The influence of halo assembly on galaxies and galaxy groups     

Tatiana Zapata  Josefa Perez  Nelson Padilla  Patricia Tissera 《Monthly notices of the Royal Astronomical Society》2009,394(4):2229-2237

In this paper, we study the variations of group galaxy properties according to the assembly history in Sloan Digital Sky Survey Data Release 6 (SDSS-DR6) selected groups. Using mock SDSS group catalogues, we find two suitable indicators of group formation time: (i) the isolation of the group, defined as the distance to the nearest neighbour in terms of its virial radius and (ii) the concentration, measured as the group inner density calculated using the fifth nearest bright galaxy to the group centre. Groups within narrow ranges of mass in the mock catalogue show increasing group age with isolation and concentration. However, in the observational data the stellar age, as indicated by the spectral type, only shows a correlation with concentration.
We study groups of similar mass and different assembly history, finding important differences in their galaxy population. Particularly, in high-mass SDSS groups, the number of members, mass-to-light ratios, red galaxy fractions and the magnitude difference between the brightest and second-brightest group galaxies, show different trends as a function of isolation and concentration, even when it is expected that the latter two quantities correlate with group age. Conversely, low-mass SDSS groups appear to be less sensitive to their assembly history.
The correlations detected in the SDSS are not consistent with the trends measured in the mock catalogues. However, discrepancies can be explained in terms of the disagreement found in the age-isolation trends, suggesting that the model might be overestimating the effects of environment. We discuss how the modelling of the cold gas in satellite galaxies could be responsible for this problem. These results can be used to improve our understanding of the evolution of galaxies in high-density environments.  相似文献   

Milky Way type galaxies in a ΛCDM cosmology     

Maria E. De Rossi  Patricia B. Tissera  Gabriella De Lucia †  Guinevere Kauffmann 《Monthly notices of the Royal Astronomical Society》2009,395(1):210-217

We analyse a sample of 52 000 Milky Way (MW) type galaxies drawn from the publicly available galaxy catalogue of the Millennium Simulation with the aim of studying statistically the differences and similarities of their properties in comparison to our Galaxy. Model galaxies are chosen to lie in haloes with maximum circular velocities in the range 200–250 km s⁻¹ and to have bulge-to-disc ratios similar to that of the MW. We find that model MW galaxies formed 'quietly' through the accretion of cold gas and small satellite systems. Only ≈12 per cent of our model galaxies experienced a major merger during their lifetime. Most of the stars formed ' in situ ', with only about 15 per cent of the final mass gathered through accretion. Supernovae (SNe) and active galactic nuclei (AGN) feedback play an important role in the evolution of these systems. At high redshifts, when the potential wells of the MW progenitors are shallower, winds driven by SNe explosions blow out a large fraction of the gas and metals. As the systems grow in mass, SNe feedback effects decrease and AGN feedback takes over, playing a more important role in the regulation of the star formation activity at lower redshifts. Although model MW galaxies have been selected to lie in a narrow range of maximum circular velocities, they nevertheless exhibit a significant dispersion in the final stellar masses and metallicities. Our analysis suggests that this dispersion results from the different accretion histories of the parent dark matter haloes. Statistically, we also find evidences to support the MW as a typical Sb/Sc galaxy in the same mass range, providing a suitable benchmark to constrain numerical models of galaxy formation.  相似文献   

Fisheries — life after UNCLOS     

Patricia Birnie 《Marine Policy》1981,5(1):2-3

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WHEN A RESIDENCE IS NOT A HOUSE: EXAMINING RESIDENCE-BASED MIGRATION DEFINITIONS     

Michelle Behr  Patricia Gober 《The Professional geographer》1982,34(2):178-184

Traditional definitions of migration contain implicit assumptions: origin and destination places are fixed residences, households inhabit one residence at any given time, and dwelling units form the center of individual activity spaces. Though these assumptions hold for certain movers, they restrict migration studies to a narrow range of movement. It is argued here that such assumptions bias present notions of migrant behavior and selectivity. A more realistic appraisal might consider integration in sets of spatial networks rather than movement between two discrete points in space.  相似文献   

Back to Dunkirk     

Patricia White 《Geology Today》2001,17(1):13-13

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33rd Meeting of the International Whaling Commission: Brighton,UK, 20–26 July 1981     

Patricia Birnie 《Marine Policy》1982,6(1):74-76

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Geochemistry of Santo Antônio da Barra Kamafugites, Goiás, Brazil     

Patricia Barbosa de Albuquerque Sgarbi  Jos Carlos Gaspar 《Journal of South American Earth Sciences》2002,14(8):236

This work focuses on the kamafugites from Santo Antônio da Barra, Minas–Goiás Alkaline Province. These rocks contain olivine, clinopyroxene, titanomagnetite, perovskite, leucite (pseudomorphs), kalsilite, nepheline, and phlogopite.The rocks investigated are ultrabasic, with high contents of CaO, FeO, and TiO₂, high to moderate contents of Al₂O₃, alkalis, and P₂O₅, and low contents of MgO. The alkaline characteristic of the rocks is reflected in TiO₂, K₂O, and Na₂O contents and in the frequent presence of normative nepheline and leucite. K₂O contents are not primary since most of the leucite was replaced by analcime.The negative K anomaly verified in the extended incompatible element distribution diagram for kamafugites seems to be mainly related to alteration. Kamafugites are characterized by a marked enrichment in incompatible and large ion lithophile elements together with other typical compatible elements.The Santo Antônio da Barra kamafugites are less enriched in titanium, niobium, zirconium, and REE than the Mata da Corda and most of the Toro-Ankole ones. San Venanzo–Cupaello rocks have much lower titanium contents.  相似文献   

Harmful algal blooms in the Chesapeake and Coastal Bays of Maryland, USA: Comparison of 1997, 1998, and 1999 events   总被引：2，自引：0，他引：2  

Patricia M. Glibert  Robert Magnien  Michael W. Lomas  Jeffrey Alexander  Chunlei Tan  Erin Haramoto  Mark Trice  Todd M. Kana 《Estuaries and Coasts》2001,24(6):875-883

Harmful algal blooms in the Chesapeake Bay and coastal bays of Maryland, USA, are not a new phenomenon, but may be increasing in frequency and diversity. Outbreaks ofPfiesteria piscicida (Dinophyceae) were observed during 1997 in several Chesapeake Bay tributaries, while in 1998,Pfiesteria-related events were not found but massive blooms ofProrocentrum minimum (Dinophyceae) occurred. In 1999,Aureococcus anophagefferens (Pelagophyceae) developed in the coastal bays in early summer in sufficient densities to cause a brown tide. In 1997, toxicPfiesteria was responsible for fish kills at relatively low cell densities. In 1998 and 1999, the blooms ofP. minimum andA. anophagefferens were not toxic, but reached sufficiently high densities to have ecological consequences. These years differed in the amount and timing of rainfall events and resulting nutrient loading from the largely agricultural watershed. Nutrient loading to the eastern tributaries of Chesapeake Bay has been increasing over the past decade. Much of this nutrient delivery is in organic form. The sites of thePfiesteria outbreaks ranked among those with the highest organic loading of all sites monitored bay-wide. The availability of dissolved organic carbon and phosphorus were also higher at sites experiencingA. anophagefferens blooms than at those without blooms. The ability to supplement photosynthesis with grazing or organic substrates and to use a diversity of organic nutrients may play a role in the development and maintenance of these species. ForP. minimum andA. anophagefferens, urea is used preferentially over nitrate.Pfiesteria is a grazer, but also has the ability to take up nutrients directly. The timing of nutrient delivery may also be of critical importance in determining the success of certain species.  相似文献   

Horizontal distribution of the rotifer plankton of Lake Aydat (France)   总被引：2，自引：0，他引：2  

Nicole Lair  Hassan Taleb  Patricia Reyes-Marchant 《Aquatic Sciences - Research Across Boundaries》1996,58(3):253-268

We investigated the causes of the shift which occurs in the horizontal distribution of planktonic rotifers of Lake Aydat. A normalised PCA was used to analyse the spatio-temporal distribution of 18 rotifer species. They are more numerous in the littoral zone, but seasonal changes remain higher than spatial changes. Results illustrate the capability of rotifers to take efficiently advantage of available space and food. The digestive tracts of fry and juvenile roach was also analysed and the index of Ivlev was used to determine prey selection. Fish predation occurs in the nearshore zone and the interactions between invertebrate predators and their prey are illustrated by the quantitative changes in community dynamics. Among the complex cascade of events varying in the course of the year, and despite of resource partitioning, predator-prey interactions as well as the exploitative competition between rotifers and crustaceans produce gradients exhibited in the horizontal distribution of rotifers in Lake Aydat.  相似文献   

Sunken wood from the Vanuatu Islands: identification of wood substrates and preliminary description of associated fauna   总被引：1，自引：0，他引：1  

Marie Pailleret  Takuma Haga  Patricia Petit  Catherine Privé-Gill  Nima Saedlou  Françoise Gaill  & Magali Zbinden 《Marine Ecology》2007,28(1):233-241

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