Optical and near-infrared velocity dispersions of early-type galaxies?

We have carried out a systematic, homogeneous comparison of optical and near-infrared dis-persions. Our magnitude-limited sample of early-type galaxies in the Fornax cluster comprises 11 elliptical and 11 lenticular galaxies more luminous than MB = −17. We were able to de-termine the central dispersions based on the near-infrared CO absorption band head for 19 of those galaxies. The velocity dispersions range from less than 70 km s−1 to over 400 km s−1. We compare our near-infrared velocity dispersions to the optical dispersions measured by Kuntschner (2000). Contrary to previous studies, we find a one-to-one correspondence with a median fractional difference of 6.4%. We examine the correlation between the relative dust mass and the fractional difference of the velocity dispersions, but find no significant trend. Our results suggest that early-type galaxies are largely optically thin, which is consistent with recent Herschel observations