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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.