The Shape of Dark Matter Halos: Dependence on Mass, Redshift, Radius, and Formation

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.Comment: 18 pages, 21 figures, submitted to MNRA

Resonant sterile neutrino dark matter in the local and high-z Universe

Sterile neutrinos comprise an entire class of dark matter models that, depending on their production mechanism, can be hot, warm, or cold dark matter (CDM). We simulate the Local Group and representative volumes of the Universe in a variety of sterile neutrino models, all of which are consistent with the possible existence of a radiative decay line at ∼3.5 keV. We compare models of production via resonances in the presence of a lepton asymmetry (suggested by Shi & Fuller 1999) to ‘thermal’ models. We find that properties in the highly non-linear regime – e.g. counts of satellites and internal properties of haloes and subhaloes – are insensitive to the precise fall-off in power with wavenumber, indicating that non-linear evolution essentially washes away differences in the initial (linear) matter power spectrum. In the quasi-linear regime at higher redshifts, however, quantitative differences in the 3D matter power spectra remain, raising the possibility that such models can be tested with future observations of the Lyman-α forest. While many of the sterile neutrino models largely eliminate multiple small-scale issues within the CDM paradigm, we show that these models may be ruled out in the near future via discoveries of additional dwarf satellites in the Local Group

Rumble Stripes and Pavement Marking Delineation

Pavement markings serve an important role on the highway and must be visible in day, night, and wet conditions. Pavement markings placed in grooved pavement are receiving considerable interest due to their potential for greater durability by providing protection from plow blades used during winter maintenance. Raised pavement markers (RPMs) are also used for roadway delineation, and the failure rate of RPMs in and in between rumble depressions is also of interest. This study evaluated white edge lines and yellow edge lines on roads in Indiana and other northern states, as well as RPMs in and in between rumble depressions. Data for pavement markings was collected by use of a hand-operated retroreflectometer as well as a mobile retroreflectometer, while RPM data collection consisted of documenting the total number of RPMs, number of missing reflectors, and castings. Results showed that for both white and yellow edge lines, grooved preformed tape has the highest durability for greater than ten winter seasons, and grooved thermoplastic could last five winter seasons. Grooved multi-component may last three or four winter seasons while non-grooved paint will last one or perhaps two winter seasons. RPMs in rumble stripes have a higher failure rate than RPMs installed between rumble stripes. Based upon data collected during this project, grooved thermoplastic and multi-component have the lowest lifetime costs for durable markings, and additional performance data should be collected to determine if either has a distinct economic advantage. It is also recommended that RPM installation in rumble depressions be further evaluated

Properties of resonantly produced sterile neutrino dark matter subhaloes

The anomalous 3.55 keV X-ray line recently detected towards a number of massive dark matter objects may be interpreted as the radiative decays of 7.1 keV mass sterile neutrino dark matter. Depending on its parameters, the sterile neutrino can range from cold to warm dark matter with small-scale suppression that differs in form from commonly adopted thermal warm dark matter. Here, we numerically investigate the subhalo properties for 7.1 keV sterile neutrino dark matter produced via the resonant Shi–Fuller mechanism. Using accurate matter power spectra, we run cosmological zoom-in simulations of a Milky Way-sized halo and explore the abundance of massive subhaloes, their radial distributions, and their internal structure. We also simulate the halo with thermal 2.0 keV warm dark matter for comparison and discuss quantitative differences. We find that the resonantly produced sterile neutrino model for the 3.55 keV line provides a good description of structures in the Local Group, including the number of satellite dwarf galaxies and their radial distribution, and largely mitigates the too-big-to-fail problem. Future searches for satellite galaxies by deep surveys, such as the Dark Energy Survey, Large Synoptic Survey Telescope, and Wide Field Infrared Survey Telescope, will be a strong direct test of warm dark matter scenarios

The Shape of Galaxy Cluster Dark Matter Haloes: Systematics of Its Imprint on Cluster Gas, and Comparison to Observations

(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...Comment: 16 pages, 9 figures; corrected typo (no result affected) submitted to MNRA

Resonant sterile neutrino dark matter in the local and high-z Universe

Sterile neutrinos comprise an entire class of dark matter models that, depending on their production mechanism, can be hot, warm, or cold dark matter (CDM). We simulate the Local Group and representative volumes of the Universe in a variety of sterile neutrino models, all of which are consistent with the possible existence of a radiative decay line at ∼3.5 keV. We compare models of production via resonances in the presence of a lepton asymmetry (suggested by Shi & Fuller 1999) to ‘thermal’ models. We find that properties in the highly non-linear regime – e.g. counts of satellites and internal properties of haloes and subhaloes – are insensitive to the precise fall-off in power with wavenumber, indicating that non-linear evolution essentially washes away differences in the initial (linear) matter power spectrum. In the quasi-linear regime at higher redshifts, however, quantitative differences in the 3D matter power spectra remain, raising the possibility that such models can be tested with future observations of the Lyman-α forest. While many of the sterile neutrino models largely eliminate multiple small-scale issues within the CDM paradigm, we show that these models may be ruled out in the near future via discoveries of additional dwarf satellites in the Local Group

Dwarf Galaxies in CDM, WDM, and SIDM: Disentangling Baryons and Dark Matter Physics

We present a suite of FIRE-2 cosmological zoom-in simulations of isolated field dwarf galaxies, all with masses of $M_\mathrm{halo} \approx 10^{10}\,$M$_\odot$ at $z=0$, across a range of dark matter models. For the first time, we compare how both self-interacting dark matter (SIDM) and/or warm dark matter (WDM) models affect the assembly histories as well as the central density structure in fully hydrodynamical simulations of dwarfs. Dwarfs with smaller stellar half-mass radii (r$_{1/2}<500$ pc) have lower $\sigma_\star/V_\mathrm{max}$ ratios, reinforcing the idea that smaller dwarfs may reside in halos that are more massive than is naively expected. The majority of dwarfs simulated with self-interactions actually experience contraction of their inner density profiles with the addition of baryons relative to the cores produced in dark-matter-only runs, though the simulated dwarfs are always less centrally dense than in $\Lambda$CDM. The V$_{1/2}-$r$_{1/2}$ relation across all simulations is generally consistent with observations of Local Field dwarfs, though compact objects such as Tucana provide a unique challenge. Spatially-resolved rotation curves in the central regions ($<400$ pc) of small dwarfs could provide a way to distinguish between CDM, WDM, and SIDM, however: at the masses probed in this simulation suite, cored density profiles in dwarfs with small r$_{1/2}$ values can only originate from dark matter self-interactions.Comment: 16 pages, 12 figures. V2: matches version accepted by MNRA

The Shape of galaxy cluster dark matter haloes: Systematics of its imprint on cluster gas, and comparison to observations

ABSTRACT We study predictions for galaxy cluster observables that can test the statistics of dark matter halo shapes expected in a flat cold dark matter (CDM) 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 (homeoidal ellipsoids). 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 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 CDM cosmology implies an X-ray ellipticity distribution with a mean X = 0.32 ± 0.01, and a scatter σ = 0.14 ± 0.01 for the mass range (1-4) × 10 14 h −1 M . We find it important to include the mass dependence of halo shape when making comparisons to observational samples. We analyse the systematics of four observational samples of cluster ellipticities and find that our results are statistically compatible with these observations. In particular, we find remarkably good agreement between two recent ROSAT samples and CDM predictions that do not include gas cooling. We also test how well our analytical model can predict Sunyaev-Zel&apos;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 that appears to be seen in observational data. Thus, dark matter haloes cannot be accurately characterized as homeoidal ellipsoids for all comparisons. Key words: cosmology: theory -dark matter -X-rays: galaxies: clusters. I N T RO D U C T I O N Clusters of galaxies are the largest bound structures in the Universe and the most recently formed ones according to the very successful cold dark matter (CDM) cosmology. As such, their dark matter (DM) haloes are expected to be less evolved and more aspherical than, say, galaxy-size haloes. Most gas in cluster DM haloes has E-mail: [email protected] (RAF); [email protected] (BA); [email protected] (AVK); [email protected] (JRP); [email protected] (DAB); [email protected] (JSB) not had time to cool, and since it is gravitationally subdominant, we can expect it to reflect the underlying 3D shape of their dark matter haloes. Indeed, large samples of X-ray clusters have been known to show a broad distribution of ellipticities in their surface brightness (SB) maps since the work of McMillan, The general expectation that in CDM-based theories DM haloes are flattened, are approximately ellipsoidal and have short-to-long axial ratios as small as s ≡ c/a ∼ 0.5 has been known for mor