Galaxy formation and evolution

Abstract:

The optical backscatter of the 4W CW laser used to produce a mesospheric sodium-layer laser guide star for the MPE adaptive optics system (ALFA) has been observed from a neighbouring 2.2 m telescope. The observations, taken at the Max Planck Observatory in Calar-Alto (Spain), in August 1998, had two aims: study the Na plume (altitude and profile variations) and the Rayleigh cone to achieve Rayleigh scattering measurements. In the framework of the network, 'Laser Guide Star for 8m class telescopes', a program of the European Commission, ESO, MPE and NUI, Galway are collaborating on studying the light pollution due to the MPE ALFA laser. The light pollution of the observatory is due to Rayleigh and Mie scattering, including water scattering, of the laser beam in the low atmosphere. With the increase of Laser Guide Star Adaptive Optics system use, there is a need for a laser management policy. In order to optimise observing time, it is important to evaluate exactly when to stop the LGS, as the line of sight of one telescope gets near the laser beam, and as a function of other telescope observing programme. In this perspective, not only the single and multiple Rayleigh scattering by atmospheric molecules but also Mie scattering by aerosols has to be taken into account. Modeling of these phenomena needs an experimental calibration in realistic circumstances, and precise measurements of single and multiple light scattering in the surroundings of the beam at various altitudes. We report here on the result of such experiments.

Abstract:

We present the results of a long-slit K-band spectroscopic search for Hα emission from eight damped Lyα absorbers (DLAs) at z > 2 with the goal of measuring the star formation rates in these systems. For each system we searched for compact sources of Hα emission within a solid angle 11 × 2.5 arcsec2 (44 × 10 h-2 kpc2, for q0 = 0.5, H0 = 100 h km s-1 Mpc-1). No Ha emission was detected above 3σ limits in the range (6.5-16) × 10-20 W m-2, equivalent to star formation rates of 5.6-18 h-2 M⊙ yr-1, for a standard initial mass function, assuming the lines are spectrally unresolved (<650km s-1 FWHM). We compare these results against the predictions of the models of Pei & Fall of the global history of star formation, under two different simplifying hypotheses: (i) the space density of DLAs at z = 2.3 is equal to the space density of spiral galaxies today (implying DLA discs were larger in the past, the 'large-disc' hypothesis); (ii) the sizes of DLAs at z = 2.3 were the same as the gas sizes of spiral galaxies today (implying DLA discs were more common in the past, the 'hierarchical' hypothesis). Compared with the previous most sensitive spectroscopic search, our sample is twice as large, our limits are a factor greater than two deeper, and the solid angle surveyed is over three times as great. Despite this, our results are not in conflict with either the large-disc hypothesis, because of the limited solid angle covered by the slit, or the hierarchical hypothesis, because of the limited sensitivity.