Dynamics of companion galaxies of early-type galaxies

We estimated the dynamical masses of 115 early-type galaxies (ETGs) by analyzing the dynamics of satellite and companion galaxies of these ETGs. We selected galaxies with absolute magnitudes between −22 and −25 in the K_s-band from the Extragalactic Distance Database. We also selected 216 spiral galaxies for comparison. We employed a simple model to simulate the observed dynamical mass from satellite galaxies at various distances. Our simulations showed that the dynamical masses derived from satellite galaxies with elliptical orbits would be smaller than those with circular orbits even though they contain the same dark mass halos. Therefore, relationships between the observed M_dyn/M_b distributions and distances would depend on orbital shapes. From the relationships between our observed M_dyn/M_b distributions and distances, we suggest that the satellite galaxies of the ETGs have relatively more elliptical orbits than those of the spiral galaxies and the M_dyn/M_b of the ETGs are greater than those of the spiral galaxies.