Revisiting the Dichotomy of Early-type Galaxies

Abstract

We study the relationship among isophotal shapes, central light profiles and kinematic properties of early-type galaxies (ETGs) based on a compiled sample of 184 ETGs. These sample galaxies are included in the Data Release 8 of Sloan Digital Sky Survey (SDSS DR8) and have central light profiles and kinematic properties available from the literature, which were measured based on Hubble Space Telescope ({\it HST}) and \atlas\ integral-field spectrograph (IFS) observations, respectively. We find that there is only a weak correlation between the isophotal shape ($a_{4}/a$) and the central light profile (within $1\,\rm kpc$) of ETGs. About two-fifths of "core" galaxies have disky isophotes, while one-third of "power-law" galaxies are boxy deviated. Our statistical results also show that there are weak correlations between galaxy luminosity and dynamical mass with $a_{4}/a$, but such correlations are tighter with central light profile. Moreover, no clear link has been found between the isophotal shape and the S\' ersic index. Comparisons show that there are similar correlations between $a_{4}/a$ and ellipticity and between $a_{4}/a$ and specific angular momentum $\lambda_{R_e/2}$ for "power-law" ETGs, but there are no such correlations for "core" ETGs. Therefore, we speculate that the bimodal classifications for ETGs are not as simple as previously thought, though we also find that the most deviated disky ETGs are "power-law", more elongated and fast rotators.