Dissertation com nov 2005 (3 résultats)

Taschenbuch. Etat : Neu. Neuware - We study the properties of star-forming galaxies at redshift z~2, anera in which a substantial fraction of the stellar mass in theuniverse formed. Using 114 near-IR spectra of the H-alpha and [N II]emission lines and model spectral energy distributions fit torest-frame UV through IR photometry, we examine the galaxies' starformation properties, dynamical masses and velocity dispersions,spatially resolved kinematics, outflow properties, and metallicitiesas a function of stellar mass and age. While the stellar masses ofthe galaxies in our sample vary by a factor of ~500, dynamical massesfrom H-alpha velocity dispersions and indirect estimates of gas massesimply that the variation of stellar mass is due as much to theevolution of the stellar population and the conversion of gas intostars as to intrinsic differences in the total masses of the galaxies.About 10% of the galaxies are apparently young starbursts with highgas fractions, caught just as they have begun to convert large amountsof gas into stars. Using the [N II]/H-alpha ratio of compositespectra to estimate the average oxygen abundance, we find a monotonicincrease in metallicity with stellar mass. From the estimated gasfractions, we conclude that the observed mass-metallicity relation isprimarily driven by the increase in metallicity as gas is converted tostars. The picture that emerges is of galaxies with a broad range instellar population properties, from young galaxies with ages of a fewtens of Myr, stellar masses M~10^9 Msun, and metallicities Z~ 1/3Zsun, to massive objects with M\*~10^11 Msun, Z~Zsun, and ages as oldas the universe allows. All, however, are rapidly star-forming, powergalactic-scale outflows, and have masses in gas and stars of at least~10^10 Msun, in keeping with their likely role as the progenitors ofelliptical galaxies seen today.…

Taschenbuch. Etat : Neu. Neuware - As late as 1995, the anticipated widespread population of primeval galaxies remained at large, lurking undetected at unknown redshifts, with undiscovered properties. We present results from our efforts to detect and characterize primeval galaxies by theirsignature high-redshift Lyman-alpha emission lines utilizing two observational techniques: serendipitous slit spectroscopy and narrowband imaging. By pushing these techniques to their utmost limits, we probe the Lyman-alpha-emitting galaxy population out to redshifts as high as z = 6.5. Galaxies at this epoch reside in a universe which is just 800 million years old, a mere 6% of its current age. As such, this work provides one account of the manner by which observational cosmology has recently shifted from merely marveling at the incredible lookback times implied by the first few high-redshift detections, to the routine assembly of high-redshift datasets designed to address specific astrophysical issues.…

Taschenbuch. Etat : Neu. Neuware - Is there a qualitative difference between memory functions in adults and children Given the relevance of memory processes for virtually every aspect of everyday life and the complex interactions between memory development and general cognitive abilities, surprisingly little is known about memory abilities in childhood. In four experiments with children aged 6-12 years and young adults, the developmental trajectories of two separate mechanisms underlying recognition memory - familiarity and recollection - were examined using event-related potentials (ERPs). Developmental changes were evident in both mechanisms of recognition memory. The data also revealed some surprising similarities across the life-span and emphasize the importance of strategic modulation of memory retrieval. Furthermore, the present investigation of memory development highlights age-related changes in cognitive processes and the maturation of the brain structures underlying these developmental changes. …