- TRIPPE, SASCHA
- Journal of the Korean astronomical society = 천문학회지
- 49, n.5
- pp.193-198
- 2016
- 원문 바로보기
Early-type galaxies (ETGs) are supposed to follow the virial relation <TEX>$M=k_e{\sigma}^2R_e/G$</TEX>, with M being the mass, σ<sub>*</sub> being the stellar velocity dispersion, R<sub>e</sub> being the effective radius, G being Newton's constant, and k<sub>e</sub> being the virial factor, a geometry factor of order unity. Applying this relation to (a) the ATLAS<sup>3D</sup> sample of Cappellari et al. (2013) and (b) the sample of Saglia et al. (2016) gives ensemble-averaged factors 〈k<sub>e</sub>〉 = 5.15 ± 0.09 and 〈k<sub>e</sub>〉 = 4.01 ± 0.18, respectively, with the difference arising from different definitions of effective velocity dispersions. The two datasets reveal a statistically significant tilt of the empirical relation relative to the theoretical virial relation such that <TEX>$M{\propto}({\sigma}^2_*R_e)^{0.92}$</TEX>. This tilt disappears when replacing R<sub>e</sub> with the semi-major axis of the projected half-light ellipse, a. All best-fit scaling relations show zero intrinsic scatter, implying that the mass plane of ETGs is fully determined by the virial relation. Whenever a comparison is possible, my results are consistent with, and confirm, the results by Cappellari et al. (2013). The difference between the relations using either a or R<sub>e</sub> arises from a known lack of highly elliptical high-mass galaxies; this leads to a scaling (1 - ϵ ) ∝ M<sup>0.12</sup>, with ϵ being the ellipticity and <TEX>$R_e=a\sqrt[]{1-{\epsilon}}$</TEX>. Accordingly, a, not R<sub>e</sub>, is the correct proxy for the scale radius of ETGs. By geometry, this implies that early-type galaxies are axisymmetric and oblate in general, in agreement with published results from modeling based on kinematics and light distributions.