the disk surface brightness profiles as well as the profile of isophotal ellipticity and position angle. The models are centrally dominated by baryons; however, the dark matter component is globally dominant. The simulated disk mass is marginally consistent with a stellar mass-to-light ratio in agreement with the observed colors. We show that non-circular motions combined with gas pressure support and projection effects results in a large underestimation of the circular velocity in the central $\sim 1$ kpc region, creating the illusion of a constant density core. Although the systematic effects mentioned above are stronger in barred systems, they are also present in axisymetric disks. Our results strongly suggest that there is no contradiction between the observed rotation curves in dwarf galaxies and the cuspy central dark matter density profiles predicted by Cold Dark Matter models.Comment: Accepted for publication in the ApJ. New discussion, figures and one appendix. High resolution version at:http://www.astro.washington.edu/octavio/N3109_paper.ps.g