51ÁÔÆæÈë¿Ú

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.

Abstract:

There is strong evidence for some kind of massive dark object in the centres of many galaxy bulges. The detection of flares from tidally disrupted stars could confirm that these objects are black holes (BHs). Here we present calculations of the stellar disruption rates in detailed dynamical models of real galaxies, taking into account the refilling of the loss cone of stars on disruptable orbits by two-body relaxation and tidal forces in non-spherical galaxies. The highest disruption rates (one star per 104 yr) occur in faint (L ≲ 1010L⊙) galaxies, which have steep central density cusps. More luminous galaxies are less dense and have much longer relaxation times and more massive BHs. Dwarf stars in such galaxies are swallowed whole by the BH and hence do not emit flares; giant stars could produce flares as often as every 105 yr, although the rate depends sensitively on the shape of the stellar distribution function. We discuss the possibility of detecting disruption flares in current supernova searches. The total mass of stars consumed over the lifetime of the galaxy is of the order of 106 M⊙, independent of galaxy luminosity; thus, disrupted stars may contribute significantly to the present BH mass in galaxies fainter than ∼ 109L⊙.