http://arxiv.org/abs/1609.07188
Early-type galaxies (ETGs) are supposed to follow the virial relation $M = k_e \sigma_*^2 R_e / G$, with $M$ being the mass, $\sigma_*$ being the stellar velocity dispersion, $R_e$ being the effective radius, $G$ being Newton’s constant, and $k_e$ being the virial factor, a geometry factor of order unity. Applying this relation to (a) the \atlas\ sample of \citet{cappellari2013a} and (b) the sample of \cite{saglia2016} gives ensemble-averaged factors $\langle k_e\rangle =5.15\pm0.09$ and $\langle k_e\rangle =4.01\pm0.18$, 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 $M\propto(\sigma_*^2R_e)^{0.92}$. This tilt disappears when replacing $R_e$ 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. The difference between the relations using either $a$ or $R_e$ arises from a known lack of highly elliptical high-mass galaxies; this leads to a scaling $(1-\epsilon) \propto M^{0.12}$, with $\epsilon$ being the ellipticity and $R_e = a\sqrt{1-\epsilon}$. Accordingly, $a$, not $R_e$, 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.
S. Trippe
Mon, 26 Sep 16
13/48
Comments: 5 pages, 3 figures; accepted by JKAS
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