[Paper Review] MaNGA DynPop -- III. Stellar dynamics versus stellar population relations in 6000 early-type and spiral galaxies: Fundamental Plane, mass-to-light ratios, total density slopes, and dark matter fractions

Kai Zhu, Shengdong Lu+5 more|arXiv (Cornell University)|Apr 23, 2023

Galaxies: Formation, Evolution, Phenomena152 references4 citations

TL;DR

This study presents dynamical scaling relations for 6,000 nearby early- and late-type galaxies from the MaNGA survey, linking mass, size, and velocity dispersion via the mass plane. It reveals a parabolic M/L–σ relation driven by dark matter at low σ, with total density slopes steepening at high σ and median dark matter fractions of 8% rising to 33% at σ < 100 km s⁻¹.

ABSTRACT

We present dynamical scaling relations, combined with the stellar population properties, for a subsample of about 6000 nearby galaxies with the most reliable dynamical models extracted from the full MaNGA sample of 10K galaxies. We show that the inclination-corrected mass plane (MP) for both early-type galaxies (ETGs) and late-type galaxies (LTGs), which links dynamical mass, projected half-light radius $R_{ m e}$, and the second stellar velocity moment $σ_{ m e}$ within $R_{ m e}$, satisfies the virial theorem and is even tighter than the uncorrected one. We find a clear parabolic relation between $\lg(M/L)_{ m e}$, the total mass-to-light ratio within a sphere of radius $R_{ m e}$, and $\lgσ_{ m e}$, with the $M/L$ increasing with $σ_{ m e}$ and for older stellar populations. However, the relation for ETGs is linear and the one for the youngest galaxies is constant. We confirm and improve the relation between average logarithmic total density slopes $\overline{γ_{ m T}}$ and $σ_{ m e}$: $\overline{γ_{ m T}}$ become steeper with increasing $σ_{ m e}$ until $\lg(σ_{ m e}/{ m km\,s^{-1}})\approx 2.2$ and then remain constant around $\overline{γ_{ m T}}\approx -2.2$. The $\overline{γ_{ m T}}-σ_{ m e}$ variation is larger for LTGs than ETGs. At fixed $σ_{ m e}$ the total density profiles steepen with galaxy age and for ETGs. We find generally low dark matter fractions, median $f_{ m DM}(

Motivation & Objective

• To establish tight dynamical scaling relations—specifically the mass plane—for a large, diverse sample of 6,000 nearby galaxies using spatially resolved kinematics from MaNGA.
• To investigate the interplay between stellar dynamics (e.g., mass-to-light ratios, density slopes) and stellar population properties (e.g., age, metallicity) across galaxy types.
• To quantify dark matter fractions within the effective radius (Rₑ) and assess their dependence on kinematic and stellar population parameters.
• To test the robustness of dynamical models under varying assumptions, including spatially varying stellar mass-to-light ratios.
• To interpret observed trends in terms of galaxy formation scenarios involving bulge growth and dry mergers.

Proposed method

• Constructed dynamical models using spatially resolved stellar velocity dispersions from MaNGA integral field spectroscopy, applying the Jeans Anisotropy Modelling (JAM) method to derive total mass and mass-to-light ratios (M/L) within Rₑ.
• Calculated the inclination-corrected mass plane using dynamical mass, projected half-light radius (Rₑ), and second-moment velocity dispersion (σₑ), confirming its tightness and virial consistency.
• Derived total density slopes (γ_T) from the radial dependence of the mass profile, using the JAM model and comparing results with NFW and gNFW dark matter halo profiles.
• Computed dark matter fractions f_DM(<Rₑ) by comparing total mass from dynamics with stellar mass from stellar population synthesis (SPS) models, incorporating M/L gradients from Paper II.
• Used LOESS smoothing and statistical fitting to model the relations between M/L, σₑ, γ_T, f_DM, and stellar population parameters, accounting for measurement uncertainties and sample selection.
• Explored evolutionary interpretations by mapping dynamical properties on the M_JAM–Rₑ^maj plane, proposing two evolutionary channels: bulge growth and dry mergers.

Experimental results

Research questions

• RQ1 How do the mass plane and fundamental plane relations behave when using dynamical mass instead of luminosity, and how does inclination correction affect their tightness?
• RQ2 What is the nature of the relation between total mass-to-light ratio (M/L)ₑ and velocity dispersion σₑ, and how is it influenced by stellar population age?
• RQ3 How do total density slopes (γ_T) vary with σₑ, and what is the role of galaxy type and age in shaping this relation?
• RQ4 What is the dependence of dark matter fraction f_DM(<Rₑ) on σₑ and stellar mass, and how does it vary across galaxy types?
• RQ5 To what extent do assumptions about spatially varying stellar M/L ratios affect the inferred dark matter fractions and dynamical relations?

Key findings

• The inclination-corrected mass plane for both early- and late-type galaxies is tighter than the uncorrected version and satisfies the virial theorem.
• A parabolic relation exists between log(M/L)ₑ and log(σₑ), with M/L increasing at high and low σₑ, driven by rising dark matter fractions at low σₑ.
• The total density slope γ_T increases with σₑ up to log(σₑ/km s⁻¹) ≈ 2.2, after which it saturates at ≈2.2, with steeper variations in late-type galaxies.
• Median dark matter fraction within Rₑ is 8%, but rises to 33% for galaxies with σₑ < 100 km s⁻¹, explaining the M/L–σₑ parabolic trend.
• Including spatially varying stellar M/L gradients increases f_DM(<Rₑ) by ~7% (NFW) and ~13% (gNFW), but does not qualitatively alter the f_DM–M* relation.
• Dynamical properties on the M_JAM–Rₑ^maj plane are consistent with two evolutionary channels: bulge growth (increasing σₑ, steepening γ_T, reducing f_DM) and dry mergers (increasing M_JAM, decreasing λ_Rₑ, flattening γ_T, constant f_DM).