The BTC40 Survey for Quasars at 4.8 < z < 6

The BTC40 Survey for high-redshift quasars is a multicolor search using images obtained with the Big Throughput Camera (BTC) on the CTIO 4-m telescope in V, I, and z filters to search for quasars at redshifts of 4.8 < z < 6. The survey covers 40 sq. deg. in B, V, & I and 36 sq. deg. in z. Limiting magnitudes (3 sigma) reach to V = 24.6, I = 22.9 and z = 22.9. We used the (V-I) vs. (I-z) two-color diagram to select high-redshift quasar candidates from the objects classified as point sources in the imaging data. Follow-up spectroscopy with the AAT and CTIO 4-m telescopes of candidates having I < 21.5 has yielded two quasars with redshifts of z = 4.6 and z = 4.8 as well as four emission line galaxies with z = 0.6. Fainter candidates have been identified down to I = 22 for future spectroscopy on 8-m class telescopes.Comment: 27 pages, 8 figures; Accepted for publication in the Astronomical Journa

The ESO UVES Advanced Data Products Quasar Sample - VI. Sub-Damped Lyman-α\alpha Metallicity Measurements and the Circum-Galactic Medium

The Circum-Galactic Medium (CGM) can be probed through the analysis of absorbing systems in the line-of-sight to bright background quasars. We present measurements of the metallicity of a new sample of 15 sub-damped Lyman-$\alpha$ absorbers (sub-DLAs, defined as absorbers with 19.0 < log N(H I) < 20.3) with redshift 0.584 < $\rm z_{abs}$ < 3.104 from the ESO Ultra-Violet Echelle Spectrograph (UVES) Advanced Data Products Quasar Sample (EUADP). We combine these results with other measurements from the literature to produce a compilation of metallicity measurements for 92 sub-DLAs as well as a sample of 362 DLAs. We apply a multi-element analysis to quantify the amount of dust in these two classes of systems. We find that either the element depletion patterns in these systems differ from the Galactic depletion patterns or they have a different nucleosynthetic history than our own Galaxy. We propose a new method to derive the velocity width of absorption profiles, using the modeled Voigt profile features. The correlation between the velocity width delta_V90 of the absorption profile and the metallicity is found to be tighter for DLAs than for sub-DLAs. We report hints of a bimodal distribution in the [Fe/H] metallicity of low redshift (z < 1.25) sub-DLAs, which is unseen at higher redshifts. This feature can be interpreted as a signature from the metal-poor, accreting gas and the metal-rich, outflowing gas, both being traced by sub-DLAs at low redshifts.Comment: 64 pages, 31 figures, 27 tables. Submitted to MNRA

Climate Stabilization at 2°C and Net Zero Carbon Emissions

The goal to stabilize global average surface temperature at lower than 2°C above pre-industrial level has been extensively discussed in climate negotiations. A number of publications state that achieving this goal will require net anthropogenic carbon emissions (defined as anthropogenic emissions minus anthropogenic sinks such as carbon capture and sequestration and reforestation) to be reduced to zero between years 2050 and 2100. At the same time, it is also shown in the literature that decreases of non-CO2 emissions can significantly affect the allowable carbon budget. In this study, we explore possible emission pathways under which surface warming will not exceed 2°C, by means of emission-driven climate simulations with an Earth System Model of Intermediate Complexity linked to an Economic Projection and Policy Analysis Model. We carried out a number of simulations from 1861 to 2500 for different values of parameters defining the strength of the climate system response to radiative forcing and the strength of the natural carbon sources and sinks under different anthropogenic emission projections. Although net anthropogenic emissions need to be reduced to zero eventually to achieve climate stabilization, the results of our simulations suggest that, by including significant reductions in non-CO2 emissions, net carbon emissions do not have to be zero by 2050 or even 2100 to meet the 2°C target because of offsets due to the natural carbon sinks in the oceans and terrestrial ecosystems. We show that net anthropogenic carbon emissions falling from today’s 9.5 GtC/year to 2.5–7 GtC/year by 2050 and then to 1–2.8 GtC/year by 2100 are consistent with a 2°C target for a range of climate sensitivities (2.0–4.5°C) similar to the IPCC likely range. Changes in the surface temperature beyond 2100 depend on the emission profiles after 2100. For post-2100 carbon emissions decreasing at a rate of about 1.5% per year along with continued decreases in non-CO2 emissions, our projections indicate that natural ecosystems will be able to absorb enough carbon to prevent surface temperature from rising further. A major reason for our results is that the land and ocean uptake rates are a function of the total atmospheric CO2 concentration and, due to the very long lifetime of CO2, this does not decrease anywhere near as fast as the imposed CO2 emissions. The required mixes of energy technologies and the overall costs to achieve the 2°C target are highly dependent on the assumptions about the future costs of low-carbon and zero-carbon emitting technologies. In all our projections, the global energy system requires substantial transformations in a relatively short time.The MIT Joint Program is funded by a consortium of government, industrial and foundation sponsors (for the complete list, see: http://globalchange.mit.edu/sponsors). Martin Haigh represents the Scenarios Team at Shell International Ltd

Modeling Regional Carbon Dioxide Flux over California using the WRF‑ACASA Coupled Model

Many processes and interactions in the atmosphere and the biosphere influence the rate of carbon dioxide exchange between these two systems. However, it is difficult to estimate the carbon dioxide flux over regions with diverse ecosystems and complex terrains, such as California. Traditional carbon dioxide measurements are sparse and limited to specific ecosystems. Therefore, accurately estimating carbon dioxide flux on a regional scale remains a major challenge. In this study, we couple the Weather Research and Forecasting Model (WRF) with the Advanced Canopy-Atmosphere-Soil Algorithm (ACASA), a high complexity land surface model. Although WRF is a state-of-the-art regional atmospheric model with high spatial and temporal resolutions, the land surface schemes available in WRF lack the capability to simulate carbon dioxide. ACASA is a complex multilayer land surface model with interactive canopy physiology and full surface hydrological processes. It allows microenvironmental variables such as air and surface temperatures, wind speed, humidity, and carbon dioxide concentration to vary vertically. Carbon dioxide, sensible heat, water vapor, and momentum fluxes between the atmosphere and land surface are estimated in the ACASA model through turbulence equations with a third order closure scheme. It therefore permits counter-gradient transports that low-order turbulence closure models are unable to simulate. A new CO2 tracer module is introduced into the model framework to allow the atmospheric carbon dioxide concentration to vary according to terrestrial responses. In addition to the carbon dioxide simulation, the coupled WRF-ACASA model is also used to investigate the interactions of neighboring ecosystems in their response to atmospheric carbon dioxide concentration. The model simulations with and without the CO2 tracer for WRF-ACASA are compared with surface observations from the AmeriFlux network.This work is supported in part by the National Science Foundation under Awards No.ATM-0619139 and EF-1137306. The Joint Program on the Science and Policy of Global Change is funded by a number of federal agencies and a consortium of 40 industrial and foundation sponsors. (For the complete list see http://globalchange.mit.edu/sponsors/current.html)