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Rachel S. Somerville Joel R. Primack S. M. Faber 《Monthly notices of the Royal Astronomical Society》2001,320(4):504-528
Using semi-analytic models of galaxy formation set within the cold dark matter (CDM) merging hierarchy, we investigate several scenarios for the nature of the high-redshift ) Lyman-break galaxies (LBGs). We consider a 'collisional starburst' model in which bursts of star formation are triggered by galaxy–galaxy mergers, and find that a significant fraction of LBGs are predicted to be starbursts. This model reproduces the observed comoving number density of bright LBGs as a function of redshift and the observed luminosity function at and with a reasonable amount of dust extinction. Model galaxies at have star formation rates, half-light radii, colours and internal velocity dispersions that are in good agreement with the data. Global quantities such as the star formation rate density and cold gas and metal content of the Universe as a function of redshift also agree well. Two 'quiescent' models without starbursts are also investigated. In one, the star formation efficiency in galaxies remains constant with redshift, while in the other, it scales inversely with disc dynamical time, and thus increases rapidly with redshift. The first quiescent model is strongly ruled out, as it does not produce enough high-redshift galaxies once realistic dust extinction is accounted for. The second quiescent model fits marginally, but underproduces cold gas and very bright galaxies at high redshift. A general conclusion is that star formation at high redshift must be more efficient than locally. The collisional starburst model appears to accomplish this naturally without violating other observational constraints. 相似文献
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Multi-object spectroscopy of the field surrounding PKS 2126−158: discovery of a z= 0.66 galaxy group
Matthew T. Whiting Rachel L. Webster Paul J. Francis 《Monthly notices of the Royal Astronomical Society》2006,368(1):341-350
The high-redshift radio-loud quasar PKS 2126−158 is found to have a large number of red galaxies in close apparent proximity. We use the Gemini Multi-Object Spectrograph (GMOS) on Gemini South to obtain optical spectra for a large fraction of these sources. We show that there is a group of galaxies at z ∼ 0.66 , coincident with a metal-line absorption system seen in the quasar's optical spectrum. The multiplexing capabilities of GMOS also allow us to measure redshifts of many foreground galaxies in the field surrounding the quasar.
The galaxy group has five confirmed members, and a further four fainter galaxies are possibly associated. All confirmed members exhibit early-type galaxy spectra, a rare situation for a Mg ii absorbing system. We discuss the relationship of this group to the absorbing gas, and the possibility of gravitational lensing of the quasar due to the intervening galaxies. 相似文献
The galaxy group has five confirmed members, and a further four fainter galaxies are possibly associated. All confirmed members exhibit early-type galaxy spectra, a rare situation for a Mg ii absorbing system. We discuss the relationship of this group to the absorbing gas, and the possibility of gravitational lensing of the quasar due to the intervening galaxies. 相似文献
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Rachel T. Buxton Caitlin A. Currey Philip O’B. Lyver Christopher J. Jones 《Marine pollution bulletin》2013
Marine plastic pollution is ubiquitous throughout the world’s oceans, and has been found in high concentrations in oceanic gyres of both the northern and southern hemispheres. The number of studies demonstrating plastic debris at seabird colonies and plastic ingestion by adult seabirds has increased over the past few decades. Despite the recent discovery of a large aggregation of plastic debris in the South Pacific subtropical gyre, the incidence of plastics at seabird colonies in New Zealand is unknown. Between 2011 and 2012 we surveyed six offshore islands on the northeast coast of New Zealand’s North Island for burrow-nesting seabird colonies and the presence of plastic fragments. We found non-research related plastic fragments (0.031 pieces/m2) on one island only, Ohinau, within dense flesh-footed shearwater (Puffinus carneipes) colonies. On Ohinau, we found a linear relationship between burrow density and plastic density, with 3.5 times more breeding burrows in areas with plastic fragments found. From these data we conclude that plastic ingestion is a potentially a serious issue for flesh-footed shearwaters in New Zealand. Although these results do not rule out plastic ingestion by other species, they suggest the need for further research on the relationship between New Zealand’s pelagic seabirds and marine plastic pollution. 相似文献
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Jennifer J. Wetz A. Denene Blackwood J. Stephen Fries Zachary F. Williams Rachel T. Noble 《Estuaries and Coasts》2014,37(2):421-435
Using a refined quantitative polymerase chain reaction assay, we compared surface and bottom water concentrations of Vibrio vulnificus with total Vibrio spp. concentrations, fecal indicator bacteria (Escherichia coli and Enterococcus sp.), and environmental parameters (salinity, temperature, chlorophyll a, and turbidity) over 4 years at a mesohaline site in the Neuse River Estuary in North Carolina. V. vulnificus was not detected when water temperature was below 20 °C and 93 % of positive samples were from salinities 10–20 psu. V. vulnificus was detected in 50 % of summer samples with peak concentrations in summer bottom waters. Significant positive correlations for V. vulnificus and temperature, salinity, and bacterial group abundance were found. Significant negative correlations with chlorophyll a were also observed. Interannual comparisons indicated a significant decrease in mean V. vulnificus since 2005, corresponding with increasing salinity due to severe drought conditions in 2007 and 2008. Total Vibrio spp. abundance also decreased in 2007 but returned to the previously observed abundance by 2008. Although a significant positive relationship between total Vibrio spp. and V. vulnificus was documented, interannual comparisons indicate that total Vibrio spp. densities may not be indicative of V. vulnificus in all environmental conditions and that long-term drought conditions may alter community composition. 相似文献