Statistics on 24 spiral galaxies having different observed arm locations using different arm tracers

The density wave theory predicted some physical offsets among different tracers of star formation. To test this prediction, here we compiled data on 40 galaxies searched observationally for a physical offset between spiral arm tracers, and found that 24 of them have a positive offset. In a spiral arm, an arm tracer in a region with a given temperature may be at a different location (offset) than an arm tracer in a region with a colder temperature.Some conditions are found to be necessary or sufficient in order to detect an offset between two arm tracers. To find the offset of a tracer from another tracer, one needs a proper linear resolution. Starting in the dust lane and going across the spiral arm, we seek the observed physical width of the star-forming zone (offset). In our sample of 24 galaxies with measured offsets, we find offsets with a median value near 326 pc and a mean near 370 pc. These offsets are comparable to those found in our Milky Way galaxy, between the cold diffuse CO 1–0 gas set at 0 pc, and the hot dust near 350 pc.Preliminary statistics are performed on the angular velocity of the gas and stars and angular velocity of the spiral pattern. Their observed orbital velocity of 200 km/s at a typical galactic radius near 4 kpc yields an angular speed of the gas and stars near 60 km/s/kpc. Their deduced angular rotation for the spiral pattern averages 36 km/s/kpc. These observational results are close to the results predicted by the shock-induced star-forming density wave theory. These mean or median property values will be useful for finding other galaxies that can support density waves.     

星系盘厚度效应的研究

在三维引力Poisson方程严格解基础上，探讨了有限厚星系盘基盘的动力学性质，并进一步讨论了盘的厚度效应对银河系所需晕质量的影响。研究了扰动盘的动力学性质，通过将扰动引力势Poisson方程的严格解与林家翘、徐遐生提出的自维持密度波理论相结合，建立了三维旋涡星系有限厚盘上密度波的色散关系。在此色散关系的基础上讨论了盘的局域稳定性，研究了旋涡星系旋臂的形态、三维盘状星系密度波的群速度。研究表明厚度是星系盘研究中不容忽略的重要参量。另外在有限厚盘星系密度波色散关系的基础上还探讨了一种确定星系厚度的新方法。     

Optical Thickness of the 2–4.5 GHz Solar Plasma Emission

For radio emission at the frequency corresponding to the second harmonic of the local plasma frequency, the optical thickness in the solar atmosphere is calculated. Three types of models are assumed: the model with radio emission from the narrow transition region, and models with radio emission from a cool and dense plasma filament embedded in hotter plasma at the transition region and in the corona. The optical thickness is computed by integration of the collisional (free–free) absorption along a radio-ray path radial in the solar atmosphere. In all models considered the optical thickness can be sufficiently low for appropriate parameters. For example, in the narrow (<100 km) transition region where the density scale height is much less than that of the pressure one, the optical thickness can be lower than 1. Furthermore, the optical thickness can be decreased if the radio emission is generated in the cool and dense plasma filament surrounded by hotter and thinner plasma. But the models differ in density scale heights and thus in distances between plasma emission levels. This difference is essential for the interpretation of high-frequency type III radio bursts.     

Effect of density inhomogeneity on YORP: The case of Itokawa

The effect of density inhomogeneity on the YORP effect for a given shape model is investigated. A density inhomogeneity will cause an offset between the center of figure and the center of mass and a re-orientation of the principal axes away from those associated with the shape alone. Both of these effects can alter the predicted YORP rate of change in angular velocity and obliquity. We apply these corrections to the Itokawa shape model and find that its YORP angular velocity rate is sensitive to offsets between its center of mass and center of figure, with a shift on the order of 15 m being able to change the sign of the YORP effect for that asteroid. Given the non-detection of YORP for Itokawa as of 2008, this can shed light on the density distribution within that body. The theory supports a shift of the asteroid center of mass towards Itokawa's neck region, where there is an accumulation of finer gravels, or towards the asteroid's “Head” region. Detection of the YORP effect for Itokawa should provide some strong constraints on its density distribution. This theory could also be applied to asteroids visited by future spacecraft to constrain density inhomogeneities.     

Biases in the polarization position angles in the NRAO-VLA sky survey point source catalogue

We have examined the statistics of the polarization position angles determined for point sources in the NRAO-VLA Sky Survey (NVSS), and find that there is a statistically significant bias towards angles which are multiples of 45°. The formal probability that the polarization angles are drawn from a uniform distribution is exponentially small. When the sample of those NVSS sources with polarizations detected with a signal-to-noise ratio ≥3 is split either around the median polarized flux density or the median fractional polarization, the effect appears to be stronger for the more highly polarized sources. Regions containing strong sources and regions at low-Galactic latitudes are not responsible for the non-uniform distribution of position angles. We identify clean bias as the probable cause of the dominant effect, coupled with small multiplicative and additive offsets on each of the Stokes parameters. Our findings have implications for the extraction of science, such as information concerning galactic magnetic fields, from large-scale polarization surveys.     

LoCuSS: the connection between brightest cluster galaxy activity, gas cooling and dynamical disturbance of X-ray cluster cores

We study the distribution of projected offsets between the cluster X-ray centroid and the brightest cluster galaxy (BCG) for 65 X-ray-selected clusters from the Local Cluster Substructure Survey, with a median redshift of   z = 0.23  . We find a clear correlation between X-ray/BCG projected offset and the logarithmic slope of the cluster gas density profile at  0.04 r ₅₀₀(α  ), implying that more dynamically disturbed clusters have weaker cool cores. Furthermore, there is a close correspondence between the activity of the BCG, in terms of detected Hα and radio emission, and the X-ray/BCG offset, with the line-emitting galaxies all residing in clusters with X-ray/BCG offsets of ≤15 kpc. Of the BCGs with  α < −0.85  and an offset <0.02 r ₅₀₀, 96 per cent (23/24) have optical emission and 88 per cent (21/24) are radio active, while none has optical emission outside these criteria. We also study the cluster gas fraction ( f _gas) within r ₅₀₀ and find a significant correlation with X-ray/BCG projected offset. The mean f _gas of the 'small offset' clusters (<0.02 r ₅₀₀) is  0.106 ± 0.005 (σ= 0.03  ) compared to  0.145 ± 0.009 (σ= 0.04  ) for those with an offset >0.02 r ₅₀₀, indicating that the total mass may be systematically underestimated in clusters with larger X-ray/BCG offsets. Our results imply a link between cool core strength and cluster dynamical state consistent with the view that cluster mergers can significantly perturb cool cores, and set new constraints on models of the evolution of the intracluster medium.     

Strength of the lithosphere of the Galilean satellites

Several approaches have been used to estimate the ice shell thickness on Callisto, Ganymede, and Europa. Here we develop a method for placing a strict lower bound on the thickness of the strong part of the shell (lithosphere) using measurements of topography. The minimal assumptions are that the strength of faults in the brittle lithosphere is controlled by lithostatic pressure according to Byerlee's law and the shell has relatively uniform density and thickness. Under these conditions, the topography of the ice provides a direct measure of the bending moment in the lithosphere. This topographic bending moment must be less than the saturation bending moment of the yield strength envelope derived from Byerlee's law. The model predicts that the topographic amplitude spectrum decreases as the square of the topographic wavelength. This explains why Europa is rugged at shorter wavelengths (∼10 km) but extremely smooth, and perhaps conforming to an equipotential surface, at longer wavelengths (>100 km). Previously compiled data on impact crater depth and diameter [Schenk, P.M., 2002. Nature 417, 419-421] on Europa show good agreement with the spectral decrease predicted by the model and require a lithosphere thicker than 2.5 km. A more realistic model, including a ductile lower lithosphere, requires a thickness greater than 3.5 km. Future measurements of topography in the 10-100 km wavelength band will provide tight constraints on lithospheric strength.     

Layering stratigraphy of eastern Coprates and northern Capri Chasmata, Mars

Distinct competent layers are observed in the slopes of eastern Coprates Chasma, part of the Valles Marineris system on Mars. Our observations indicate that the stratigraphy of Coprates Chasma consists of alternating thin strong layers and thicker sequences of relatively weak layers. The strong, competent layers maintain steeper slopes and play a major role in controlling the overall shape and geomorphology of the chasmata slopes. The topmost competent layer in this area is well preserved and easy to identify in outcrops on the northern rim of Coprates Chasma less than 100 m below the southern Ophir Planum surface. The volume of the topmost emplaced layer is at least 70 km³ and may be greater than 2100 km³ if the unit underlies most of Ophir Planum. The broad extent of this layer allows us to measure elevation offsets within the north rim of the chasma and in a freestanding massif within Coprates Chasma where the layer is also observed. Rim outcrop morphology and elevation differences between Ophir and Aurorae Plana may be indicative of the easternmost extent of the topmost competent layer. These observations allow an insight into the depositional processes that formed the stratigraphic stack into which this portion of the Valles Marineris is carved, and they present a picture of some of the last volcanic activity in this area. Furthermore, the elevation offsets within the layer are evidence of significant subsidence of the massif and surrounding material.     

基于VLBI资料的ERP高频变化求解方法的研究

在天测与测地VLBI资料分析软件CALC／SOLVE中，对地球自转参数(ERP)高频变化的解算采用了附加约束的连续分段线性拟合方法，即要求在两个历元节点之间ERP变化率小于某事先约定值，并要求ERP在历元节点上连续．实测资料分析表明，当资料点密度较低时，引入约束条件和要求连续均有助于提高解的稳定性，但也人为降低了解的客观性，使各历元节点ERP解算结果之间相关．为此，基于CALC／SOLVE的用户偏导功能，实现了ERP高频变化的直接求解模块，不附加约束，也不要求连续．实测资料分析表明对ERP高频变化的直接求解模式更为可取．对于长时段VLBI资料的ERP高频变化求解，需要考虑岁差和章动模型偏差(天极偏移)的影响，编写相应的求解模块，并成功实现了1979至2003年长时段ERP高频变化求解．比较表明，在考虑了岁差章动模型偏差的影响时能够显著提高解的精度．为此，在基于VLBI资料解算ERP高频变化时，建议采用直接求解模式，并考虑岁差章动模型偏差的影响．     

Extinction techniques and impact on dust property determination

The near-infrared (NIR) extinction power-law index (β) and its uncertainty is derived from three different techniques based on star counts, colour excess and a combination of them. We have applied these methods to Two Micron All Sky Survey (2MASS) data to determine maps of β and NIR extinction of the small cloud IC 1396 W. The combination of star counts and colour excess results in the most reliable method to determine β. It is found that the use of the correct β map to transform colour excess values into extinction is fundamental for column density profile analysis of clouds. We describe how artificial photometric data, based on the model of stellar population synthesis of the Galaxy, can be used to estimate uncertainties and derive systematic effects of the extinction methods presented here. We find that all colour excess based extinction determination methods are subject to small but systematic offsets, which do not affect the star counting technique. These offsets occur since stars seen through a cloud do not represent the same population as stars in an extinction-free control field.     

The strength of the lunar lithosphere

A self-gravitating, elastic, spherical thick shell model is used to derive the present state of the lateral variations of density and stress differences within the lunar lithosphere. The model is allowed to deform under the load of an initial surface topography and internal density distribution, such that the resulting deformed body gives rise to the observed surface topography and gravity specified by the spherical harmonics of degree up to 70. Two main models are considered, Model A and Model B, with elastic lithospheres of thickness 300 and 210 km, respectively. Model A displays density perturbations of generally less than ±200 kg/m³ within the crustal layers, reducing rapidly to less than ±20 kg/m³ at the base of the lithosphere. The density perturbations in Model B are similar in the crust and marginally higher at the base of the lithosphere. The major stress differences in the mantle are associated with the mascon basins and are found to reach maximums of 8-10 MPa within the lower lithosphere (150-270 km) of Model A and maximums of 12-16 MPa at 150 to 180 km depth for Model B. A moderate correlation exists between the modeled stress distributions and shallow moonquake epicenters. However, the overall results of this study imply that other remnant stresses, due to processes other than density perturbations, exist and play a critical role in the large shallow moonquakes.     

Ancient heat flow and crustal thickness at Warrego rise, Thaumasia highlands, Mars: Implications for a stratified crust

Heat flow calculations based on geological and/or geophysical indicators can help to constrain the thickness, and potentially the geochemical stratification, of the martian crust. Here we analyze the Warrego rise region, part of the ancient mountain range referred to as the Thaumasia highlands. This region has a crustal thickness much greater than the martian average, as well as estimations of the depth to the brittle-ductile transition beneath two scarps interpreted to be thrust faults. For the local crustal density (2900 kg m⁻³) favored by our analysis of the flexural state of compensation of the local topography, the crustal thickness is at least 70 and 75 km at the scarp locations. However, for one of the scarp locations our nominal model does not obtain heat flow solutions permitting a homogeneous crust as thick as required. Our results, therefore, suggest that the crust beneath the Warrego rise region is chemically stratified with a heat-producing enriched upper layer thinner than the whole crust. Moreover, if the mantle heat flow (at the time of scarp formation) was higher than 0.3 of the surface heat low, as predicted by thermal history models, then a stratified crust rise seems unavoidable for this region, even if local heat-producing element abundances lower than average or hydrostatic pore pressure are considered. This finding is consistent with a complex geological history, which includes magmatic-driven activity.     

自由地核章动的时变特性

对VLBI观测确定的IAU1980章动模型的天极偏移序列进行分析,结果显示自由地核章动在1990年以前的幅值比其后为强,其时变强度比周年受迫章动的为大．另外,小波变换的时频谱分析结果显示在天极偏移序列中存在一幅值约0．1毫角秒的准两年周期信号．仅从目前的数据分析结果尚不足以确定此信号与顺向自由地核章动之间的关系,进一步的观测检,验和深入的内核动力学研究是非常必要的．     

Rings of Uranus: Not so thick,not so black

The optical thickness of the rings of Uranus has been thoroughly measured by many stellar occultations. However, we show that the optical thickness so obtained is larger by an extinction efficiency factor of 2 than the fractional area physically filled by particles which is commonly used to infer both particle reflectivity and particle size. By neglecting this factor, previous work has overestimated particle packing density and therefore underestimated individual particle reflectances and sizes (as well as overestimated collision frequencies). This has led to concern as to why the ring particles seemed unusually black and small. We present new estimates of particle reflectance which include both this effect and an improved radiative transfer treatment, and show that the ring particles, while still quite dark, are no longer mysteriously so. Particle sizes, while not strongly constrained, could easily lie in the macroscopic size range characteristic of other planetary rings.     

Electrolytic currents in Europa

Electrical currents should flow in Europa because of its presence in Jupiter's corotating magnetosphere. The possible magnitudes of these currents are calculated assuming that Europa is a differentiated body consisting of an outer H₂O layer and a silicate core. Two types of models are considered here: one in which the water is completely frozen and a second in which there is an intermediate liquid layer. For the transverse electric mode (eddy currents), the calculated current density in a liquid layer is approximately 10^?5 A m^?2. For the transverse magnetic mode (unipolar generator), the calculated current density in the liquid is severely constrained by the ice layer to only 10^?10 to 10^?11 A m^?2, for a total H₂O thickness of 100 km, provided that neither layer is less than 4 km thick. The current density is less for a completely frozen H₂O layer. If transient cracks were to appear in the ice layer, exposing liquid, the calculated current density could rise to a range of 10^?6 to 10^?5 A m^?2, depending on layer thicknesses, requiring an exposed area of 10^?9 to 10^?8 of the Europa surface. Electrical heating would be significant only if the ice layer thickness were on the order of 1 m, such as might occur if an exposed liquid surface were to freeze over; the heating under this condition could hinder the thickening of the ice layer.     

Stability of particulate discs

Collisionally-induced amplification of density fluctuations can also produce non-axisymmetric local condensations in particulate discs if the optical thickness is between definite values. Gravitational instability occurs above this interval. The theory of both phenomena is derived from collisional equations. The conventional criterion for gravitational instability in a gaseous medium cannot be used for particulate discs, in which the equilibrium depends on the collisional energy loss. These instabilities can produce an unbounded growth in density or a gravitational coagulation of particles, but the typical consequence is the formation of highly elongated clouds which are denser than the background matter and have a relatively long lifetime before decay. The third type of instability, the thermal one, appears at low values of velocity dispersion. It only affects the random motion of particles without producing condensations.     

Comparison of Ground-Based and Space-Based Longitudinal Magnetograms

We compare photospheric line-of-sight magnetograms from the Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM) instrument with observations from the 150-foot Solar Tower at Mt. Wilson Observatory (MWO), the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), and the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). We find very good agreement between VSM and the other data sources for both disk-averaged flux densities and pixel-by-pixel measurements. We show that the VSM mean flux density time series is of consistently high signal-to-noise ratio with no significant zero offsets. We discuss in detail some of the factors ?C spatial resolution, flux dependence, and position on the solar disk ?C affecting the determination of scaling between VSM and SOHO/MDI or SDO/HMI magnetograms. The VSM flux densities agree well with spatially smoothed data from MDI and HMI, although the scaling factors show a clear dependence on flux density. The factor to convert VSM to HMI increases with increasing flux density (from ??1 to ??1.5). The nonlinearity is smaller for the VSM vs. SOHO/MDI scaling factor (from ??1 to ??1.2).     

Nearby galaxies. II. Luminosity function and spatial distribution

A sample of nearby galaxies (Schmidt and Boller 1992, paper I) has been analysed with regard to the luminosity function and the spatial distribution. The main results are as follows: (i) a relation between the slope of the luminosity function of the members of a group of galaxies and the earliest morphological type in this group has been detected which possibly is a new kind of environmental effects; (ii) the virial masses of the groups of galaxies are, on the average, by a factor 3 to 4 greater, only, than the luminous masses derived from the individual galaxy masses; (iii) the nearby galaxies are concentrated in a thin disk-like layer around the supergalactic plane the thickness of which is a few hundred kiloparsecs, only; (iv) a population of field galaxies with nearly constant density (about 3 per cent of the number density in the supergalactic plane) is extended into the voids on both the supergalactic hemispheres.     

Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites

Data from the recent gravity measurements by the Galileo mission are used to construct wide ranges of interior structure and composition models for the Galilean satellites of Jupiter. These models show that mantle densities of Io and Europa are consistent with an olivine-dominated mineralogy with the ratios of Mg to Fe components depending on mantle temperature for Io and on ice shell thickness for Europa. The mantle density and composition depend relatively little on core composition. The size of the core is largely determined by the core's composition with core radius increasing with the concentration of a light component such as sulfur. For Io, the range of possible core sizes is between 38 and 53% of the satellite's radius. For Europa, there is also a substantial effect of the thickness of the ice layer which is varied between 120 and 170 km on the core size. Core sizes are between 10 and 45% of Europa's radius. The core size of Ganymede ranges between one-quarter and one-third of the surface radius depending on its sulfur content and the thickness of the ice shell. A subset of the Ganymede models is consistent with an olivine-dominated mantle mineralogy. The thickness of the silicate mantle above the core varies between 900 and 1100 km. The outermost ice shell is about 900 km in thickness and is further subdivided by pressure-induced phase transitions into ice I, ice III, ice V, and ice VI layers. Callisto should be differentiated, albeit incompletely. It is proposed that this satellite was never molten at a large scale but differentiated through the convective gradual unmixing of the ice and the metal/rock component. Bulk iron-to-silicon ratios Fe/Si calculated for the inner pair of satellites, Io and Europa, are less than the CI carbonaceous chondrite value of 1.7±0.1, whereas ratios for the outer pair, Ganymede and Callisto, cover a broad range above the chondritic value. Although the ratios are uncertain, in particular for Ganymede and Callisto, the values are sufficiently distinct to suggest a difference in composition between these two pairs of satellites. This may indicate a difference in iron-silicon fractionation during the formation of both classes of satellites in the protojovian nebula.     

Density waves in Cassini UVIS stellar occultations: 1. The Cassini Division

We analyze density waves in the Cassini Division of Saturn's rings revealed by multiple stellar occultations by Saturn's rings observed with the Cassini Ultraviolet Imaging Spectrograph. The dispersion and damping of density waves provide information on the local ring surface mass density and viscosity. Several waves in the Cassini Division are on gradients in the background optical depth, and we find that the dispersion of the wave reflects a change in the underlying surface mass density. We find that over most of the Cassini Division the ring opacity (the ratio of optical depth to surface mass density) is nearly constant and is ∼5 times higher than the opacity in the A ring where most density waves are found. However, the Cassini Division ramp, a 1100-km-wide, nearly featureless region of low optical depth that connects the Cassini Division to the inner edge of the A ring, has an opacity like that of the A ring and significantly less than that in the rest of the Cassini Division. This is consistent with particles in the ramp originating in the A ring and being transported into the Cassini Division through ballistic transport processes. Damping of the waves in the Cassini Division suggests a vertical thickness of 3–6 m. Using a mean opacity of 0.1 cm²/g we find the mass of the Cassini Division, excluding the ramp, is 3.1×¹⁰¹⁶ kg while the mass of the Cassini Division ramp, with an opacity of 0.015 cm²/g, is 1.1×¹⁰¹⁷ kg. Assuming a power-law size distribution for the ring particles, the larger opacity of the main Cassini Division is consistent with the largest ring particles there being ∼5 times smaller than the largest particles in the ramp and A ring.