Dust and the Infrared Kinematic Properties of Early-Type Galaxies

Abstract

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 to over 300 km/s) using the near-infrared CO absorption bandhead at 2.29 microns. Our median uncertainty for the dispersions is ~10%. We examine the effects of dust on existing optical kinematic measurements. We find that the near-infrared velocity dispersions are in general smaller than optical velocity dispersions, with differences as large as 30%. The median difference is 11% smaller, and the effect 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 ellipticals (7 out of 25) are consistent between the two wavelength regimes. If uniformly distributed dust causes these differences, we would expect to find a correlation between the relative amount of dust in a galaxy and the fractional change in dispersion, but we do not find such a correlation. We do see correlations both between velocity dispersion and CO bandhead equivalent width, and velocity dispersion and Mg2 index. The differences 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 suggets that dust does not have a similar distribution from galaxy to galaxy. The CO equivalent widths of these galaxies are quite high (>10 angstroms for almost all), requiring the light at these wavelengths to be dominated by very cool stars.