Ricardo A. Flores’s research while affiliated with University of Missouri–St. Louis and other places

Using six high-resolution dissipationless simulations with a varying box size in a flat Lambda cold dark matter (ΛCDM) universe, we study the mass and redshift dependence of dark matter halo shapes for Mvir= 9.0 × 1011− 2.0 × 1014 h−1 M⊙, over the redshift range z= 0–3, and for two values of σ8= 0.75 and 0.9. Remarkably, we find that the redshift, mass and σ8 dependence of the mean smallest-to-largest axis ratio of haloes is well described by the simple power-law relation 〈s〉= (0.54 ± 0.02)(Mvir/M*)−0.050±0.003, where s is measured at 0.3Rvir, and the z and σ8 dependences are governed by the characteristic non-linear mass, M*=M*(z, σ8). We find that the scatter about the mean s is well described by a Gaussian with σ∼ 0.1, for all masses and redshifts. We compare our results to a variety of previous works on halo shapes and find that reported differences between studies are primarily explained by differences in their methodologies. We address the evolutionary aspects of individual halo shapes by following the shapes of the haloes through ∼100 snapshots in time. We determine the formation scalefactor ac as defined by Wechsler et al. and find that it can be related to the halo shape at z= 0 and its evolution over time.

(Abridged) We study predictions for galaxy cluster observables that can test the statistics of dark matter halo shapes expected in a flat LCDM universe. We present a simple analytical model for the prediction of cluster-scale X-ray observations, approximating clusters as isothermal systems in hydrostatic equilibrium, and dark matter haloes as ellipsoids with uniform axial ratios. We test the model against high-resolution, hydrodynamic cluster simulations to gauge its reliability. We find that this simple prescription does a good job of predicting the distribution of cluster X-ray ellipticities compared to the simulations as long as one focuses on cluster regions that are less sensitive to recent mergers. Based on this simple model, the distribution of cluster-size halo shapes expected in the concordance LCDM cosmology implies an X-ray ellipticity distribution with a mean of 0.32 +- 0.01 and a scatter of 0.14 +- 0.01 for the mass range (1-4)x10^{14} Msun/h. We find it important to include the mass dependence of halo shape to make comparisons to observational samples that contain many, very massive clusters. We analyse the systematics of four observational samples of cluster ellipticities and find that our results are statistically compatible with observations. In particular, we find remarkably good agreement between two recent ROSAT samples and LCDM predictions that DO NOT include gas cooling. We also test how well our analytical model can predict Sunyaev-Zel'dovich decrement maps and find that it is less successful although still useful; the model does not perform as well as a function of flux level in this case because of the changing triaxiality of dark matter haloes as a function of radial distance. Both this effect and the changing alignment of isodensity shells of dark matter haloes leave an imprint on cluster gas...

Using six high resolution dissipationless simulations with a varying box size in a flat LCDM universe, we study the mass and redshift dependence of dark matter halo shapes for M_vir = 9.0e11 - 2.0e14, over the redshift range z=0-3, and for two values of sigma_8=0.75 and 0.9. Remarkably, we find that the redshift, mass, and sigma_8 dependence of the mean smallest-to-largest axis ratio of halos is well described by the simple power-law relation = (0.54 +- 0.02)(M_vir/M_*)^(-0.050 +- 0.003), where s is measured at 0.3 R_vir and the z and sigma_8 dependences are governed by the characteristic nonlinear mass, M_*=M_*(z,sigma_8). We find that the scatter about the mean s is well described by a Gaussian with sigma ~ 0.1, for all masses and redshifts. We compare our results to a variety of previous works on halo shapes and find that reported differences between studies are primarily explained by differences in their methodologies. We address the evolutionary aspects of individual halo shapes by following the shapes of the halos through ~100 snapshots in time. We determine the formation scalefactor a_c as defined by Wechsler et al. (2002) and find that it can be related to the halo shape at z = 0 and its evolution over time.

The degeneracy between the disk and the dark matter contribution to galaxy rotation curves remains an important uncertainty in our understanding of disk galaxies. Here we discuss a new method for breaking this degeneracy using gravitational lensing by spiral galaxies, and apply this method to the spiral lens B1600+434 as an example. The combined image and lens photometry constraints allow models for B1600+434 with either a nearly singular dark matter halo, or a halo with a sizable core. A maximum disk model is ruled out with high confidence. Further information, such as the circular velocity of this galaxy, will help break the degeneracies. Future studies of spiral galaxy lenses will be able to determine the relative contribution of disk, bulge, and halo to the mass in the inner parts of galaxies.

The frequency with which background galaxies appear as long arcs as a result of gravitational lensing by foreground clusters of galaxies has recently been found to be a very sensitive probe of cosmological models by Bartelmann et al. (1998). They have found that such arcs would be expected far less frequently than observed (by an order of magnitude) in the currently favored model for the universe, with a large cosmological constant $\Omega_\Lambda \sim 0.7$. Here we analyze whether including the effect of cluster galaxies on the likelihood of clusters to generate long-arc images of background galaxies can change the statistics. Taking into account a variety of constraints on the properties of cluster galaxies, we find that there are not enough sufficiently massive galaxies in a cluster for them to significantly enhance the cross section of clusters to generate long arcs. We find that cluster galaxies typically enhance the cross section by only $\lesssim 15$. Comment: 19 pages, 1 figure, uses aasms4.sty, submitted to ApJ

Spheroidal components of spiral galaxies have been considered the only dynamically important component in gravitational lensing studies thus far. Here we point out that including the disk component can have a significant effect, depending on the disk inclination, on a variety of lensing properties that are relevant to present studies and future surveys. As an example, we look at the multiple image system B1600+434, recently identified as being lensed by a spiral galaxy. We find that including the disk component one can understand the fairly large image separation as being due to the inclination of a typical spiral, rather than the presence of a very massive halo. The fairly low magnification ratio can also be readily understood if the disk is included. We also discuss how such lensed systems might allow one to constrain parameters of spiral galaxies such as a disk-to-halo mass ratio, and disk mass scale length. Another example we consider is the quasar multiple-lensing cross section, which we find can increase many-fold at high inclination for a typical spiral. Finally, we discuss the changes in the gravitational lensing effects on damped Lyman alpha systems (DLAS) when disk lensing is included. Comment: 13 pages, 6 figures. To appear in ApJ v.486, September 10, 1997

Supersymmetric model contributions to the neutron electric dipole moment arise via quark electric dipole moment operators, whose matrix elements are usually calculated using the Naive Quark Model (NQM). However, experiments indicate that the NQM does not describe well the quark contributions Δq to the nucleon spin, and so may provide misleading estimates of electric dipole operator matrix elements. Taking the Δq from experiment, we indeed find consistently smaller estimates of the neutron electric dipole moment for given values of the supersymmetric model parameters. This weakens previous constraints on CP violation in supersymmetric models, which we exemplify analytically in the case where the lightest supersymmetric particle (LSP) is a U(1) gaugino , and display numerically for other LSP candidates.

Supersymmetric model contributions to the neutron electric dipole moment arise via quark electric dipole moment operators, whose matrix elements are usually calculated using the Naive Quark Model (NQM). However, experiments indicate that the NQM does not describe well the quark contributions $\Delta q$ to the nucleon spin, and so may provide misleading estimates of electric dipole operator matrix elements. Taking the $\Delta q$ from experiment, we indeed find consistently smaller estimates of the neutron electric dipole moment for given values of the supersymmetric model parameters. This weakens previous constraints on CP violation in supersymmetric models, which we exemplify analytically in the case where the lightest supersymmetric particle (LSP) is a U(1) gaugino $\tilde{B}$, and display numerically for other LSP candidates.

The distribution of dark matter around galactic or cluster halos has usually been assumed to be approximately isothermal with a non-zero core radius, which is expected to be of the order of the size of the visible matter distribution. Recently, the possibility has been raised that dark matter halos might be singular in the sense that the dark matter density $\rho$ could increase monotonically with radius r down to a very small distance from the center of galaxies or clusters. Such central cusps in the dark matter density could lead to a high flux of gamma rays from WIMP dark matter annihilation. Here we analyze two possibilities that have been discussed in the literature, $\rho \propto r^{-n}$ with $n \approx 1\ {\rm or}\ 2$, and point out that such density profiles are excluded by gravitional lensing analyses on cluster scales and by the rotation curves of gas-rich, halo-dominated dwarf spirals on small scales. We also point out that if spiral galaxies form by gas infall inside dark matter halos, as they are expected to do in any hierarchical clustering model, such profiles almost always lead to falling rotation curves after infall, contrary to observations. Comment: 15 pages incl 4 figs, submitted to Astrophys J Lett, uuencoded compressed postscript, preprint SCIPP 93/01-rev

We discuss how the parameter space of the Minimal Supersymmetric extension of the Standard Model could be probed by future searches for neutralinos with laboratory Dark Matter detectors of a given level of sensitivity. We take into account all the constraints already imposed on model parameters by negative searches for supersymmetry at LEPI, restricting our attention to neutralinos lighter than the W-boson. We find that at a level of sensitivity of 0.1 (kg · day)−1 targets of heavy nuclei would probe much more of the remaining parameter space than would targets of light nuclei. However, at a level of sensitivity of 0.01 (kg · day)−1, a target of heavy nuclei combined with a target of light nuclei such as 19F would allow exploration of more parameter space than either one alone. We then consider the impact that a negative search for supersymmetry at LEPII would have on these conclusions. We find that when all the constraints that such searches would imply are imposed on model parameters, at a level of sensitivity of 0.1 (kg · day)−1 even targets of heavy nuclei would probe only a small fraction of the allowed parameter space, and at a level of sensitivity of 0.01 (kg · day)−1 a target of heavy nuclei would allow exploration of much more parameter space than a target of light nuclei. Our conclusions are genetic to the extent that the large variety of nuclei that we consider cover the range of possibilities.