The Infrared Diameter-Velocity Dispersion Relation for Elliptical Galaxies

Abstract

Using single channel K -band photometric data from the literature, a provisional infrared diameter-velocity dispersion relation for elliptical galaxies in the Coma and Virgo clusters is derived. The Coma cluster relation has ~1.5 times lower scatter at K band than in B or V. Excluding 4 outliers, the rms scatter at K for 24 galaxies in Coma is only 4.8% in distance, close to the limit implied by the observational errors. Distance estimates based on the IR D - σ, relation will be more accurate than those derived from optical data. The improvement in the infrared is attributed to a decrease in sensitivity to stellar population parameters (age, metallicity, and slope of the IMF), as well as lower internal extinction from dust compared to the optical. That the D - σ, relation has a larger scatter in the optical indicates that there are detectable, but small, stellar population or dust content differences among the Coma ellipticals. Since the D -σ relations are based on the fundamental plane, this result promises that the fundamental plane is thinner in the infrared than it is in the optical. Infrared photometric data available for Virgo are limited to just 13 objects; the spread in distance due to the depth of the cluster precludes any significant improvement over B and V. A relative Coma-Virgo distance of 5.56 is derived from the K band data, in agreement with estimates in other colors and using other techniques, indicating that there is no significant age difference between Virgo and Coma ellipticals.