Uranus at equinox: Cloud morphology and dynamics     

L.A. Sromovsky  P.M. Fry  W.M. Ahue  K.A. Rages  M.A. van Dam 《Icarus》2009,203(1):265-286

As the 7 December 2007 equinox of Uranus approached, collaboration between ring and atmosphere observers in the summer and fall of 2007 produced a substantial collection of ground-based observations using the 10-m Keck telescope with adaptive optics and space-based observations with the Hubble Space Telescope. Both near-infrared and visible-wavelength imaging and spatially resolved near-infrared spectroscopic observations were obtained. We used observations spanning the period from 7 June 2007 through 9 September 2007 to identify and track cloud features, determine atmospheric motions, characterize cloud morphology and dynamics, and define changes in atmospheric band structure. Atmospheric motions were obtained over a wider range of latitudes than previously was possible, extending to 73°N, and for 28 cloud features we obtained extremely high wind-speed accuracy through extended tracking times. We confirmed the existence of the suspected northern hemisphere prograde jet, locating its peak near 58°N. The new results confirm a small N-S asymmetry in the zonal wind profile, and the lack of any change in the southern hemisphere between 1986 (near solstice) and 2007 (near equinox) suggests that the asymmetry may be permanent rather than seasonally reversing. In the 2007 images, we found two prominent groups of discrete cloud features with very long lifetimes. The one near 30°S has departed from its previous oscillatory motion and started a significant northward drift, accompanied by substantial morphological changes. The complex of features near 30°N remained at a nearly fixed latitude, while exhibiting some characteristics of a dark spot accompanied by bright companion features. Smaller and less stable features were used to track cloud motions at other latitudes, some of which lasted over many planet rotations, though many could not be tracked beyond a single transit. A bright band has developed near 45°N, while the bright band near 45°S has begun to decline, both events in agreement with the idea that the asymmetric band structure of Uranus is a delayed response to solar forcing, but with a surprisingly short delay of only a few years.  相似文献   

Investigation of water signatures at gully-exposed sites on Mars by hyperspectral image analysis     

Chaojun Fan  Dirk Schulze-Makuch 《Planetary and Space Science》2009,57(1):93-104

High-resolution images from the Mars Obiter Camera (MOC) onboard the Mars Global Surveyor (MGS) show a variety of gully features on sloped surfaces of Mars. The mechanism of gully formation is still under debate, although a majority of studies tend to favor a mechanism related to liquid water flow based on geomorphology and fluid mechanics considerations. In this study, we examined four known gully sites using Visible and Infrared Mineralogical Mapping Spectrometer (OMEGA) imagery. In particular, we analyzed the absorption depths of the water-associated absorption bands and concluded that there are stronger water signatures at the gully-exposed sites than in the surrounding areas. This implies that the water signatures, most likely representing water ice, isolated water molecules, and/or hydroxyl molecules incorporated into minerals, are still present in the shallow unconsolidated soils. This study provides additional evidence that water was likely involved in the formation of the gully features and is still locally active on the Martian surface in the present time.  相似文献   

Evidence for Amazonian acidic liquid water on Mars—A reinterpretation of MER mission results     

Alberto G. Fairén  Dirk Schulze-Makuch  Wolfgang Fink  Esther R. Uceda  Ricardo Amils 《Planetary and Space Science》2009,57(3):276-318

The Mars Exploration Rover (MER) missions have confirmed aqueous activity on Mars. Here we review the analyses of the field-based MER data, and conclude that some weathering processes in Meridiani Planum and Gusev crater are better explained by late diagenetic water-rock interactions than by early diagenesis only. At Meridiani, the discovery of jarosite by MER-1 Opportunity indicates acidic aqueous activity, evaporation, and desiccation of rock materials. MER-based information, placed into the context of published data, point to local and limited aqueous activity during geologically recent times in Meridiani. Pre-Amazonian environmental changes (including important variations in the near-surface groundwater reservoirs, impact cratering, and global dust storms and other pervasive wind-related erosion) are too extreme for pulverulent jarosite to survive over extended time periods, and therefore we argue instead that jarosite deposits must have formed in a climatically more stable period. Any deposits of pre-existent concretionary jarosite surviving up to the Amazonian would not have reached completion in the highly saline and acidic brines occurring at Meridiani. MER-2 Spirit has also revealed evidence for local and limited Amazonian aqueous environmental conditions in Gusev crater, including chemical weathering leading to goethite and hematite precipitation, rock layering, and chemical enhancement of Cl, S, Br, and oxidized iron in rocks and soils. The estimated relative age of the impact crater materials in Gusev indicates that these processes have taken place during the last 2 billion years. We conclude that minor amounts of shallow acidic liquid water have been present on the surface of Mars at local scales during the Amazonian Period.  相似文献   

Possibilities for the detection of hydrogen peroxide-water-based life on Mars by the Phoenix Lander     

Joop M. Houtkooper  Dirk Schulze-Makuch 《Planetary and Space Science》2009,57(4):449-453

The Phoenix Lander landed on Mars on 25 May 2008. It has instruments on board to explore the geology and climate of subpolar Mars and to explore if life ever arose on Mars. Although the Phoenix mission is not a life detection mission per se, it will look for the presence of organic compounds and other evidence to support or discredit the notion of past or present life.The possibility of extant life on Mars has been raised by a reinterpretation of the Viking biology experiments [Houtkooper, J. M., Schulze-Makuch, D., 2007. A possible biogenic origin for hydrogen peroxide on Mars: the Viking results reinterpreted. International Journal of Astrobiology 6, 147-152]. The results of these experiments are in accordance with life based on a mixture of water and hydrogen peroxide instead of water. The near-surface conditions on Mars would give an evolutionary advantage to organisms employing a mixture of H₂O₂ and H₂O in their intracellular fluid: the mixture has a low freezing point, is hygroscopic and provides a source of oxygen. The H₂O₂-H₂O hypothesis also explains the Viking results in a logically consistent way. With regard to its compatibility with cellular contents, H₂O₂ is used for a variety of purposes in terran biochemistry. The ability of the anticipated organisms to withstand low temperatures and the relatively high water vapor content of the atmosphere in the Martian arctic, means that Phoenix will land in an area not inimical to H₂O₂-H₂O-based life. Phoenix has a suite of instruments which may be able to detect the signatures of such putative organisms.  相似文献   

Satellite kinematics – II. The halo mass–luminosity relation of central galaxies in SDSS     

Surhud More †  Frank C. van den Bosch  Marcello Cacciato  H. J. Mo  Xiaohu Yang  Ran Li 《Monthly notices of the Royal Astronomical Society》2009,392(2):801-816

The kinematics of satellite galaxies reflect the masses of the extended dark matter haloes in which they orbit, and thus shed light on the mass–luminosity relation (MLR) of their corresponding central galaxies. In this paper, we select a large sample of centrals and satellites from the Sloan Digital Sky Survey and measure the kinematics (velocity dispersions) of the satellite galaxies as a function of the r -band luminosity of the central galaxies. Using the analytical framework presented in More, van den Bosch & Cacciato, we use these data to infer both the mean and the scatter of the MLR of central galaxies, carefully taking account of selection effects and biases introduced by the stacking procedure. As expected, brighter centrals on average reside in more massive haloes. In addition, we find that the scatter in halo masses for centrals of a given luminosity,  σ_{log  M}   , also increases with increasing luminosity. As we demonstrate, this is consistent with  σ_{log  L}   , which reflects the scatter in the conditional probability function   P ( L _c| M )  , being independent of halo mass. Our analysis of the satellite kinematics yields  σ_{log  L} = 0.16  ±  0.04  , in excellent agreement with constraints from clustering and group catalogues, and with predictions from a semi-analytical model of galaxy formation. We thus conclude that the amount of stochasticity in galaxy formation, which is characterized by  σ_{log  L}   , is well constrained, independent of halo mass and in a good agreement with current models of galaxy formation.  相似文献   

STAGES: the Space Telescope A901/2 Galaxy Evolution Survey     

Meghan E. Gray  Christian Wolf  Marco Barden  Chien Y. Peng  Boris Häußler  Eric F. Bell  Daniel H. McIntosh  Yicheng Guo  John A. R. Caldwell  David Bacon  Michael Balogh  Fabio D. Barazza  Asmus Böhm  Catherine Heymans  Knud Jahnke  Shardha Jogee  Eelco van Kampen  Kyle Lane  Klaus Meisenheimer  Sebastian F. Sánchez  y Taylor  Lutz Wisotzki  Xianzhong Zheng  David A. Green  R. J. Beswick  D. J. Saikia  Rachel Gilmour  Benjamin D. Johnson  Casey Papovich 《Monthly notices of the Royal Astronomical Society》2009,393(4):1275-1301

We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). STAGES is a multiwavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity. A complex multicluster system at   z ∼ 0.165  has been the subject of an 80-orbit F606W Hubble Space Telescope (HST) /Advanced Camera for Surveys (ACS) mosaic covering the full     span of the supercluster. Extensive multiwavelength observations with XMM–Newton , GALEX, Spitzer , 2dF, Giant Metrewave Radio Telescope and the 17-band COMBO-17 photometric redshift survey complement the HST imaging. Our survey goals include simultaneously linking galaxy morphology with other observables such as age, star formation rate, nuclear activity and stellar mass. In addition, with the multiwavelength data set and new high-resolution mass maps from gravitational lensing, we are able to disentangle the large-scale structure of the system. By examining all aspects of an environment we will be able to evaluate the relative importance of the dark matter haloes, the local galaxy density and the hot X-ray gas in driving galaxy transformation. This paper describes the HST imaging, data reduction and creation of a master catalogue. We perform the Sérsic fitting on the HST images and conduct associated simulations to quantify completeness. In addition, we present the COMBO-17 photometric redshift catalogue and estimates of stellar masses and star formation rates for this field. We define galaxy and cluster sample selection criteria, which will be the basis for forthcoming science analyses, and present a compilation of notable objects in the field. Finally, we describe the further multiwavelength observations and announce public access to the data and catalogues.  相似文献   

The correlation of star formation quenching with internal galaxy properties and environment     

Taysun Kimm  Rachel S. Somerville  Sukyoung K. Yi  Frank C. van den Bosch  Samir Salim  Fabio Fontanot  Pierluigi Monaco  Houjun Mo  Anna Pasquali  R. M. Rich  Xiaohu Yang 《Monthly notices of the Royal Astronomical Society》2009,394(3):1131-1147

We investigate the correlation of star formation quenching with internal galaxy properties and large-scale environment (halo mass) in empirical data and theoretical models. We make use of the halo-based group catalogue of Yang and collaborators, which is based on the Sloan Digital Sky Survey. Data from the Galaxy evolution explorer are also used to extract the recent star formation rate. In order to investigate the environmental effects, we examine the properties of 'central' and 'satellite' galaxies separately. For central galaxies, we are unable to conclude whether star formation quenching is primarily connected with halo mass or stellar mass, because these two quantities are themselves strongly correlated. For satellite galaxies, a nearly equally strong dependence on halo mass and stellar mass is seen. We make the same comparison for five different semi-analytic models based on three independently developed codes. We find that the models with active galactic nuclei feedback reproduce reasonably well the dependence of the fraction of central red and passive galaxies on halo mass and stellar mass. However, for satellite galaxies, the same models badly overproduce the fraction of red/passive galaxies and do not reproduce the empirical trends with stellar mass or halo mass. This satellite overquenching problem is caused by the too-rapid stripping of the satellites' hot gas haloes, which leads to rapid strangulation of star formation.  相似文献   

Metal-rich carbon stars in the Sagittarius dwarf spheroidal galaxy     

Eric Lagadec  Albert A. Zijlstra  G. C. Sloan  Peter R. Wood  Mikako Matsuura  Jeronimo Bernard-Salas  J. A. D. L. Blommaert  M.-R. L. Cioni  M. W. Feast  M. A. T. Groenewegen  Sacha Hony  J. W. Menzies  J. Th. van Loon  P. A. Whitelock 《Monthly notices of the Royal Astronomical Society》2009,396(1):598-608

We present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C₂H₂ and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range  1.0–3.3 × 10⁻⁸ M_⊙ yr⁻¹  . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample.  相似文献   

The impact of feedback on disc galaxy scaling relations     

Aaron A. Dutton  Frank C. van den Bosch 《Monthly notices of the Royal Astronomical Society》2009,396(1):141-164

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Future evolution of bound superclusters in an accelerating Universe     

Pablo A. Araya-Melo  reas Reisenegger  rés Meza  Rien van de Weygaert  Rolando Dünner  Hernán Quintana 《Monthly notices of the Royal Astronomical Society》2009,399(1):97-120

The evolution of marginally bound supercluster-like objects in an accelerating Λ cold dark matter (ΛCDM) Universe is followed, by means of cosmological simulations, from the present time to an expansion factor   a = 100  . The objects are identified on the basis of the binding density criterion introduced by Dünner et al. Superclusters are identified with the ones whose mass   M > 10¹⁵  h ⁻¹ M_⊙  , the most massive one with   M ∼ 8 × 10¹⁵  h ⁻¹ M_⊙  , comparable to the Shapley supercluster. The spatial distribution of the superclusters remains essentially the same after the present epoch, reflecting the halting growth of the cosmic web as Λ gets to dominate the expansion of the Universe. The same trend can be seen in the stagnation of the development of the mass function of virialized haloes and bound objects. The situation is considerably different when looking at the internal evolution, quantified in terms of their shape, compactness and density profile, and substructure in terms of their multiplicity function. We find a continuing evolution from a wide range of triaxial shapes at   a = 1  to almost perfect spherical shapes at   a = 100  . We also find a systematic trend towards a higher concentration. Meanwhile, we see their substructure gradually disappearing, as the surrounding subclumps fall in and merge to form one coherent, virialized system.  相似文献