Structural evolution in elliptical galaxies: the age–shape relation     

Barbara S. Ryden  Duncan A. Forbes  A.I. Terlevich 《Monthly notices of the Royal Astronomical Society》2001,326(3):1141-1148

We test the hypothesis that the apparent axial ratio of an elliptical galaxy is correlated with the age of its stellar population. We find that old ellipticals (with estimated ages t >7.5 Gyr) are rounder on average than younger ellipticals. The statistical significance of this shape difference is greatest at small radii; a Kolmogorov–Smirnov test comparing the axial ratios of the two populations at R = R _e/16 yields a statistical significance greater than 99.96 per cent. The relation between age and apparent shape is linked to the core/power-law surface brightness profile dichotomy. Core ellipticals have older stellar populations, on average, than power-law ellipticals and are rounder in their inner regions. Our findings are consistent with a scenario in which power-law ellipticals are formed in gas-rich mergers, while core ellipticals form in dissipationless mergers, with cores formed and maintained by the influence of a binary black hole.  相似文献   

Self-consistent equilibrium ocean tides   总被引：4，自引：0，他引：4  

Duncan Carr Agnew  W. E. Farrell 《Geophysical Journal International》1978,55(1):171-181

Summary. We compute the static response of the world ocean to an external zonal gravitational potential. The computation includes the effects of the self-attraction of the ocean, and the yielding of the Earth caused both by the external potential and the change in ocean load. We compare the computed tide with measurements of the fortnightly and monthly ocean tides. The short-wavelength departures from equilibrium found by Wunsch are still present. An average of observations at Pacific islands shows that the fortnightly tide departs significantly from equilibrium but the monthly may not. We have also calculated the effects of our computed tide on measurements of tidal gravity and tidal fluctuations in the length of day. Existing tidal gravity data are too imprecise to enable us to determine whether or not the spatial average of the ocean tides departs from equilibrium. The length of day data suggest that the monthly tide is farther from equilibrium than the fortnightly. We have not been able to resolve the apparent discrepancy between the length of day and ocean tide data.  相似文献   

The connection between globular cluster systems and the host galaxies     

Duncan A. Forbes  Juan C. Forte 《Monthly notices of the Royal Astronomical Society》2001,322(2):257-261

A large number of early-type galaxies are now known to possess blue and red subpopulations of globular clusters. We have compiled a data base of 28 such galaxies exhibiting bimodal globular cluster colour distributions. After converting to a common V – I colour system, we investigate correlations between the mean colour of the blue and red subpopulations with galaxy velocity dispersion. We support previous claims that the mean colours of the blue globular clusters are unrelated to their host galaxy. They must have formed rather independently of the galaxy potential they now inhabit. The mean blue colour is similar to that for halo globular clusters in our Galaxy and M31. The red globular clusters, on the other hand, reveal a strong correlation with galaxy velocity dispersion. Furthermore, in well-studied galaxies the red subpopulation has similar, and possibly identical, colours to the galaxy halo stars. Our results indicate an intimate link between the red globular clusters and the host galaxy; they share a common formation history. A natural explanation for these trends would be the formation of the red globular clusters during galaxy collapse.  相似文献   

Spectrophotometry of Jupiter     

Willem Wamsteker  Theodore D. Fay  Bill J. Duncan 《Icarus》1976,29(2):315-319

Spectrophotometric maps of Jupiter were made between 24 and 27(UT) November 1974 on the McMath Solar Telescope at Kitt Peak National Observatory. We report a comparison between observed scaled reflectivities of the Jovian North Tropical Zone and the North Equatorial Belt at System II longitudes between 195 and 205 degrees. The belt/zone reflectivity ratios between 430 and 740nm are related to the optical transmission curves of the organic and/or sulfur polymers synthesized by Khare and Sagan in a simulated Jovian atmosphere.  相似文献   

Correlation of side-scan backscatter intensity with grain-size distribution of shelf sediments, New Jersey margin   总被引：2，自引：0，他引：2  

J. A. Goff  H. C. Olson  C. S. Duncan 《Geo-Marine Letters》2000,20(1):43-49

The dependence of acoustic backscatter variations on sediment grain-size distribution is examined using a combined analysis of 95 kHz side-scan data and approx. 300 grab samples from the New Jersey margin. We show that, under well-controlled circumstances, an unprecedented degree of correlation can be obtained between backscatter intensity and mean grain size. However, backscatter is disproportionately affected by larger grain sizes. Addition of just a few extra weight percentage of the larger grain sizes (>4 mm), which in our study is typically represented by more abundant shell hash among the older sediments at greater water depths, can completely degrade this correlation. Received: 29 September 1999 / Revision accepted: 14 February 2000  相似文献   

Imaging of the protoelliptical NGC 1700 and its globular cluster system     

Richard J. N. Brown  Duncan A. Forbes  Markus Kissler-Patig  Jean P. Brodie 《Monthly notices of the Royal Astronomical Society》2000,317(2):406-420

An excellent candidate for a young elliptical, or 'protoelliptical' galaxy is NGC 1700. Here we present new B -, V - and I -band imaging using the Keck telescope, and reanalyse existing V - and I -band images from the Hubble Space Telescope . After subtracting a model of the galaxy from the Keck images, NGC 1700 reveals two symmetric tidal tail-like structures. If this interpretation is correct, it suggests a past merger event involving two spiral galaxies. These tails are largely responsible for the 'boxiness' of the galaxy isophotes observed at a radius of ∼13 kpc.
We also show that the B − I colour distribution of the globular cluster system is bimodal. The mean colour of the blue population is consistent with that of old Galactic globular clusters. Relative to this old, metal-poor population, we find that the red population is younger and more metal-rich. This young population has an age and metallicity similar to that inferred for the central stars, suggesting that both populations are associated with an episode of star formation triggered by the merger that may have formed the galaxy. We find that, although they have large errors, the majority of the age estimates of NGC 1700 are reasonably consistent and we adopt a 'best estimate' for the age of 3.0±1.0 Gyr. This relatively low age places NGC 1700 within the age range where there is a notable lack of obvious candidates for protoellipticals. The total globular cluster specific frequency is rather low for a typical elliptical, even after taking into account the fading of the galaxy over the next 10 Gyr. We speculate that NGC 1700 will eventually form a relatively 'globular cluster poor' elliptical galaxy.  相似文献   

Are spatial patterns of soil moisture at plot scales generalisable across catchments,climates, and other characteristics? A synthesis of synoptic soil moisture across the Mid-Atlantic     

Charles I. Scaife  Jonathan M. Duncan  Laurence Lin  Christina Tague  Colin D. Bell  Lawrence Band 《水文研究》2021,35(9):e14313

The spatial variation of soil moisture over very small areas (<100 m²) can have nonlinear impacts on cycling and flux rates resulting in bias if it is not considered, but measuring this variation is difficult over extensive temporal and spatial scales. Most studies examining spatial variation of soil moisture were conducted at hillslope (0.01 km²) to multi-catchment spatial scales (1000 km²). They found the greatest variation at mid wetness levels and the smallest variation at wet and dry wetness levels forming a concave down relationship. There is growing evidence that concave down relationships formed between spatial variation of soil moisture and average soil moisture are consistent across spatial scales spanning several orders of magnitude, but more research is needed at very small, plot scales (<100 m²). The goal of this study was to characterise spatial variation in shallow soil moisture at the plot scale by relating the mean of measurements collected in a plot to the standard deviation (SD). We combined data from a previous study with thousands of new soil moisture measurements from 212 plots in eight catchments distributed across the US Mid-Atlantic Region to (1) test for a generalisable mean–SD relationship at plot scales, (2) characterise how landcover, land use, season, and hillslope position contribute to differences in mean–SD relationships, and (3) use these generalised mean–SD relationships to quantify their impacts on catchment scale nitrification and denitrification potential. Our study found that 98% of all measurements formed a generalised mean–SD relationship like those observed at hillslope and catchment spatial scales. The remaining 2% of data comprised a mean–SD relationship with greater spatial variation that originated from two riparian plots reported in a previous study. Incorporating the generalised mean–SD relationship into estimates of nitrification and denitrification potential revealed strong bias that was even greater when incorporating mean–SD observations from the two riparian plots with significantly greater spatial variation.  相似文献   

Probabilistic spatial risk assessment of heat impacts and adaptations for London     

Katie Jenkins  Jim Hall  Vassilis Glenis  Chris Kilsby  Mark McCarthy  Clare Goodess  Duncan Smith  Nick Malleson  Mark Birkin 《Climatic change》2014,124(1-2):105-117

High temperatures and heatwaves can cause large societal impacts by increasing health risks, mortality rates, and personal discomfort. These impacts are exacerbated in cities because of the Urban Heat Island (UHI) effect, and the high and increasing concentrations of people, assets and economic activities. Risks from high temperatures are now widely recognised but motivation and implementation of proportionate policy responses is inhibited by inadequate quantification of the benefits of adaptation options, and associated uncertainties. This study utilises high spatial resolution probabilistic projections of urban temperatures along with projections of demographic change, to provide a probabilistic risk assessment of heat impacts on urban society. The study focuses on Greater London and the surrounding region, assessing mortality risk, thermal discomfort in residential buildings, and adaptation options within an integrated framework. Climate change is projected to increase future heat-related mortality and residential discomfort. However, adjusting the temperature response function by 1–2 °C, to simulate adaptation and acclimatisation, reduced annual heat related mortality by 32–69 % across the scenarios tested, relative to a no adaptation scenario. Similar benefits of adaptation were seen for residential discomfort. The study also highlights additional benefits in terms of reduced mortality and residential discomfort that mitigating the urban heat island, by reducing albedo and anthropogenic heat emissions, could have.  相似文献   

An application of the response function technique to profiles of bathymetry and gravity in the Gulf of Aden   总被引：1，自引：0，他引：1  

Duncan Tamsett 《Geophysical Journal International》1984,78(2):349-369

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On the atmospheric limitations of ground-based submillimetre astronomy using array receivers     

E. N. Archibald  T. Jenness  W. S. Holland  I. M. Coulson  N. E. Jessop  J. A. Stevens  E. I. Robson  R. P. J. Tilanus  W. D. Duncan  J. F. Lightfoot 《Monthly notices of the Royal Astronomical Society》2002,336(1):1-13

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