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Observations of H2O Absorption and the Coolest Stellar Component of E and S0 Galaxies


Multiaperture observations of the H2O absorption feature near 2.0 microns are presented for the nuclei of 37 early-type galaxies, 5 globular clusters, and for a selection of stars. The H2O absorption is a sensitive function of effective temperature, and provides a strong constraint on the contribution of the very coolest stars to integrated galaxian light. In combination with the luminosity-sensitive CO absorption, the large H2O absorption found in galaxies indicates that at two microns a significant contribution from a stellar giant component at least as late as M5 in spectral type is present. For our limited data sample, the amount of H2O absorption shows no dependence on absolute magnitude for galaxies with MV leq - 20, and no dependence on projected aperture size in the range 1.0 geq A / D(0) geq 0.1. The observations are compared with recently published synthesis models of Tinsley and Gunn and of O'Connell; better agreement is found with the latter author's models. It does not appear that a significant contribution of carbon stars can account for the discrepancy between the infrared data and the models of Tinsley and Gunn.