The SAURON project--IV. The mass-to-light ratio, the virial mass estimator and the Fundamental Plane of elliptical and lenticular galaxies

Abstract

We investigate the well‐known correlations between the dynamical mass‐to‐light ratio (M/L) and other global observables of elliptical (E) and lenticular (S0) galaxies. We construct two‐integral Jeans and three‐integral Schwarzschild dynamical models for a sample of 25 E/S0 galaxies with SAURON integral‐field stellar kinematics to about one effective (half‐light) radius Re. They have well‐calibrated I‐band Hubble Space Telescope WFPC2 and large‐field ground‐based photometry, accurate surface brightness fluctuation distances, and their observed kinematics is consistent with an axisymmetric intrinsic shape. All these factors result in an unprecedented accuracy in the M/L measurements. We find a tight correlation of the form (M/L) = (3.80 ± 0.14) × (σe/200 km s−1)0.84±0.07 between the M/L (in the I band) measured from the dynamical models and the luminosity‐weighted second moment σe of the LOSVD within Re. The observed rms scatter in M/L for our sample is 18 per cent, while the inferred intrinsic scatter is ∼13 per cent. The (M/L)–σe relation can be included in the remarkable series of tight correlations between σe and other galaxy global observables. The comparison of the observed correlations with the predictions of the Fundamental Plane (FP), and with simple virial estimates, shows that the ‘tilt’ of the FP of early‐type galaxies, describing the deviation of the FP from the virial relation, is almost exclusively due to a real M/L variation, while structural and orbital non‐homology have a negligible effect. When the photometric parameters are determined in the ‘classic’ way, using growth curves, and the σe is measured in a large aperture, the virial mass appears to be a reliable estimator of the mass in the central regions of galaxies, and can be safely used where more ‘expensive’ models are not feasible (e.g. in high‐redshift studies). In this case the best‐fitting virial relation has the form (M/L)vir= (5.0 ± 0.1) ×Reσ2e/(LG), in reasonable agreement with simple theoretical predictions. We find no difference between the M/L of the galaxies in clusters and in the field. The comparison of the dynamical M/L with the (M/L)pop inferred from the analysis of the stellar population, indicates a median dark matter fraction in early‐type galaxies of ∼30 per cent of the total mass inside one Re, in broad agreement with previous studies, and it also shows that the stellar initial mass function varies little among different galaxies. Our results suggest a variation in M/L at constant (M/L)pop, which seems to be linked to the galaxy dynamics. We speculate that fast‐rotating galaxies have lower dark matter fractions than the slow‐rotating and generally more‐massive ones. If correct, this would suggest a connection between the galaxy assembly history and the dark matter halo structure. The tightness of our correlation provides some evidence against cuspy nuclear dark matter profiles in galaxies.

Initial Mass Function for Massive Galaxies at z ~ 1

“…assumptions are supported by results of realistic simulations carried out by Lablanche et al (2012) and previous works (Cappellari et al 2006;Cappellari 2008) that have shown that the errors introduced by the uncertainty in these quantities are insignificant compared to the errors on the stellar kinematics of our galaxies.…”

Comparison of simple mass estimators for slowly rotating elliptical galaxies

“…Dynamical modeling using the orbitsuperposition method is considered to be the state-of-theart technique and allows one to recover both the galaxy gravitational potential and the orbital structure with an accuracy of 15% (Thomas et al 2005;Krajnovic et al 2005). Schwarzschild modeling is now widely used for mass measurements of supermassive black holes, for determination of the total mass profile and its decomposition into luminous and dark matter components as well as for constraining the orbital structure (e.g., Gebhardt et al 2003;Cappellari et al 2006; Thomas et al 2007bThomas et al , 2011McConnell et al 2012McConnell et al , 2013Rusli et al 2013). As such an approach only makes sense with high quality observational data allowing the determination of the high order lineof-sight velocity moments (namely, in addition to velocity and projected velocity dispersion also the third and the fourth order Gauss-Hermite moments (e.g., Gerhard 1993;van der Marel and Franx 1993) or better yet the complete line-of-sight velocity distribution), it can only be applied to nearby galaxies.…”

The Kinematics of Core and Cusp Galaxies: Comparing HST Imaging and Integral-Field Observations

“…2, we show the same trends but now as a function of the total mass of the galaxies. We have derived the mass values assuming it approximates to the virial mass derived from the best-fitting M vir − σ relation presented in [1]. The figure displays a trend with λ R such that smaller values are found in more massive galaxies.…”