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We present the analytical framework for converting projected light distributions with a Sérsic profile into three-dimensional light distributions for stellar systems of arbitrary triaxial shape. The main practical result is the definition of a simple yet robust measure of intrinsic galaxy size: the median radius rmed, defined as the radius of a sphere that contains 50% of the total luminosity or mass, that is, the median distance of a star to the galaxy center. We examine how rmed depends on projected size measurements as a function of Sérsic index and intrinsic axis ratios, and demonstrate its relative independence of these parameters. As an application we show that the projected semimajor axis length of the ellipse enclosing 50% of the light is an unbiased proxy for rmed, with small galaxy-to-galaxy scatter of ∼10% (1σ), under the condition that the variation in triaxiality within the population is small. For galaxy populations with an unknown or a large range in triaxiality an unbiased proxy for rmed is 1.3 × Re, where Re is the circularized half-light radius, with galaxy-to-galaxy scatter of 20%–30% (1σ). We also describe how inclinations can be estimated for individual galaxies based on the measured projected shape and prior knowledge of the intrinsic shape distribution of the corresponding galaxy population. We make the numerical implementation of our calculations available. [ABSTRACT FROM AUTHOR]

We use the stellar kinematics for 2458 galaxies from the MaNGA survey to explore a universal fundamental plane (UFP) described by the equation , defined by the logarithms of effective radius, Re; the surface brightness within Re, Ie; the dynamical mass-to-light ratio within Re, Ye; and the total velocity parameter at Re, , where is the rotation velocity and is the velocity dispersion. The surface brightness is within Re, Ie, and the dynamical mass-to-light ratio is within Re, ϒe. We use sophisticated Schwarzschild dynamical models for a subsample of 300 galaxies from the CALIFA survey to calibrate the universal fundamental plane. This calibration allows us to propose both (i) a parameterization to estimate the difficult-to-measure averaged dynamical mass-to-light ratio within Re, , once the internal kinematics, surface brightness, and effective radius are known; and (ii) a new, more robust dynamical mass proxy consistent with dynamical models within 0.09 dex. We reproduce the relation between the dynamical mass and the stellar mass in the inner regions of galaxies with lower scatter. We use the estimated from our analysis to explore the UFP with the MaNGA data set. We find that all galaxies, from spheroids to disks, follow the UFP with a scatter significantly smaller (0.05 dex) than the one reported for the fundamental plane (∼0.09 dex) and comparable with Tully–Fisher studies (∼0.05 dex), but for a wider range of galaxy types. We also confirm that spheroidal and spiral galaxies follow the same M⋆–S0.5 scaling relation, with lower scatter than the and ones, which is in agreement with previous studies found in the literature. [ABSTRACT FROM AUTHOR]

To constrain the Galactic gravitational potential near the Sun (~1.5 kpc),we derive and model the spatial and velocity distributions for a sample of 9000 K-dwarfs with spectra from SDSS/SEGUE, which yield radial velocities and abundances ([Fe/H] and [α/Fe]). We first derive the spatial density distribution for three abundance-selected subpopulations of stars accounting for the survey's selection function. The vertical profiles of these sub-populations are simple exponentials and their vertical dispersion profile is nearly isothermal. To model these data, we apply the "vertical" Jeans equation, which relates the observable tracer number density and vertical velocity dispersion to the gravitational potential or vertical force. We explore a number of functional forms for the vertical force law, fit the dispersion and density profiles of all abundance-selected sub-populations simultaneously in the same potential, and explore all parameter co-variances using a Markov Chain Monte Carlo technique. Our fits constrain a disk mass scale height ≲ 300 pc and the total surface mass density to be 67 ± 6 M⊙ pc-2 at ∣z∣ = 1.0 kpc of which the contribution from all stars is 42 ± 5 M⊙ pc-2 (assuming a contribution from cold gas of 13 M⊙ pc-2). We find significant constraints on the local darkmatter density of 0.0065±0.0023 M⊙ pc-3 (0.25±0.09 GeV cm-3). Together with recent experiments this firms up the best estimate of 0.0075 ± 0.0021 M⊙ pc-3 (0.28 ± 0.08 GeV cm-3), consistent with global fits of approximately round dark matter halos to kinematic data in the outskirts of the Galaxy. [ABSTRACT FROM AUTHOR]

Using data from the Wide-field Infrared Survey Explorer mission, we have measured near infra-red (NIR) photometry of a diverse sample of dust-free stellar systems (globular clusters, dwarf and giant early-type galaxies) which have metallicities that span the range -2.2 < [Fe/H] (dex) < 0.3. This dramatically increases the sample size and broadens the metallicity regime over which the 3.4 (W1) and 4.6 μm (W2) photometry of stellar populations have been examined. We find that the W1 – W2 colors of intermediate and old (>2 Gyr) stellar populations are insensitive to the age of the stellar population, but that the W1 – W2 colors become bluer with increasing metallicity, a trend not well reproduced by most stellar population synthesis (SPS) models. In common with previous studies, we attribute this behavior to the increasing strength of the CO absorption feature located in the 4.6 μm bandpass with metallicity. Having used our sample to validate the efficacy of some of the SPS models, we use these models to derive stellar mass-to-light ratios in the W1 and W2 bands. Utilizing observational data from the SAURON and ATLAS3D surveys, we demonstrate that these bands provide extremely simple, yet robust stellar mass tracers for dust free older stellar populations that are freed from many of the uncertainties common among optical estimators. [ABSTRACT FROM AUTHOR]