Evaluating galaxy dynamical masses from kinematics and jeans equilibrium in simulations. Issue 4 (22nd March 2021)

• Record Type: Journal Article
• Authors: Kretschmer, Michael; Dekel, Avishai; Freundlich, Jonathan; Lapiner, Sharon; Ceverino, Daniel; Primack, Joel
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 503:Issue 4(2021)
• Page Start: 5238
• Page End: 5253
• Publication Date: 2021-03-22
• Subjects: galaxies: evolution -- galaxies: formation -- galaxies: high-redshift -- galaxies: kinematics and dynamics; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stab833
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We provide prescriptions to evaluate the dynamical mass ( M dyn ) of galaxies from kinematic measurements of stars or gas using analytic considerations and the VELA suite of cosmological zoom-in simulations at z = 1–5. We find that Jeans or hydrostatic equilibrium is approximately valid for galaxies of stellar masses above M ⋆ ∼ 10 9.5 M⊙ out to 5 effective radii ( R e ). When both measurements of the rotation velocity v ϕ and of the radial velocity dispersion σ r are available, the dynamical mass $M_{\rm dyn} \!\simeq \! G^{-1} V_{\rm c}^2 r$, can be evaluated from the Jeans equation $V_{\rm c}^2= v_\phi ^2 + \alpha \sigma _{\rm r}^2$ assuming cylindrical symmetry and a constant, isotropic σ r . For spheroids, α is inversely proportional to the Sérsic index n and α ≃ 2.5 within R e, stars for the simulated galaxies. The prediction for a self-gravitating exponential disc, α = 3.36( r / R e ), is invalid in the simulations, where the dominant spheroid causes a weaker gradient from α ≃ 1 at R e, gas to 4 at 5 R e, gas . The correction in α for the stars due to the gradient in σ r ( r ) is roughly balanced by the effect of the aspherical potential, while the effect of anisotropy is negligible. When only the effective projected velocity dispersion σ l is available, the dynamical mass can be evaluated as $M_{\rm dyn} = K G^{-1} R_{\rm e} \sigma _{\rm l}^2$, where the virial factor K is derived from α, given the inclination and v ϕ / σ r . We find that the standard value K = 5 is approximately valid only when averaged over inclinations and for compact and thick discs, as it ranges from 4.5 to above 10 between edge-on and face-on projections.