Events at Physics |
Events on Tuesday, September 30th, 2008
- Chaos & Complex Systems Seminar
- Indicators of regime shifts in ecosystems
- Time: 12:05 pm
- Place: 4274 Chamberlin (Refreshments will be served)
- Speaker: Steve Carpenter, UW Center for Limnology
- Abstract: Ecosystems occasionally undergo rapid massive changes - desertification, algae blooms of lakes, replacement of coral reefs by fleshy algae turf, trophic cascades, economic collapse of fisheries, and shrub invasion of rangelands are a few examples. Some regime shifts have big impacts on human life-support and are therefore important for environmental policy. Thus the detection and prediction of regime shifts has emerged as a research topic in basic and applied ecology. Theory shows that certain regular changes in time series should be measurable before an incipient regime shift - autoregression coefficients near one, variance spectra shifted to low frequencies, and rising variance, skewness and kurtosis, for example. However, not all regime shifts show these indicators, and some show opposite responses. While leading indicators show promise as tools for field science and ecosystem management, at present it is difficult to diagnose the characteristics of incipient regime shifts from the indicators alone. Field trials, and modeling to connect theory with the characteristics of particular ecosystems in the field, are needed to improve our understanding of these signals.
- Astronomy Colloquium
- The Assembly of Galaxy Disks and Evolution of Galactic Structures in COSMOS - Reconstructing the Hubble Sequence
- Time: 3:30 pm
- Place: 6515 Sterling Hall
- Speaker: Kartik Sheth, CALTECH
- Abstract: We have analyzed the redshift-dependent fraction of galactic bars over 0.2<z<0.84 in 2,157 luminous face-on spiral galaxies from the COSMOS2-square degree field. Our sample is an order of magnitude larger than that used in any previous investigation, and is based on substantially deeper imaging data than that available from earlier wide-area studies of high-redshift galaxy morphology. We find that the fraction of barred spirals declines rapidly with redshift. Whereas in the local Universe about 65% of luminous spiral galaxies contain bars (SB+SAB), at z~0.84 this fraction drops to about 20%. Over this redshift range the fraction of strong (SB) bars drops from about 30% to under 10%. It is clear that when the Universe was half its present age, the census of galaxies on the Hubble sequence was fundamentally different from that of the present day. A major clue to understanding this phenomenon has also emerged from our analysis, which shows that the bar fraction in spiral galaxies is a strong function of stellar mass, integrated color and bulge prominence. The bar fraction in very massive, luminous spirals is about constant out to z~0.84 whereas for the low mass, blue spirals it declines significantly with redshift beyond z=0.3. There is also a slight preference for bars in bulge dominated systems at high redshifts which may be an important clue towards the co-evolution of bars, bulges and black holes. Our results thus have important ramifications for the processes responsible for galactic downsizing, suggesting that massive galaxies matured early in a dynamical sense, and not just as a result of the regulation of their star formation rate. <br>
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- Host: Professor Eric Wilcots