Dust and the infrared kinematic properties of early-type galaxies

We have obtained spectra and measured the stellar kinematics in a sample of 25 nearby early- type galaxies (with velocity dispersions from less than 100 km s(-1) to over 300 km s(-1)) using the near- infrared CO absorption band head at 2.29 mum. Our median uncertainty for the dispersions is similar to10%. We examine the effects of dust on existing optical kinematic measurements. We. nd that the near- infrared velocity dispersions are, in general, smaller than optical velocity dispersions, with di. erences as large as 30%. The median di. erence is 11% smaller, and the e. ect is of greater magnitude for higher dispersion galaxies. The lenticular galaxies (18 out of 25) appear to be causing the shift to lower dispersions, while the classical elliptical galaxies (7 out of 25) are consistent between the two wavelength regimes. If uniformly distributed dust causes these di. erences, we would expect to. nd a correlation between the relative amount of dust in a galaxy and the fractional change in dispersion, but we do not. nd such a correlation. We do see correlations between both velocity dispersion and CO band head equivalent width, and velocity dispersion and Mg-2 index. The di. erences in dispersion are not well explained by current models of dust absorption. The lack of correlation between the relative amount of dust and shift in dispersion possibly suggests that dust does not have a similar distribution from galaxy to galaxy. The CO equivalent widths of these galaxies are quite high (greater than or similar to10 Angstrom for almost all), requiring the light at these wavelengths to be dominated by very cool stars.