Michele Cappellari

Abstract:

The stellar mass-to-light ratio gradient in SDSS r-band ∇(M_*/L_r) of a galaxy depends on its mass assembly history, which is imprinted in its morphology and gradients of age, metallicity, and stellar initial mass function (IMF). Taking a MaNGA sample of 2051 galaxies with stellar masses ranging from 10⁹ to 10¹²M_⊙ released in SDSS DR15, we focus on face-on galaxies, without merger and bar signatures, and investigate the dependence of the 2D ∇(M_*/L_r) on other galaxy properties, including M_*/L_r-colour relationships by assuming a fixed Salpeter IMF as the mass normalization reference. The median gradient is ∇M_*/L_r ∼ −0.1 (i.e. the M_*/L_r is larger at the centre) for massive galaxies, becomes flat around M_* ∼ 10¹⁰M_⊙ and change sign to ∇M_*/L_r ∼ 0.1 at the lowest masses. The M_*/L_r inside a half-light radius increases with increasing galaxy stellar mass; in each mass bin, early-type galaxies have the highest value, while pure-disc late-type galaxies have the smallest. Correlation analyses suggest that the mass-weighted stellar age is the dominant parameter influencing the M_*/L_r profile, since a luminosity-weighted age is easily affected by star formation when the specific star formation rate (sSFR) inside the half-light radius is higher than 10⁻³ Gyr⁻¹. With increased sSFR gradient, one can obtain a steeper negative ∇(M_*/L_r). The scatter in the slopes of M_*/L-colour relations increases with increasing sSFR, for example, the slope for post-starburst galaxies can be flattened to 0.45 from the global value 0.87 in the M_*/L versus g − r diagram. Hence converting galaxy colours to M_*/L should be done carefully, especially for those galaxies with young luminosity-weighted stellar ages, which can have quite different star formation histories.

Abstract:

We construct a dynamical model of the Milky Way disk from a data set that combines Gaia EDR3 and APOGEE data throughout galactocentric radii in the range 5.0 kpc ≤ R ≤ 19.5 kpc. We make use of the spherically aligned Jeans anisotropic method to model the stellar velocities and their velocity dispersions. Building upon our previous work, our model is now fitted to kinematic maps that have been extended to larger galactocentric radii due to the expansion of our data set, probing the outer regions of the Galactic disk. Our best-fitting dynamical model suggests a logarithmic density slope of αDM = −1.602 ± 0.079syst for the dark matter halo and a dark matter density of ρDM(R⊙) = (8.92 ± 0.56syst) × 10−3 M⊙ pc−3 (0.339 ± 0.022syst GeV cm3). We estimate a circular velocity at the solar radius of vcirc = (234.7 ± 1.7syst) km s−1 with a decline toward larger radii. The total mass density is ρtot(R⊙) = (0.0672 ± 0.0015syst) M⊙ pc−3 with a slope of αtot = −2.367 ± 0.047syst for 5 kpc ≤ R ≤ 19.5 kpc, and the total surface density is Σ(R⊙, ∣z∣ ≤ 1.1 kpc) = (55.5 ± 1.7syst) M⊙ pc−2. While the statistical errors are small, the error budget of the derived quantities is dominated by the three to seven times larger systematic uncertainties. These values are consistent with our previous determination, but the systematic uncertainties are reduced due to the extended data set covering a larger spatial extent of the Milky Way disk. Furthermore, we test the influence of nonaxisymmetric features on our resulting model and analyze how a flaring disk model would change our findings.